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1. External and Internal Causes of the Stormtime Intensification of the Dawnside Westward Auroral Electrojet

   https://doi.org/10.1029/2023JA031457  

   Ohtani, S. ; Sorathia, K. ; Merkin, V. G. ; Frey, H. U. ; Gjerloev, J. W. ( October 2023 , Journal of Geophysical Research: Space Physics)

   The dawn‐dusk asymmetry of magnetic depression is a characteristic feature of the storm main phase. Recently Ohtani (2021,https://doi.org/10.1029/2021JA029643) reported that its magnitude is correlated with the dawnside westward auroral electrojet (AEJ) intensity, and suggested that the dawnside AEJ intensification is a fundamental process of the stormtime magnetosphere‐ionosphere coupling. In this study we observationally address the cause of the dawnside AEJ intensification in terms of four scenarios. That is, the dawnside AEJ intensifies because (a) the external driving of global convection strengthens, (b) solar wind compression enhances energetic electron precipitation, and therefore, ionospheric conductance, through wave‐particle interaction, (c) the substorm current wedge forms in the dawn sector, and (d) energetic electrons injected by nightside substorms drift dawnward, and the subsequent precipitation enhances ionospheric conductance. We find an event that fits each scenario, and therefore, none of these scenarios can be precluded. However, the result of a superposed epoch analysis shows that some causes are more prevalent than others. More specifically, (a) although the enhancement of external driving may precondition the dawnside AEJ intensification, it is rarely the direct cause; (b) external compression probably explains only a small fraction of the events; (c) prior to the dawnside AEJ intensification, the westward AEJ tends to intensify in the midnight sector along with mid‐latitude positive bays, which suggests that the substorm injection of energetic electrons is the most prevalent cause. This last result may also be explained by the dawnside expansion of the substorm current wedge, which, however, is arguably far less common.

    

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2. Validation of Inner Magnetosphere Particle Transport and Acceleration Model (IMPTAM) With Long‐Term GOES MAGED Measurements of keV Electron Fluxes at Geostationary Orbit

   https://doi.org/10.1029/2018SW002028  

   Ganushkina, N. Yu ; Sillanpää, I. ; Welling, D. ; Haiducek, J. ; Liemohn, M. ; Dubyagin, S. ; Rodriguez, J. V. ( May 2019 , Space Weather)

   Surface charging by keV (kiloelectron Volt) electrons can pose a serious risk for satellites. There is a need for physical models with the correct and validated dynamical behavior. The 18.5‐month (2013–2015) output from the continuous operation online in real time as a nowcast of the Inner Magnetosphere Particle Transport and Acceleration Model (IMPTAM) is compared to the GOES 13 MAGnetospheric Electron Detector (MAGED) data for 40, 75, and 150 keV energies. The observed and modeled electron fluxes were organized by Magnetic Local Time (MLT) and IMPTAM driving parameters; the observed Interplanetary Magnetic Field (IMF)B_Z,B_Y, and |B|; the solar wind speedV_SW; the dynamic pressureP_SW; andKpandSYM‐Hindices. The peaks for modeled fluxes are shifted toward midnight, but the ratio between the observed and modeled fluxes at around 06 MLT is close to 1. All the statistical patterns exhibit very similar features with the largest differences of about 1 order of magnitude at 18–24 MLT. Based on binary event analysis, 20–78% of threshold crossings are reproduced, but Heidke skill scores are low. The modeled fluxes are off by a factor of 2 in terms of the median symmetric accuracy. The direction of the error varies with energy: overprediction by 50% for 40 keV, overprediction by 2 for 75 keV, and underprediction by 18% for 150 keV. The revealed discrepancies are due to the boundary conditions developed for ions but used for electrons, absence of substorm effects, representations of electric and magnetic fields which can result in not enough adiabatic acceleration, and simple models for electron lifetimes.

    

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3. MMS Observations of the Multiscale Wave Structures and Parallel Electron Heating in the Vicinity of the Southern Exterior Cusp

   https://doi.org/10.1029/2019JA027698  

   Nykyri, K. ; Ma, X. ; Burkholder, B. ; Rice, R. ; Johnson, J. R. ; Kim, E‐K. ; Delamere, P. ; Michael, A. ; Sorathia, K. ; Lin, D. ; et al ( March 2021 , Journal of Geophysical Research: Space Physics)

   Understanding the physical mechanisms responsible for the cross‐scale energy transport and plasma heating from solar wind into the Earth's magnetosphere is of fundamental importance for magnetospheric physics and for understanding these processes in other places in the universe with comparable plasma parameter ranges. This paper presents observations from the Magnetosphere Multiscale (MMS) mission at the dawn‐side high‐latitude dayside boundary layer on February 25, 2016 between 18:55 and 20:05 UT. During this interval, MMS encountered both the inner and outer boundary layers with quasiperiodic low frequency fluctuations in all plasma and field parameters. The frequency analysis and growth rate calculations are consistent with the Kelvin‐Helmholtz instability (KHI). The intervals within the low frequency wave structures contained several counter‐streaming, low‐ (0–200 eV) and mid‐energy (200 eV–2 keV) electrons in the loss cone and trapped energetic (70–600 keV) electrons in alternate intervals. The counter‐streaming electron intervals were associated with large‐magnitude field‐aligned Poynting fluxes. Burst mode data at the large Alfvén velocity gradient revealed a strong correlation between counter streaming electrons, enhanced parallel electron temperatures, strong anti‐field aligned wave Poynting fluxes, and wave activity from sub‐proton cyclotron frequencies extending to electron cyclotron frequency. Waves were identified as Kinetic Alfvén waves but their contribution to parallel electron heating was not sufficient to explain the >100 eV electrons.

    

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4. Mesoscale and Small-Scale Structure of the Subauroral Geospace

   https://doi.org/10.1002/9781119815617.ch8  

   E.V. Mishin and A.V. Streltsov ( March 2021 , Ionosphere Dynamics and Applications)

   :Chaosong Huang, Gang Lu (Ed.)

   A review is given of the current state-of-the-art of experimental studies and the theoretical understanding of meso-scale and small-scale structure of the subauroral geospace, connecting ionospheric structures to plasma wave processes in the turbulent plasmasphere boundary layer (TPBL). Free energy for plasma waves comes from diamagnetic electron and ion currents in the entry layer near the plasma sheet boundary and near the TPBL inner boundary, respectively, and anisotropic distributions of energetic ions inside the TPBL and interior to the inner boundary. Collisionless heating of the plasmaspheric particles gives downward heat and suprathermal electron fluxes sufficient to provide the F-region electron temperature greater than 6000 K. This leads to the formation of specific density troughs in the ionospheric regions in the absence of strong electric fields and upward plasma flows. Small-scale MHD wave structures (SAPSWS) and irregular density troughs emerge on the duskside, coincident with the substorm current wedge development. Numerical simulations show that the ionospheric feedback instability significantly contributes to the SAPSWS formation. Antiparallel temperature and density gradients inside the subauroral troughs lead to the temperature gradient instability. The latter and the gradient-drift instability lead to enhanced decameter-scale irregularities responsible for subauroral HF radar backscatter 

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5. Modeling the Dynamics of Radiation Belt Electrons With Source and Loss Driven by the Solar Wind

   https://doi.org/10.1029/2020JA028988  

   Xiang, Zheng ; Li, Xinlin ; Kapali, Sudha ; Gannon, Jennifer ; Ni, Binbin ; Zhao, Hong ; Zhang, Kun ; Khoo, Leng Ying ( May 2021 , Journal of Geophysical Research: Space Physics)

   A radial diffusion model directly driven by the solar wind is developed to reproduce MeV electron variations betweenL = 2–12 (LisL* in this study) from October 2012 to April 2015. The radial diffusion coefficient, internal source rate, quick loss due to EMIC waves, and slow loss due to hiss waves are all expressed in terms of the solar wind speed, dynamic pressure, and interplanetary magnetic field (IMF). The model achieves a prediction efficiency (PE) of 0.45 atL = 5 and 0.51 atL = 4 after converting the electron phase space densities to differential fluxes and comparing with Van Allen Probes measurements of 2  and 3 MeV electrons atL = 5 andL = 4, respectively. Machine learning techniques are used to tune parameters to get higher PE. By tuning parameters for every 60‐day period, the model obtains PE values of 0.58 and 0.82 atL = 5 andL = 4, respectively. Inspired by these results, we divide the solar wind activity into three categories based on the condition of solar wind speed, IMF Bz, and dynamic pressure, and then tune these three sets of parameters to obtain the highest PE. This experiment confirms that the solar wind speed has the greatest influence on the electron flux variations, particularly at higherL, while the dynamic pressure has more influence at lower L. Also, the PE atL = 4 is mostly higher than those atL = 5, suggesting that the electron loss due to the magnetopause shadowing combined with the outward radial diffusion is not well captured in the model.

    

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