SPECTRAL CHARACTERISTICS OF PLASMA SHEET ION AND ELECTRON POPULATIONS DURING DISTURBED GEOMAGNETIC CONDITIONS

We have determined the spectral characteristics of central plasma sheet ions and electrons observed during 71 hours when geomagnetic activity was at moderate to high levels (AE greater-than-or-equal-to 100 nT). Particle data from the low-energy proton and electron differential energy analyzer and the medium energy particle instrument on ISEE 1 are combined to obtain differential energy spectra (measured in units of particles/cm2 s sr keV) in the kinetic energy range approximately 30 eV/e to approximately 1 MeV at geocentric radial distances > 12 R(e). Nearly isotropic central plasma sheet total ion and electron populations were chosen for analysis and were measured to be continuous particle distributions from our lowest to highest energies. During these high AE periods the > 24 keV particle fluxes and the temperature of the entire particle distribution kT are significantly higher than during low AE periods (AE < 100 nT). The temperatures kT(i) and kT(e) are highly correlated during both quiet and disturbed periods. The active period spectral shape appears softer for ions and somewhat harder for electrons than during quiet periods. We find that the observed active period spectrum typically is complex and cannot be represented in general by a single functional form, as during quiet periods when it can be represented by the kappa distribution function. Although a power-law shape is observed at higher energies, ion and electron spectral shapes deviate from a strictly kappalike form in different ways. In a limited energy range near the knee of the ion spectra (the knee is that portion of the spectrum at energies E less-than-or-similar-to E(o) where the flux starts to decrease swiftly with increasing energy), the spectral shape can often be fit with a Maxwellian form, thus rolling over faster than the typical quiet time spectrum. At higher energies this shape merges into a harder nonthermal power-law tail. Electron spectra also display this spectral characteristic, although at a lower occurrence frequency than for ions. The electron spectra are predominantly kappalike at energies near and above the knee. At energies below the knee, both ions and electrons often have an excess of flux with respect to the functional form that best fits the shape for energies at or above the knee, be it a kappa distribution oR a Maxwellian distribution; the electron flux excess is significantly greater than the ion flux excess. We conclude that both ions and electrons participate in at least two separate acceleration mechanisms as geomagnetic activity evolves from low AE to high AE values. We suggest that both spectrum-preserving and spectrum-altering heating processes (possibly involving nonlocal betatron acceleration and crosstail current sheet acceleration, respectively) participate in overall particle energization during geomagnetic active periods. Observations are compared to model predictions.