Energy spectra of the major ion species in the ring current during geomagnetic storms

Abstract

Using the University of Maryland/Max‐Planck‐Institut für Aeronomie charge‐energy‐mass (CHEM) spectrometer on the AMPTE Charge Composition Explorer (CCE) spacecraft, we have examined the nearly equatorial storm time energy spectra of four major magnetospheric ions, H+, O+, He+, and He++, over the energy range 1–300 keV/e in the L range 3–6. The data were obtained during the main and early recovery phases of all geomagnetic storms with minimum Dst less than −50 nT in the time period September 1984 to November 1985. When the spectra are organized by local time, certain features emerge. In particular, there is a dip in the spectra of all ions at 5–20 keV/e in the dawn‐to‐noon sector, while in the noon‐to‐dusk sector the proton phase space density drops off sharply below ∼5 keV. We have compared these spectra with those predicted by a model of ion drift and loss in the magnetosphere. The model calculates the drift paths in a Volland‐Stern electric field and dipole magnetic field and determines the losses due to charge exchange and strong pitch angle diffusion along the paths. We find that the spectra are most consistent with a Volland‐Stern electric field with γ = 2 and with a rotation of the nominal dawn‐to‐dusk electric field eastward by 2 hours local time. Charge exchange is found to be the dominant loss process during the main phase of the storm, producing qualitative agreement with the observed spectra for all species. There are some quantitative disagreements, particularly in the prenoon sector, which may be explained either by an additional loss process or by a modified drift model.

Modeling the electromagnetic ion cyclotron wave‐induced formation of detached subauroral proton arcs

“…To investigate this further we simulated ring current dynamics during this event using an eastward offset of 2 hours in the convection electric field. Previous studies [e.g., Gussenhoven et al, 1981;Matsui et al, 2004] have indicated that the symmetry axis of the plasma convection pattern may be rotated by a few hours from the dawn-dusk direction during disturbed periods, and better agreement between measured and observed ion fluxes can be obtained if an offset is applied [e.g., Kistler et al, 1989;Jordanova et al, 1999]. A rotation of the axis of symmetry and further distortion of the electric field may be due to disruptions of electric field shielding and fieldaligned currents, as well as changes in the near-terminator conductivity gradient or in the magnetic configuration [Wolf et al, 2005, and references therein].…”

Evolution of CIR storm on 22 July 2009

“…[17] O + ions can be a significant contribution to the trapped ion population in the ring current during geomagnetic storms [Kistler et al, 1989]. Mitchell et al [2003] showed how O + could be distinguished from H + in ENA images.…”

Testing electric field models using ring current ion energy spectra from the Equator-S ion composition (ESIC) instrument

“…The ion spectra observed on the dayside typically contain one or more minima which correspond to the drift paths on which the greatest losses occur, either because they have the longest drift time, or because they drift the closest to the Earth, where losses are greater. Many studies (e.g., Kistler et al, 1989;Fok et al, 1996;Jordanova et al, 1999) have shown qualitative agreement between observed and predicted spectra based on these principles. However, there has been di culty in getting quantitative agreement.…”