Flapping current sheet with superposed waves seen in space and on the ground

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AbstractA wavy current sheet event observed on 15 October 2004 between 1235 and 1300 UT has been studied by using Cluster and ground‐based magnetometer data. Waves propagating from the tail center to the duskside flank with a period ~30 s and wavelength ~1 RE are superimposed on a flapping current sheet, accompanied with a bursty bulk flow. Three Pi2 pulsations, with onset at ~1236, ~1251, and ~1255 UT, respectively, are observed at the Tixie station located near the foot points of Cluster. The mechanism creating the Pi2 (period ~40 s) onset at ~1236 UT is unclear. The second Pi2 (period ~90 s, onset at ~1251 UT) is associated with a strong field‐aligned current, which has a strong transverse component of the magnetic field, observed by Cluster with a time delay ~60 s. We suggest that it is caused by bouncing Alfvén waves between the northern and southern ionosphere which transport the field‐aligned current. For the third Pi2 (period ~60 s) there is almost no damping at the first three periods. They occur in conjunction with periodic field‐aligned currents one‐on‐one with 72 s delay. We suggest that it is generated by these periodic field‐aligned currents. We conclude that the strong field‐aligned currents generated in the plasma sheet during flapping with superimposed higher‐frequency waves can drive Pi2 pulsations on the ground, and periodic field‐aligned currents can even control the period of the Pi2s.