Particle acceleration in disruption of the tail current sheet

Abstract

The current sheet in the Earth's magnetotail is a favorite site for consideration of particle acceleration. One of the major findings from the AMPTE/CCE program is the occurrence of highly fluctuating magnetic field and particle acceleration in the near-Earth tail current sheet at substorm onsets. This phenomenon, called current disruption, signals a substantial reduction of the cross-tail current locally in association with the formation of the substorm current wedge. This high level of field fluctuation subsides with the magnetic field returning to a more dipolar configuration. The study of current disruption has rekindled interest in the near-Earth tail (|X| < ∼ 12 RE) current sheet as a substorm initiation site rather than the mid-tail (|X| ∼ 15–20 RE) as advocated by the traditional near-Earth neutral line model. Furthermore, the study of this phenomenon has led to the development of a substorm onset model based on a current-driven plasma instability termed the cross-field current instability. We review here observations of particle acceleration during current disruption. We show that at least some current disruptions are initiated locally at |X| < 12 RE and not by a disturbance travelling from the mid-tail region. Furthermore, we show observations indicating that local reversals in the north-south component of the magnetic field in the neutral sheet are not necessarily sites of intense particle energization, contrary to the expectation from magnetic reconnection theories. Comparison between observations and preliminary theoretical predictions of the cross-field current instability model gives reasonably good agreement. Local current reduction can lead to loss of equilibrium and a global field reconfiguration from a stretched state before current disruption to a relaxed state afterwards. This reconfiguration can lead to additional particle energization through the induced electric fields arising from this global field reconfiguration.