We analyze 127 one-hour average samples of central plasma sheet ions and electrons in order to determine spectral characteristics of these magnetotail particle populations during periods of low geomagnetic activity (AE12 RE. We find that, for even the longest periods sampled, the nearly isotropic central plasma sheet total ion and electron populations were measured to be continuous particle distributions from our lowest energy of tens of eV/e to a few hundred keV. The kappa distribution function (f~[1+E/&kgr;Eo>-&kgr;-1, where E0, the energy of the peak differential number flux (measured in particles/cm2 s sr keV), is related to the temperature through &kgr;, a constant) most often reproduces the observed differential energy spectra. Spectra dominated by a single kappa functional form are observed during 83 (99) hours for ions (electrons). Spectra which are not dominated by a single kappa functional form can usually be closely approximated by superposed kappa functional forms. For both ions and electrons &kgr; is typically in the range 4--8, with a most probable value between 5 and 6, so that the spectral shape is distinctly non-Maxwellian. Eoi and Eoe are highly correlated, whereas &kgr;i and &kgr;e are not correlated; &kgr;i is roughly proportional to Eoi1/2, whereas &kgr;e is not correlated with Eoe. We statistically investigate the importance of flux and energy contributions from extramagnetospheric sources by separately analyzing intervals when simultaneously measured interplanetary particle fluxes are either enhanced or at low levels.

A linear superposition of plasma sheet fluxes and interplanetary fluxes that have entered the magnetosphere is observed. The presence of interplanetary particles does not affect the average values of plasma sheet Eo or &kgr;. We conclude that for AE<100 nT the nonthermal shape of plasma sheet particle distributions results from ongoing magnetospheric processes which are probably independent of geomagnetic activity as measured by AE. ¿ American Geophysical Union 1989