HR: 0830h
AN: SM51A-05    [Abstracts]
TI: Contributions from Different Sources to Semiannual Variation of Geomagnetic Activity
AU: * Lyatskaya, S
EM: sonya_lyatsky@usa.com
AF: Alabama A&M University, 4900 Meridian St., Normal, AL 35762 United States
AU: Lyatsky, W
EM: lyatsky@cspar.uah.edu
AF: Alabama A&M University, 4900 Meridian St., Normal, AL 35762 United States
AU: Tan, A
EM: arjun.tan@email.aamu.edu
AF: Alabama A&M University, 4900 Meridian St., Normal, AL 35762 United States
AB: Historically, the three possible causes for semiannual variation of geomagnetic activity were proposed: 1) an inclination of the ecliptic plane to the solar equatorial plane (while moving around the Sun, the Earth attains its highest heliospheric latitudes, where solar wind speed increases, in equinoctial months); 2) the semiannual variation of the angle between the Earth's axis and the y-axis in the solar-ecliptic (SE) coordinate system (this angle attains its minimum in equinoctial months that results in increasing the contribution from IMF By in the SE coordinate system to IMF Bz in the solar-magnetospheric coordinate system, which is responsible for geomagnetic activity; this effect is known as the Russell-McPherron effect); and 3) the semiannual variation of solar luminosity of high-latitude conjugate ionospheres (in summer-winter months one of the polar caps is in sunlit conditions while in equinoxes both nightside high-latitude ionospheres are in darkness that is favor for the generation of substorm activity). The last mechanism is not dependent on solar wind conditions while two first mechanisms are dependent. This allowed us to estimate the contributions from these two possible mechanisms to the semiannual variation of geomagnetic activity. For this purpose we investigated the semiannual variation of the Dst index for ten years, 1995-2004. We found that after exclusion of the first mechanism, the amplitude decreases by less than 10%, after excluding the second mechanisms (the Russell-McPherron effect) the amplitude of the semiannual variation decreases by ~ 20%. Although both dependence of solar wind speed on heliospheric latitude and the Russell-McPherron effect are evidently seen in the data, our study showed that these effects contribute in total to the semiannual variation of Dst index not more than 30%.
DE: 2435 Ionospheric disturbances
DE: 2784 Solar wind/magnetosphere interactions
DE: 2788 Storms and substorms
SC: SPA-Magnetospheric Physics [SM]
MN: 2005 Joint Assembly