Planetary Magnetosphere Branch, Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, MD 20771

Eleven traversals of the magnetotail at 30-40 R_E by the IMP 7 and IMP 8 spacecraft occurred while IMP 6 was making magnetotail measurements inside 33 R_E. Combined magnetic field and plasma data from these spacecraft provide the first demonstration of pressure balance between the high b plasma of the plasma sheet and the strong magnetic field of the tail lobes. The time variations of this pressure support the view that a southward interplanetary magnetic field enhances the accumulation of energy in a magnetotail energy reservoir. Sometimes the accumulated magnetic energy is rapidly dissipated during a magnetospheric substorm. At other times the dissipation can occur more gradually during ongoing magnetic activity. At such times the energy supplied by the solar wind may even exceed that being dissipated, thus causing the tail energy to increase. In addition to plasma sheet thinnings seen prior to substorm onsets inside ~ 15 R_E a gradual decrease in plasma b is detected in the deep tail which precedes onset and the more prominent plasma disappearance that typically accompanies it. The frequency of thinnings and the regions over which they occur indicated that drastic changes in plasma sheet thickness are common features of substorms which occur at all locations across the tail. Several magnetically quiet periods correspond to thick plasma sheets with enhanced B_Z components and gradually cooling plasmas.

J. Geophys. Res., 86, No. A3, 1396-1414, Mar. 1981