Observational Evidence and Formation Mechanism of Low-Density Cells in the Upper Thermosphere on September 8, 2017

The low-density cell structure in the high-latitude thermosphere is referred to as the density depletion with respect to the adjacent area. Based on Gravity Recovery and Climate Experiment (GRACE) accelerometer data during the September 2017 geomagnetic storms, the thermospheric mass density at about 350 km are estimated and further investigated especially in the high-latitude regions. At least two kinds of low-density cells over the Southern Hemisphere (SH) are observed along the GRACE orbit. To understand the low-density cell structures over the SH observed by GRACE, we investigate the underlying physical mechanism based on thermosphere-ionosphere numerical simulations using Thermosphere-Ionosphere Electrodynamic General Circulation Model and Global Ionosphere Thermosphere Model. According to the simulation results, the formation mechanism of the low-density cell is attributed to the storm-time vertical advection and horizontal velocity divergence driven by the auroral ion convection. The critical height of observable low-density cells is shown to be not less than 350 km. The meridional spatial scale of observed low-density cells over the SH are approximately or slightly larger than 1,500 km. Three low-density cells, including two in the dawn sector and one in the night sector were observed about 1 hour after the direction of interplanetary magnetic field BY component reversed. The occurrence of thermospheric low-density structure is essential to be included in the empirical model during geomagnetic storm time.