Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsm33b0360o&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #SM33B-0360
Physics
2790 Substorms
Scientific paper
It is well known that the flux and energy density of energetic ions increase in the near-Earth plasma sheet during substorm associated dipolarizations. Some studies have focused on the mass dependence of the ion flux increases and found that the flux and energy density of O+ ions are more enhanced than those of H+ ions. They have suggested that O+ ions are accelerated non-adiabatically during dipolarization because the O+ ion gyro-period is comparable to the time-scale of the magnetic field variations. Whereas these earlier studies used just a few events for analysis, we analyze 95 events from 10-years of data (from 1995 to 2004) collected by the Energetic Particles and Ion Composition/ Suprathermal Ion Composition Spectrometer (EPIC/STICS) on board the Geotail spacecraft, which measures 9-210 keV/e ions. We compare the rate of O+ ion energy density before and after a substorm onset to that of H+ ion energy density. We find that, for 9-36 keV/e ions, the rate of O+ energy density increase is larger than that of H+. However, there is no sharp difference between the two rates for 55-210 keV/e O+ and H+ ions. In order to examine whether O+ ions are accelerated non-adiabatically or not, we compare the gyro-period of O+ ions (Tp) to the time-scale of the magnetic field variation (Tm) generated by substorm associated dipolarization. If they were close, the ions should be accelerated non-adiabatically. However, since Tm is found to be much larger than Tp in most of the events, we conclude that dipolarization cannot accelerate O+ ions non-adiabatically and that previous studies observe rare events. We then separate our events into two groups, storm-time substorms and non-storm-time substorms, and perform the same analysis independently in each group. The main results are as follows: (1) The increasing rate of O+ with respect to that of H+ is generally larger during non-storm-time than during the storm-time. (2) The increasing rate of the O+ energy density in the dusk side of the plasma sheet is larger than that in the dawn side during the non-storm-time. (3) The energy density of low energy O+ ions increases in the dawn side but decreases in the dusk side during the storm-time.
Christon Stephen P.
Nagata D.
Nose Mikiha
Ono Yohei
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