Overwhelming O+ contribution to the plasma sheet energy density during the October 2003 superstorm:: Geotail/EPIC and IMAGE/LENA observations -: art. no. A09S24

[1] We studied dynamics of O+ ions during the superstorm that occurred on 29 - 31 October 2003, using energetic (9 - 210 keV/e) ion flux data obtained by the energetic particle and ion composition (EPIC) instrument on board the Geotail satellite and neutral atom data in the energy range of 10 eV to a few keV acquired by the low-energy neutral atom (LENA) imager on board the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. Since the low-energy neutral atoms are created from the outflowing ionospheric ions by the charge exchange process, we could examine variations of ionospheric ion outflow with the IMAGE/LENA data. In the near-Earth plasma sheet of X-GSM similar to - 6 R-E to - 8.5 R-E, we found that the H+ energy density showed no distinctive differences between the superstorm and quiet intervals ( 1 - 10 keV cm(-3)), while the O+ energy density increased from 0.05 - 3 keV cm(-3) during the quiet intervals to similar to 100 keV cm(-3) during the superstorm. The O+/ H+ energy density ratio reached 10 - 20 near the storm maximum, which is the largest ratio in the near-Earth plasma sheet ever observed by Geotail, indicating more than 90% of O+ in the total energy density. We argued that such extreme increase of the O+/ H+ energy density ratio during the October 2003 superstorm was due to mass-dependent acceleration of ions by storm-time substorms as well as an additional supply of O+ ions from the ionosphere to the plasma sheet. We compared the ion composition between the ring current and the near-Earth plasma sheet reported by previous studies and found that they are rather similar. On the basis of the similarity, we estimated that the ring current had the O+/ H+ energy density ratio as large as 10 - 20 for the October 2003 superstorm.