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Plasma flows and magnetic field vectors in the plasma sheet during substorms

Simultaneous observations of plasma flow and the magnetic field in the plasma sheet from the Imp 6 satellite during magnetospheric substorms are examined to assess whether plasma flow is predominantly perpendicular or along the magnetic field line. For the period from September 1971 to December 1972, simultaneous magnetic field data are available for a total of 119 measurements of plasma flow during plasma sheet thinnings and expansions within the downstream distance of X/sub SM/approx. =-32R/sub E/. It is found that the observed plasma flow has a large component along the magnetic field line and is not simply the E x B drift (steady or transient convection flow) nor any cross-field flow. In general, the higher the flow speed, the closer the observed plasma flow direction is aligned with the magnetic field line. Further, there is no clear association between the plasma flow direction and the sign of the B/sub ZSM/ component of the magnetic field. These results suggest that in general the rapid plasma flows observed in the magnetotail during substorms are predominantly field-aligned flows and are likely to be due to a magnetic-field-aligned pressure gradient or magnetic-field-aligned electric field.