Sources of geomagnetic activity during nearly three solar cycles

mass ejections (CMEs), shocks, and postshock flows) to averages of the aa geomagnetic index and the interplanetary magnetic field (IMF) strength in 1972–2000 during nearly three solar cycles. A prime motivation is to understand the influence of solar cycle variations in solar wind structure on long-term (e.g., approximately annual) averages of these parameters. We show that high-speed streams account for approximately two-thirds of long-term aa averages at solar minimum, while at solar maximum, structures associated with transients make the largest contribution (50%), though contributions from streams and slow solar wind continue to be present. Similarly, high-speed streams are the principal contributor (55%) to solar minimum averages of the IMF, while transientrelated structures are the leading contributor (40%) at solar maximum. These differences between solar maximum and minimum reflect the changing structure of the near-ecliptic solar wind during the solar cycle. For minimum periods, the Earth is embedded in high-speed streams 55% of the time versus 35% for slow solar wind and 10% for CME-associated structures, while at solar maximum, typical percentages are as follows: high-speed streams 35%, slow solar wind 30%, and CME-associated 35%. These compositions show little cycle-to-cycle variation, at least for the interval considered in this paper. Despite the change in the occurrences of different types of solar wind over the solar cycle (and less significant changes from cycle to cycle), overall, variations in the averages of the aa index and IMF closely follow those in corotating streams. Considering solar cycle averages, we show that high-speed streams account for 44%, 48%, and 40% of the solar wind composition, aa, and the IMF strength,, respectively, with corresponding figures of 22%, 32%, and 25% for CME-related structures, and 33%, 19%, and 33% for slow solar wind. INDEXTERMS: 2134 Interplanetary Physics: Interplanetary magnetic fields; 2162 Interplanetary Physics: Solar cycle variations (7536); 2164 Interplanetary Physics: Solar wind plasma; 2111 Interplanetary Physics: Ejecta, driver gases, and magnetic clouds; 2788 Magnetospheric Physics: Storms and substorms; KEYWORDS: geomagnetic activity, solar cycle variation, solar wind, interplanetary magnetic field

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