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

On 20 April 2007, four Pi2 pulsation bursts occurred successively and simultaneously in the premidnight sector at the E spacecraft and ground-based observatories for the THEMIS mission, while the AE index was less than 100 nT. Especially for the last three onsets, both ground-based and GOES 12 magnetometers sensed magnetic perturbations, as expected from the formation of a substorm current wedge (SCW). Moreover, LANL 1994-084 detected an enhancement of energetic particle flux. Spectral analysis shows a matched wave frequency similar to 6-8 mHz and another harmonic frequency similar to 17 mHz for the fourth burst. The orientation of the major axis of the wave polarization hodogram points toward the SCW location. The first burst has both latitudinal and longitudinal polarization changes from counterclockwise (CCW) to clockwise (CW), in contrast to the other three that have a latitudinal reversal only. The longitudinal CW to CCW change at low latitudes signifies that hydromagnetic waves propagate westward and eastward from the longitude of the impulsive source responsible for SCW. The latitudinal CW to CCW reversal is consistent with induction by a westward moving upward field-aligned current carried by Alfven waves leading to field line oscillations. Consequently, they can be explained by the coupling of a fast magnetospheric cavity mode driven by fast compressional waves to field line resonances as expected from braking bursty bulk flows, resulting from magnetotail reconnection, triggered by a preceding northward interplanetary magnetic field turning. This event shows that the source mechanism of consecutive Pi2 onsets at times of weak geomagnetic activity is the same as during substorm times.