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2023

A Statistical Analysis of the Auroral Streamer Current System

Homayon Aryan, James Michael Weygand, Jacob Bortnik, and 3 more authors

Journal of Geophysical Research: Space Physics, 2023

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Magnetosphere-ionosphere coupling is an important process that involves the transport of vast quantities of mass, energy, and momentum in the Earth’s near-space environment. This transport involves extended, global sheets of electrical current that flows along the magnetic field lines. Many studies have discussed the magnetosphere-ionosphere coupling between the fast-moving earthward flows and the north-south oriented streamers in the ionosphere. However, only recently, the temporal evolution of the ionospheric current pattern, from the start to the end of the streamer, has been investigated through case studies, by combining the ionospheric current pattern with the magnetotail fast-moving earthward flows. In this study, we show the statistical spatial and temporal evolution of the ionospheric current pattern associated with north-south oriented streamers using simultaneous THEMIS (Time History of Events and Macroscale Interaction during Substorms) observations of fast-moving earthward flows within the magnetotail, Geostationary Operational Environmental Satellite observations, auroral all-sky images, and ionospheric equivalent current maps derived from ground magnetometer measurements. We show that the streamer related ionospheric field-aligned currents (FACs) are statistically almost equivalent in magnitude for the 38 events that were analyzed. Results show that the streamer related ionospheric FACs are statistically approximately similar in magnitude for 38 events. The duration of these events ranges from 2 to 21 min with a median value of 9 min.
Distribution and Evolution of Chorus Waves Modeled by a Neural Network: The Importance of Imbalanced Regression

Xiangning Chu, Jacob Bortnik, Wen Li, and 5 more authors

Space Weather, 2023

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Whistler-mode chorus waves play an essential role in the acceleration and loss of energetic electrons in the Earth’s inner magnetosphere, with the more intense waves producing the most dramatic effects. However, it is challenging to predict the amplitude of strong chorus waves due to the imbalanced nature of the data set, that is, there are many more non-chorus data points than strong chorus waves. Thus, traditional models usually underestimate chorus wave amplitudes significantly during active times. Using an imbalanced regressive (IR) method, we develop a neural network model of lower-band (LB) chorus waves using 7-year observations from the EMFISIS instrument onboard Van Allen Probes. The feature selection process suggests that the auroral electrojet index alone captures most of the variations of chorus waves. The large amplitude of strong chorus waves can be predicted for the first time. Furthermore, our model shows that the equatorial LB chorus’s spatiotemporal evolution is similar to the drift path of substorm-injected electrons. We also show that the chorus waves have a peak amplitude at the equator in the source MLT near midnight, but toward noon, there is a local minimum in amplitude at the equator with two off-equator amplitude peaks in both hemispheres, likely caused by the bifurcated drift paths of substorm injections on the dayside. The IR-based chorus model will improve radiation belt prediction by providing chorus wave distributions, especially storm-time strong chorus. Since data imbalance is ubiquitous and inherent in space physics and other physical systems, imbalanced regressive methods deserve more attention in space physics.
Evaluating the performance of empirical models of total electron density and whistler-mode wave amplitude in the Earth’s inner magnetosphere

Qianli Ma, Xiangning Chu, Donglai Ma, and 4 more authors

Frontiers in Astronomy and Space Sciences, 2023

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Empirical models have been previously developed using the large dataset of satellite observations to obtain the global distributions of total electron density and whistler-mode wave power, which are important in modeling radiation belt dynamics. In this paper, we apply the empirical models to construct the total electron density and the wave amplitudes of chorus and hiss, and compare them with the observations along Van Allen Probes orbits to evaluate the model performance. The empirical models are constructed using the Hp30 and SME (or SML) indices. The total electron density model provides an overall high correlation coefficient with observations, while large deviations are found in the dynamic regions near the plasmapause or in the plumes. The chorus wave model generally agrees with observations when the plasma trough region is correctly modeled and for modest wave amplitudes of 10–100 pT. The model overestimates the wave amplitude when the chorus is not observed or weak, and underestimates the wave amplitude when a large-amplitude chorus is observed. Similarly, the hiss wave model has good performance inside the plasmasphere when modest wave amplitudes are observed. However, when the modeled plasmapause location does not agree with the observation, the model misidentifies the chorus and hiss waves compared to observations, and large modeling errors occur. In addition, strong (\textgreater200 pT) hiss waves are observed in the plumes, which are difficult to capture using the empirical model due to their transient nature and relatively poor sampling statistics. We also evaluate four metrics for different empirical models parameterized by different indices. Among the tested models, the empirical model considering a plasmapause and controlled by Hp* (the maximum Hp30 during the previous 24 h) and SME* (the maximum SME during the previous 3 h) or Hp* and SML has the best performance with low errors and high correlation coefficients. Our study indicates that the empirical models are applicable for predicting density and whistler-mode waves with modest power, but large errors could occur, especially near the highly-dynamic plasmapause or in the plumes.
The Response of Ionospheric Currents to External Drivers Investigated Using a Neural Network-Based Model

Xin Cao, Xiangning Chu, Jacob Bortnik, and 4 more authors

Space Weather, 2023

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A predictive model for the variation of ionospheric currents is of great scientific and practical importance to our modern industrial society. To study the response of ionospheric currents to external drivers including geomagnetic indices and solar radiation, we developed a feedforward neural network model trained on the Equivalent Ionospheric Current (EIC) data from 1st January 2007 to 31st December 2019. Due to the highly imbalanced nature of the ionospheric currents data, which means that the data of extreme events are much less than those of quiet times, we utilized different loss functions to improve the model performance. Our model demonstrates the potential to predict the active events of ionospheric currents reasonably well (e.g., EICs during substorms) within a timescale of a few minutes. Although the data used for training are measurements over the North American and Greenland sectors, our model is not only able to predict EICs within this region, but is also able to provide a promising out-of-sample prediction on a global scale.
Deep learning model of hiss waves in the plasmasphere and plumes and their effects on radiation belt electrons

Sheng Huang, Wen Li, Qianli Ma, and 7 more authors

Frontiers in Astronomy and Space Sciences, 2023

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Hiss waves play an important role in removing energetic electrons from Earth’s radiation belts by precipitating them into the upper atmosphere. Compared to plasmaspheric hiss that has been studied extensively, the evolution and effects of plume hiss are less understood due to the challenge of obtaining their global observations at high cadence. In this study, we use a neural network approach to model the global evolution of both the total electron density and the hiss wave amplitudes in the plasmasphere and plume. After describing the model development, we apply the model to a storm event that occurred on 14 May 2019 and find that the hiss wave amplitude first increased at dawn and then shifted towards dusk, where it was further excited within a narrow region of high density, namely, a plasmaspheric plume. During the recovery phase of the storm, the plume rotated and wrapped around Earth, while the hiss wave amplitude decayed quickly over the nightside. Moreover, we simulated the overall energetic electron evolution during this storm event, and the simulated flux decay rate agrees well with the observations. By separating the modeled plasmaspheric and plume hiss waves, we quantified the effect of plume hiss on energetic electron dynamics. Our simulation demonstrates that, under relatively quiet geomagnetic conditions, the region with plume hiss can vary from L = 4 to 6 and can account for up to an 80% decrease in electron fluxes at hundreds of keV at L \textgreater 4 over 3 days. This study highlights the importance of including the dynamic hiss distribution in future simulations of radiation belt electron dynamics.
Machine Learning Interpretability of Outer Radiation Belt Enhancement and Depletion Events

Donglai Ma, Jacob Bortnik, Qianli Ma, and 2 more authors

2023

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We investigate the response of outer radiation belt electron fluxes to different solar wind and geomagnetic indices using an interpretable machine learning method. We reconstruct the electron flux variation during 19 enhancement and 7 depletion events and demonstrate a feature attribution analysis on the superposed epoch results for the first time. We find that the intensity and duration of the substorm sequence following an initial dropout determine the overall enhancement or depletion of electron fluxes, while the solar wind pressure drives the initial dropout in both types of events. Further statistical results from a dataset with 71 events confirm this and show a significant correlation between the resulting flux levels and the average AL index, indicating that the observed "depletion" event can be more accurately described as a "non-enhancement" event. Our novel SHAP-Enhanced Superposed Epoch Analysis (SHESEA) method can be used as an insight discovery tool in various physical systems.
Cold ion heating in Earth’s magnetosphere

Maria Usanova, Gian Luca Delzanno, Xiangning Chu, and 16 more authors

Bulletin of the AAS, 2023

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Whitepaper #405 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: space weather applications; basic research. Additional topics: space weather research/operations/research loop; system science; space-based missions/projects; […]
Science Return of Probing Magnetospheric Systems of Ice Giants

Xin Cao, Xiangning Chu, Hsiang-Wen Hsu, and 31 more authors

Bulletin of the AAS, 2023

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Whitepaper #045 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: basic research. Additional topics: planetary magnetospheres; system science.
Modeling Ring Current Proton Fluxes Using Artificial Neural Network and Van Allen Probe Measurements

Jinxing Li, Jacob Bortnik, Xiangning Chu, and 5 more authors

Space Weather, 2023

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Terrestrial ring current dynamics are a critical part of the near-space environment, in that they directly drive geomagnetic field variations that control particle drifts, and define geomagnetic storms. The present study aims to specify a global and time-varying distribution of ring current proton using geomagnetic indices and solar wind parameters with their history as input. We train an artificial neural network (ANN) model to reproduce proton fluxes measured by the Radiation Belt Storm Probes Ion Composition Experiment instrument onboard Van Allen Probes. By choosing optimal feature parameters and their history length, the model results show a high correlation and a small error between model specifications and satellite measurements. The modeled results well capture energy-dependent proton dynamics in association with geomagnetic storms, including inward radial diffusion, acceleration and decay. Our ANN model produces proton fluxes with their corresponding 3D spatiotemporal variations, capturing the latitudinal distribution and local time asymmetry that are consistent with observations and that can further inform theory.
Opening the Black Box of the Radiation Belt Machine Learning Model

Donglai Ma, Jacob Bortnik, Xiangning Chu, and 3 more authors

Space Weather, 2023

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Many Machine Learning (ML) systems, especially deep neural networks, are fundamentally regarded as black boxes since it is difficult to fully grasp how they function once they have been trained. Here, we tackle the issue of the interpretability of a high-accuracy ML model created to model the flux of Earth’s radiation belt electrons. The Outer RadIation belt Electron Neural net (ORIENT) model uses only solar wind conditions and geomagnetic indices as input features. Using the Deep SHAPley additive explanations (DeepSHAP) method, for the first time, we show that the “black box” ORIENT model can be successfully explained. Two significant electron flux enhancement events observed by Van Allen Probes during the storm interval of 17–18 March 2013 and non-storm interval of 19–20 September 2013 are investigated using the DeepSHAP method. The results show that the feature importance calculated from the purely data-driven ORIENT model identifies physically meaningful behavior consistent with current physical understanding. This work not only demonstrates that the physics of the radiation belt was captured in the training of our previous model, but that this method can also be applied generally to other similar models to better explain the results and to potentially discover new physical mechanisms.
Modeling of the cold electron plasma density for radiation belt physics

J.-F. Ripoll, V. Pierrard, G. S. Cunningham, and 8 more authors

Frontiers in Astronomy and Space Sciences, 2023

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This review focusses strictly on existing plasma density models, including ionospheric source models, empirical density models, physics-based and machine-learning density models. This review is framed in the context of radiation belt physics and space weather codes. The review is limited to the most commonly used models or to models recently developed and promising. A great variety of conditions is considered such as the magnetic local time variation, geomagnetic conditions, ionospheric source regions, radial and latitudinal dependence, and collisional vs. collisionless conditions. These models can serve to complement satellite observations of the electron plasma density when data are lacking, are for most of them commonly used in radiation belt physics simulations, and can improve our understanding of the plasmasphere dynamics.
Follow the mass: the Science Case for Transformational Multi-scale Observations of Mass and Energy Flow Dynamics in Earth’s Magnetosphere

David Malaspina, Robert Ergun, Jerry Goldstein, and 31 more authors

Bulletin of the AAS, 2023

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Whitepaper #261 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: basic research. Additional topics: planetary magnetospheres; space-based missions/projects; system science.

2022

Can We Forecast And Detect Earthquakes From Heterogeneous Multivariate Time Series Data?

Luke Cullen, Asadullah Galib, Andrew Smith, and 5 more authors

2022

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Earthquake forecasting is a topic of utmost societal importance, yet has represented one of the greatest challenges to date. Case studies from the past show that seismic activity may lead to changes in the local geomagnetic and ionospheric field, which may operate as potential precursors and postcursors to large-magnitude earthquakes. However, detailed and data-driven research has yet to support the existence of precursors and postcursors. This work makes an attempt to build data-driven deep learning networks that can learn the temporal changes in geo-physical phenomena before and after large magnitude earthquake events. First, we do numerous experiments using various machine learning and deep learning models, but none of them are sufficiently generalizable to forecast earthquakes from potential precursors. Our negative findings may make sense as there is not any conclusive and comprehensive evidence yet supporting the existence of earthquake precursors. We, therefore consider detecting earthquakes from postcursors data to spot potential pitfalls and outline the scope of possibility. Our tests indicate that while detecting earthquakes from postcursor data might be promising, it would fall short. Poor performance could be brought on by a lack of data and extremely complex relationships. However, we are leaving room for future research with deeper networks and data augmentation.
Unraveling the Critical Geomagnetic Conditions Controlling the Upper Limit of Electron Fluxes in the Earth’s Outer Radiation Belt

Man Hua, Jacob Bortnik, Xiangning Chu, and 2 more authors

Geophysical Research Letters, 2022

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We investigate the statistical distribution of the upper limit of outer radiation belt electron fluxes during active times using 5-year measurements from Van Allen Probes. The upper flux limit at different energies peaks around L ∼ 4.7, with larger flux variation at higher energies \textgreater1 MeV compared to energies at hundreds of keV. Based on the correlation analysis between maximum fluxes and various time-integrated geomagnetic indices, including SYM-H, AE, and AL, we reveal that maximum fluxes strongly correlate to cumulative effects of substorms instead of storms, with the strongest dependence on the time-integrated AL (Int(AL)). We investigate the correlation coefficients (CC) between maximum fluxes and time-integrated substorm indices at different time lags and identify the time lag corresponding to the best CC. Finally, we develop a prediction model of maximum fluxes using only Int(AL) as input, which shows CC of ∼0.80 and allows future forecasts of extreme cases of outer belt electrons.
The Space Physics Environment Data Analysis System in Python

Eric W. Grimes, Bryan Harter, Nick Hatzigeorgiu, and 13 more authors

Frontiers in Astronomy and Space Sciences, 2022

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In this article, we describe the free, open-source Python-based Space Physics Environment Data Analysis System (PySPEDAS), a platform for multi-mission, multi-instrument retrieval, analysis, and visualization of Heliophysics data. PySPEDAS currently contains load routines for data from 23 space missions, as well as a variety of data from ground-based observatories. The load routines are built from a common set of general routines that provide access to datasets in different ways (e.g., downloading and caching CDF files or accessing data hosted on web services), making the process of adding additional datasets simple. In addition to load routines, PySPEDAS contains numerous analysis tools for working with the dataset once it is loaded. We describe how these load routines and analysis tools are built by utilizing other free, open-source Python projects (e.g., PyTplot, cdflib, hapiclient, etc.) to make tools for space and solar physicists that are extremely powerful, yet easy-to-use. After discussing the code in detail, we show numerous examples of code using PySPEDAS, and discuss limitations and future plans.
Magnetosphere-Ionosphere Coupling Between North-South Propagating Streamers and High-Speed Earthward Flows

J. M. Weygand, J. Bortnik, X. Chu, and 4 more authors

Journal of Geophysical Research: Space Physics, 2022

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The magnetosphere-ionosphere coupling between the high-speed earthward flows and the north-south oriented streamers in the ionosphere has been discussed for decades, but to date, no one has examined the formation of that coupling from the start of the streamer to the end. We investigate the formation and development of the magnetosphere-ionosphere coupling using THEMIS observations of high-speed earthward flows within the magnetotail, simultaneous auroral all-sky images, and ionospheric equivalent current maps derived from ground magnetometer measurements. We show the formation of a downward field-aligned current on the dawnside of the north-south streamer and the upward current on the duskside, as well as the vortices within the equivalent currents around the field-align-like currents from the poleward boundary to the equatorward edge of the auroral oval for two substorm streamer events. By removing the background Birkeland current system, we can determine the current densities uniquely associated with the streamer current wedge and demonstrate that the downward and upward currents within the streamer are approximately balanced for one event. Furthermore, we find that the longitudinal size of the streamer current wedge is more transient and localized, and does not change, whereas the substorm current wedge is larger and expands during the first part of the substorm.
Application of Recurrent Neural Network to Modeling Earth’s Global Electron Density

Sheng Huang, Wen Li, Xiao-Chen Shen, and 7 more authors

Journal of Geophysical Research: Space Physics, 2022

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The total electron density is a fundamental quantity in the Earth’s magnetosphere and plays an important role in a number of physical processes, but its dynamic global evolution is not fully quantified yet. We present an implementation of a specific type of recurrent neural network (encoder-decoder), which is distinct from previous models, to construct global electron density based on the multiyear data from Van Allen Probes. The history of geomagnetic indices is first encoded into a hidden state H, then together with auxiliary information (satellite location), they are decoded into the quantity of interest (total electron density in this study). In this process the input of historical geomagnetic indices is detangled from the satellite location and is processed chronologically by the encoder. As a result, time evolution of geomagnetic indices is explicitly embedded in the structure and the encoded hidden state H can be viewed as the representation of the inner magnetospheric state. The magnetospheric state is then decoded to predict global electron density evolution. Our results show that the model can capture the dynamical evolution of total electron density with the formation and evolution of stable and evident plume configurations that roughly agree with global observations. Our findings demonstrate the importance of applying recurrent neural networks to specify the inner magnetospheric state in a novel way, which will potentially improve our fundamental understanding of wave and particle dynamics in the Earth’s magnetosphere.
Multiple conjugate observations of magnetospheric fast flow bursts using THEMIS observations

Homayon Aryan, Jacob Bortnik, Jinxing Li, and 3 more authors

Annales Geophysicae, 2022

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Magnetotail earthward fast flow bursts can transport most magnetic flux and energy into the inner magnetosphere. These fast flow bursts are generally an order of magnitude higher than the typical convection speeds that are azimuthally localised (1–3 RE) and are flanked by plasma vortices, which map to ionospheric plasma vortices of the same sense of rotation. This study uses a multipoint analysis of conjugate magnetospheric and ionospheric observations to investigate the magnetospheric and ionospheric responses to fast flow bursts that are associated with both substorms and pseudobreakups. We study in detail what properties control the differences in the magnetosphere–ionosphere responses between substorm fast flow bursts and pseudobreakup events, and how these differences lead to different ionospheric responses. The fast flow bursts and pseudobreakup events were observed by the Time History of Events and Macroscale Interaction during Substorms (THEMIS), while the primary ionospheric observations were made by all-sky cameras and magnetometer-based equivalent ionospheric currents. These events were selected when the satellites were at least 6 RE from the Earth in radial distance and a magnetic local time (MLT) region of ± 5 h from local midnight. The results show that the magnetosphere and ionosphere responses to substorm fast flow bursts are much stronger and more structured compared to pseudobreakups, which are more likely to be localised, transient and weak in the magnetosphere. The magnetic flux in the tail is much stronger for strong substorms and much weaker for pseudobreakup events. The Blobe decreases significantly for substorm fast flow bursts compared to pseudobreakup events. The curvature force density for pseudobreakups are much smaller than substorm fast flow events, indicating that the pseudobreakups may not be able to penetrate deep into the inner magnetosphere. This association can help us study the properties and activity of the magnetospheric earthward flow vortices from ground data.
Modeling the Dynamic Variability of Sub-Relativistic Outer Radiation Belt Electron Fluxes Using Machine Learning

Donglai Ma, Xiangning Chu, Jacob Bortnik, and 6 more authors

Space Weather, 2022

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We present a set of neural network models that reproduce the dynamics of electron fluxes in the range of 50 keV ∼1 MeV in the outer radiation belt. The Outer Radiation belt Electron Neural net model for Medium energy electrons uses only solar wind conditions and geomagnetic indices as input. The models are trained on electron flux data from the Magnetic Electron Ion Spectrometer instrument onboard Van Allen Probes, and they can reproduce the dynamic variations of electron fluxes in different energy channels. The model results show high coefficient of determination (R2 ∼ 0.78–0.92) on the test data set, an out-of-sample 30-day period from 25 February to 25 March in 2017, when a geomagnetic storm took place, as well as an out-of-sample one year period after March 2018. In addition, the models are able to capture electron dynamics such as intensifications, decays, dropouts, and the Magnetic Local Time dependence of the lower energy (∼\textless100 keV) electron fluxes during storms. The models have reliable prediction capability and can be used for a wide range of space weather applications. The general framework of building our model is not limited to radiation belt fluxes and could be used to build machine learning models for a variety of other plasma parameters in the Earth’s magnetosphere.
Plasma Imaging, LOcal Measurement, and Tomographic Experiment (PILOT): A Mission Concept for Transformational Multi-Scale Observations of Mass and Energy Flow Dynamics in Earth’s Magnetosphere

David Malaspina, Robert Ergun, Jerry Goldstein, and 14 more authors

Frontiers in Astronomy and Space Sciences, 2022

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We currently do not understand the fundamental physical processes that govern mass and energy flow through the Earth’s magnetosphere. Knowledge of these processes is critical to understanding the mass loss rate of Earth’s atmosphere, as well as for determining the role that a planetary magnetic field plays in atmospheric retention, and therefore habitability, for Earth-like planets beyond the solar system. Mass and energy flow processes are challenging to determine at Earth in part because Earth’s planetary magnetic field creates a complex “system of systems” composed of interdependent plasma populations and overlapping spatial regions that perpetually exchange mass and energy across a broad range of temporal and spatial scales. Further, the primary mass carrier in the magnetosphere is cold plasma (as cold as ∼0.1 eV), which is invisible to many space-borne instruments that operate in the inner magnetosphere. The Plasma Imaging LOcal and Tomographic experiment (PILOT) mission concept, described here, provides the transformational multi-scale observations required to answer fundamental open questions about mass and energy flow dynamics in the Earth’s magnetosphere. PILOT uses a constellation of spacecraft to make radio tomographic, remote sensing, and in-situ measurements simultaneously, fully capturing cold plasma mass dynamics and its impact on magnetospheric systems over an unprecedented range of spatial and temporal scales. This article details the scientific motivation for the PILOT mission concept as well as a potential mission implementation.

2021

Relativistic Electron Model in the Outer Radiation Belt Using a Neural Network Approach

Xiangning Chu, Donglai Ma, Jacob Bortnik, and 10 more authors

Space Weather, 2021

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We present a machine-learning-based model of relativistic electron fluxes \textgreater1.8 MeV using a neural network approach in the Earth’s outer radiation belt. The Outer RadIation belt Electron Neural net model for Relativistic electrons (ORIENT-R) uses only solar wind conditions and geomagnetic indices as input. For the first time, we show that the state of the outer radiation belt can be determined using only solar wind conditions and geomagnetic indices, without any initial and boundary conditions. The most important features for determining outer radiation belt dynamics are found to be AL, solar wind flow speed and density, and SYM-H indices. ORIENT-R reproduces out-of-sample relativistic electron fluxes with a correlation coefficient of 0.95 and an uncertainty factor of ∼2. ORIENT-R reproduces radiation belt dynamics during an out-of-sample geomagnetic storm with good agreement to the observations. In addition, ORIENT-R was run for a completely out-of-sample period between March 2018 and October 2019 when the AL index ended and was replaced with the predicted AL index (lasp.colorado.edu/home/personnel/xinlin.li). It reproduces electron fluxes with a correlation coefficient of 0.92 and an out-of-sample uncertainty factor of ∼3. Furthermore, ORIENT-R captured the trend in the electron fluxes from low-earth-orbit (LEO) SAMPEX, which is a completely out-of-sample data set both temporally and spatially. In sum, the ORIENT-R model can reproduce transport, acceleration, decay, and dropouts of the outer radiation belt anywhere from short timescales (i.e., geomagnetic storms) and very long timescales (i.e., solar cycle) variations.
Reconstructing Substorms via Historical Data Mining: Is It Really Feasible?

N. A. Tsyganenko, V. A. Andreeva, M. I. Sitnov, and 4 more authors

Journal of Geophysical Research: Space Physics, 2021

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The evolution of the low-latitude magnetosphere over the substorm cycle is reconstructed based on a new high-resolution 3D representation of the magnetic field and nearest-neighbor data mining. The study covers radial distances 2.5–25 and employs a record-large pool of spacecraft data taken during 1995–2019. The magnetospheric state is quantified by four indices, representing the ground geomagnetic activity and its temporal trends in the entire range of geomagnetic latitude: the SuperMAG SMR, the midlatitude positive bay MPB, the auroral SML, and the polar cap PC index. The developed technique has been tested on specific substorm events, with the results presented in the form of 5-min cadence diagrams and animations of the magnetic field line configurations and electric current distributions. In all the analyzed events, the initial intensification and radial expansion of the inner tail current is accompanied by a gradual stretching of the magnetic field, followed by its sudden collapse, dramatic depletion of the current beyond , and a large-scale dipolarization of the field around the time of MPB peak, after which the system recovers and tends to its pre-substorm state.
Realistic Electron Diffusion Rates and Lifetimes Due to Scattering by Electron Holes

Yangyang Shen, Ivan Y. Vasko, Anton Artemyev, and 4 more authors

Journal of Geophysical Research: Space Physics, 2021

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Plasma sheet electron precipitation into the diffuse aurora is critical for magnetosphere-ionosphere coupling. Recent studies have shown that electron phase space holes can pitch-angle scatter electrons and may produce plasma sheet electron precipitation. These studies have assumed identical electron hole parameters to estimate electron scattering rates (Vasko et al., 2018, https://doi.org/10.1063/1.5039687). In this study, we have re-evaluated the efficiency of this scattering by incorporating realistic electron hole properties from direct spacecraft observations into computing electron diffusion rates and lifetimes. The most important electron hole properties in this evaluation are their distributions in velocity and spatial scale and electric field root-mean-square intensity (). Using direct measurements of electron holes during a plasma injection event observed by the Van Allen Probe at , we find that when 4 mV/m electron lifetimes can drop below 1 h and are mostly within strong diffusion limits at energies below 10 keV. During an injection observed by the THEMIS spacecraft at , electron holes with even typical intensities (1 mV/m) can deplete low-energy (a few keV) plasma sheet electrons within tens of minutes following injections and convection from the tail. Our results confirm that electron holes are a significant contributor to plasma sheet electron precipitation during injections.
Ionospheric Electron Density and Conductance Changes in the Auroral Zone During Substorms

N. A. Stepanov, V. A. Sergeev, M. A. Shukhtina, and 3 more authors

Journal of Geophysical Research: Space Physics, 2021

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Enhanced precipitation of magnetospheric energetic electrons during substorms increases ionospheric electron density and conductance. Such enhancements, which have timescales of a few hours, are not reproduced by the existing ionospheric models. We use the linear prediction filter (LPF) method to reconstruct the substorm-related response of electron densities and integral conductances from long-term ionospheric observations made by the European Incoherent SCATer radar located at Tromsø. To characterize the intensity of substorm dipolarization at a 5 min time step, we use the midlatitude positive bay index. We build response functions (LP filters) as a function of substorm time between T0−1 h and T0 + 4 h (T0 is a substorm onset time) in different magnetic local time (MLT) sectors to estimate the magnitude and delays of the ionospheric density response at different altitudes. Systematic and large relative changes are mostly observed in the D- and E regions. The duration of the response is about 3 h. It starts and reaches maximum magnitude near midnight, propagating from there toward the east and decaying after passing into the noon-evening sector. The reliability of LPF results is confirmed by the consistency of D-region response with independently derived response of the auroral absorption. Whereas strong ionization increases are seen in both E- and D-regions on the nightside, the D-region response is stronger in the morning-dayside sector. Such MLT variation corresponds to the drift motion and precipitation of the high-energy electrons injected in the nightside magnetosphere during substorm dipolarization. The inferred ionization changes result in strong enhancements of integral Hall (and Pedersen) conductance in the nightside auroral zone, where intense auroral currents are known to occur during substorms
Characteristics of Substorm-Onset-Related and Nonsubstorm Earthward Fast Flows and Associated Magnetic Flux Transport: THEMIS Observations

Jinxing Li, Xiangning Chu, Jacob Bortnik, and 5 more authors

Journal of Geophysical Research: Space Physics, 2021

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Substorms are closely associated with fast flows; however, many fast flows are not associated with substorm onsets and only lead to a localized, transient, and weak response in the magnetosphere and ionosphere. This study uses the midlatitude positive bay index to identify substorms. A case study investigates the magnetospheric and ionospheric responses to substorm-onset-related fast flows and nonsubstorm fast flows using observations from the THEMIS spacecraft and ionospheric currents. Statistics based on THEMIS observations made over 11 years show that substorm-onset-related fast flows are more likely to penetrate closer to the Earth and spread over a wider range of MLTs compared to nonsubstorm fast flows. The substorm-onset-related fast flow durations are slightly longer than nonsubstorm fast flow on average, and there is no significant difference between their peak velocity probability distributions. However, substorm-onset-related fast flows are statistically shown to be accompanied by substantially larger Bz increases that persist for longer periods of time, and hence result in 80% larger earthward-directed magnetic flux transport rates.
Plasmaspheric dynamics studied using a three-dimensional machine learning-based plasma density model in the inner magnetosphere and the ionosphere

Hannah Ace

UVM Honors College Senior Theses, 2021

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Magnetotail Flux Accumulation Leads to Substorm Current Wedge Formation: A Case Study

Xiangning Chu, Robert McPherron, Tung-Shin Hsu, and 4 more authors

Journal of Geophysical Research: Space Physics, 2021

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Reconnection-generated earthward flows, magnetic field dipolarizations, and auroral expansions are related to substorm current wedge (SCW) development. It has been suggested that field-aligned currents (FACs) within the SCW can be generated by flow vortices, pressure gradients, or both. Observations related to these generation mechanisms differ from one event to another, due to their different locations relative to SCW’s central meridian and timing relative to the SCW’s evolutionary state. A pattern of in situ observations consistent with these generation mechanisms has yet to emerge. Obtaining such a pattern of in situ observations relies on the satellite locations relative to the FAC driver regions, which are hard to determine because coincident magnetotail observations are sparse. To solve this problem, an SCW inversion technique was used to model the FAC locations and determine the connections between magnetospheric and ionospheric phenomena. Using this technique, the magnetic flux, a parameter that is relatively insensitive to FAC locations, was analyzed during an isolated substorm on February 13, 2008. We compared the temporal variations of the accumulated flux that caused magnetic dipolarization in the SCW and the flux within the auroral poleward boundary. We found them to be in good agreement with the flux transported by earthward flows. This agreement suggests that the accumulation of the magnetic flux leads to the generation of the SCW, causing magnetic dipolarization and auroral poleward expansion. The amount of accumulated flux was found to be positively correlated with the amplitudes of these substorm-related phenomena.

2020

Morphological Characteristics of Strong Thermal Emission Velocity Enhancement Emissions

Xiangning Chu, Lukas Wolter, David Malaspina, and 4 more authors

Journal of Geophysical Research: Space Physics, 2020

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Morphological characteristics of Strong Thermal Emission Velocity Enhancement (STEVE) are investigated during an event on July 17, 2018. We calibrate photographs from citizen scientists for scientific purposes. We determine the altitude profiles for STEVE emissions. The spectral continuum purplish STEVE emission peaks at 200 km, which is in the altitude range where redline-only SAR emission could be generated. The green picket fence peaks at 110 km, similar to that of the typical green aurora. For both emissions, their altitudes of the peak emissions and the shapes of the altitude profiles are similar across different longitudes. In regions of two-layer purplish STEVE emissions, the lower layer STEVE peaks at ∼130 km with slightly different peak altitudes at different longitudes. The green picket fence structures are separated by 14 km in longitude with a full width of half maximum of ∼5.3 km. They move westward at a roughly constant speed of ∼250 m/s, although they sometimes disappear and reappear. The purplish STEVE and green picket fence emissions are latitudinally narrow, and whether they are simultaneously collocated on the same magnetic field line depends on their latitudinal offset relative to their width. We demonstrate that the green picket fence and purplish STEVE emissions are located on magnetic field lines within a tenth (0.02°) of the latitudinal width of STEVE (0.2°). The fact suggests that different STEVE emissions are driven by the same narrow region either in the ionosphere or the magnetosphere, although their generation mechanisms differ. The morphological characteristics have important implications in determining how STEVE is generated.
How whistler mode hiss waves and the plasmasphere drive the quiet decay of radiation belts electrons following a geomagnetic storm

J.-F. Ripoll, M. Denton, V. Loridan, and 18 more authors

Journal of Physics: Conference Series, 2020

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We show how an extended period of quiet solar wind conditions contributes to a quiet state of the plasmasphere that expands up to L ∼ 5.5, which creates the perfect conditions for wave-particle interactions between the radiation belt electrons and whistler-mode hiss waves. The correlation between the hiss waves and the plasma density is direct with hiss wave power increasing with plasma density, while it was generally assumed that these quantities can be specified independently. Whistler-mode hiss waves pitch angle diffuse and ultimately scatter freshly injected electrons into the atmosphere until the slot region is formed between the inner and outer belt and the outer belt is drastically reduced. In this study, we use and combine Van Allen Probes observations and Fokker-Planck numerical simulations. The Fokker-Planck model uses consistent event-driven pitch angle diffusion coefficients from whistler-mode hiss waves. Observations and simulations allow us to reach a global understanding of the variations in the trapped electron population with time, space, energy, and pitch angle that is based on the existing theory of quasi-linear wave-particle interactions. We show, for instance, the outer belt is pitch-angle homogeneous, which is explained by the event-driven diffusion coefficients that are roughly constant for equatorial pitch angle α 0∼\textless60°, E\textgreater100 keV, 3.5\textlessL\textlessLpp∼6. The impact of this work is to bring an improved understanding of the belt evolution based on the integration of high quality and highly temporally and spatially resolved measurements that are integrated in modern computations. We also propose the event-driven method as an accurate method (within ×2) to predict the electron flux decay after storms.
Using Multiple Signatures to Improve Accuracy of Substorm Identification

John D. Haiducek, Daniel T. Welling, Steven K. Morley, and 2 more authors

Journal of Geophysical Research: Space Physics, 2020

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We have developed a new procedure for combining lists of substorm onset times from multiple sources. We apply this procedure to observational data and to magnetohydrodynamic (MHD) model output from 1–31 January 2005. We show that this procedure is capable of rejecting false positive identifications and filling data gaps that appear in individual lists. The resulting combined onset lists produce a waiting time distribution that is comparable to previously published results, and superposed epoch analyses of the solar wind driving conditions and magnetospheric response during the resulting onset times are also comparable to previous results. Comparison of the substorm onset list from the MHD model to that obtained from observational data reveals that the MHD model reproduces many of the characteristic features of the observed substorms, in terms of solar wind driving, magnetospheric response, and waiting time distribution. Heidke skill scores show that the MHD model has statistically significant skill in predicting substorm onset times.
Inward Propagation of Flow-Generated Pi2 Waves From the Plasma Sheet to the Inner Magnetosphere

Chih-Ping Wang, Xiaoyan Xing, Jacob Bortnik, and 1 more author

Journal of Geophysical Research: Space Physics, 2020

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To understand the inward propagation of the Pi2 waves generated by bursty bulk flows (BBFs) from the plasma sheet to the inner magnetosphere, we statistically investigate 137 midnight conjunction events with one satellite of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) in the plasma sheet and one satellite of Geostationary Operational Environmental Satellite (GOES) in the inner magnetosphere. All events have Pi2 wave enhancements associated with BBFs at THEMIS. In response, larger Pi2 wave enhancements at GOES are correlated with larger dipolarization associated with each BBF at THEMIS but not with stronger waves and flows at THEMIS. The larger wave enhancements at GOES occur preferentially under stronger solar wind driving. We discuss an explanation considering that BBFs are an earthward moving plasma sheet bubble having relatively lower flux tube entropy compared to that of the background plasma and that Pi2 waves are generated by the slowing down of the bubble. The stronger dipolarization of BBFs suggests a bubble of smaller entropy, while the stronger solar wind driving leads to higher background entropy in the inner magnetosphere, both favorable for a bubble to penetrate deeper, and thus, Pi2 waves generated by the bubble are more likely to propagate to the GOES location with less damping to result in larger wave enhancements. In addition, our analyses using empirical density and magnetic field models suggest that the strong Pi2 wave enhancements at GOES are less likely to be a cavity resonance driven inside the plasmasphere.

2019

Optical Spectra and Emission Altitudes of Double-Layer STEVE: A Case Study

Jun Liang, E. Donovan, M. Connors, and 6 more authors

Geophysical Research Letters, 2019

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We report an event study of STEVE on 17 July 2018, with focus on the optical spectra and emission altitudes of STEVE. We find that the STEVE comprises two traces, one at a higher elevation angle and the other at a lower elevation angle. The two traces merge into one when viewed near the zenith. Spectrograph measurements show that both STEVE traces are characterized by enhancements over broadband wavelengths, that is, an airglow continuum, but they differ in their red-line (630 nm) component: The higher-elevation STEVE contains substantial red-line enhancement over background, while the lower-elevation STEVE does not. Based upon triangulation analyses using multiple optical instruments, we evaluate that the two STEVE traces are likely emitted from distinctly different altitudes: The higher-elevation STEVE comes from 250-km altitude, while the lower-elevation one is from ≤150-km altitude. Our results impose implications and constraints on the possible underlying mechanisms of STEVE.
Identifying STEVE’s Magnetospheric Driver Using Conjugate Observations in the Magnetosphere and on the Ground

Xiangning Chu, David Malaspina, Bea Gallardo-Lacourt, and 18 more authors

Geophysical Research Letters, 2019

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The magnetospheric driver of strong thermal emission velocity enhancement (STEVE) is investigated using conjugate observations when Van Allen Probes’ footprint directly crossed both STEVE and stable red aurora (SAR) arc. In the ionosphere, STEVE is associated with subauroral ion drift features, including electron temperature peak, density gradient, and westward ion flow. The SAR arc at lower latitudes corresponds to regions inside the plasmapause with isotropic plasma heating, which causes redline-only SAR emission via heat conduction. STEVE corresponds to the sharp plasmapause boundary containing quasi-static subauroral ion drift electric field and parallel-accelerated electrons by kinetic Alfvén waves. These parallel electrons could precipitate and be accelerated via auroral acceleration processes powered by Alfvén waves propagating along the magnetic field with the plasmapause as a waveguide. The electron precipitation, superimposed on the heat conduction, could explain multiwavelength continuous STEVE emission. The green picket-fence emissions are likely optical manifestations of electron precipitation associated with wave structures traveling along the plasmapause.
How Sudden, Intense Energetic Electron Enhancements Correlate With the Innermost Plasmapause Locations Under Various Solar Wind Drivers and Geomagnetic Conditions

L.-Y. Khoo, X. Li, H. Zhao, and 3 more authors

Journal of Geophysical Research: Space Physics, 2019

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In this report, the relationship between innermost plasmapause locations (Lpp) and initial electron enhancements during both storm and nonstorm (Dst \textgreater −30 nT) periods are examined using data from the Van Allen Probes. The geomagnetic storms are classified into coronal mass ejection (CME)-driven and corotating interaction region (CIR)-driven storms to explore their influences on the initial electron enhancements, respectively. We also study nonstorm time electron enhancements and observe frequent, sudden (within two consecutive orbital passes) \textless400-keV electron enhancements during quiet periods. Our analysis reveals an incredibly cohesive observation that holds regardless of electron energies ( 30 keV–2.5 MeV) or geomagnetic conditions: the innermost Lpp is the innermost boundary of the initial energetic electron enhancements. Interestingly, the quantified energy-dependent relationship of the sudden, intense energetic electron enhancements, with respect to the innermost Lpp, also exhibit a very similar trend during both storm and nonstorm periods. In summary, the goal of this report is to provide a comprehensive quantification of this consistent relationship under various geomagnetic conditions, which will also enable better forecast and specification of energetic electrons in the inner magnetosphere.
Kinetic Equilibrium and Stability Analysis of Dipolarization Fronts

Alex C. Fletcher, Chris Crabtree, Gurudas Ganguli, and 3 more authors

Journal of Geophysical Research: Space Physics, 2019

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Dipolarization fronts are typically observed with a density gradient of scale size comparable to an ion gyroradius, which naturally results in an ambipolar electric field in the direction of the gradient. Prevailing models ignore this ambipolar electric field, the separation of ion and electron scale physics, and consequent non-Maxwellian plasma distributions with strong spatial gradients in velocity, all of which we investigate in this paper. We examine two dipolarization front events observed by the Magnetospheric Multiscale mission (one with low plasma beta, one with high plasma beta), develop a rigorous kinetic equilibrium for dipolarization fronts, analyze the linear stability, and explore the nonlinear evolution and observable signatures with kinetic simulations. There are two major drivers of instability in the lower-hybrid frequency range: the density gradient (lower-hybrid drift instability) and the velocity shear (electron-ion hybrid instability). We argue the electron-ion hybrid mode is dominant, and consequently a dipolarization front approaches a steady or saturated state through the emission of waves that relax the velocity shear. A key aspect of these shear-driven waves is a broadband frequency spectrum that is consistent with satellite observation.
On the Generation of Probabilistic Forecasts From Deterministic Models

E. Camporeale, X. Chu, O. V. Agapitov, and 1 more author

Space Weather, 2019

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Most of the methods that produce space weather forecasts are based on deterministic models. In order to generate a probabilistic forecast, a model needs to be run several times sampling the input parameter space, in order to generate an ensemble from which the distribution of outputs can be inferred. However, ensemble simulations are costly and often preclude the possibility of real-time forecasting. We introduce a simple and robust method to generate uncertainties from deterministic models, that does not require ensemble simulations. The method is based on the simple consideration that a probabilistic forecast needs to be both accurate and well calibrated (reliable). We argue that these two requirements are equally important, and we introduce the Accuracy-Reliability cost function that quantitatively measures the trade-off between accuracy and reliability. We then define the optimal uncertainties as the standard deviation of the Gaussian distribution that minimizes the cost function. We demonstrate that this simple strategy, implemented here by means of a deep neural network, produces accurate and well-calibrated forecasts, showing examples both on synthetic and real-world space weather data.

2018

Variation in Plasmaspheric Hiss Wave Power With Plasma Density

David M. Malaspina, Jean-Francois Ripoll, Xiangning Chu, and 2 more authors

Geophysical Research Letters, 2018

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Plasmaspheric hiss waves are commonly observed in the inner magnetosphere. These waves efficiently scatter electrons, facilitating their precipitation into the atmosphere. Predictive inner magnetosphere simulations often model hiss waves using parameterized empirical maps of observed hiss power. These maps nearly always include parameterization by magnetic L value. In this work, data from the Van Allen Probes are used to compare variation in hiss wave power with variation in both L value and cold plasma density. It is found that for L\textgreater 2.5, plasmaspheric hiss wave power increases with plasma density. For L\textgreater 3, this increase is stronger and occurs regardless of L value and for all local times. This result suggests that the current paradigm for parameterizing hiss wave power in many magnetospheric simulations may need to be revisited and that a new parameterization in terms of plasma density rather than L value should be explored.
The Current System of Dipolarizing Flux Bundles and Their Role as Wedgelets in the Substorm Current Wedge

Jiang Liu, V. Angelopoulos, Zhonghua Yao, and 3 more authors

In Electric Currents in Geospace and Beyond, 2018

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In this chapter, we review recent observations of the current system of dipolarizing flux bundles (DFBs), small magnetotail flux tubes (typically \textless 3 RE in X GSM and Y GSM) with a significantly more dipolar magnetic field than their background. When propagating, a DFB or a group of several consecutive DFBs is known as a bursty bulk flow. A DFB’s current system consists of a duskward current at the dipolarization front (DF, the leading edge of a DFB), a dawnward current immediately earthward of the DFB, field-aligned currents (FACs) at the DF and earthward of the DFB, and cross-tail current reduction inside the DFB. The FACs and cross-tail current reduction resemble a substorm current wedge (SCW), but are of much smaller scale. Therefore, each DFB possibly serves as a wedgelet, the building block of an SCW. We review a suggested way in which many wedgelets may collectively form a large-scale SCW. We also present some new results that help us to understand the DFB current system.
The Midlatitude Positive Bay Index and the Statistics of Substorm Occurrence

Robert L. McPherron, and Xiangning Chu

Journal of Geophysical Research: Space Physics, 2018

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The auroral breakup and expansion are essential features of a magnetospheric substorm. Negative bays beneath the aurora and positive bays at midlatitudes accompany the expansion. These effects are caused by the substorm current wedge. The negative bay strength is provided by the auroral electrojet lower (AL) index. At midlatitudes the positive bay is caused by the field-aligned currents connected to the electrojet. The midlatitude positive bay (MPB) index measures these effects. Both AL and MPB can be used to time auroral expansion onset. This work creates lists of MPB onsets and compares them with lists from other sources including onsets derived from IMAGE and Polar spacecraft and ground all-sky camera data. The lists show that these measurements are nearly simultaneous with satellite expansion onsets. Substorms occur most frequently during the declining phase of the solar cycle, at the equinoxes, and after the passage of a stream interface in a corotating interaction region. Both MPB and SML indices reveal three substorm phases: growth, expansion, and recovery. In addition, the MPB onset list, the SML onset list, and the IMAGE auroral onset list reveal the presence of a 45-min and a 2 3/4-hr quasi-periodicity in substorm occurrence. A comparison of the various lists reveals radical difference in their waiting time distributions and relatively low levels of association between events in the lists. The results suggest that none of the lists adequately capture the complexity of substorm activity and further refinements are needed in every algorithm used for onset detection.
A Census of Plasma Waves and Structures Associated With an Injection Front in the Inner Magnetosphere

David M. Malaspina, Aleksandr Ukhorskiy, Xiangning Chu, and 1 more author

Journal of Geophysical Research: Space Physics, 2018

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Now that observations have conclusively established that the inner magnetosphere is abundantly populated with kinetic electric field structures and nonlinear waves, attention has turned to quantifying the ability of these structures and waves to scatter and accelerate inner magnetospheric plasma populations. A necessary step in that quantification is determining the distribution of observed structure and wave properties (e.g., occurrence rates, amplitudes, and spatial scales). Kinetic structures and nonlinear waves have broadband signatures in frequency space, and consequently, high-resolution time domain electric and magnetic field data are required to uniquely identify such structures and waves as well as determine their properties. However, most high-resolution fields data are collected with a strong bias toward high-amplitude signals in a preselected frequency range, strongly biasing observations of structure and wave properties. In this study, an ∼45 min unbroken interval of 16,384 samples/s field burst data, encompassing an electron injection event, is examined. This data set enables an unbiased census of the kinetic structures and nonlinear waves driven by this electron injection, as well as determination of their “typical” properties. It is found that the properties determined using this unbiased burst data are considerably different than those inferred from amplitude-biased burst data, with significant implications for wave-particle interactions due to kinetic structures and nonlinear waves in the inner magnetosphere.
Relation of Field-Aligned Currents Measured by the Network of Iridium® Spacecraft to Solar Wind and Substorms

R. L. McPherron, B. J. Anderson, and Xiangning Chu

Geophysical Research Letters, 2018

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The strength of field-aligned currents coupling the magnetosphere to the ionosphere was obtained by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) using the network of Iridium® spacecraft. The distribution of current was integrated giving total current in and out of the ionosphere on the dayside and nightside of the Earth in both hemispheres. The onset of auroral zone negative bays and midlatitude positive bays corresponds to an increase in nightside upward current. The total outward current tends toward saturation with increasing solar wind driver strength. The optimum solar wind coupling function for AL index predicts 73% of the variance in nightside upward current. The dayside and nightside predictors of upward current rise to a peak at 30–45 min and decay slowly over 2.5 hr. Nightside response is delayed relative to dayside.
Quantitative Evaluation of Radial Diffusion and Local Acceleration Processes During GEM Challenge Events

Q. Ma, W. Li, J. Bortnik, and 13 more authors

Journal of Geophysical Research: Space Physics, 2018

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We simulate the radiation belt electron flux enhancements during selected Geospace Environment Modeling (GEM) challenge events to quantitatively compare the major processes involved in relativistic electron acceleration under different conditions. Van Allen Probes observed significant electron flux enhancement during both the storm time of 17–18 March 2013 and non–storm time of 19–20 September 2013, but the distributions of plasma waves and energetic electrons for the two events were dramatically different. During 17–18 March 2013, the SYM-H minimum reached −130 nT, intense chorus waves (peak Bw 140 pT) occurred at 3.5 \textless L \textless 5.5, and several hundred keV to several MeV electron fluxes increased by 2 orders of magnitude mostly at 3.5 \textless L \textless 5.5. During 19–20 September 2013, the SYM-H remained higher than −30 nT, modestly intense chorus waves (peak Bw 80 pT) occurred at L \textgreater 5.5, and electron fluxes at energies up to 3 MeV increased by a factor of 5 at L \textgreater 5.5. The two electron flux enhancement events were simulated using the available wave distribution and diffusion coefficients from the GEM focus group Quantitative Assessment of Radiation Belt Modeling. By comparing the individual roles of local electron heating and radial transport, our simulation indicates that resonant interaction with chorus waves is the dominant process that accounts for the electron flux enhancement during the storm time event particularly near the flux peak locations, while radial diffusion by ultralow-frequency waves plays a dominant role in the enhancement during the non–storm time event. Incorporation of both processes reasonably reproduces the observed location and magnitude of electron flux enhancement.
Artificial Neural Networks for Determining Magnetospheric Conditions

Jacob Bortnik, Xiangning Chu, Qianli Ma, and 11 more authors

In Machine Learning Techniques for Space Weather, 2018

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This chapter presents a neural-network-based technique that allows for the reconstruction of the global, time-varying distribution of some physical quantity Q, that has been sparsely sampled at various locations within the magnetosphere, and at different times. We begin with a general introduction to the problem of prediction and specification, and why it is important and difficult to achieve with existing methods. We then provide a basic introduction to neural networks, and describe our technique using the specific example of reconstructing the electron plasma density in the Earth’s inner magnetosphere on the equatorial plane. We then show more advanced uses of the technique, including 3D reconstruction of the plasma density, specification of chorus and hiss waves, and energetic particle fluxes. We summarize and conclude with a general discussion of how machine learning techniques might be used to advance the state-of-the-art in space weather prediction, and insight discovery.

2017

The Mid-Latitude Positive Bay and the MPB Index of Substorm Activity

Robert L. McPherron, and Xiangning Chu

Space Science Reviews, 2017

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Substorms are a major source of magnetic activity. At substorm expansion phase onset a westward current flows through the expanding aurora. This current is the ionospheric closure of the substorm current wedge produced by diversion of tail current along magnetic field lines. At low latitudes the field-aligned currents create a systematic pattern in the north (X) and east (Y) components of the surface magnetic field. The rise and decay in X is called a midlatitude positive bay whose start is a proxy for expansion onset. In this paper we describe a new index called the midlatitude positive bay index (MPB) which monitors the power in the substorm perturbations of X and Y. The index is obtained by removing the main field, storm time variations, and the solar quiet (Sq) variation from the measured field. These are estimated with spline fits and principal component analysis. The residuals of X and Y are high pass filtered to eliminate variations with period longer than 3 hours. The sum of squares of the X and Y power is determined at each of 35 midlatitude stations. The average power in night time stations is the MPB index. The index series is standardized and intervals above a fixed threshold are taken as possible bay signatures. Post processing constrains these to have reasonable values of rise time, strength, and duration. Minima in the index before and after the peak are taken as the start and end of the bay. The MPB and AL indices can be used to identify quiet intervals in the magnetic field.
Ultralow Frequency Waves Deep Inside the Inner Magnetosphere Driven by Dipolarizing Flux Bundles

Jiang Liu, V. Angelopoulos, X.-J. Zhang, and 7 more authors

Journal of Geophysical Research: Space Physics, 2017

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Dipolarizing flux bundles (DFBs) are small flux tubes (typical cross-tail scale of 1–3 RE) in the nightside magnetosphere that have magnetic field more dipolar than the background. They are generated at or beyond 20 RE downtail and then travel earthward. Although DFBs usually stop before reaching the geosynchronous orbit (GEO), they may still transfer some portion of their energy into the inner magnetosphere by radiating ULF waves. We show the clearest evidence of this process, to date, using in situ data from a fleet of spacecraft and ground stations. We illustrate that a typical DFB stopped before reaching GEO but excited Pi2 waves that filled a volume of space extending from the plasma sheet to deep inside the plasmasphere. This event provides the first in situ, direct link between stopped DFBs and ULF waves deep inside the inner magnetosphere (as deep as L = 3). The waves were attenuated when traveling away from the DFB, but even at L = 3 (XGSM = −2.2), they were still traveling with a significant earthward Poynting flux. In addition, we observed evidence of interaction between these waves and electrons at GEO.
A neural network model of three-dimensional dynamic electron density in the inner magnetosphere

X. Chu, J. Bortnik, W. Li, and 10 more authors

Journal of Geophysical Research: Space Physics, 2017

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A plasma density model of the inner magnetosphere is important for a variety of applications including the study of wave-particle interactions, and wave excitation and propagation. Previous empirical models have been developed under many limiting assumptions and do not resolve short-term variations, which are especially important during storms. We present a three-dimensional dynamic electron density (DEN3D) model developed using a feedforward neural network with electron densities obtained from four satellite missions. The DEN3D model takes spacecraft location and time series of solar and geomagnetic indices (F10.7, SYM-H, and AL) as inputs. It can reproduce the observed density with a correlation coefficient of 0.95 and predict test data set with error less than a factor of 2. Its predictive ability on out-of-sample data is tested on field-aligned density profiles from the IMAGE satellite. DEN3D’s predictive ability provides unprecedented opportunities to gain insight into the 3-D behavior of the inner magnetospheric plasma density at any time and location. As an example, we apply DEN3D to a storm that occurred on 1 June 2013. It successfully reproduces various well-known dynamic features in three dimensions, such as plasmaspheric erosion and recovery, as well as plume formation. Storm time long-term density variations are consistent with expectations; short-term variations appear to be modulated by substorm activity or enhanced convection, an effect that requires further study together with multispacecraft in situ or imaging measurements. Investigating plasmaspheric refilling with the model, we find that it is not monotonic in time and is more complex than expected from previous studies, deserving further attention.
Erosion and refilling of the plasmasphere during a geomagnetic storm modeled by a neural network

X. N. Chu, J. Bortnik, W. Li, and 3 more authors

Journal of Geophysical Research: Space Physics, 2017

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We present a history-dependent model of the equatorial plasma density of the inner magnetosphere using a feedforward neural network with two hidden layers. As the model inputs, we take locations and time series of SYM-H, AL, and F10.7 indices. By considering not only the instantaneous values but also the past values of geomagnetic and solar indices, the model is history dependent on levels of geomagnetic and solar activity. The modeled electron density is continuous both spatially and temporally so that the evolution of the density can be studied (such as plasmaspheric refilling). The model is trained using the electron density inferred from the spacecraft potential from three THEMIS probes. The equatorial electron density is shown to be accurately reconstructed with a correlation coefficient of r 0.953 between data and model target. Since the model is history dependent, it succeeds in reconstructing various density features and dynamic behaviors, such as the quiet time plasmasphere, erosion and recovery of the plasmasphere, as well as the plume formation during a storm on 4 February 2011. Our model may provide unprecedented insight into the behavior of the equatorial density at any time and location; as an example we show the inferred refilling rate from our model and compare it to previous estimates.
Effects of solar wind ultralow-frequency fluctuations on plasma sheet electron temperature: Regression analysis with support vector machine

Chih-Ping Wang, Hee-Jeong Kim, Chao Yue, and 3 more authors

Journal of Geophysical Research: Space Physics, 2017

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To investigate whether ultralow-frequency (ULF) fluctuations from 0.5 to 8.3 mHz in the solar wind and interplanetary magnetic field (IMF) can affect the plasma sheet electron temperature (Te) near geosynchronous distances, we use a support vector regression machine technique to decouple the effects from different solar wind parameters and their ULF fluctuation power. Te in this region varies from 0.1 to 10 keV with a median of 1.3 keV. We find that when the solar wind ULF power is weak, Te increases with increasing southward IMF Bz and solar wind speed, while it varies weakly with solar wind density. As the ULF power becomes stronger during weak IMF Bz ( 0) or northward IMF, Te becomes significantly enhanced, by a factor of up to 10. We also find that mesoscale disturbances in a time scale of a few to tens of minutes as indicated by AE during substorm expansion and recovery phases are more enhanced when the ULF power is stronger. The effect of ULF powers may be explained by stronger inward radial diffusion resulting from stronger mesoscale disturbances under higher ULF powers, which can bring high-energy plasma sheet electrons further toward geosynchronous distance. This effect of ULF powers is particularly important during weak southward IMF or northward IMF when convection electric drift is weak.
Coherently modulated whistler mode waves simultaneously observed over unexpectedly large spatial scales

Jinxing Li, Jacob Bortnik, Wen Li, and 10 more authors

Journal of Geophysical Research: Space Physics, 2017

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Utilizing simultaneous twin Van Allen Probes observations of whistler mode waves at variable separations, we are able to distinguish the temporal variations from spatial variations, determine the coherence spatial scale, and suggest the possible mechanism of wave modulation. The two probes observed coherently modulated whistler mode waves simultaneously at an unexpectedly large distance up to 4.3 RE over 3 h during a relatively quiet period. The modulation of 150–500 Hz plasmaspheric hiss was correlated with whistler mode waves measured outside the plasmasphere across 3 h in magnetic local time and 3 L shells, revealing that the modulation was temporal in nature. We suggest that the coherent modulation of whistler mode waves was associated with the coherent ULF waves measured over a large scale, which modulate the plasmaspheric density and result in the modulation of hiss waves via local amplification. In a later period, the 500–1500 Hz periodic rising-tone whistler mode waves were strongly correlated when the two probes traversed large spatial regions and even across the plasmapause. These periodic rising-tone emissions recurred with roughly the same period as the ULF wave, but there was no one-to-one correspondence, and a cross-correlation analysis suggests that they possibly originated from large L shells although the actual cause needs further investigation.

2016

Relation of the auroral substorm to the substorm current wedge

Robert L. McPherron, and Xiangning Chu

Geoscience Letters, 2016

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The auroral substorm is an organized sequence of events seen in the aurora near midnight. It is a manifestation of the magnetospheric substorm which is a disturbance of the magnetosphere brought about by the solar wind transfer of magnetic flux from the dayside to the tail lobes and its return through the plasma sheet to the dayside. The most dramatic feature of the auroral substorm is the sudden brightening and poleward expansion of the aurora. Intimately associated with this expansion is a westward electrical current flowing across the bulge of expanding aurora. This current is fed by a downward field-aligned current (FAC) at its eastern edge and an upward current at its western edge. This current system is called the substorm current wedge (SCW). The SCW forms within a minute of auroral expansion. FAC are created by pressure gradients and field line bending from shears in plasma flow. Both of these are the result of pileup and diversion of plasma flows in the near-earth plasma sheet. The origins of these flows are reconnection sites further back in the tail. The auroral expansion can be explained by a combination of a change in field line mapping caused by the substorm current wedge and a tailward growth of the outer edge of the pileup region. We illustrate this scenario with a complex substorm and discuss some of the problems associated with this interpretation.
Distribution of Region 1 and 2 currents in the quiet and substorm time plasma sheet from THEMIS observations

Jiang Liu, V. Angelopoulos, Xiangning Chu, and 1 more author

Geophysical Research Letters, 2016

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Although Earth’s Region 1 and 2 currents are related to activities such as substorm initiation, their magnetospheric origin remains unclear. Utilizing the triangular configuration of THEMIS probes at 8–12 RE downtail, we seek the origin of nightside Region 1 and 2 currents. Our statistical study reveals that both kinds of currents exist in the plasma sheet during quiet and active times. Region 2 currents are deep inside the plasma sheet; Region 1 currents, which are farther away from the neutral sheet, extend to the plasma sheet boundary layer. At geomagnetic quiet times, the separation between the two currents is located 2.5 RE from the neutral sheet. During substorms, the separation migrates toward (away from) the neutral sheet as the plasma sheet thins (thickens). These findings suggest that the plasma sheet is a source of Region 1 and 2 currents and its deformation is associated with redistribution of FAC sources in the magnetotail.

2015

The February 24, 2010 substorm: a refined view involving a pseudobreakup/expansive phase/poleward boundary intensification sequence

Martin Connors, Christopher T. Russell, Xiangning Chu, and 1 more author

Earth, Planets and Space, 2015

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A substorm on February 24, 2010 was chosen for study by Connors et al. (Geophys. Res. Lett. 41:4449–4455, 2014) due to simple symmetric subauroral magnetic perturbations observed in North America. It was shown that a substorm current wedge (SCW) three-dimensional current model could represent these perturbations well, gave a reasonable representation of auroral zone perturbations, and matched field-aligned currents determined in space from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) project. The conclusion was that substorm onset was at approximately 4:30 UT and that the substorm current wedge (SCW) formed in the region 1 (more poleward) current system.
A physical explanation for the magnetic decrease ahead of dipolarization fronts

Z. H. Yao, J. Liu, C. J. Owen, and 9 more authors

Annales Geophysicae, 2015

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Recent studies have shown that the ambient plasma in the near-Earth magnetotail can be compressed by the arrival of a dipolarization front (DF). In this paper we study the variations in the characteristics of currents flowing in this compressed region ahead of the DF, particularly the changes in the cross-tail current, using observations from the THEMIS satellites. Since we do not know whether the changes in the cross-tail current lead to a field-aligned current formation or just form a current loop in the magnetosphere, we thus use redistribution to represent these changes of local current density. We found that (1) the redistribution of the cross-tail current is a common feature preceding DFs; (2) the redistribution of cross-tail current is caused by plasma pressure gradient ahead of the DF and (3) the resultant net current redistributed by a DF is an order of magnitude smaller than the typical total current associated with a moderate substorm current wedge (SCW). Moreover, our results also suggest that the redistributed current ahead of the DF is closed by currents on the DF itself, forming a closed current loop around peaks in plasma pressure, what is traditionally referred to as a banana current.
Solar cycle dependence of substorm occurrence and duration: Implications for onset

Xiangning Chu, Robert L. McPherron, Tung-Shin Hsu, and 1 more author

Journal of Geophysical Research: Space Physics, 2015

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AbstractMagnetospheric substorms represent a major energy release process in Earth’s magnetosphere. Their duration and intensity are coupled to solar wind input, but the precise way the solar wind energy is stored and then released is a matter of considerable debate. Part of the observational difficulty has been the gaps in the auroral electrojet index traditionally used to study substorm properties. In this study, we created a midlatitude positive bay (MPB) index to measure the strength of the substorm current wedge. Because this index is based on midlatitude magnetometer data that are available continuously over several decades, we can assemble a database of substorm onsets lasting 31 years (1982–2012). We confirmed that the MPB onsets have a good agreement (±2 min) with auroral onsets as determined by optical means on board the IMAGE mission and that the MPB signature of substorms is robust and independent of the stations’ position relative to ionospheric currents. Using the MPB onset, expansion, and recovery as a proxy of the respective substorm quantities, we found that the solar cycle variation of substorm occurrence depends on solar wind conditions and has a most probable value of 80 min. In contrast, the durations of substorm expansion and recovery phases do not change with the solar cycle. This suggests that the frequency of energy unloading in the magnetosphere is controlled by solar wind conditions through dayside reconnection, but the unloading process related to flux pileup in the near-Earth region is controlled by the magnetosphere and independent of external driving.
An optimum solar wind coupling function for the AL index

Robert L. McPherron, Tung-Shin Hsu, and Xiangning Chu

Journal of Geophysical Research: Space Physics, 2015

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We define a coupling function as a product of solar wind factors that partially linearizes the relation between it and a magnetic index. We consider functions that are a product of factors of solar wind speed V, density N, transverse magnetic field B⊥, and interplanetary magnetic field (IMF) clock angle θc each raised to a different power. The index is the auroral lower (AL index) which monitors the strength of the westward electrojet. Solar wind data 1995–2014 provide hour averages of the factors needed to calculate optimum exponents. Nonlinear inversion determines both the exponents and linear prediction filters of short data segments. The averages of all exponents are taken as optimum exponents and for V, N, B⊥, and sin(θc/2) are [1.92, 0.10, 0.79, 3.67] with errors in the second decimal. Hohtmly values from 1966 to 2014 are used next to calculate the optimum function (opn) and the functions VBs (eys), epsilon (eps), and universal coupling function (ucf). A yearlong window is advanced by 27 days calculating linear prediction filters for the four functions. The functions eps, eys, ucf, and opn, respectively, predict 43.7, 61.2, 65.6, and 68.3% of AL variance. The opn function is 2.74% better than ucf with a confidence interval 2.60–2.86%. Coupling strength defined as the sum of filter weights (nT/mV/m) is virtually identical for all functions and varies systematically with the solar cycle being strongest (188 nT/mV/m) at solar minimum and weakest (104) at solar maximum. Saturation of the polar cap potential approaching solar maximum may explain the variation.
Substorm current wedge composition by wedgelets

Jiang Liu, V. Angelopoulos, Xiangning Chu, and 2 more authors

Geophysical Research Letters, 2015

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Understanding how a substorm current wedge (SCW) is formed is crucial to comprehending the substorm phenomenon. One SCW formation scenario suggests that the substorm time magnetosphere is coupled to the ionosphere via “wedgelets,” small building blocks of an SCW. Wedgelets are field-aligned currents (FACs) carried by elemental flux transport units known as dipolarizing flux bundles (DFBs). A DFB is a magnetotail flux tube with magnetic field stronger than that of the ambient plasma. Its leading edge, known as a “dipolarization front” or “reconnection front,” is a product of near-Earth reconnection. Dipolarizing flux bundles, and thus wedgelets, are localized—each is only \textless3 RE wide. How these localized wedgelets combine to become large-scale (several hours of magnetic local time) region-1-sense SCW FACs is unclear. To determine how this occurs, we investigated wedgelets statistically using Time History of Events and Macroscale Interactions during Substorms (THEMIS) data. The results show wedgelet asymmetries: in the dawn (dusk) sector of the magnetotail, a wedgelet has more FAC toward (away from) the Earth than away from (toward) the Earth, so the net FAC is toward (away from) the Earth. The combined effect of many wedgelets is therefore the same as that of large-scale region-1-sense SCW, supporting the idea that they comprise the SCW.
Configuration and Generation of Substorm Current Wedge

Xiangning Chu

2015

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The substorm current wedge (SCW), a core element of substorm dynamics coupling the magnetotail to the ionosphere, is crucial in understanding substorms. It has been suggested that the field-aligned currents (FACs) in the SCW are caused by either pressure gradients or flow vortices, or both. Our understanding of FAC generations is based predominately on numerical simulations, because it has not been possible to organize spacecraft observations in a coordinate system determined by the SCW. This dissertation develops an empirical inversion model of the current wedge and inverts midlatitude magnetometer data to obtain the parameters of the current wedge for three solar cycles. This database enables statistical data analysis of spacecraft plasma and magnetic field observations relative to the SCW coordinate. In chapter 2, a new midlatitude positive bay (MPB) index is developed and calculated for three solar cycles of data. The MPB index is processed to determine the substorm onset time, which is shown to correspond to the auroral breakup onset with at most 1-2 minutes difference. Substorm occurrence rate is found to depend on solar wind speed while substorm duration is rather constant, suggesting that substorm process has an intrinsic pattern independent of external driving. In chapter 3, an SCW inversion technique is developed to determine the strength and locations of the FACs in an SCW. The inversion parameters for FAC strength and location, and ring current strength are validated by comparison with other measurements. In chapter 4, the connection between earthward flows and auroral poleward expansion is examined using improved mapping, obtained from a newly-developed dynamic magnetospheric model by superimposing a standard magnetospheric field model with substorm current wedge obtained from the inversion technique. It is shown that the ionospheric projection of flows observed at a fixed point in the equatorial plane map to the bright aurora as it expands poleward, suggesting that auroral poleward expansion is mainly a consequence of magnetic dipolarization caused by the SCW. Chapter 5 shows that increased plasma pressure caused by flow braking has a temporal pattern similar to that of the currents in the SCW. In contrast, flow vortices vanish quickly, suggesting that pressure gradient is an important factor in generating the SCW. The measured pressure gradients are found to be organized relative to SCW central meridian. Nonalignment between pressure gradient and flux tube volume gradient lead to the generation of an SCW with quadrupole FACs (inner and outer loop of FACs). Because the inner current loop is weaker than the outer loop, the combined magnetic effect of the two current loops is similar to a classic SCW. The final chapter studies the magnetic flux transport by earthward flows, and accumulated inside the SCW and enclosed within auroral poleward boundary. Their good agreement suggests that flux accumulation causes magnetic dipolarization and auroral poleward expansion. The strength of the SCW is positively correlated with the amount of magnetic flux accumulated.
Magnetic mapping effects of substorm currents leading to auroral poleward expansion and equatorward retreat

Xiangning Chu, Robert L. McPherron, Tung-Shin Hsu, and 5 more authors

Journal of Geophysical Research: Space Physics, 2015

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Magnetotail fast flows, magnetic field dipolarization, and its relaxation are linked to auroral brightening, poleward expansion, and equatorward motion during substorm onset, expansion, and recovery, respectively. While auroral brightening is often attributed to the field-aligned currents produced by flow vorticity and pressure redistribution, the physical causes of auroral poleward expansion and equatorward retreat are not fully understood. Simplistically, such latitudinal changes can be directly associated to the tailward motion of the flux pileup region and the earthward flux transport toward the dayside that depletes the near-Earth plasma sheet. However, because the equatorial magnetic field profile and the magnetospheric field-aligned current system change significantly, mapping is severely distorted. To investigate this distortion, we superimpose a substorm current wedge model (dynamically driven by ground-based observations) on the global Tsyganenko model T96 during an isolated substorm on 13 February 2008, observed by the Time History of Events and Macroscale Interactions during Substorms and GOES 10 spacecraft and by ground all-sky imagers. We validate our model by showing that the timing and ionospheric projection of the flux pileup region and flow bursts observed at the spacecraft match auroral activations. We then use the improved mapping enabled by the model to demonstrate that in this event, auroral poleward expansion and equatorward retreat are mainly caused by substorm-current-wedge-induced mapping changes.

2014

Pressure gradient evolution in the near-Earth magnetotail at the arrival of BBFs

Zhonghua Yao, Zuyin Pu, Aimin Du, and 8 more authors

Chinese Science Bulletin, 2014

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Using in situ observations from THEMIS A, D and E during the 2008–2011 tail season, we present a statistical study of the evolution of pressure gradients in the near-Earth tail during bursty bulk flow (BBF) convection. We identified 138 substorm BBFs and 2,197 non-substorm BBFs for this study. We found that both the pressure and the BZ component of the magnetic field were enhanced at the arrival of BBFs at the spacecraft locations. We suggest that the increase of BZ during non-substorm BBFs is associated with flux pile-up. However, the much stronger enhancement of BZ during substorm BBFs implies the occurrence of magnetic field dipolarization which is caused by both the flux pile-up process and near-Earth current disruption. Furthermore, a bow-wave-like high pressure appears to be formed at the arrival of substorm BBFs, which is responsible for the formation of region-1-sense FACs. The azimuthal pressure gradient associated with the arrival of substorm BBFs lasts for about 5 min. The enhanced pressure gradient associated with the bow wave is caused by the braking and diversion of the Earthward flow in the inner plasma sheet. The results from this statistical study suggest that the braking and azimuthal diversion of BBFs may commonly create azimuthal pressure gradients, which are related to the formation of the FAC of the substorm current wedge.
Current reduction in a pseudo-breakup event: THEMIS observations

Z. H. Yao, Z. Y. Pu, C. J. Owen, and 12 more authors

Journal of Geophysical Research: Space Physics, 2014

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Pseudo-breakup events are thought to be generated by the same physical processes as substorms. This paper reports on the cross-tail current reduction in an isolated pseudo-breakup observed by three of the THEMIS probes (THEMIS A (THA), THEMIS D (THD), and THEMIS E (THE)) on 22 March 2010. During this pseudo-breakup, several localized auroral intensifications were seen by ground-based observatories. Using the unique spatial configuration of the three THEMIS probes, we have estimated the inertial and diamagnetic currents in the near-Earth plasma sheet associated with flow braking and diversion. We found the diamagnetic current to be the major contributor to the current reduction in this pseudo-breakup event. During flow braking, the plasma pressure was reinforced, and a weak electrojet and an auroral intensification appeared. After flow braking/diversion, the electrojet was enhanced, and a new auroral intensification was seen. The peak current intensity of the electrojet estimated from ground-based magnetometers, 0.7 × 105 A, was about 1 order of magnitude lower than that in a typical substorm. We suggest that this pseudo-breakup event involved two dynamical processes: a current-reduction associated with plasma compression ahead of the earthward flow and a current-disruption related to the flow braking/diversion. Both processes are closely connected to the fundamental interaction between fast flows, the near-Earth ambient plasma, and the magnetic field.
Electric currents of a substorm current wedge on 24 February 2010

Martin Connors, Robert L. McPherron, Brian J. Anderson, and 3 more authors

Geophysical Research Letters, 2014

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The three-dimensional “substorm current wedge” (SCW) was postulated by McPherron et al. (1973) to explain substorm magnetic perturbations. The origin and coherence as a physical system of this important paradigm of modern space physics remained unclear, however, with progress hindered by gross undersampling, and uniqueness problems in data inversion. Complementing AMPERE (Active Magnetosphere and Planetary Electrodynamics Response Experiment) space-derived radial electric currents with ground magnetic data allowing us to determine currents from the ionosphere up, we overcome problems of uniqueness identified by Fukushima (1969, 1994). For a substorm on 24 February 2010, we quantify SCW development consistently from ground and space data. Its westward electrojet carries 0.5 MA in the more poleward part of the auroral oval, in Region 1 (R1) sense spanning midnight. The evening sector electrojet also feeds into its upward current. We thus validate the SCW concept and obtain parameters needed for quantitative study of substorms.
Development and validation of inversion technique for substorm current wedge using ground magnetic field data

Xiangning Chu, Tung-Shin Hsu, Robert L. McPherron, and 8 more authors

Journal of Geophysical Research: Space Physics, 2014

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The classic substorm current wedge model represents ground and space magnetic perturbations measured during substorms. We have developed an inversion technique to calculate parameters determining the intensity and geometry of the current system using magnetic field data at midlatitudes. The current wedge consists of four segments: a sheet-like field-aligned current downward to the ionosphere postmidnight, a westward current across the auroral bulge, an upward sheet-like current from the westward surge premidnight, and an eastward current in the equatorial plane. The model has five parameters including the current strength, the locations, and breadths of the two field-aligned current sheets. Simultaneous changes in the ring current are represented by the superposition of a symmetric ring current and a partial ring current characterized by three additional parameters. Parameters of the model are determined as a function of time based on midlatitude ground magnetometers, using realistic field lines and accounting for Earth’s induction. The model is validated by a variety of techniques. First, the model predicts more than 80% of the variance in the observations. Second, the intensity of the current wedge and the ring current follows the same trends of the westward electrojet and the ring current indices. Third, the intensity of the westward electrojet agrees extremely well with the intensity of the current wedge. Finally, spacecraft observations of the aurora correspond with the evolution deduced from the model. This model of the substorm current wedge provides a valuable tool for the study of substorm development and its relation to phenomena in space.

2013

Current structures associated with dipolarization fronts

Zhonghua Yao, W. J. Sun, S. Y. Fu, and 8 more authors

Journal of Geophysical Research: Space Physics, 2013

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AbstractRecently, observational results on currents around the dipolarization fronts (DFs) of earthward flow bursts have attracted much research attention. These currents are found to have close association with substorm intensifications. This paper devotes to further study of the current system ahead and within the DFs with high-resolution magnetic field measurements from Cluster constellation in 2003. The separation of four spacecraft is much smaller than the scales of spatial structures ahead and within the DF layer so that the currents can be reliably obtained. Based on features of the magnetic field variations prior to the fronts, we categorized the DFs into two types: DFs with magnetic dips immediate ahead of the fronts (type I) and DFs without magnetic dips (type II). For type I DFs, it is found that dawnward currents along the DFs exist in the dip region; duskward currents exist within the fronts. Furthermore, the dawnward currents in the dip region are found to be mainly parallel to the local magnetic field with a spatial scale of 1000 km, whereas the duskward currents within the fronts have both significant parallel and perpendicular components. On the other hand, for type II DFs, only significant duskward and mainly perpendicular currents show up within the fronts; no dawnward currents exist ahead of DFs. The dawnward and mainly parallel current in the type I DFs is important in the current coupling process between magnetosphere and ionosphere and may lead to local current disruptions for substorm initiations.
Conjugate observations of flow diversion in the magnetotail and auroral arc extension in the ionosphere

Z. H. Yao, V. Angelopoulos, Z. Y. Pu, and 10 more authors

Journal of Geophysical Research: Space Physics, 2013

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AbstractWe present a case study of a plasma flow diversion event in the inner magnetosphere to illustrate the effects of near-Earth pressure evolution, field-aligned current (FAC) generation, and associated auroral evolution during conjugate observations from THEMIS satellites and All-Sky Imagers (ASIs) on 29 January 2008. At 0831 UT, an earthward flow at THC, at X 18 Re, was observed which was likely to be associated with an auroral streamer observed at the ASI station FSMI. Near-Earth satellites THD and THE, separated in the Y-direction by 1 Re, and magnetically mapping onto the preexisting aurora arc, also observed the earthward flow but with a small Alfvén transit time of 20 s between the magnetotail and the ionosphere. As the earthward flow arrived at THD and THE, the aurora arc near the footprints of THD and THE brightened. A duskward pressure gradient developed between THD and THE in the equatorial magnetotail, corresponding to an upward FAC and consistent with the auroral brightening. At 0834 UT, an eastward diversion of the flow was observed first at THE and later at THD, further to the East. At the head of that flow, pressure variations, coherent between THE and THD, were observed to propagate Eastward at 400 km/s, consistent with an eastward auroral expansion near the footprints of THE and THD. Concurrent perturbations of the By component of the magnetic field signify eastward motion of a FAC pair at the two spacecraft locations. We interpret the pressure variations as the generator of the observed FAC that feeds the arc and its eastward expansion.
Changes in solar wind–magnetosphere coupling with solar cycle, season, and time relative to stream interfaces

Robert L. McPherron, Daniel N. Baker, T. I. Pulkkinen, and 3 more authors

Journal of Atmospheric and Solar-Terrestrial Physics, 2013

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Geomagnetic activity depends on a variety of factors including solar zenith angle, solar UV, strength of the interplanetary magnetic field, speed and density of the solar wind, orientation of the Earth’s dipole, distance of the Earth from Sun, occurrence of CMEs and CIRs, and possibly other parameters. We have investigated some of these using state-dependant linear prediction filters. For a given state a prediction filter transforms a coupling function such as rectified solar wind electric field (VBs) to an output like the auroral electrojet index (AL). The area of this filter calculated from the sum of the filter coefficients measures the strength of the coupling. When the input and output are steady for a time longer than the duration of the filter the ratio of output to input is equal to this area. We find coupling strength defined in this way for Es=VBs to AL (and AU) is weakest at solar maximum and strongest at solar minimum. AL coupling displays a semiannual variation being weakest at the solstices and strongest at the equinoxes. AU coupling has only an annual variation being strongest at summer solstice. AL and AU coupling also vary with time relative to a stream interface. Es coupling is weaker after the interface, but ULF coupling is stronger. Total prediction efficiency remains about constant at the interface. The change in coupling strength with the solar cycle can be explained as an effect of more frequent saturation of the polar cap potential causing a smaller ratio of AL to Es. Stronger AL coupling at the equinoxes possibly indicates some process that makes magnetic reconnection less efficient when the dipole axis is tilted along the Earth–Sun line. Strong AU coupling at summer solstice is likely due to high conductivity in northern summer. Coupling changes at a stream interface are correlated with the presence of strong wave activity in ground and satellite measurements and may be an artifact of the method by which solar wind data are propagated.

2012

A statistical analysis of the association between fast plasma flows and Pi2 pulsations

Tung-Shin Hsu, R. L. McPherron, Vassilis Angelopoulos, and 5 more authors

Journal of Geophysical Research: Space Physics, 2012

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What is the energy source for Pi2 pulsations? Some researchers have suggested that Pi2 pulsations are caused by plasma flows. Others have suggested that Pi2s are caused by a plasma instability initiated in the near-Earth region. In this study we use Time History of Events and Macroscale Interactions during Substorms (THEMIS) data to examine the relationship between plasma flows and Pi2s. We first identify plasma flows in the tail and then associate these flows with Pi2s. We find that the overall probability of association is nearly 70%. If we further examine the spatial distribution of association we find that in the near-Earth region (8–15RE) 90% of the flows are associated with Pi2s. This suggests either that plasma flow in the tail is the cause of Pi2 on the ground or that both are caused by the same process. More importantly, as the flows approach the Earth, the increasing magnetic field causes the flow speed to decrease, that is, the flows are decelerated. In this process of flow braking, the local magnetic field becomes more dipolar. The region of strongest flow braking corresponds to the region of strongest association between flows and Pi2s (8–15 RE). The GSM X component of plasma flows become zero at around 8 RE where the most significant magnetic dipolarization occurs. We also find that most plasma flows are associated with a sudden enhancement of the westward electrojet. These results suggest that plasma flows, Pi2 pulsations, and magnetic field dipolarization are essential components of the substorm expansion onset.
Necessity of substorm expansions in the initiation of steady magnetospheric convection

J. Kissinger, R. L. McPherron, T.-S. Hsu, and 2 more authors

Geophysical Research Letters, 2012

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Steady magnetospheric convection (SMC) events occur during enhanced solar wind driving of the magnetosphere and are characterized by quasi-stable convection, without any substorm expansions, for several hours. Previous research has hinted that a substorm onset may be required before the magnetosphere can enter the SMC mode of response. For the first time, we show statistically that nearly all SMC events are preceded by a substorm. Only 1% of SMC events occur without any preceding substorm signatures. The typical magnetospheric reaction to enhanced solar wind driving is a substorm. After this occurs, we find that the duration and stability of southward IMF determines whether a particular substorm will transition into an SMC event or not. The initial substorm sets up a high pressure region in the inner magnetosphere that causes flux diversion to the flanks, which allows closed flux from the nightside x-line to return to balance the dayside reconnection rate.

2011

Characteristics of plasma flows at the inner edge of the plasma sheet

R. L. McPherron, T.-S. Hsu, J. Kissinger, and 2 more authors

Journal of Geophysical Research: Space Physics, 2011

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All known types of auroral zone magnetic activity are associated with closure of open magnetic flux in the magnetotail. As closure is caused by magnetic reconnection we expect to observe fast flows during geomagnetic activity. We have scanned the ion flow data during the first pass of the THEMIS D spacecraft through the tail (December 2007 to May 2008), identifying all flows with ∣V⊥x∣ \textgreater 150 km/s. These flows generally occur in a sequence of several short bursts (bursty bulk flows). Earthward flows are much more common than tailward flows and are faster than tailward flows. Earthward flows have a longer duration; tailward flows are seen alone or after an earthward flow. Both directions of flow are associated with an increase in tail Bz (dipolarization). Fast flows in either direction are rarely seen inside of 9 RE. Earthward flows are strongly localized in the local time sector 2100–0100 and have a probability distribution identical to that seen in auroral substorm expansions by the IMAGE spacecraft. Tailward flows are also localized but with a peak shifted to 2330 LT. Very close to midnight the flows are slowed and reflected. At other local times they appear to be deflected around the Earth. Fast flows often follow a reduction in Es (GSM VBs) and occur close to the time of a sudden decrease in the AL index. Generally, the first flow burst in a sequence is most closely associated with the AL onset, and its peak follows the AL onset by about 2 min. The probability of observing a fast flow at THEMIS D during steady magnetospheric convection (SMC) events is quite low compared with the probability during an interval before the SMC. Since most of the fast flows carry magnetic flux earthward and are associated with substorm onset seen in the aurora by IMAGE and in the AL index, we interpret them as evidence that magnetic reconnection has occurred in the tail. Near 30 RE in the tail plasmoid ejection has also been associated with substorm onset, so we conclude that the fast flows are created by a new X line formed outside the 11.9 apogee of THEMIS D some time earlier than they are seen at THEMIS D. During SMC it appears that fast flows due to reconnection are deflected around the Earth outside the apogee of the satellite.

2010

THEMIS observations of two substorms on February 26, 2008

XiangNing Chu, ZuYin Pu, Xin Cao, and 21 more authors

Science China Technological Sciences, 2010

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Two substorms occurred at ∼04:05 and ∼04:55 UT on February 26, 2008 are studied with the in-situ observations of THEMIS satellites and ground-based aurora and magnetic field measurements. Angelopoulos et al. have made a comprehensive study of the 04:55 UT event. We showed detailed features of the two substorms with much attention to the first event and to the relationship between mid-tail magnetic reconnection (MR) and substorm activities. It was found that in the earlier stage of each substorm, a first auroral intensification occurred 2–3 min soon after the start of mid-tail MR, followed by a slow and very limited expansion. The auroral arcs were weak, short-lived, and localized, characterizing all features of a pseudobreakup. We regarded the first auroral brightening as the initial onset of the substorms. A few minutes later, a second stronger auroral intensification appeared, followed by quick and extensive expansions. It was interesting to papernote that the second brightening and related poleward expansion happened almost simultaneously (within a couple of minutes) with the onset of earthward flow and dipolarization in the near-Earth tail and other phenomenon of the substorm expansion phase. We thus regarded the second auroral brightening as the major onset of the substorms. Furthermore, it was seen that during the growth phase of the two substorms, the polar cap open flux Ψ kept increasing, while it quickly reduced during the substorm expansion and recovery phase. These variations of Ψ implied that the evolution of the two substorm expansion phases were closely related to MR of tail lobe open field lines. Analysis of substorm activities revealed that the two events studied were small substorms; while estimate of MR rate indicated that the MR processes in the two substorms were weak. The aforementioned observations suggested that mid-tail MR initiated the pseudobreakup first; the earthward flow generated by MR transported magnetic flux and energy to the near-Earth tail to cause the formation of SCW and CD, which induced near-Earth dipolarization and major auroral brightening, and eventually led to the onset of the substorm expansion phase. These results were clearly consistent with the picture of NENL and RCS models and supported the two step initiation scenario of substorms.
THEMIS observations of substorms on 26 February 2008 initiated by magnetotail reconnection

Z. Y. Pu, X. N. Chu, X. Cao, and 25 more authors

Journal of Geophysical Research: Space Physics, 2010

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On 26 February 2008, the THEMIS satellites observed two substorms that occurred at about 0405 and 0455 UT. Angelopoulos et al. (2008) made a comprehensive study of the second event. In this paper we display detailed features of the two substorms with emphasis on the first. In both substorms, a distinct auroral intensification occurred during the earliest stage of onset, about 1 to 2 min after midtail reconnection began. This initial intensification was weak and localized and thus had the signatures of a pseudobreakup. In both substorms, a second, major intensification occurred next in the substorm onset sequence, followed by rapid and extensive poleward expansion. This second intensification had the features of the major expansion onset and was nearly coincident with observations of earthward flows and magnetic dipolarization in the near-Earth tail. During the growth phase of the two substorms, open magnetic flux accumulated in the polar cap; in the expansion/recovery phase the polar cap open flux was quickly reduced. These observations are in agreement with the assertion that tail reconnection initiates the initial pseudobreakup and the ensuing major expansion and releases and transports energy to eventually cause near-Earth dipolarization and the expansion phase onset of these two substorms.

2009

Westward ionospheric electric field perturbations on the dayside associated with substorm processes

Y. Wei, Z. Pu, M. Hong, and 8 more authors

Journal of Geophysical Research: Space Physics, 2009

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A controversy has risen in the direction of ionospheric electric field perturbation on the dayside caused by substorm expansion phase in recent years, i.e., eastward or westward. To exclude the effect of interplanetary magnetic field northward turning, the substorms without interplanetary field (IMF) trigger are required to investigate this issue. Previous works, such as that by Huang et al. (2004), showed that the eastward electric field perturbations caused by substorms can be observed. However, our case suggests that some substorms can produce strong westward electric field perturbations and drive westward equatorial electrojets on the dayside ionosphere. This westward electric field is created by an overshielding-like imbalance state of field-aligned currents (FACs), Region 2 (R2) FAC greater than Region 1 (R1) FAC, which is built up through R2 FAC enhancement rather than R1 FAC reduction due to IMF northward turning. The substorm processes should be responsible for the westward electric field especially through polar cap shrinkage and magnetic field dipolarization.