Abstract
Abstract. During a storm recovery phase on 15 May 2005, the Geotail spacecraft repeatedly observed high-energy (>180 keV) oxygen ions in the dayside magnetosheath near the equatorial plane. We focused on the time period from 11:20 UT to 13:00 UT, when Geotail observed the oxygen ions and the interplanetary magnetic field (IMF) was constantly northward. The magnetic reconnection occurrence northward and duskward of Geotail is indicated by the Walén analysis and convective flows in the magnetopause boundary layer. Anisotropic pitch angle distributions of ions suggest that high-energy oxygen ions escaped from the northward of Geotail along the reconnected magnetic field lines. From the low-energy particle precipitation in the polar cap observed by DMSP, which is consistent with magnetic reconnection occurring between the magnetosheath field lines and the magnetospheric closed field lines, we conclude that these oxygen ions are of ring current origin. Our results thus suggest a new escape route of oxygen ions during northward IMF. In the present event, this escape mechanism is more dominant than the leakage via the finite Larmor radius effect across the dayside equatorial magnetopause.
Highlights
Escape of the Earth’s oxygen ions (O+ and O++) is a longstanding issue in magnetospheric physics
We present for the first time evidence of the escape of high-energy (>180 keV) oxygen ions along magnetic field lines produced by magnetic reconnection under northward interplanetary magnetic field (IMF)
We examined the dayside magnetopause skimming by Geotail around the equatorial plane, during a storm recovery phase on 15 May 2005
Summary
Observations of energetic ions (including oxygen ions) in the magnetosheath have often been reported. One exception is the study by Onsager et al (2001) who showed anisotropic phase space densities of ions (without mass discrimination) observed by the Polar spacecraft during northward IMF, and suggested that the ion escape was due to high-latitude reconnection with the closed magnetospheric field lines. Their observation, was limited to ions with energy 180 keV) oxygen ions along magnetic field lines produced by magnetic reconnection under northward IMF
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