Abstract
Geomagnetic activity during three days before and after heliospheric current sheet crossings is examined. It is shown that the expression for the index , obtained by Murayama [1979], is applicable to such geomagnetic activity, where (in nT) and V (in km/sec) stand for the southward component of the interplanetary magnetic field and solar wind bulk velocity, respectively. The effects of and V on AL are also examined separately. It is found that short term (∼hours) variations of AL are controlled by fluctuations of while long term (∼days) variations of AL are controlled by V.