The auroral electron precipitation during extremely quiet geomagnetic conditions

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Abstract
The electron precipitation over the polar regions during extremely quiet geomagnetic conditions is examined based on data from 5 years of DMSP observations. A total of 12 periods were selected for this study on the basis of prolonged, extremely low values of the Kp index which persisted for at least 2 consecutive days. The interesting electron precipitation features can be summarized as follows: (1) At all times, precipitation indeed occurred over both the northern and southern polar regions with significant intensity. The precipitating intensities were 1–2 orders of magnitude below the level for nominal, quiet (Kp ≲2) auroral oval precipitations. (2) The measured precipitating electrons were very soft, most having energies below 1 keV. The observed fluxes of low‐energy electrons between 50 eV to a few hundred eV were often merely the high‐energy tail of an extremely soft precipitation. (3) Electron precipitation with a most probable energy of a few keV (harder than that of the auroral oval) can form a band detached from and equatorward of the morning auroral oval. (4) A dramatic feature of the electron precipitation of the extremely quiet magnetosphere is the unexpectedly wide latitudinal extent of the low‐energy electron precipitation. It spreads into the high‐latitude polar regions to at least ∼85° geomagnetic latitude.