Solar Cycle Effects on Different Levels of Geomagnetic Storms

A yearly check on the correlation between the pressure corrected Dst and interplanetary parameters for the whole OMNI data set (1974--1996) reveals that apparently the main controlling parameter producing Dst variations is the solar wind speed v, however it is clear that the driving parameters during the main and recovery phases of approximately 300 magnetic storm events extracted from the OMNI database are the southward IMF B_S and the dawn-dusk component of the interplanetary electric field vB_S. To analyze solar cycle effects the selected set of storm events were separated in two subsets using, as a reference, a baseline given by a sunspot number of ∼ 100. We found that the correlation is enhanced for those lying under the baseline and reduced for those events lying over the baseline. Apparently, the correlation is higher for storms occurring near minima of the solar cycle than those surrounding the solar cycle maxima. In a little further analysis we separate the selected data base in four subsets, two for the events occurring on opposite phases of the solar cycle and the other two for those occurring on the ascending and descending parts of the solar cycle. In this case, the correlations between the absolute value of Dst^* and B_S are similar for the opposite phases of the solar cycle (R=0.72, with standard deviations of 30.7 and 40.5, respectively). The best correlation (R=0.79) is found for the ascending phase of the solar cycle marking a significant difference with the descending phase (R=0.69).