Seasonal, Solar Cycle and Universal Time Variations of the Auroral Electrojets
Abstract
It has been reported that magnetic activity shows pronounced seasonal and universal time (UT) variations. Various attempts have been made to explain the reasons. As one of such attempts we analyze the hourly mean auroral electroject indices obtained during the last 20 years. Recognizing that the AU and AL indices play different roles in the solar wind-magnetosphere coupling, we examine the two indices separately instead of examining their combined index, AE. The AU and AL indices maximize during summer and equinoctial months, respectively. By normalizing the solar conductivity contribution to the AU index, we are able to estimate the electric field contribution to the AU index. It is found that the AU and AL exhibit the same semiannual variation pattern, indicating that the semiannual magnetic variations are controlled by the electric field. The variation pattern of the yearly mean AL index follows the mirror image of the AU index, providing another indication that the electric field is the main modulator of magnetic disturbances. We also confirm that the Dst index shows the same seasonal variation pattern. The pronounced UT variations of the auroral electrojet indices are also examined. To determine the variations as functions of the electrojet intensity, the probability of observing a given level of the indices during each hour of UT, or the number of occurrence of a given level of AU and AL during each UT, is obtained. The probability distribution of the AL index is characterized by a maximum appearing around 12-18 UT and a minimum around 00-08 UT, while the AU shows two peaks. This feature is particularly clear during disturbed periods, indicating that the latitudinal "mismatch" between the AE stations and the auroral electrojet belt is its main cause. It is also confirmed that the seasonal change of the ionospheric conductivity due to the solar EUV radiation contributes to the UT variation, particularly of the AL index during winter. Thus, the combined effect resulting from the unfavorable distribution of the AE stations and the seasonal change of the ionospheric conductivity seems to contribute more effectively to the UT variation of the auroral elctrojet indices during disturbed periods than any physical reason, for example, such as either the Russell-McPherron effect or the equinoctial effect.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFMSM22B..07A
- Keywords:
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- 2407 Auroral ionosphere (2704);
- 2409 Current systems (2708);
- 2411 Electric fields (2712);
- 2778 Ring current;
- 2784 Solar wind/magnetosphere interactions