Magnetotail and boundary layer plasmas at a geocentric distance of approx.18R/subE/: Vela 5 and 6 observations

At the distance of the Vela satellite orbits (r ≈ 18 RE) there are three distinct plasma environments associated with the earth's magnetotail: the plasma sheet, the high-latitude tail (or tail lobes), and the magnetotail boundary layer. This paper reports measurements of the plasma properties in these three regions made with electrostatic analyzers on Vela satellites 5A, 5B, 6A, and 6B. Evidence presented suggests that the plasma temperature profile across the thickness of the plasma sheet may be nearly uniform and very cool (proton average energy ≈1 kev) after long geomagnetically quiet periods; the profile may be nearly uniform and very hot (proton average energy ≈20–30 kev) immediately after a substorm. Except during these extreme cases the central part of the plasma sheet is hottest, and there is a gradual but substantial cooling toward the upper and lower edges. The magnetotail boundary layer, containing plasma less dense than that in the magnetosheath and flowing antisunward at less than magnetosheath flow speeds, is often very thin near the tail's magnetic midplane, but it is thousands of kilometers thick away from the midplane. We suggest that it may extend completely around the top and the bottom surfaces of the tail. The very low density plasma in the tail lobes is rather uniform over that portion of the height of the lobes explored with the Vela satellites. Comparisons of these observations with a magnetospheric model suggest that the boundary layer is the magnetic projection of the dayside cusps and may be a site of injection of magnetosheath plasma into the plasma sheet.