Detailed observations of the plasma sheet during a substorm on April 24, 1979

This paper reports a detailed study of plasma, magnetic field, and energetic particle data acquired by the ISEE 1 and 2 satellites, about 20 RE from earth in the magnetotail plasma sheet during a well-defined substorm on April 24, 1979. The paper addresses, particularly, the plasma flows and energetic particle anisotropies that were encountered during the various stages of the substorm's evolution and compares these to predictions of the neutral line model of substorms. We find that the observations support that model in considerable detail. The interval from substorm onset at earth to plasma dropout was only 4½ min, and we stress the importance of high time resolution in all measurements if a full understanding of substorm initiation is to be developed. We found that the energetic ions (up to 125 keV) and plasma ions, although they sometimes showed somewhat different angular distributions in the plasma rest frame, typically shared a common bulk speed, i.e., the bulk flow speed of the plasma, parallel and perpendicular to the magnetic field. Thus the anisotropies of the energetic ion fluxes that have sometimes led to their characterization as earthward or tailward “beams” were found to be largely a consequence of their bulk motion with the plasma. We identify the region at the lobe-plasma sheet interface, which has been called the plasma sheet boundary layer, with the separatrix layer in the neutral line model. This is the region of field lines including, and just equatorward of, the lobe-plasma sheet separatrix, which should carry the most recent dynamic signatures of reconnection. Our results seem to support that expectation. Several features of our observations were inconsistent with the substorm picture, called the boundary layer dynamics model, in which a satellite's observation of fast plasma and particle flow is due primarily to its envelopment by a preexisting boundary layer of flowing plasma.

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