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

Abstract

The Arase satellite observed clear dipolarization signatures at r~4.3–4.6 RE, 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.