Acceleration of oxygen ions of ionospheric origin in the near-Earth magnetotail during substorms

Measurements from the suprathermal ion composition spectrometer (STICS) sen sor of the energetic particle and ion composition (EPIC) instrument on the Geotail spacecraft were used to investigate dynamics of O+ ions of ionosphe ric origin at energies of 9 keV to 210 keV in the near-Earth plasma, sheet during the substorm expansion phase. Substorm signatures were clearly obser ved on the ground at 1850 UT on May 17, 1995. In the expansion phase of thi s substorm, Geotail stayed in the plasma sheet at X similar to-10.5 R-E and observed a local dipolarization signature accompanied by strong disturbanc es of the magnetic field. From the energetic ion flux data of EPIC/STICS, w e obtained the following results: (1) energetic flux enhancement was more p ronounced for O+ than for H+; (2) the flux was enhanced almost simultaneous ly with local dipolarization; (3) the enhancement factor of O+ ions (EO+), which represents the enhancement of the O+ flux ratio (after and before sub storm onset) relative to the H+ flux ratio, was Bs large as 1.31; and (4) t hermal energy increased from 8.9 keV to 42.8 keV for O+ ions and from 9.4 k eV to 15.9 keV for H+ ions. We also performed statistical analysis for 35 e vents of local dipolarization found in the near-Earth region (X similar to- 6 to -16 R-E). We found that EO+ is larger than unity in all ranges of radi al distance and that the average value of EO+ is 1.37. These results sugges t that O+ ions are commonly more energized than H+ ions during the substorm expansion phase. To interpret these observational results, we propose a me chanism in which ions are accelerated in a non-adiabatic way during substor m-associated field reconfiguration.