Ion temperature drop and quasi‐electrostatic electric field at the current sheet boundary minutes prior to the local current disruption

Abstract

From a survey of the cross‐tail current disruption (CD) events in the near‐Earth plasma sheet collected from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission, we identify a highly repeatable class of event occurring at the current sheet boundary in a few minutes before the local CD onset. Salient features of this class of event include (1) a precipitous drop of the ion temperature, (2) concurrent growth of a neutral sheet‐pointing electric field, and (3) ULF wave activations at Pi1/Pi2 bands. We interpret the ion temperature drop as a manifestation of the extremely thinning of the local current sheet prior to its disruption. This thinning process is inferred as nonadiabatic by nature. Particularly, when the current sheet thickness is down to ion kinetic scales, the ions are demagnetized, and a quasi‐electrostatic neutral sheet‐pointing electric field emerges owing to the charge separation. The ULF fluctuations of electric/magnetic field have a two‐band structure. The lower‐frequency band with a period of 50 to 80 s is interpreted as an Alfvenic mode coupled from other preonset wave modes excited at the equatorial plasma sheet such as the ballooning. The higher‐frequency wave at 10‐ to 20‐s periods is attributed to some instability mode, directly leading to the disruption of the thin current sheet (TCS). We suggest that an extremely thinned non‐Harris TCS and the emergence of quasi‐electrostatic field constitute the conducive conditions for a local CD to occur. A companion paper by Liu et al. (2009) covers some theoretical aspects of this study.