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

Using the University of Maryland/Max-Planck-Institut fuer 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 /similar to/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. copyright American Geophysical Union 1989