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

Authors:
  • DFD. H. Fairfield
    D. H. Fairfield
  • EHE. W. Hones
    E. W. Hones
  • CMC.‐I. Meng
    C.‐I. Meng
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Abstract
High‐resolution magnetic field, plasma, and energetic particle data from the IMP 8 spacecraft have been used to study multiple crossings of the earth's magnetotail plasma sheet when it becomes thin during magnetospheric substorms. IMP 8 is found to traverse the entire thickness of the β > 1 plasma sheet in times that are as short as 10 s. Such 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. We argue that these observations can be explained by rapid oscillations of a plasma sheet that is only a few thousand kilometers thick, a dimension that is comparable to the gyroradius of energetic protons. When ions of 50 keV and 290 keV are detected at a location on tail lobe field lines near the plasma sheet, they are often seen to be moving in the dawn to dusk direction and may be undergoing serpentine cross tail motion. Differences in the angular distributions of the two energies indicate that the higher energy ions are preferentially located on field lines deeper in the tail lobe. This result is consistent with ion acceleration at a neutral line with subsequent drift in a cross‐tail electric field that moves the lower energy particles closer to the equatorial plane. A neutral line acceleration model is also supported by our observation that tailward streaming energetic electrons are occasionally present at the lobe‐plasma sheet interface.

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