LONG-DURATION PC-5 COMPRESSIONAL PULSATIONS INSIDE THE EARTH MAGNETOTAIL LOBES

Pc 5-type magnetic held pulsations are detected by the IMP-8 spacecraf t well inside the Earth's magnetotail lobes. The three studied events with an average duration of 3 h and mean amplitude of Delta B/B = 6.6% show a strong longitudinal oscillation. The clockwise polarization se nse of the magnetic held arrowheads in the north lobe (as well as the counterclockwise in the south lobe) on the XZ plane is consistent with that expected when periodic solar wind lateral pressures squeeze the magnetotail axisymmetrically while moving tailward. In the two case st udies, the latter property has been found to concur with quasi-periodi c upstream density fluctuations detected by ISEE-3 and/or ISSE-1. The lobe magnetic field oscillations are classified in two distinct modes. The manifestations of the first mode are tailward-travelling waves de tectable along the B-y and B-z magnetic field traces (i.e., with regar d to the B-z the spacecraft encounters constantly the same conspicuous signature of south-then-north tilting of field lines around each loca l compression region). The second mode is associated with prolonged pe riods of extremely low geomagnetic activity and exhibits a signature a long the B-y component inconsistent with travelling waves. Thus, the m axima of compressions occur simultaneously with the maxima of By excur sions a feature that is explained in terms of tail-aligned current den sity flowing at the boundary which separates the stable magnetic field in the tail lobe from the very irregular in the magnetosheath. In thi s case, the spacecraft was located in the vicinity of the high-latitud e tail boundary and the observed B-y excursions are consistent with th ose anticipated by the tail-aligned current polarity, which is determi ned by the dominant B-y-component of the in terplanetary magnetic fiel d (IMF). On the plane YZ we observe an almost linear and circular pola rization sense of the vector magnetic field for the first and second m ode, respectively.