Response of the Ionosphere-Plasmasphere Coupling to the September 2017 Storm: What Erodes the Plasmasphere so Severley?
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Response of the Ionosphere-Plasmasphere Coupling to the September 2017 Storm: What Erodes the Plasmasphere so Severley?
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    Space Weatehr (2019) 17(6): 861-876
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    We report an extreme erosion of the plasmasphere arising from the September 2017 storm. The cold electron density is identified from the upper limit frequency of upper hybrid resonance waves observed by the Plasma Wave Experiment instrument onboard the Exploration of energization and Radiation in Geospace/Arase satellite. The electron density profiles reveal that the plasmasphere was severely eroded during the recovery phase of the storm and the plasmapause was located at L = 1.6-1.7 at 23 UT 8 September 2017. This is the first report of deep erosion of the plasmasphere (L-pp < 2) with the in situ observation of the electron density. The degree of the severity is much more than what is expected from the relatively moderate value of the SYM-H minimum (-146 nT). We attempt to find a possible explanation for the observed severe depletion by using both observational evidence and numerical simulations. Our results suggest that the middle latitude electric field had penetrated from the high-latitude storm time convection for several hours. Such an unusually long-lasting penetration event can cause this observed degree of severity. Plain Language Summary The plasmasphere is the region of cold, relatively dense ionized gas (mostly protons and helium ions) that resides on the magnetic field lines close to the Earth. It is understood that the plasmasphere is threaded by magnetic field flux tubes that are persistently "closed," so that plasma from the Earth's ionosphere has filled the flux tubes. The typical location of the outer boundary of the plasmasphere, known as the plasmapause, is usually 40,000-50,000 km from the Earth. Here we report that a magnetic storm during 7-10 September 2017 dramatically displaced the outer boundary of the plasmasphere inwards, to only similar to 4,000 km from Earth's surface. Our study suggests that the remarkable deformation is caused by the unusually long-lasting leakage of the convection electric field deep within the plasmasphere.
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