Characteristics of plasmoidlike structures in the distant magnetotail

Abstract

The average characteristics of 20 energetic particle burst events in the distant geomagnetic tail have been determined. The measurements were made with two particle sensors of the Max‐Planck‐Institut/University of Maryland experiment on ISEE 3 during the recent Geotail Mission. The events are characterized initally by beamlike energetic electron distributions which are followed by isotropic distributions. Assuming that the energetic protons (≳30 keV) are convected with the local plasma flow (which is generally tailward), we can determine the flow speed from the measured angular distributions and the differential energy spectral shape. We find that the average velocities range from ∼300 to ∼1000 km/s. The size of these structures, most probably plasmoids, along the convective flow direction is observed to be between ∼50 and ∼100 RE in most of the cases, with a mean value of ∼80 RE. The energy spectra (in the plasma frame) are consistently steeper than power law, being closer to exponentials in energy. The differential proton intensity in these events, on the average ∼500 protons cm−2 s−1 sr−1 keV−1 at 30 keV, is an order of magnitude smaller than in the near‐earth plasma sheet.