Inverse cascade feature in current disruption

Abstract

We investigate the origin of waves leading to current disruption and dipolarization observed by a THEMIS satellite at XGSM ∼ −8 RE in the magnetotail near a substorm expansion onset on 29 January 2008. Continuous wavelet transform of the magnetic activity associated with current disruption shows a clear inverse cascade feature of waves starting at frequencies slightly below the ion cyclotron frequency to waves at low frequencies that correspond to the time scale of dipolarization. There was no other low‐frequency wave associated with the time scale of dipolarization. On the basis of this inverse cascade feature, we provide a preliminary theoretical analysis to propose that the initial excited waves causing eventual dipolarization in this event may originate from the drift‐driven electromagnetic ion cyclotron instability, which is an electromagnetic version of the Weibel instability, or an ordinary mode instability similarly driven by the cross‐field ion drift.