Multiple crossings of a very thin plasma sheet in the Earth's magnetotail

High resolution magnetic field, plasma and energetic particle data from the IMP-8 spacecraft were studied for multiple crossings of the Earth's magnetotail plasma sheet when it becomes thin during magnetospheric substorms. Traversals recur on a time scale of several minutes and they are associated with high velocity plasma flows that are usually directed tailward but are occasionally directed earthward for brief intervals. Observations are explained by rapid oscillations of a plasma sheet that is only a few thousand km thick, a dimension comparable to the gyroradius of energetic protons. Differences in the angular distributions of the two energies indicate that the higher energy protons are preferentially located on field lines deeper in the tail lobe. A neutral line acceleration model is supported tailward streaming energetic electrons which are occasionally present at the lobe plasma sheet interface.