ANGEO Annales Geophysicae ANGEO Ann. Geophys. 1432-0576 Copernicus Publications Göttingen, Germany 10.5194/angeo-26-3955-2008 Escape of high-energy oxygen ions through magnetopause reconnection under northward IMF Kasahara S. 1 Hasegawa H. 1 Keika K. 2 Miyashita Y. 1 Nishino M. N. 1 Sotirelis T. 3 Saito Y. 1 Mukai T. 1 Institute of Space and Astronautical Science, JAXA, sagamihara, Kanagawa, Japan Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, Graz, Austria Applied Physics Laboratory, 11100 Johns Hopkins Road Laurel, MD, USA 05 12 2008 26 12 3955 3966 Copyright: © 2008 S. Kasahara et al. 2008 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://angeo.copernicus.org/articles/26/3955/2008/angeo-26-3955-2008.html The full text article is available as a PDF file from https://angeo.copernicus.org/articles/26/3955/2008/angeo-26-3955-2008.pdf

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.