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 relatively dense flowing plasmas in the magnetotail lobes. Within these flowing plasmas, density discontinuities of factors of 2 to 4 were observed. 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 across which the magnetosheath plasma had direct entry into the tail lobes. Within the lobes near the rotational discontinuities, the spacecraft typically crossed a second boundary at which the plasma density decreased from its magnetosheath value by a typical factor of 1.5 to 3 while the magnetic field strength and flow speed increased. Equatorward of this density discontinuity, the plasma density and flow remained appreciable. At least 2 of the 18 magnetopause crossings may have been tangential discontinuities across which the density and magnetic field strength both changed. Within the lobes near these tangential discontinuities, significant densities of flowing plasma were observed. Since II of the 18 discontinuities were crossed within a time interval of less than 7 seconds, the boundary current layer was well-defined and very thin.