Polar cap arcs and their relationship to IMF and geomagnetic activity

概要

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From four months DMSP auroral image data, various kinds of polar cap arcs are examined and they may be categorized fundamentally four typical polar cap arcs. The first one is the bright arcs which occur during strong northward IMF period. The second one is poleward leap arcs which occur during the recovery phase of a substorm. The third one is multiple faint arcs which occur during the strong northward IMF period and the fourth one is very faint sun-aligned arcs which occur during a magnetically quiet period. The first and the second polar cap arcs may be originated from the plasma sheet particles. On the other hand, the third and the fourth arcs may not be related to plasma sheet particles but magnetosheath particles. Although the first and the third arcs are seen during the period of a strong northward IMF, the magnitude of geomagnetic activity for the first one is larger than that for the third one. It seems that magnetospheric energy release may be also another important factor for the auroral dynamics.