First Simultaneous Multi-Point Observation of the Local-Time Asymmetry of keV Ions in the Dayside Magnetosphere During the Main Phase of the Geomagnetic Storm

Abstract

Our study presents the first simultaneous multi-point observation of the local-time asymmetry of 10's–100's of keV energy ion fluxes during the main phase of the geomagnetic storm that occurred on 7 September 2017. During this event, Van Allen Probe-A and Van Allen Probe-B observed two different tendencies. The ion fluxes increased by an order of magnitude in the noon-dusk sector, while decreasing by one order or more in the dawn-noon sector, offering a unique opportunity to investigate this asymmetry. Numerical simulations employing the Comprehensive Inner Magnetosphere-Ionosphere model with time-dependent electric fields from Global Magnetohydrodynamic (MHD) simulations revealed that the local time asymmetry in ion fluxes is associated with a sharp southward turning of the interplanetary magnetic field (IMF) and long-duration persistent westward electric field. These factors cause ions to drift toward the dusk sector, while preexisting ions on the dayside drift sunward and escape the inner magnetosphere. Our findings provide the first direct observational evidence of ring current asymmetry, complementing and supporting prior statistical studies and simulation results.