Geomagnetic activity during three days before and after helioshperic current sheet crossings is examined. It is shown that the expression for the index AL=60 (Bs+0.5)(V/400)2, obtained by Murayama [1979>, is applicable to such geomagnetic activity, where Bs (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 Bs and V on AL are also examined separately. It is found that short term (~hours) variations of AL are controlled by fluctuations of Bs while long term (~days) variations of AL are controlled by V.