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The geospace plume, referring to the combined processes of the plasmaspheric and the ionospheric storm‐enhanced density (SED)/total electron content (TEC) plumes, is one of the unique features of geomagnetic storms. The apparent spatial overlap and joint temporal evolution between the plasmaspheric plume and the equatorial mapping of the SED/TEC plume indicate strong magnetospheric‐ionospheric coupling. However, a systematic modeling study of the factors contributing to geospace plume development has not yet been performed due to the lack of a sufficiently comprehensive model including all the relevant physical processes. In this paper, we present a numerical simulation of the geospace plume in the 31 March 2001 storm using the Multiscale Atmosphere‐Geospace Environment model. The simulation reproduces the observed linkage of the two plumes, which, we interpret as a result of both being driven by the electric field that maps between the magnetosphere and the ionosphere. The model predicts two velocity channels of sunward plasma drift at different latitudes in the dusk sector during the storm main phase, which are identified as the sub‐auroral polarization stream (subauroral polarization streams (SAPS)) and the convection return flow, respectively. The SAPS is responsible for the erosion of the plasmasphere plume and contributes to the ionospheric TEC depletion in the midlatitude trough region. We further find the spatial distributions of the magnetospheric ring current ions and electrons, determined by a delicate balance of the energy‐dependent gradient/curvature drifts and the E × B drifts, are crucial to sustain the SAPS electric field that shapes the geospace plume throughout the storm main phase. Key Points: The first whole geospace simulation to demonstrate coherent storm‐time evolution of plasmaspheric and total electron content (TEC) plumesThe model demonstrates plasmasphere erosion and TEC depletion by the subauroral polarization streams (SAPS)SAPS is sustained by magnetospheric ion and electron distributions formed by a delicate balance of energy‐dependent and E × B drifts [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Subauroral polarization streams (SAPS) are a mesoscale (∼100–500 km) plasma flow channel frequently observed in the duskside subauroral ionosphere. This study investigates how diffuse electron precipitation affects the location and structure of SAPS, using the newly developed, state‐of‐the‐art geospace model of Multiscale Atmosphere‐Geospace Environment (MAGE). The MAGE model has the capability to directly simulate diffuse precipitation using particle drift physics. MAGE numerical experiments show that SAPS exhibit as a separate flow channel when diffuse precipitation is included in the simulation, but subauroral and auroral convections become one broad channel when diffuse precipitation is turned off. SAPS are produced in the gap region between the low latitude boundaries of electron aurora and downward field‐aligned current (FAC) on the duskside, where the ionospheric conductance is low due to lack of ionization while downward region‐2 FAC closes through this low conductance region. Strong poleward electric fields are generated to drive large westward ion drifts in the SAPS channel. Tracing back to the magnetosphere, the gap between the inner boundaries is formed because the ring current protons, whose distribution primarily determines the downward FAC, penetrate deeper than the electrons. Thus diffuse aurora, with plasma sheet as the source population, occurs poleward of the lower boundary of the downward Region‐2 current. This study demonstrates the importance of including diffuse precipitation in coupled geospace models to understand the dynamics of SAPS. Key Points: A fully coupled geospace model captures the key physical mechanism and distinct spatial structure of subauroral polarization streamsThe separation of the flow channel from the main auroral convection is determined by diffuse electron precipitation boundaryThe evolution of the SAPS in the ionosphere and magnetosphere is reproduced by the simulation in accordance with observations [ABSTRACT FROM AUTHOR]

The pursuit of realistic simulation of the physics of plasma transport, ring current formation and storm‐triggered Earth magnetic and electric field is an ongoing challenge in magnetospheric physics. To this end, we have implemented a coupling of the Lyon‐Fedder‐Mobarry (LFM) global magnetohydrodynamic model with the Rice convection model‐equilibrium (RCM‐E) of the inner‐magnetosphere and plasma sheet. This one‐way coupling scheme allows continuous update of the RCM‐E boundary conditions from the plasma moments calculated by the LFM while preserving entropy conservation. This results in a model that has the high‐resolution self‐consistent description of the inner magnetosphere and includes the effects of time‐dependent outer‐magnetospheric electromagnetic fields and plasma configurations. In addition, driving the RCM‐E in this way resolves the issue of having a plasma‐β‐constrained region in the coupled model of LFM‐RCM and expands the RCM‐E simulation region farther out into plasma sheet where the storm‐time plasma transportation takes place. In the ionosphere, the RCM‐E replaces the ionospheric electric field model of LFM with the one used by the RCM. The electric potential produced, along with the realistic ionospheric precipitation patterns shows strong consistency with the transportation patterns in the plasma sheet featured with well‐resolved bubbles and bursty bulk flows. Results from the simulations of an idealized event will be presented and discussed. Plain Language Summary: Understanding the important phenomena in the inner magnetosphere such as plasma transport, ring current formation, storm‐triggered Earth electromagnetic field changes and related ionospheric signatures is of great importance to space weather research. We implement a new coupling scheme of two models: the Lyon‐Fedder‐Mobarry (LFM) global magnetohydrodynamic model that simulates the global evolution of the magnetosphere and the Rice convection model‐equilibrium (RCM‐E) which self‐consistently describes the dynamics in the inner magnetosphere. Compared with the coupled model of LFM‐RCM, this new coupling scheme expands the RCM simulation region significantly farther out into plasma sheet, so the trajectory and evolution of the plasma flows can be tracked. In addition, the built‐in potential solver of RCM‐E allows us to more accurately connect the plasma distribution to the ionospheric potential by the Birkeland currents. The resulting electric potential better resolves the ionospheric features that correspond to the flow patterns in the plasma sheet than that in LFM‐RCM. The simulated precipitation patterns on the polar cap resemble the aurora observations during the injection events. Key Points: Self‐consistent inner‐magnetosphere model is driven by inputs from the Lyon‐Fedder‐Mobarry global magnetohydrodynamic modelThe expanded inner magnetospheric modeling region captures high‐resolution bursty bulk flows in the plasma sheetRealistic bursty bulk flows induced aurora patterns are simulated [ABSTRACT FROM AUTHOR]

The 17 March 2015 St. Patrick's Day Storm is the largest geomagnetic storm to date of Solar Cycle 24, with a D s t of −223 nT. The magnetopause moved inside geosynchronous orbit under high solar wind dynamic pressure and strong southward interplanetary magnetic field B z causing loss; however, a subsequent drop in pressure allowed for rapid rebuilding of the radiation belts. The 17 March 2013 storm also shows similar effects on outer zone electrons: first, a rapid dropout due to inward motion of the magnetopause followed by rapid increase in flux above the prestorm level early in the recovery phase and a slow increase over the next 12 days. These phases can be seen in temporal evolution of the electron phase space density measured by the Energetic Particle, Composition, and Thermal Plasma Suite (ECT) instruments on Van Allen Probes. Using the Lyon-Fedder-Mobarry global MHD model driven by upstream solar wind measurements, we simulated both St. Patrick's Day 2013 and 2015 events, analyzing Lyon-Fedder-Mobarry electric and magnetic fields to calculate radial diffusion coefficients. These coefficients have been implemented in a radial diffusion code, using the measured electron phase space density following the local heating as the initial radial profile and outer boundary condition for subsequent temporal evolution over the next 12 days, beginning 18 March. Agreement with electron phase space density at 1000 MeV/G measured by the MagEIS component of the ECT instrument suite on Van Allen Probes was much improved using radial diffusion coefficients from the MHD simulations relative to coefficients parameterized by a global geomagnetic activity index. [ABSTRACT FROM AUTHOR]

Prompt enhancement of relativistic electron flux at L = 3-5 has been reported from Van Allen Probes Relativistic Electron Proton Telescope (REPT) measurements associated with the 17 March 2015 interplanetary shock compression of the dayside magnetosphere. Acceleration by ∼1 MeV is inferred on less than a drift timescale as seen in prior shock compression events, which launch a magnetosonic azimuthal electric field impulse tailward. This impulse propagates from the dayside around the flanks accelerating electrons in drift resonance at the dusk flank. Such longitudinally localized acceleration events produce a drift echo signature which was seen at >1 MeV energy on both Van Allen Probe spacecraft, with sustained observations by Probe B outbound at L = 5 at 2100 MLT at the time of impulse arrival, measured by the Electric Fields and Waves instrument. MHD test particle simulations are presented which reproduce drift echo features observed in the REPT measurements at Probe B, including the energy and pitch angle dependence of drift echoes observed. While the flux enhancement was short lived for this event due to subsequent inward motion of the magnetopause, stronger events with larger electric field impulses, as observed in March 1991 and the Halloween 2003 storm, produce enhancements which can be quantified by the inward radial transport and energization determined by the induction electric field resulting from dayside compression. [ABSTRACT FROM AUTHOR]