Seasonal Variations of Large-scale Field-aligned Currents
Abstract
Auroral electron acceleration is closely related to upward-flowing field-aligned currents (FACs), and therefore interhemispherical comparison of FACs should provide important clues for understanding (the lack of) auroral conjugacy. In the present study we statistically examine the seasonal dependence of large-scale FACs. We previously developed an automatic procedure to identify the structures of large-scale FACs [Higuchi and Ohtani, JGR, 105, 25,305, 2000], and the present study is based on ~185,000 FAC crossings we identified by applying this procedure to magnetometer data from the DMSP-F7 and F12-15 spacecraft. The result confirms that the dayside FAC tends to be more intense in the summer hemisphere and weaker in the winter hemisphere showing annual variations of its intensity. It is also found that the average latitude of midday FAC systems is higher in summer than in winter; the difference can reach as much as 5 degrees at solstices. This displacement can be explained in terms of asymmetric magnetospheric configurations. The degree of the winter-summer asymmetry diminishes away from the midday sector. On the nightside, the average FAC intensity tends to be larger around equninoxes and smaller around solstices, which is consistent with the well-known semiannual variations of geomagnetic activity. It is therefore concluded that the primary cause of the seasonal variation of large-scale FACs depends on local time. It is the ionospheric conductance owing to the solar illumination for dayside FAC systems, whereas for nightside FAC systems, it is the efficiency of energy coupling between the solar wind and the magnetosphere.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSMSM24A..04O
- Keywords:
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- 2704 Auroral phenomena (2407);
- 2708 Current systems (2409);
- 2736 Magnetosphere/ionosphere interactions;
- 2740 Magnetospheric configuration and dynamics;
- 2784 Solar wind/magnetosphere interactions