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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, A11104, doi:10.1029/2003JA009953, 2004

Acceleration sites of energetic ions upstream of the Earth's bow shock and in the magnetosheath: Statistical study on charge states of heavy ions

K. Keika

Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto, Japan


M. Nosé

Data Analysis Center for Geomagnetism and Space Magnetism, Graduate School of Science, Kyoto University, Kyoto, Japan


S. P. Christon

Focused Analysis and Research, Columbia, Maryland, USA


R. W. McEntire

Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA


Abstract

In order to identify acceleration sites of energetic ions upstream of the Earth's bow shock and in the dayside magnetosheath, we investigated energetic ion events observed by the Energetic Particles and Ion Composition (EPIC) instrument on board the Geotail spacecraft from 1995 to 2001. Energetic ion data obtained by the Ion Composition System (ICS) sensor were used to select “events,” that is, intervals when the 77–107 keV ion flux increased by more than two orders of magnitude within 10 min, and examine their spatial distribution and its geomagnetic activity dependence. Heavy ions with energy range of 9–210 keV/e detected by the Suprathermal Ion Composition Spectrometer (STICS) sensor were chosen to calculate the percentage of low-charge-state heavy ions (P LCS = (O+ + N+)/(O+ + N+ + O6+ + O7+)) in each event, and the events were classified into three groups: high-charge-state (HCS) events (P LCS ≤ 25%), intermediate (IM) events (25% < P LCS < 75%), and low-charge-state (LCS) events (P LCS ≥ 75%). The HCS and LCS events occurred at almost the same rate in geomagnetically quiet conditions (SYM-H ≥ 0 nT), while the LCS events were observed more frequently than the HCS events in geomagnetically disturbed conditions (SYM-H ≤ −30 nT). The percentage of the IM and LCS events summed was higher in disturbed conditions than in quiet conditions. The HCS and LCS events were frequently observed in the dawnside upstream region and in the duskside magnetosheath, respectively. The IM events occurred both in the dawnside upstream region and in the magnetosheath around noon. We conclude that energetic ion events including ions accelerated in the magnetosphere are dominant. Ions accelerated in the magnetosphere leak out from the duskside magnetopause, in particular when geomagnetic activity is high, and then most likely mix with ions accelerated at the bow shock on the way to the dawnside upstream region. The acceleration site of the HCS events is also discussed.

Received 24 March 2003; accepted 27 August 2004; published 5 November 2004.

Keywords: origin of upstream energetic ions; ion escape from the magnetosphere; bow shock acceleration; charge states of heavy ions.

Index Terms: 2116 Interplanetary Physics: Energetic particles, planetary; 2154 Interplanetary Physics: Planetary bow shocks; 2788 Magnetospheric Physics: Storms and substorms; 2778 Magnetospheric Physics: Ring current; 7807 Space Plasma Physics: Charged particle motion and acceleration.

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Citation: Keika, K., M. Nosé, S. P. Christon, and R. W. McEntire (2004), Acceleration sites of energetic ions upstream of the Earth's bow shock and in the magnetosheath: Statistical study on charge states of heavy ions, J. Geophys. Res., 109, A11104, doi:10.1029/2003JA009953.