Proton flow measurements in the magnetotail plasma sheet made with Imp 6

Patterns of proton flow in the magnetotail plasma sheet are described. They are derived from observed anisotropies of the flux of protons measured with the Los Alamos Scientific Laboratory (LASL) plasma probe on Imp 6. These observations, made in the geocentric distance range rapprox. =25--32 R/subE/, are entirely consistent with those made at rapprox. =18 R/subE/ with Vela satellites and reported earlier. In particular, the substorm thinning and thickening of the plasma sheet are seen and are accompanied by antisunward, then sunward, flow of plasma as they are at the Vela orbit. During one substorm the Goddard Space Flight Center magnetometer on the satellite clearly showed the Z component of the magnetic field in the plasma sheet to be southward throughout the expansion phase, while antisunward flow prevailed, and then to become suddenly northward again as the flow turned sunward at the substorm's recovery. This observation of the magnetic field behavior provides important confirmation of the view, regarding neutral line formation and movement during substorms, that has been established earlier on the basis of plasma flow measurements made by the Vela satellites. A dawn-to-dusk cross-tail proton flow comprising a proton current of approx.107 protons/cm2 -s(approx.1.6x10-12 A/cm2) was observed as the magnetometer indicated that the satellite was crossing the magnetic 'neutral sheet.' This observation further supports an earlier suggestion that dawn-to-dusk cross-tail proton flows, measured occasionally with the Vela satellites, constituted the magnetotail neutral sheet current