Ion temperature drop and quasi-electrostatic electric field on edge of the current sheet minutes prior to the local current disruption

Cross-tail current disruption (CD) in the near-Earth plasma sheet has been well recognized as the key process at the substorm expansion phase onset. However, some outstanding questions such as: what is the principal local magnetospheric parameters that control the stableness/unstableness of a thin current sheet (TCS), what is the dominating plasma instability mode leading to the CD, remain to be answered. Recent theoretical studies indicated that the stableness of the TCS is particularly sensitive to the distribution of ion drift velocity across the thickness of the current sheet, and that the quasi-electrostatic electric field developed in an ion-kinetic-scale TCS may play a key role in driving the TCS to instability. This motivates an investigation of the variations of plasma parameters at the edge of the TCS during the final minutes prior to the local CD. On a statistical survey of the near-Earth CD events during substorm intervals collected form the first tail season of the THEMIS mission, we identify a consistent behavior of precipitous drops of the thermal ion temperature at the edge of the current sheet in a couple of minutes before the local CD. Such ion temperature drop is interpreted as associated with the extreme thinning of an embedded TCS structure that leads to its ultimate disruption. The observed ion cooling is always accompanied with significant electric and magnetic field perturbations containing both systematic shift and wave oscillations. In particular we also reveal the concurrent growth of a neutral sheet-pointing quasi-electrostatic electric field minutes prior to the local CD. We discuss and simulate the role of such quasi-electrostatic field in leading to the ion temperature drop and the subsequent CD process. The wave components of the observed electric/magnetic perturbations are interpreted as Alfvenic effects of the kinetic ballooning mode.