Observations of kinetic ballooning/interchange instability signatures in the magnetotail

[1] Stimulated by a recent study of a kinetic ballooning/interchange instability by Pritchett and Coroniti (2010), we present THEMIS events that confirm the predictions of this mechanism. In these events the probes were situated in the plasma sheet at 11 Re, near the presumed location of a B minimum. Prior to substorm onset, they observed strong magnetic oscillations with periods 20–100 s and δBX about 10–20 nT. Associated with these were oscillations of the electric field δEY ∼ 1 mV/m and the field-aligned electron velocity of several hundreds of km/s. No comparable perturbations in the ion velocity were observed. For two cases cross-correlation analyses proved duskward propagation of the elongated spatial structures with a cross-tail width of a few ion gyroradii and a propagation velocity of about the ion drift velocity. In one case THEMIS probes confirmed a sausage-like geometry of the structures.