Magnetic Field Dipolarization and Its Associated Ion Flux Variations in the Dawnside Deep Inner Magnetosphere: Arase Observations

The Arase satellite observed clear dipolarization signatures at r 4.3-4.6 R_E, GMLAT 16°-18°, and MLT 5.5-5.7 hr around 15:00 UT on 27 March 2017 when Dst -70 nT. Strong magnetic field fluctuations were embedded and their characteristic frequency was close to the local gyrofrequency of O⁺ ions. After the dipolarization, O⁺ flux was enhanced at ≤15 keV, while H⁺ flux showed no clear variations. These observations provide evidence for the direct supply of O⁺ ions from the ionosphere. There were no clear signatures for the nonadiabatic local acceleration of O⁺ ions. We consider that a bump-on-tail structure in the energy spectrum around 30-50 keV due to a combination of charge exchange loss and drift motion of ions masks the nonadiabatic acceleration. Occurrence of the magnetic field dipolarization at dawn, which is far from the well-known premidnight occurrence peak, may be due to an eastward skewing of partial ring current during the storm main phase.