Observations of Slow-Mode Shocks in Earth's Distant Magnetotail with the GEOTAIL Spacecraft.

Details Observations of Slow-Mode Shocks in Earth's Distant Magnetotail with the GEOTAIL Spacecraft. Observations of Slow-Mode Shocks in Earth's Distant Magnetotail with the GEOTAIL Spacecraft.

Jan 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996phdt.........9s&link_type=abstract

Thesis (PH.D.)--THE UNIVERSITY OF IOWA, 1996.Source: Dissertation Abstracts International, Volume: 57-05, Section: B, page: 327

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Scientific paper

Four crossings of the lobe-plasma sheet boundaries encountered within Earth's distant magnetotail with the Geotail spacecraft are analyzed in terms of slow-mode shocks. Measurements of the three-dimensional velocity distributions of protons and electrons and of the magnetic fields allow a quantitative comparison with theoretical predictions for a steady-state, one-dimensional shock. The observed plasma moments and magnetic field upstream from the shock are successfully used to predict these parameters in the downstream region. Slow -mode Mach numbers in the upstream and downstream regions are also shown to be above and below unity, respectively, as required for the slow-mode shock transition. However, consideration of the relative uncertainties for the upstream and downstream slow-mode Mach numbers allows convincing identifications of only two of the boundaries as slow-mode shocks. Low number densities of plasmas for the other boundaries are responsible for the large relative uncertainties for the Mach numbers. The pressure tensor as calculated from the measured velocity distributions indicates that the plasmas are highly anisotropic in the downstream region. A modification to the procedure for the identification of the slow-mode shocks is developed in order to take into account the observed large anisotropy. The modification is made with an assumption of gyrotropic plasmas that is supported by the diagonalization of the pressure tensor in magnetic field coordinates. The predictions for the downstream plasma moments and magnetic fields from the modified procedure are in sufficient agreement with the measured parameters to provide considerably greater confidence that the boundaries are slow-mode shocks. Also observed in the ion velocity distributions are accelerated ions escaping from the downstream region. Observations of the electron velocity distributions allow identification of the separatrix in Earth's magnetotail. The separatrix in the upstream region is identified with the first appearance of hot electrons streaming away from the shock. Flat-topped electron velocity distributions are found in the downstream region. These comprehensive observations of plasmas and magnetic fields with the Geotail spacecraft provide firm evidence for the existence of slow-mode shocks in the distant magnetotail.

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