Acceleration sites of energetic ions upstream of the Earth's bow shock and in the magnetosheath:: Statistical study on charge states of heavy ions -: art. no. A11104

[1] 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 less than or equal to 25%), intermediate ( IM) events ( 25% < P-LCS < 75%), and low- charge- state ( LCS) events ( P-LCS greater than or equal to 75%). The HCS and LCS events occurred at almost the same rate in geomagnetically quiet conditions ( SYM- H greater than or equal to 0 nT), while the LCS events were observed more frequently than the HCS events in geomagnetically disturbed conditions ( SYM- H less than or equal to - 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.