Electron dynamics in the current disruption region

Abstract

Turbulent magnetic field observed at substorm onsets in the near‐Earth magnetotail is often regarded as a manifestation of the disruption of the local tail current. The present study examines the associated dynamics of electrons using data from the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer. Two well‐known events were selected, which took place on 28 August 1986 and 1 June 1985. The results are summarized as follows: (1) during the early period of the 28 August 1986 event, the average perpendicular energy changed very little even though the total magnetic field strength (BT) fluctuated by an order of magnitude; (2) except the period of (1), tends to change in proportion to BT in both events; and (3) and the density N tend to be anticorrelated, whereas such a tendency cannot be found for the parallel average energy. The successive trigger of the Ion Weibel instability (IWI) and the modified two‐stream instability (MTSI) is a possible explanation for (1). As the plasma sheet thins, the IWI may be triggered first, followed by the MTSI, which becomes unstable at higher frequencies than the IWI and energizes electrons. The observed sequence of magnetic turbulence and electron energization in the 28 August event is consistent with this idea. On the other hand, (1) can also be explained as far as magnetic turbulence is localized near the neutral sheet irrespective of its trigger mechanism. In this case electrons stay mostly outside of the turbulent region, and accordingly the variations of the average electron energy do not have to be correlated with the variations of magnetic field in the turbulence region. In contrast, when the source current of the magnetic turbulence is not located closely (compared with its characteristic scale), the resultant magnetic disturbance is more coherent and is distributed widely along the flux tube. In this case the observed magnetic field can represent the magnetic variation that electrons actually undergo, which explains (2). Result (3) indicates that for the full understanding, the electron dynamics need to be addressed in a three‐dimensional configuration. As the present study is based on two events, it remains to be understood how common the observed features are among current disruption events. However, the results of the present study as well as their implications for the magnetic turbulence, such as its confinement near the neutral sheet, provide additional information for further understanding of the current disruption process.