Plasmas on the surface of the plasma sheet at its interface with the high‐latitude lobes are studied at ∼30–40 earth radii (RE) 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 RE 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 RE in the magnetotail.

Three field‐aligned current sheets are directly observed with plasma and magnetometer instrumentation on board the ISEE spacecraft located at ∼20 RE in the postmidnight sector of the magnetotail. These current sheets are encountered at the northern plasma sheet boundary as the plasma sheet expanded past the spacecraft positions during the recovery phase of a magnetic substorm. This expansion speed, Vz in solar magnetospheric coordinates, is 14 km/s. The corresponding convection electric fields E⊥ are derived from the three‐dimensional proton velocity distributions as measured with the quadrispherical Lepedea plasma instrumentation. The average current densities within the three field‐aligned current sheets are +3.3 × 10−9, −1.3 × 10−8, and +1.1 × 10−8 A/m², in order of decreasing distance to the plasma sheet. Current densities in the first and third sheets are directed into the ionosphere, and the current carriers are ionospheric electrons drifting away from the ionosphere. The second, or central, current sheet is directed away from the ionosphere; however, the source of the current‐carrying electrons is unclear. This central current sheet is thought to be associated with ionospheric electron precipitation producing discrete auroral arcs via an acceleration mechanism at intermediate altitudes. The thicknesses of the three magnetotail current sheets are 0.36, 0.55, and 0.74 RE, again in order of decreasing distance from the plasma sheet. Current intensities, the products of current densities and sheet thicknesses, of the field‐aligned current sheets are measured simultaneously with magnetometers and are +0.010, −0.028, and +0.018 A/m, respectively. These values agree reasonably well with those derived from the direct plasma measurements. Electron drift velocities VD within the current sheets are in the range 0.05 to 0.1 Ve, where Ve is the electron thermal velocity. The ion sound velocity Cs ≃VD. These plasma parameters, including the fact that the protons are hot with Tp ≃3Te, appear to provide an unfavorable situation for any substantial steady state anomalous resistivity at this location of the magnetotail. Current‐driven ion cyclotron and/or ion cyclotron drift instabilities may be responsible for broadband electrostatic noise previously observed at the boundary of the plasma sheet.

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