Interplanetary shock–induced current sheet disturbances leading to auroral activations: THEMIS observations

Abstract

We present a unique case of shock‐generated tail current sheet (CS) disturbances measured by five THEMIS spacecraft in a tail‐aligned configuration from −11 to −17 Re on 3 March 2009 when an interplanetary shock passed by the Earth. The spacecraft were ideally near the tail CS center. The IMF Bz was weakly southward with a 2 h average of −0.4 nT in the shock upstream. At 0602 UT when the shock impinged at the subsolar magnetopause, compressional waves were launched and propagated at a speed of ~2900 km s−1 into the tail. Their arrival presented a discontinuity in the magnetic field and plasma density around a THEMIS outer probe. Then, the CS thickness started decreasing and the tail reconnection rate started increasing. About 3–4 min later when the shock in the solar wind compressed the tail magnetopause at the down‐tail location of the THEMIS spacecraft, the CS thinning became more significant and abrupt. Consequently, the CS thickness reduced by ~80% from ~16 × 103 to ~3 × 103 km in ~7 min X = −11 Re. At the same time, earthward fast flows and multiple dipolarization fronts were detected in the tail CS and auroral activity similar to a small substorm on the ground. We found that the tail CS and plasma sheet experienced a two‐step evolution, which is attributed to the much faster propagation of compressional waves inside the magnetosphere than the shock in the solar wind. During the first step, the local magnetic field and plasma varied with low fluctuations. In the second step, the CS and plasma sheet became very disturbed with fast flows and significant waves presented. Observations also indicated that the magnetotail is not in pressure equilibrium during the shock compression.