The Sector Boundary of IMF and the Geomagnetic Disturbance

The variation of the geomagnetic indices AE, D_st, the frequency of occurrence of disturbance day N_D and quiet day NQduring the time before and after the earth’s passage through the sector boundary of the interplanetary magnetic field (IMP) is studied. According to the change of the polarity of IMF at the sector boundary, the sector boundaries can be divided into the following types: (I) from negative to positive polarity; (II) from positive to negative polarity; (III) from negative to a varied polarity, and (IV) from positive to a varied polarity. The results are:The sector boundary type Iand IIcan be regarded as one of the sources of the geomagnetic disturbance. After the earth’s passage through the sector boundary the geomagnetic indices increase (but NQdecreases) and remain constant in three days.For sector boundary type III and IV, the variation of the IMF and geomagnetic indices are rather complicated. It is found that before the earth’s passage through the sector boundary, AE, D_st and N_D have already increased and N_Q decreased.