Identification of Dipolarization-Associated High Frequency Waves That Exhibit an Inverse Cascade Feature

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. Continuous wavelet transform of the magnetic activity associated with the current disruption shows a clear inverse cascade feature of waves starting at frequencies slightly below the ion cyclotron frequency and gradually evolving 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. We developed the diagnostics of wave polarization based on cold plasma theory in order to identify the wave propagation angle and the normal mode characteristics. A preliminary theoretical analysis is carried out to account for the initial wave characteristics identified through wavelet and wave polarization procedures. From these results, we propose that the excited waves causing eventual dipolarization in this event may originate from the drift-driven electromagnetic ion cyclotron instability propagating at an oblique angle with respect to the magnetic field and approximately parallel to the cross-field current flow. On the basis of such information we have also solved the two-fluid theory of oblique Alfvén-ion-cyclotron instability driven by the cross-field ion motion.