Long duration Pc 5 compressional pulsations inside the Earth’s magnetotail lobes

Abstract

Pc 5-type magnetic field pulsations are detected by the IMP-8 spacecraft 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 sense of the magnetic field 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 studies, the latter property has been found to concur with quasi-periodic 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 detectable along the B _y and B _z magnetic field traces (i.e., with regard to the B _z the spacecraft encounters constantly the same conspicuous signature of south-then-north tilting of field lines around each local compression region). The second mode is associated with prolonged periods of extremely low geomagnetic activity and exhibits a signature along the B _y component inconsistent with travelling waves. Thus, the maxima of compressions occur simultaneously with the maxima of B _y excursions: a feature that is explained in terms of tail-aligned current density flowing at the boundary which separates the stable magnetic field in the tail lobe from the very irregular in the magnetosheath. In this case, the spacecraft was located in the vicinity of the high-latitude tail boundary and the observed B _y excursions are consistent with those anticipated by the tail-aligned current polarity, which is determined by the dominant B _y -component of the interplanetary magnetic field (IMF). On the plane YZ we observe an almost linear and circular polarization sense of the vector magnetic field for the first and second mode, respectively.