Implications of large flow velocity signatures in nearly isotropic ion distributions

We have compared ISEE-1 energetic (≥24 keV) ion distributions with previously published, lower-energy (≤17 keVe) ISEE-1 H+ and He++ distributions [Peterson et al., 1981] in the quiet-time plasma sheet at ∼15 R/sub E/ radial distance, ∼0430 local time, and within ∼1 R/sub E/ of the expected neutral sheet position. Moment calculations carried out over the nearly isotropic H+ and He++ plasma distributions yield low (approx. <14 kmsec) bulk velocities. However, the spectral shapes of these same distributions are indicative of high flow speeds (∼500--700 kmsec). We argue that these seemingly contradictory results within a given measurement of the plasma distribution are consistent and can be explained by the evolution of plasma distributions flowing along a geomagnetic field line. Isotropization of the (observed) distributions while preserving their spectral signatures can result from the effects of single-particle motion and pitch angle diffusion. Whether the dual properties of isotropy and high-velocity spectra are observed depends on the position of the observation point along the field line and on the ambient plasma number density, bulk velocity, and temperature. For the case presented we conclude that H+ and He++ ions of solar wind origin were accelerated through an electrostatic potential of ∼2.4 kV prior to their observation by ISEE-1 in the plasma sheet on a closed magnetospheric field line. From energetic ion observations we infer that a second, independent plasma sheet population existed at this ISEE-1 position in the geomagnetic field. copyright American Geophysical Union 1988