Identification of the poleward boundary of the auroral oval using characteristics of ion precipitation

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Abstract
Electron and ion precipitation data from DMSP F6 and F7 spacecraft are compared in the polar region for BZ < 0 and BZ > 0. We used the electron and ion number fluxes averaged over 0.5° latitude, normalized by the respective flux maxima in each crossing. Analysis of these averaged and normalized fluxes shows that two regions of large‐scale precipitation are present. The first is the “regular” auroral oval, where electron and ion precipitation is highly correlated irrespective of interplanetary magnetic field (IMF) polarity. The other is the polar cap, wherein for BZ > 0 the diffuse electron and ion precipitation is one or more orders of magnitude weaker than in the oval, although isolated spikes of accelerated electrons occur, and for BZ < 0, only an unstructured flux of low‐energy polar rain electrons with no easily resolvable ion precipitation is observed. For northward IMF the poleward boundary of the regular oval can be identified as a boundary where the normalized ion flux falls below 0.2. At the same boundary, diffuse electrons of Ee < 1 keV disappear when BZ > 0. Using this criterion, the poleward boundary of the polar cap can be defined computationally. It is suggested that for northward IMF, the poleward boundary of the oval may represent the mapping of a separatrix which, according to recent research results, can exist inside a closed magnetosphere at large distances (∼ X = −100 RE), namely, where a divergence exists between plasma flowing sunward and antisunward.