DIRECT ENTRY OF DENSE FLOWING PLASMAS IRATE THE DISTANT TAIL LOBES

Electric field, magnetic field, and spacecraft potential measurements on the Geotail satellite have confirmed earlier observations of relati vely dense flowing plasmas in the magnetotail lobes. Within these flow ing plasmas, density discontinuities of factors of 2 to 4 were observe d. At least 15 of the Is magnetopause crossings that occurred during a 24-hour interval when the spacecraft was at GSM coordinates near X = -140 R(e), Y = 0, and Z = 15 R(e), were rotational discontinuities acr oss which the magnetosheath plasma had direct entry into the tail lobe s. Within the lobes near the rotational discontinuities, the spacecraf t typically crossed a second boundary at which the plasma density decr eased from its magnetosheath value by a typical factor of 1.5 to 3 whi le the magnetic field strength and flow speed increased. Equatorward o f this density discontinuity, the plasma density and flow remained app reciable. At least 2 of the 18 magnetopause crossings may have been ta ngential discontinuities across which the density and magnetic field s trength both changed. Within the lobes near these tangential discontin uities, significant densities of flowing plasma were observed. Since I I of the 18 discontinuities were crossed within a time interval of les s than 7 seconds, the boundary current layer was well-defined and very thin.