The Solar Cycle Dependence of the Solar Wind Sources of Geomagnetic Activity

Details The Solar Cycle Dependence of the Solar Wind Sources of Geomagnetic Activity The Solar Cycle Dependence of the Solar Wind Sources of Geomagnetic Activity

May 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agusmsm43d..01r&link_type=abstract

American Geophysical Union, Spring Meeting 2005, abstract #SM43D-01

Physics

2111 Ejecta, Driver Gases, And Magnetic Clouds, 2134 Interplanetary Magnetic Fields, 2162 Solar Cycle Variations (7536), 2784 Solar Wind/Magnetosphere Interactions, 2788 Storms And Substorms

Scientific paper

Variations of average geomagnetic activity levels occur during the solar cycle, as measured by long term (much greater than a solar rotation) averages of geomagnetic indices such as aa. These variations are generally not dominated by energetic solar events (which may be intense but brief) but by changes in the background solar wind, including the effects of changes in the solar open magnetic flux and solar wind speed. They also do not closely follow the solar activity cycle. In particular, activity tends to be enhanced during the declining phase of the cycle due to the presence of corotating solar wind streams. In addition, the period right at solar (sunspot) maximum is often marked by a decrease in activity levels, apparently associated with the weaker interplanetary magnetic fields around the time of solar field reversal, a temporary lull in the occurrence of energetic solar events, and the general absence of fast solar wind. Considering the sources of geomagnetic storms, the most intense storms as almost invariably associated with the passage of interplanetary coronal mass ejections, the associated shocks and compressed post-shock plasma. The presence of a strong southward magnetic field is an important parameter determining the storm size; speed shows a much weaker dependence. Such events predominantly occur at higher activity levels, and typically show two occurrence rate peaks, before and after a temporary decline right at solar maximum. Weaker storms are produced by both ICMEs and corotating streams at high activity levels, and predominantly by corotating streams at lower activity levels. We illustrate these points using observations from 1972 to present from an analysis of solar wind structures inferred from in-situ data.

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