First composition measurement of the bulk of the storm-time ring current (1 to 300 keV/e) with AMPTE-CCE

Abstract

Mass and charge-state measurements of major ion species have been made for the first time in the bulk of the ring current preceding and during the strongly asymmetric geomagnetic storm of Sept. 4-7, 1984. Using data from the Charge-Energy-Mass (CHEM) Spectrometer on the CCE spacecraft we derive the differential energy spectra, energy densities (u) and number densities (n) in the energy range ∼1 to 315 keV/e for H+, O+, He+, He2+, O2+ and [C+N+O] with charge states above 3. We find that the prestorm, quiet-time ring current consists primarily of ions of solar wind origin (nHe2+/nH+ = 0.017) with O+ contributing only 2.7% to the total energy density of ∼4·10−8 ergs/cm³ at L ∼3 to ∼8. In contrast, O+ supplies a substantial fraction (29%) of the total storm-time ring current energy density (2.9·10−7 ergs/cm³), with O2+, He+, He2+ and [CNO] ≥3+ contributing 2.4, 1.6, 0.96 and ∼0.04% respectively. The storm-time differential energy spectra, number and energy densities are best organized in terms of energy per charge with typical average energies (E ≡ u/n) ranging from ∼41 keV/e for O+ to 64 keV for protons. We observe 1-300 keV/e, high charge state heavy ions (CNO, Si, Fe) with abundances similar to solar wind ions, implying that their entry and acceleration does not depend strongly on the mass or charge state of the ions. We conclude that while the storm-time increases in the number densities of ring current H+ and He2+ may be roughly accounted for by the decreased volume of the compressed magnetosphere, the large jumps in the number and energy densities of O+ require injection of energetic ionospheric ions.