Inverse velocity dispersion of energetic particle bursts inside the plasma sheet

Fine-time resolution (∼ 10 s) observations of bursts of energetic particles inside the Earth's plasma sheet by the IMP-8 spacecraft, reveal that the detailed structure of the time-intensity profiles of the ion bursts often display the ''inverse velocity dispersion'' effect, whereby the low energy (∼ 300 keV) ion intensity enhancements are detected before the higher energy (∼ 1 MeV) ones. In particular, it is shown that long duration (several minutes or more) bursts of energetic particles are composed of a series of highly anisotropic, short duration impulsive bursts, which last a few tens of seconds and display the inverse velocity dispersion characteristic. Furthermore, it is demonstrated that the inverse velocity dispersion effect is observed: (a) both inside the plasma sheet and at its high latitude boundary, (b) during periods of thinning as well as expansion of the plasma sheet and (c) for both tailward and earthward streaming of the energetic ions. (author)