Auroral kilometric radiation activity during magnetically quiet periods

摘要

The behavior of auroral kilometric radiation (AKR) during magnetically quiet periods is investigated. A clear AKR onset sometimes occurs without any typical substorm signatures characterized by auroral electrojet enhancement on the ground, tail current disruption in the near-Earth plasma sheet, and energetic particle injection from the plasma sheet. However, the onset is accompanied by auroral brightening on the contracted oval, Pi2 pulsation on the ground, and fast plasma flow in the plasma sheet. These features of quiet-time AKR seem consistent with a contracted oval substorm, indicating that the onset of AKR is one of the elementary components of the contracted substorm. Unexpected features showing that AKR spectra do not always depend on the substorm magnitude are also presented: (1) the AKR intensity during the magnetically quiet period was sometimes comparable with the usual substorm AKR, and (2) the frequency-bandwidth expansion of AKR at the time of onset, which is considered to be related with the vertical development of the field-aligned potential during the substorm, was sometimes comparable between quiet- and disturbed-period AKR.