Change of the plasma sheet ion composition during magnetic storm development observed by the Geotail spacecraft

Abstract

[1] The present study aims to investigate how and where ions of ionospheric origin are accelerated to the ring current energy (a few tens to a few hundreds of keV) and how they are supplied to the ring current. We examined the plasma sheet ion composition during magnetic storm development, using energetic (9-210 keV) ion flux data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on board the Geotail spacecraft. We selected two magnetic storms, that is, the 16-17 May 2000 storm and the 25 December 1998 storm, for which the energy density ratios of O + /H + and He + /H + in the plasma sheet were calculated from the EPIC/STICS data. These magnetic storms had a minimum of the SYM-H index (the 1-min Dst index) less than À50 nT and a duration of the main phase shorter than 6 hours. We obtained the following results: (1) Both the O + /H + and He + /H + energy density ratios were anticorrelated with the SYM-H index (jrj = 0.73-0.88); (2) The O + /H + energy density ratio was rather constant at $0.1 before storms, but reached 0.3-1.0 at the storm maximum; and (3) The He + /H + energy density ratio increased from 0.01-0.02 before storms to 0.04-0.1 at the storm maximum. These ion composition changes are comparable to those in the ring current, which have been reported by previous studies, indicating that ions of ionospheric origin are possibly convected to the ring current via the plasma sheet. A close inspection of ion energy spectra revealed that the observed ion composition changes can be attributed to the mass-dependent acceleration of ions by the dawn-to-dusk electric field in the current sheet and the additional transport of ionospheric ions into the plasma sheet.INDEX TERMS: 2764 Magnetospheric Physics: Plasma sheet; 2778 Magnetospheric Physics: Ring current; 2788 Magnetospheric Physics: Storms and substorms; 2451 Ionosphere: Particle acceleration; KEYWORDS: plasma sheet, ion composition, magnetic storms, particle acceleration Citation: Nosé, M., R. W. McEntire, and S. P. Christon, Change of the plasma sheet ion composition during magnetic storm development observed by the Geotail spacecraft,

Long-term variations in the plasma sheet ion composition and substorm occurrence over 23 years

“…We note that the transient enhancements of M mostly occur during the time intervals of the orange bars. This supports the idea that sudden increases in M to 1.8-2.7 amu are caused by strong magnetic activities, and is consistent with the results for individual magnetic storms reported by previous studies (e.g., Hamilton et al 1988;Daglis 1997;Nosé et al 2003Nosé et al , 2005. On the other hand, the changes in the baseline levels of M seem follow those of Mg II; M remains at ~1.1 amu during the solar minimum (1994-1997 and 2006-2010) and increases to ~1.5 amu during the solar maximum (1998-2004 and 2012-2015).…”

Storm‐time electron density enhancement in the cleft ion fountain

“…Most magnetospheric heavy ions (e.g., O + ) observed by satellites [e.g., Shelley et al, 1972;Frank et al, 1977;Seki et al, 1996] are of ionospheric origin, since the solar wind does not contain sufficient heavy ions. Satellite observations have shown that the energy density of O + ions sometimes dominates that of H + ions in the ring current region and the plasma sheet during geomagnetic storms [e.g., Daglis, 1997;Greenspan and Hamilton, 2002;Nosé et al, 2001Nosé et al, , 2003Nosé et al, , 2005Zong et al, 2008]. Similar enhancements of ionospheric O + ions in the magnetosphere during intense geomagnetic storms have also been demonstrated in recent global simulations [Winglee et al, 2005;Peroomian et al, 2006Peroomian et al, , 2007Peroomian et al, , 2011Harnett et al, 2008;Glocer et al, 2009aGlocer et al, , 2009bMoore et al, 2010;Fok et al, 2011].…”

Evidence for significantly greater N2 Lyman‐Birge‐Hopfield emission efficiencies in proton versus electron aurora based on analysis of coincident DMSP SSUSI and SSJ/5 data

“…We have not seen a clear indication of this during highly disturbed periods in regions dominated by ion precipitation. Some indication of SSJ/5 ion composition should come from the behavior of H + and O + fluxes within the plasma sheet [e.g., Nosé et al , 2003, 2005], the source of most particle precipitation. During disturbed times, there will be significant increases in plasma sheet O + whose source is upstreaming O + from the auroral F region.…”