Occurrence Rate of Magnetic Substorms during the Northward IMF

The occurrence rate of magnetic substorms during the northward IMF was investigated statistically. The number of the substorm onsets was counted by searching the Pi2 magnetic pulsations on the rapid-run magnetograms from three mid-latitude stations. Main results obtained are as follows : (1) The occurrence rate during the northward IMF is rather high and decreases with time after the northward turning of the IMF. This indicates that the southward IMF before the northward turning has an important effect over more than several hours and causes a significant portion of the substorms during the northward IMF. (2) The condition where a substorm can be easily triggered comes with a period of three or four hours. Some oscillation in the plasma sheet may play an essential role in triggering a substorm. (3) An example shows that the tail lobe magnetic field data do not reveal any increase of the strength before the onset of a substorm during the northward IMF. (4) Two different roles of the southward IMF for a substorm, i.e. magnitude control and excitation of triggering mechanism, should be distinguished