Average electron precipitation patterns and visual aurora characteristics during geomagnetic quiescence

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Abstract
The pattern of polar region electron precipitation during geomagnetically quiet periods (i.e., mostly under the positive IMF Bz condition) is examined on the basis of measurements from the Defense Meteorological Satellite Program (DMSP) F2, F3, and F4 satellites. The auroral electron precipitation during quiet times consists of two distinct types that differ from those found in active times. The high‐latitude part of the precipitation region has average electron energies lower than 500 eV and has bursty spatial structures, but the average energies of the low‐latitude part are generally greater than 500 eV with smoother spatial structures. The boundary between the hard and soft regions is called the transition boundary. It is found that during quiet times the average location of the poleward boundary of electron precipitation is at particularly high geomagnetic latitudes: about 82° to 84° in the morning, noon, and evening sectors and at about 81° to 82° in the midnight sector. The equatorward edge is at about 70° in the morning, noon, and evening sectors and about 69° in the midnight sector. The latitudinal width of the region of auroral electron precipitation is unusually large, indicating a widening of the auroral oval in periods of geomagnetic quiescence. DMSP auroral imaging data further reveal that there are no bright, discrete optical auroras in the region of low average energy electron precipitation. Faint, stable auroral arcs are sometimes seen in the equatorial region of the high average energy electron precipitation. The location of the poleward boundary is affected by the magnitude of the northward interplanetary magnetic field (IMF) component; however, the location of the equatorward and transition boundaries of precipitation are not related to the positive IMF Bz magnitude. These results indicate that the unusually high latitude of the poleward boundary of precipitation is the consequence of a very small number of geomagnetic flux lines interconnected with the IMF during a northward IMF condition. The results also imply that the size of the polar cap is controlled inversely by the magnitude of the northward Bz component. The insensitivity of the transition and equatorward precipitation boundaries to the northward IMF Bz can be used to imply that the boundary dynamics are dominated by internal magnetospheric processes and not by direct solar‐magnetospheric interactions.