Semiannual variation of the geomagnetic activity and solar wind parameters

The semiannual variation of the geomagnetic activity is investigated in connection with a large set of solar wind and interplanetary magnetic field data (4494 daily averages from 1965 to 1987). Our analysis confirms that the geomagnetic activity (described by the aa index), is mainly modulated by the southward component of the magnetic field (Bs), as suggested by Russell and McPherron. On the other hand, it is also found that the solar wind velocity (V) has a relevant role in this phenomenon. In fact, the amplitude of the aa modulation is best correlated with the function BsV². We also explore the linkage between the annual trend of aa and the sunspot activity (1868–1989), showing that the modulation of the geomagnetic activity follows a more regular pattern during the descending phase of the solar cycle than during the rising and maximum parts.