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An important question that is being increasingly studied across subdisciplines of Heliophysics is “how do mesoscale phenomena contribute to the global response of the system?” This review paper focuses on this question within two specific but interlinked regions in Near-Earth space: the magnetotail’s transition region to the inner magnetosphere and the ionosphere. There is a concerted effort within the Geospace Environment Modeling (GEM) community to understand the degree to which mesoscale transport in the magnetotail contributes to the global dynamics of magnetic flux transport and dipolarization, particle transport and injections contributing to the storm-time ring current development, and the substorm current wedge. Because the magnetosphere-ionosphere is a tightly coupled system, it is also important to understand how mesoscale transport in the magnetotail impacts auroral precipitation and the global ionospheric system response. Groups within the Coupling, Energetics and Dynamics of Atmospheric Regions Program (CEDAR) community have also been studying how the ionosphere-thermosphere responds to these mesoscale drivers. These specific open questions are part of a larger need to better characterize and quantify mesoscale “messengers” or “conduits” of information—magnetic flux, particle flux, current, and energy—which are key to understanding the global system. After reviewing recent progress and open questions, we suggest datasets that, if developed in the future, will help answer these questions. 

Abstract The Starlink satellites launched on 3 February 2022 were lost before they fully arrived in their designated orbits. The loss was attributed to two moderate geomagnetic storms that occurred consecutively on 3–4 February. We investigate the thermospheric neutral mass density variation during these storms with the Multiscale Atmosphere‐Geospace Environment (MAGE) model, a first‐principles, fully coupled geospace model. Simulated neutral density enhancements are validated by Swarm satellite measurements at the altitude of 400–500 km. Comparison with standalone TIEGCM and empirical NRLMSIS 2.0 and DTM‐2013 models suggests better performance by MAGE in predicting the maximum density enhancement and resolving the gradual recovery process. Along the Starlink satellite orbit in the middle thermosphere (∼200 km altitude), MAGE predicts up to 150% density enhancement near the second storm peak while standalone TIEGCM, NRLMSIS 2.0, and DTM‐2013 suggest only ∼50% increase. MAGE also suggests altitudinal, longitudinal, and latitudinal variability of storm‐time percentage density enhancement due to height dependent Joule heating deposition per unit mass, thermospheric circulation changes, and traveling atmospheric disturbances. This study demonstrates that a moderate storm can cause substantial density enhancement in the middle thermosphere. Thermospheric mass density strongly depends on the strength, timing, and location of high‐latitude energy input, which cannot be fully reproduced with empirical models. A physics‐based, fully coupled geospace model that can accurately resolve the high‐latitude energy input and its variability is critical to modeling the dynamic response of thermospheric neutral density during storm time. 

Abstract Solar eruptions cause geomagnetic storms in the near‐Earth environment, creating spectacular aurorae visible to the human eye and invisible dynamic changes permeating all of geospace. Just equatorward of the aurora, radars and satellites often observe intense westward plasma flows called subauroral polarization streams (SAPS) in the dusk‐to‐midnight ionosphere. SAPS occur across a narrow latitudinal range and lead to intense frictional heating of the ionospheric plasma and atmospheric neutral gas. SAPS also generate small‐scale plasma waves and density irregularities that interfere with radio communications. As opposed to the commonly observed duskside SAPS, intense eastward subauroral plasma flows in the morning sector were recently discovered to have occurred during a super storm on 20 November 2003. However, the origin of these flows termed “dawnside SAPS” could not be explained by the same mechanism that causes SAPS on the duskside and has remained a mystery. Through real‐event global geospace simulations, here we demonstrate that dawnside SAPS can only occur during major storm conditions. During these times, the magnetospheric plasma convection is so strong as to effectively transport ions to the dawnside, whereas they are typically deflected to the dusk by the energy‐dependent drifts. Ring current pressure then builds up on the dawnside and drives field‐aligned currents that connect to the subauroral ionosphere, where eastward SAPS are generated. The origin of dawnside SAPS explicated in this study advances our understanding of how the geospace system responds to strongly disturbed solar wind driving conditions that can have severe detrimental impacts on human society and infrastructure. 

Abstract The role of diffuse electron precipitation in the formation of subauroral polarization streams (SAPS) is investigated with the Multiscale Atmosphere‐Geospace Environment (MAGE) model. Diffuse precipitation is derived from the distribution of drifting electrons. SAPS manifest themselves as a separate mesoscale flow channel in the duskside ionosphere, which gradually merges with the primary auroral convection toward dayside as the equatorward auroral boundary approaches the poleward Region‐2 field‐aligned currents (FACs) boundary. SAPS expand to lower latitudes and toward the nightside during the main phase of a geomagnetic storm, associated with magnetotail earthward plasma flows building up the ring current and intensifying Region‐2 FACs and electron precipitation. SAPS shrink poleward and sunward as the interplanetary magnetic field turns northward. When diffuse precipitation is turned off in a controlled MAGE simulation, ring current and duskside Region‐2 FACs become weaker, but subauroral zonal ion drifts are still comparable to auroral convection. However, subauroral and auroral convection manifest as a single broad flow channel without showing any mesoscale structure. SAPS overlap with the downward Region‐2 FACs equatorward of diffuse precipitation, where poleward electric fields are strong due to a low conductance in the subauroral ionosphere. The Region‐2 FACs extend to latitudes lower than the diffuse precipitation because the ring current protons penetrate closer to the Earth than the electrons do. This study reproduces the key physics of SAPS formation and their evolution in the coupled magnetosphere‐ionosphere during a geomagnetic storm. Diffuse electron precipitation is demonstrated to play a critical role in determining SAPS location and structure.