TY  - JOUR
A1  - Guo, Yingjie
A1  - Ni, Binbin
A1  - Fu, Song
A1  - Wang, Dedong
A1  - Shprits, Yuri Y.
A1  - Zhelavskaya, Irina S.
A1  - Feng, Minghang
A1  - Guo, Deyu
T1  - Identification of controlling geomagnetic and solar wind factors for magnetospheric chorus intensity using feature selection techniques
JF  - Journal of geophysical research : A, Space physics
N2  - Using over-5-year EMFISIS wave measurements from Van Allen Probes, we present a detailed survey to identify the controlling factors among the geomagnetic indices and solar wind parameters for the 1-min root mean square amplitudes of lower band chorus (LBC) and upper band chorus (UBC). 
A set of important features are automatically determined by feature selection techniques, namely, Random Forest and Maximum Relevancy Minimum Redundancy. Our analysis results indicate the AE index with zero-time-delay dominates the intensity evolution of LBC and UBC, consistent with the evidence that chorus waves prefer to occur and amplify during enhanced substorm periods. Regarding solar wind parameters, solar wind speed and IMF B-z are identified as the controlling factors for chorus wave intensity. Using the combination of all these important features, a predictive neural network model of chorus wave intensity is established to reconstruct the temporal variations of chorus wave intensity, for which application of Random Forest produces the overall best performance. 

Plain Language Summary
Whistler mode chorus waves are electromagnetic waves observed in the low-density region near the geomagnetic equator outside the plasmapause. The dynamics of Earth's radiation belts are largely influenced by chorus waves owing to their dual contributions to both radiation belt electron acceleration and loss. In this study, we use feature selection techniques to identify the controlling geomagnetic and solar wind factors for magnetospheric chorus waves. Feature selection techniques implement the processes which can select the features most influential to the output. 
In this study, the inputs are geomagnetic indices and solar wind parameters and the output is the chorus wave intensity. The results indicate that AE index with zerotime delay dominates the chorus wave intensity. Furthermore, solar wind speed and IMF B-z are identified as the most important solar wind drivers for chorus wave intensity. 
On basis of the combination of all these important geomagnetic and solar wind controlling factors, we develop a neural network model of chorus wave intensity, and find that the model with the inputs identified using the Random Forest method produces the overall best performance.
Y1  - 2021
U6  - https://doi.org/10.1029/2021JA029926
SN  - 2169-9380
SN  - 2169-9402
VL  - 127
IS  - 1
PB  - Wiley
CY  - Hoboken, NJ
ER  - 
TY  - JOUR
A1  - Drozdov, Alexander
A1  - Allison, Hayley J.
A1  - Shprits, Yuri Y.
A1  - Usanova, Maria E.
A1  - Saikin, Anthony
A1  - Wang, Dedong
T1  - Depletions of Multi-MeV Electrons and their association to Minima in Phase Space Density
JF  - Geophysical research letters
N2  - Fast-localized electron loss, resulting from interactions with electromagnetic ion cyclotron (EMIC) waves, can produce deepening minima in phase space density (PSD) radial profiles. Here, we perform a statistical analysis of local PSD minima to quantify how readily these are associated with radiation belt depletions. The statistics of PSD minima observed over a year are compared to the Versatile Electron Radiation Belts (VERB) simulations, both including and excluding EMIC waves. The observed minima distribution can only be achieved in the simulation including EMIC waves, indicating their importance in the dynamics of the radiation belts. By analyzing electron flux depletions in conjunction with the observed PSD minima, we show that, in the heart of the outer radiation belt (L* < 5), on average, 53% of multi-MeV electron depletions are associated with PSD minima, demonstrating that fast localized loss by interactions with EMIC waves are a common and crucial process for ultra-relativistic electron populations.
KW  - radiation belts
KW  - EMIC
KW  - VERB
KW  - PSD
Y1  - 2022
U6  - https://doi.org/10.1029/2021GL097620
SN  - 0094-8276
SN  - 1944-8007
VL  - 49
IS  - 8
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Shprits, Yuri Y.
A1  - Allison, Hayley J.
A1  - Wang, Dedong
A1  - Drozdov, Alexander
A1  - Szabo-Roberts, Matyas
A1  - Zhelavskaya, Irina
A1  - Vasile, Ruggero
T1  - A new population of ultra-relativistic electrons in the outer radiation zone
JF  - Journal of geophysical research : Space physics
N2  - Van Allen Probes measurements revealed the presence of the most unusual structures in the ultra-relativistic radiation belts. Detailed modeling, analysis of pitch angle distributions, analysis of the difference between relativistic and ultra-realistic electron evolution, along with theoretical studies of the scattering and wave growth, all indicate that electromagnetic ion cyclotron (EMIC) waves can produce a very efficient loss of the ultra-relativistic electrons in the heart of the radiation belts. Moreover, a detailed analysis of the profiles of phase space densities provides direct evidence for localized loss by EMIC waves. The evolution of multi-MeV fluxes shows dramatic and very sudden enhancements of electrons for selected storms. Analysis of phase space density profiles reveals that growing peaks at different values of the first invariant are formed at approximately the same radial distance from the Earth and show the sequential formation of the peaks from lower to higher energies, indicating that local energy diffusion is the dominant source of the acceleration from MeV to multi-MeV energies. Further simultaneous analysis of the background density and ultra-relativistic electron fluxes shows that the acceleration to multi-MeV energies only occurs when plasma density is significantly depleted outside of the plasmasphere, which is consistent with the modeling of acceleration due to chorus waves.
KW  - radiation belts
KW  - ultra-relativistic electrons
KW  - EMIC waves
KW  - modeling;
KW  - plasma density
KW  - chorus waves
Y1  - 2022
U6  - https://doi.org/10.1029/2021JA030214
SN  - 2169-9380
SN  - 2169-9402
VL  - 127
IS  - 5
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Haas, Bernhard
A1  - Shprits, Yuri Y.
A1  - Allison, Hayley
A1  - Wutzig, Michael
A1  - Wang, Dedong
T1  - Which parameter controls ring current electron dynamics
JF  - Frontiers in astronomy and space sciences
N2  - Predicting the electron population of Earth's ring current during geomagnetic storms still remains a challenging task. 
In this work, we investigate the sensitivity of 10 keV ring current electrons to different driving processes, parameterised by the Kp index, during several moderate and intense storms. 
Results are validated against measurements from the Van Allen Probes satellites. Perturbing the Kp index allows us to identify the most dominant processes for moderate and intense storms respectively. 
We find that during moderate storms (Kp < 6) the drift velocities mostly control the behaviour of low energy electrons, while loss from wave-particle interactions is the most critical parameter for quantifying the evolution of intense storms (Kp > 6). Perturbations of the Kp index used to drive the boundary conditions at GEO and set the plasmapause location only show a minimal effect on simulation results over a limited L range. 
It is further shown that the flux at L & SIM; 3 is more sensitive to changes in the Kp index compared to higher L shells, making it a good proxy for validating the source-loss balance of a ring current model.
KW  - ring current
KW  - magnetosphere
KW  - electron lifetimes
KW  - electrons
KW  - van allen probes (RBSP)
KW  - ring current model
KW  - verb
Y1  - 2022
U6  - https://doi.org/10.3389/fspas.2022.911002
SN  - 2296-987X
VL  - 9
PB  - Frontiers Media
CY  - Lausanne
ER  - 
TY  - JOUR
A1  - Smirnov, Artem
A1  - Shprits, Yuri Y.
A1  - Allison, Hayley
A1  - Aseev, Nikita
A1  - Drozdov, Alexander
A1  - Kollmann, Peter
A1  - Wang, Dedong
A1  - Saikin, Anthony
T1  - Storm-Time evolution of the Equatorial Electron Pitch Angle Distributions in Earth's Outer Radiation Belt
JF  - Frontiers in astronomy and space sciences
N2  - In this study we analyze the storm-time evolution of equatorial electron pitch angle distributions (PADs) in the outer radiation belt region using observations from the Magnetic Electron Ion Spectrometer (MagEIS) instrument aboard the Van Allen Probes in 2012-2019. The PADs are approximated using a sum of the first, third and fifth sine harmonics. Different combinations of the respective coefficients refer to the main PAD shapes within the outer radiation belt, namely the pancake, flat-top, butterfly and cap PADs. We conduct a superposed epoch analysis of 129 geomagnetic storms and analyze the PAD evolution for day and night MLT sectors. PAD shapes exhibit a strong energy-dependent response. At energies of tens of keV, the PADs exhibit little variation throughout geomagnetic storms. Cap PADs are mainly observed at energies < 300 keV, and their extent in L shrinks with increasing energy. The cap distributions transform into the pancake PADs around the main phase of the storm on the nightside, and then come back to their original shapes during the recovery phase. At higher energies on the dayside, the PADs are mainly pancake during pre-storm conditions and become more anisotropic during the main phase. The quiet-time butterfly PADs can be observed on the nightside at L> 5.6. During the main phase, butterfly PADs have stronger 90 degrees-minima and can be observed at lower L-shells (down to L = 5), then transitioning into flat-top PADs at L similar to 4.5 - 5 and pancake PADs at L < 4.5. The resulting PAD coefficients for different energies, locations and storm epochs can be used to test the wave models and physics-based radiation belt codes in terms of pitch angle distributions.
KW  - pitch angle
KW  - pitch angle distributions
KW  - electrons
KW  - radiation belts
KW  - magnetosphere
KW  - van alien probes
Y1  - 2022
U6  - https://doi.org/10.3389/fspas.2022.836811
SN  - 2296-987X
VL  - 9
PB  - Frontiers Media
CY  - Lausanne
ER  - 
TY  - JOUR
A1  - Smirnov, Artem
A1  - Shprits, Yuri Y.
A1  - Allison, Hayley
A1  - Aseev, Nikita
A1  - Drozdov, Alexander
A1  - Kollmann, Peter
A1  - Wang, Dedong
A1  - Saikin, Anthony
T1  - An empirical model of the equatorial electron pitch angle distributions in earth's outer radiation belt
JF  - Space Weather: the International Journal of Research and Applications
N2  - In this study, we present an empirical model of the equatorial electron pitch angle distributions (PADs) in the outer radiation belt based on the full data set collected by the Magnetic Electron Ion Spectrometer (MagEIS) instrument onboard the Van Allen Probes in 2012-2019. The PADs are fitted with a combination of the first, third and fifth sine harmonics. The resulting equation resolves all PAD types found in the outer radiation belt (pancake, flat-top, butterfly and cap PADs) and can be analytically integrated to derive omnidirectional flux. We introduce a two-step modeling procedure that for the first time ensures a continuous dependence on L, magnetic local time and activity, parametrized by the solar wind dynamic pressure. We propose two methods to reconstruct equatorial electron flux using the model. The first approach requires two uni-directional flux observations and is applicable to low-PA data. The second method can be used to reconstruct the full equatorial PADs from a single uni- or omnidirectional measurement at off-equatorial latitudes. The model can be used for converting the long-term data sets of electron fluxes to phase space density in terms of adiabatic invariants, for physics-based modeling in the form of boundary conditions, and for data assimilation purposes.
KW  - pitch angle
KW  - radiation belt
KW  - model
KW  - magnetosphere
KW  - van allen probes;
KW  - electrons
Y1  - 2022
U6  - https://doi.org/10.1029/2022SW003053
SN  - 1542-7390
VL  - 20
IS  - 9
PB  - American Geophysical Union
CY  - Washington, DC
ER  - 
TY  - JOUR
A1  - Wang, Dedong
A1  - Shprits, Yuri Y.
A1  - Zhelayskaya, Irina S.
A1  - Agapitov, Oleksiy
A1  - Drozdov, Alexander
A1  - Aseev, Nikita
T1  - Analytical chorus wave model derived from van Allen Probe Observations
JF  - Journal of geophysical research : Space physics
N2  - Chorus waves play an important role in the dynamic evolution of energetic electrons in the Earth's radiation belts and ring current. Using more than 5 years of Van Allen Probe data, we developed a new analytical model for upper‐band chorus (UBC; 0.5fce < f < fce) and lower‐band chorus (LBC; 0.05fce < f < 0.5fce) waves, where fce is the equatorial electron gyrofrequency. By applying polynomial fits to chorus wave root mean square amplitudes, we developed regression models for LBC and UBC as a function of geomagnetic activity (Kp), L, magnetic latitude (λ), and magnetic local time (MLT). Dependence on Kp is separated from the dependence on λ, L, and MLT as Kp‐scaling law to simplify the calculation of diffusion coefficients and inclusion into particle tracing codes. Frequency models for UBC and LBC are also developed, which depends on MLT and magnetic latitude. This empirical model is valid in all MLTs, magnetic latitude up to 20°, Kp ≤ 6, L‐shell range from 3.5 to 6 for LBC and from 4 to 6 for UBC. The dependence of root mean square amplitudes on L are different for different bands, which implies different energy sources for different wave bands. This analytical chorus wave model is convenient for inclusion in quasi‐linear diffusion calculations of electron scattering rates and particle simulations in the inner magnetosphere, especially for the newly developed four‐dimensional codes, which require significantly improved wave parameterizations.
KW  - chorus waves
KW  - radiation belt electrons
KW  - ring current electrons
KW  - analytical model
KW  - wave-particle interactions
KW  - diffusion coefficients
Y1  - 2019
U6  - https://doi.org/10.1029/2018JA026183
SN  - 2169-9380
SN  - 2169-9402
VL  - 124
IS  - 2
SP  - 1063
EP  - 1084
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Aseev, Nikita
A1  - Shprits, Yuri Y.
A1  - Wang, Dedong
A1  - Wygant, John
A1  - Drozdov, Alexander
A1  - Kellerman, Adam C.
A1  - Reeves, Geoffrey D.
T1  - Transport and loss of ring current electrons inside geosynchronous orbit during the 17 March 2013 storm
JF  - Journal of geophysical research : Space physics
N2  - Ring current electrons (1–100 keV) have received significant attention in recent decades, but many questions regarding their major transport and loss mechanisms remain open. In this study, we use the four‐dimensional Versatile Electron Radiation Belt code to model the enhancement of phase space density that occurred during the 17 March 2013 storm. Our model includes global convection, radial diffusion, and scattering into the Earth's atmosphere driven by whistler‐mode hiss and chorus waves. We study the sensitivity of the model to the boundary conditions, global electric field, the electric field associated with subauroral polarization streams, electron loss rates, and radial diffusion coefficients. The results of the code are almost insensitive to the model parameters above 4.5 RERE, which indicates that the general dynamics of the electrons between 4.5 RE and the geostationary orbit can be explained by global convection. We found that the major discrepancies between the model and data can stem from the inaccurate electric field model and uncertainties in lifetimes. We show that additional mechanisms that are responsible for radial transport are required to explain the dynamics of ≥40‐keV electrons, and the inclusion of the radial diffusion rates that are typically assumed in radiation belt studies leads to a better agreement with the data. The overall effect of subauroral polarization streams on the electron phase space density profiles seems to be smaller than the uncertainties in other input parameters. This study is an initial step toward understanding the dynamics of these particles inside the geostationary orbit.
KW  - ring current electrons
KW  - magnetospheric convection
KW  - ensemble modeling
KW  - inner magnetosphere
KW  - electron transport
KW  - wave-particle interactions
Y1  - 2019
U6  - https://doi.org/10.1029/2018JA026031
SN  - 2169-9380
SN  - 2169-9402
VL  - 124
IS  - 2
SP  - 915
EP  - 933
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Wang, Dedong
A1  - Shprits, Yuri Y.
T1  - On How High-Latitude Chorus Waves Tip the Balance Between Acceleration and Loss of Relativistic Electrons
JF  - Geophysical research letters
N2  - Modeling and observations have shown that energy diffusion by chorus waves is an important source of acceleration of electrons to relativistic energies. By performing long-term simulations using the three-dimensional Versatile Electron Radiation Belt code, in this study, we test how the latitudinal dependence of chorus waves can affect the dynamics of the radiation belt electrons. Results show that the variability of chorus waves at high latitudes is critical for modeling of megaelectron volt (MeV) electrons. We show that, depending on the latitudinal distribution of chorus waves under different geomagnetic conditions, they cannot only produce a net acceleration but also a net loss of MeV electrons. Decrease in high-latitude chorus waves can tip the balance between acceleration and loss toward acceleration, or alternatively, the increase in high-latitude waves can result in a net loss of MeV electrons. Variations in high-latitude chorus may account for some of the variability of MeV electrons.
KW  - radiation belts
KW  - chorus waves
KW  - high latitude
KW  - acceleration
KW  - loss
KW  - modeling
Y1  - 2019
U6  - https://doi.org/10.1029/2019GL082681
SN  - 0094-8276
SN  - 1944-8007
VL  - 46
IS  - 14
SP  - 7945
EP  - 7954
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Castillo, Angelica M.
A1  - Shprits, Yuri Y.
A1  - Ganushkina, Natalia
A1  - Drozdov, Alexander
A1  - Aseev, Nikita
A1  - Wang, Dedong
A1  - Dubyagin, Stepan
T1  - Simulations of the inner magnetospheric energetic electrons using the IMPTAM-VERB coupled model
JF  - Journal of Atmospheric and Solar-Terrestrial Physics
N2  - In this study, we present initial results of the coupling between the Inner Magnetospheric Particle Transport and Acceleration Model (IMPTAM) and the Versatile Electron Radiation Belt (VERB-3D) code. IMPTAM traces electrons of 10-100 keV energies from the plasma sheet (L = 9 Re) to inner L-shell regions. The flux evolution modeled by IMPTAM is used at the low energy and outer L* computational boundaries of the VERB code (assuming a dipole approximation) to perform radiation belt simulations of energetic electrons. The model was tested on the March 17th, 2013 storm, for a six-day period. Four different simulations were performed and their results compared to satellites observations from Van Allen probes and GOES. The coupled IMPTAM-VERB model reproduces evolution and storm-time features of electron fluxes throughout the studied storm in agreement with the satellite data (within similar to 0.5 orders of magnitude). Including dynamics of the low energy population at L* = 6.6 increases fluxes closer to the heart of the belt and has a strong impact in the VERB simulations at all energies. However, inclusion of magnetopause losses leads to drastic flux decreases even below L* = 3. The dynamics of low energy electrons (max. 10s of keV) do not affect electron fluxes at energies >= 900 keV. Since the IMPTAM-VERB coupled model is only driven by solar wind parameters and the Dst and Kp indexes, it is suitable as a forecasting tool. In this study, we demonstrate that the estimation of electron dynamics with satellite-data-independent models is possible and very accurate.
KW  - Electron populations
KW  - Radiation belts
KW  - IMPTAM
KW  - VERB
Y1  - 2019
U6  - https://doi.org/10.1016/j.jastp.2019.05.014
SN  - 1364-6826
SN  - 1879-1824
VL  - 191
PB  - Elsevier
CY  - Oxford
ER  -