SEMIANNUAL VARIATION OF THE GEOMAGNETIC-ACTIVITY AND SOLAR-WIND PARAMETERS

The semiannual variation of the geomagnetic activity is investigated i n connection with a large set of solar wind and interplanetary magneti c field data (4494 daily averages from 1965 to 1987). Our analysis con firms that the geomagnetic activity (described by the aa index), is ma inly modulated by the southward component of the magnetic field (Bs), as suggested by Russell and McPherron. On the other hand, it is also f ound that the solar wind velocity (V) has a relevant role in this phen omenon. In fact, the amplitude of the aa modulation is best correlated with the function BsV2. We also explore the linkage between the annua l trend of aa and the sunspot activity (1868-1989), showing that the m odulation of the geomagnetic activity follows a more regular pattern d uring the descending phase of the solar cycle than during the rising a nd maximum parts.