Observations pertaining to the dynamics of the plasma sheet

Plasmas on the surface of the plasma sheet at its interface with the high‐latitude lobes are studied at ∼30–40 earth radii ( R E ) in the tail of the earth's magnetosphere with Lepedea plasma instrumentation on board the earth‐orbiting Imp 7 and Imp 8 satellites. Protons at this interface are frequently found to be flowing toward earth with velocities greater than 400 km s −1 . Proton bulk speeds measured near the plasma sheet surface are typically greater than those at locations deeper within the plasma sheet. Ten of the twenty cases studied exhibit plasma sheet cooling. Simultaneous magnetotail crossings by Imp 7 and Imp 8 occurred on October 31, 1975, during which the plasma sheet first engulfs Imp 7 and ∼74 min later is detected by Imp 8, located 7.2 R E farther from the neutral sheet. Earthward‐streaming protons are detected at the boundary of the plasma sheet by both spacecraft. Typical plasma sheet conditions are observed to prevail deep within the plasma sheet coincident with the earthward streaming at the boundary of the plasma sheet. Simultaneous magnetic field measurements with Imp 8 are used to verify that the observed proton streaming is parallel to the magnetic field. The AE index indicates that a large magnetic substorm is nearing recovery as the plasma sheet expands past the two satellites. This event is interpreted as the buildup of the plasma sheet by the plasma flowing on its surface, and the velocity of this expansion of 11 km s −1 , perpendicular to the neutral sheet, is provided by our two‐point measurements. The plasma sheet cools subsequent to passage of the expanding surface beyond each of the two observing spacecraft. Characteristic anisotropic proton velocity distributions are commonly measured for the earthward‐jetting plasma. A three‐parameter model is presented that explains these velocity distributions and that places a field‐aligned acceleration region at geocentric radial distances exceeding 30–40 R E in the magnetotail.