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

Abstract

[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 $ À6 R E to À8.5 R E , we found that the H + energy density showed no distinctive differences between the superstorm and quiet intervals (1-10 keV cm À3 ), while the O + energy density increased from 0.05-3 keV cm À3 during the quiet intervals to $100 keV cm À3 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

Modeling the superstorm in November 2003

“…[20] This result is somewhat surprising and diverges from recent observations, for example those reported by Nosé et al [2005] or Mitchell et al [2003]. From those results, and from the substantial contributions of oxygen in our earlier work on substorms and smaller storms [Fok et al, 2006;Moore et al, 2008], we anticipated that we might find that oxygen would essentially take over the ring current for such a large event, but that is not the case here.…”

Atmospheric loss from the dayside open polar region and its dependence on geomagnetic activity: implications for atmospheric escape on evolutionary timescales

“…The O + density close to the mid-latitude magnetopause was shown by Bouhram et al (2005) to also increase exponentially with Kp. A consequence of increased ion outflow is an enhancement of the plasma feed into the plasma sheet during geomagnetic storms (Nose et al, 2005;Kistler et al, 2010;Haaland et al, 2012). The plasma sheet in turn feeds the ring current and its relative O + content and energy density increases significantly with geomagnetic activity.…”

The role of ULF waves interacting with oxygen ions at the outer ring current during storm times

“…The abundance of O + in terms of both number density and energy density increases with increasing geomagnetic activity ]. During extremely large storms, Nosé et al [2005] showed that the O + ions were dominant in the magnetotail plasma sheet [Zong et al, 2008] and the O + /H + energy density ratio was as large as 10-20, suggesting more than 90% of the total ring current energy density contributed from O + ions. It appears that the source and energization of O + ions are more responsive to disturbed activity than H + ions.…”