Publication Date:
2011-09-20
Description:
We have studied successive substorm expansions that occurred in spite of a period of prolonged northward interplanetary magnetic field (IMF). After the northward IMF persisted for almost ∼19 h, a series of 11 very weak to moderate substorm expansions occurred during the following 6-h interval of northward IMF on 19 January 1998, separated from each other by a few tens of minutes. Most of these substorm expansions did not accompany significant changes of the solar wind and the IMF. As for typical substorms that occur under southward IMF conditions, spacecraft and ground-based instruments observed auroral breakups and enhancements of the westward auroral electrojet and large-scale convection in the ionosphere, together with plasmoids and dipolarizations in the magnetotail, in association with the substorm expansion onsets. These signatures suggest that the mechanism of substorm expansion onsets is the same as for typical substorms, even under prolonged northward IMF conditions. We suggest that the large IMF ∣By∣ affects large-scale convection and energy accumulation in the magnetotail and that the enhanced convection due to the large IMF ∣By∣ effect and substorm expansions can be an important factor for the promotion of succeeding substorm expansion onsets.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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