THEMIS observations of consecutive bursts of Pi2 pulsations:The 20 April 2007 event

摘要

[1] On 20 April 2007, four Pi2 pulsation bursts occurred successively and simultaneouslyin the premidnight sector at the E spacecraft and ground-based observatories for theTHEMIS mission, while the AE index was less than 100 nT. Especially for the last threeonsets, both ground-based and GOES 12 magnetometers sensed magnetic perturbations,as expected from the formation of a substorm current wedge (SCW). Moreover, LANL1994-084 detected an enhancement of energetic particle flux. Spectral analysis shows amatched wave frequency ~6-8 mHz and another harmonic frequency ~17 mHz forthe fourth burst. The orientation of the major axis of the wave polarization hodogrampoints toward the SCW location. The first burst has both latitudinal and longitudinalpolarization changes from counterclockwise (CCW) to clockwise (CW), in contrast to theother three that have a latitudinal reversal only. The longitudinal CW to CCW changeat low latitudes signifies that hydromagnetic waves propagate westward and eastwardfrom the longitude of the impulsive source responsible for SCW. The latitudinal CW toCCW reversal is consistent with induction by a westward moving upward field-alignedcurrent carried by Alfven waves leading to field line oscillations. Consequently, they canbe explained by the coupling of a fast magnetospheric cavity mode driven by fastcompressional waves to field line resonances as expected from braking bursty bulk flows,resulting from magnetotail reconnection, triggered by a preceding northwardinterplanetary magnetic field turning. This event shows that the source mechanism ofconsecutive Pi2 onsets at times of weak geomagnetic activity is the same as duringsubstorm times.