Overwhelming O+ contribution to the plasma sheet energy density during the October 2003 superstorm: Geotail/EPIC and IMAGE/LENA observations

Abstract

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 XGSM ∼ −6 RE to −8.5 RE, 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 ∼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.