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  1. Abstract

    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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  3. Abstract

    Earth's magnetotail plays a critical role in the dynamics of the magnetosphere, particularly during intervals of geomagnetic activity. To improve our understanding of the ion dynamics in this region, energetic neutral atom (ENA) imaging can provide global measurements to place in situ measurements in context and validate simulations. The NASA Two Wide‐angle Imaging Neutral‐atom Spectrometers mission provided near‐continuous observations using ENA imagers. ENA data can be used to calculate maps of equatorial ion temperatures that often show observations of regions of enhanced temperatures associated with phenomena in the magnetotail such as magnetic reconnection and narrow flow channels. We present an algorithm that can be used to search through a collection of these maps to identify intervals with such enhancements for further study. The algorithm results are validated against two sets of related phenomena: (a) a database of dipolarizing flux bundle (DFB) measurements from THEMIS and (b) a list of substorm onsets from SuperMAG. We demonstrate that the algorithm is very good at identifying intervals when there are DFB measurements or substorm onsets as long as there sufficient ENA data. We discuss some potential scientific studies that can result from use of the algorithm. We also show a preliminary application of the algorithm to simulation output to demonstrate the usefulness for other datasets, facilitate comparative studies, and introduce a new method for model validation.

     
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  4. Abstract

    Electrons with energies ≥40 keV can be found at low density in many different regions of Earth's magnetosphere. A litany of fundamental questions in space physics have focused on the acceleration mechanism of these particles, given that the sources of plasma are the relatively cool ionosphere and solar wind (∼1–100s eV). Upgraded global solar wind‐magnetosphere simulations which can resolve mesoscale dynamics have the ability to enhance our understanding of these high energy particles. This is because the energization of particles often takes the form of a sequence of discrete steps, potentially occurring in different regions of the magnetosphere and due to both meso‐ and global‐scale processes. First, brief results are presented from the Grid Agnostic MHD for Extended Research Applications (GAMERA) global simulation on the structure of the cusp diamagnetic cavity for northward and southward IMF. Then, the Conservative Hamiltonian Integrator for Magnetospheric Particles (CHIMP) framework, with both guiding center and full Lorentz integrators, evolves necessary parameters such as the energy and pitch angle of electron test particles to investigate particle acceleration inside the cavity, as well as the ultimate fate of electrons accelerated inside the cavity. The simulation shows that particles can gain ≥ 10 keV inside the cavity and subsequently leak into the magnetosheath or onto dipolar field lines where they execute different types of bounce motion. The distribution of test particles initialized inside the cavity is compared with Magnetospheric Multi‐Scale (MMS) observations.

     
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  5. Abstract

    This paper addresses the question of the contribution of azimuthally localized flow channels and magnetic field dipolarizations embedded in them in the global dipolarization of the inner magnetosphere during substorms. We employ the high‐resolution Lyon‐Fedder‐Mobarry global magnetosphere magnetohydrodynamic model and simulate an isolated substorm event, which was observed by the geostationary satellites and by the Magnetospheric Multiscale spacecraft. The results of our simulations reveal that plasma sheet flow channels (bursty bulk flows, BBFs) and elementary dipolarizations (dipolarization fronts, DFs) occur in the growth phase of the substorm but are rare and do not penetrate to the geosynchronous orbit. The substorm onset is characterized by an abrupt increase in the occurrence and intensity of BBFs/DFs, which penetrate well earthward of the geosynchronous orbit during the expansion phase. These azimuthally localized structures are solely responsible for the global (in terms of the magnetic local time) dipolarization of the inner magnetosphere toward the end of the substorm expansion. Comparison with the geostationary satellites and Magnetospheric Multiscale data shows that the properties of the BBFs/DFs in the simulation are similar to those observed, which gives credence to the above results. Additionally, the simulation reveals many previously observed signatures of BBFs and DFs, including overshoots and oscillations around their equilibrium position, strong rebounds and vortical tailward flows, and the corresponding plasma sheet expansion and thinning.

     
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  6. Abstract

    Thermospheric mass density perturbations are commonly observed during geomagnetic storms and fundamental to upper atmosphere dynamics, but the sources of these perturbations are not well understood. Large neutral density perturbations during storms greatly affect the drag experienced by low Earth orbit. We investigated the thermospheric density perturbations at all latitudes observed along the CHAMP and GRACE satellite trajectories during the August 24–25, 2005 geomagnetic storm. Observations show that large neutral density enhancements occurred not only at high latitudes, but also globally. Large density perturbations were seen in the equatorial regions away from the high‐latitude, magnetospheric energy sources. We used the high‐resolution Multiscale Atmosphere Geospace Environment (MAGE) model to simulate consecutive neutral density changes observed by satellites during the storm. The MAGE simulation, which resolved mesoscale high‐latitude convection electric fields and field‐aligned currents, and included physics‐based specification of auroral precipitation, was contrasted with a standalone ionosphere‐thermosphere simulation driven by a high‐latitude electrodynamics empirical model. The comparison demonstrates that first‐principles representations of highly dynamic and localized Joule heating events in a fully coupled whole geospace model is critical to accurately capture both generation and propagation of traveling atmospheric disturbances (TADs) that produce neutral density perturbations globally. The MAGE simulation shows that larger density peaks in the equatorial region observed by CHAMP and GRACE are the result of TADs generated at high‐latitudes in both hemispheres, and intersect at low‐latitudes. This study reveals the importance of investigating thermospheric density variations at all latitudes in a fully coupled geospace model with sufficiently high resolving power.

     
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