The absence of region 1/Region 2 field‐aligned currents during prolonged quiet times

Abstract

Using magnetic field and precipitating particle data from the DMSP/F7 satellite, we have found that, during extended periods of geomagnetic quiet, the large‐scale field‐aligned current (FAC) system known as Region 1/Region 2 either does not exist or is too small to be observable. A clear dayside cleft FAC system is observed on the preferred side of local noon for the indicated by the IMF By component and the cleft electron precipitation extends far into the region normally considered to be the polar cap. These quiet periods represent times when energy input from the solar wind is low; the IMF is maintained at near zero or slightly northward levels and the solar wind speed is low. For such times, the Kp index is ≤ 1 for several consecutive 3 hour periods. In these cases, the auroral zone is marked by diffuse electron precipitation, and only small‐scale or fine‐scale FAC's. In the polar cap, electron precipitation is characterized by polar rain and there are no FAC systems (including the NBZ system). Some previous reports imply that one should always find this large‐scale Region 1/Region 2 system during quiet times. We suggest that the Region 1/Region 2 system vanishes 10's of mins to hrs after the IMF has turned northward.