MAGE simulation of storm-time ionospheric structures: Geospace plume and the post-sunset equatorial density depletion

During geomagnetic storm times, multiscale dynamic processes establish throughout the geospace in response to the solar wind driving. In the dusk sector sub-auroral region, strong electric field exists and leads to the subauroral polarization stream (SAPS). Below the SAPS region, penetration electric field exists when the Region-1 and Region-2 are imbalanced. We present case studies focusing on two ionospheric structures that are closely related to SAPS and the penetration electric field respectively. The first is the geospace plume, an enhanced-density structure referring to the combined processes of the plasmaspheric plume and the ionospheric total electron content (TEC) plume that extends from the midlatitude to the polar cap. The apparent joint temporal evolution between the two plumes indicates strong magnetosphere-ionosphere coupling. The second storm-time phenomenon is a depleted-density structure, the post-sunset equatorial density depletion. The steamer-like plasma density depletion initiates at the equator and expand poleward to the midlatitude during the storm main phase. 

To comprehensively investigate the driving factors for these two phenomena in the geospace system, we employ the two-way coupled Magnetosphere-Ionosphere-Thermosphere (M-I-T) model, Multiscale Atmosphere Geospace Environment (MAGE) in two storm event studies. We present the numerical simulation of the geospace plume in March 31, 2001 superstorm. The simulation unveils the major role of SAPS in shaping the geospace plume and the self-consistent relations among important storm-time dynamics including the ring current buildup, SAPS and the geospace plume development. In the simulation of Sept 8, 2017 storm, MAGE successfully reproduces the post-sunset plasma density depletion. According to the simulation, the post-sunset low density region is seeded by the prereversal enhancement, an enhancement of the vertical E × B drift at the evening terminator at the magnetic equator, and depleted by the ExB transport effect as it expands toward the midlatitude. Numerical experiments demonstrate the penetration electric field is the major cause of the westward and poleward expansion of the depletion. The depletion channel then merges into the plasma density trough contributed by the SAPS. These two phenomena depict the storm-time ionospheric dynamics in the dusk sector.