A Severe Erosion of the Plasmasphere Arising from the September 2017 Storm

A severe depletion of the plasmasphere was studied by using the comprehensive observations including the PWE/Arase data and the Ionosphere-Plasmasphere Electrodynamics (IPE) model simulations. The electron number density given by the Arase/PWE shows that the plasmapause was located at L = 1.6-1.7 during the recovery phase of the September 2017 storm. The degree of the severity of the erosion is much more than what would be expected from the relatively normal value of the SYM-H min (-146 nT). The midlatitude ionospheric trough shown in the ratio of difference ATEC widely expanded equatorward and reached ~48° in the dusk side ionosphere. The nightside EEJ in the East Asian longitude sector indicates that the penetration of the convection electric field lasted from 1200 to 1800 UT on 8 September. It is surprising that the shielding process seems to be ineffective even though more than 12 hr had passed since the first major southward turning at 2300 UT on 7 September. The Hokkaido East and West radars also registered enhancement of the northward electric field to penetrate to unusually low latitudes (<50° magnetic latitude). We attempted to find a possible explanation for the observed severe depletion by using both observational evidence and numerical simulations. The IPE model simulations were run in two modes: (i) the default setting run where the potential was set to 0 at the lower-latitude boundary as given by the Weimer (1996) model and (ii) the penetration electric field run where the zonal electric field was artificially increased in the middle latitudes. Only the penetrated electric field run reproduced the severe plasmasphere erosion. Our results suggest that the middle-latitude electric field penetrated from the high-latitude storm time convection that lasted for several hours can explain such a degree of severity. Our observation is the first report of such a deeply eroded plasmasphere using in situ observation of electron density in the magnetosphere. Arase observation, especially spectrograms of plasma waves in a wide (10 kHz to 10 MHz) frequency range, is a strong tool for research on deep erosion of the plasmasphere.