Dynamics of the field-aligned current during substorms with a sharp onset of the expansion phase

By using satellite magnetic field measurements it is shown that before the substorm expansion phase onset the field-aligned currents of zone 2 type are not always observed on the geosynchronous orbits at the midnight sector. The relationship between the appearance of the field-aligned currents of zone 2 types before the beginning of the substorm expansion phase and the active phase duration and the preceding magnetic disturbance level has been found. It is found that if the expansion phase lasts less than 11 min, the appearance probability of the field-aligned current of zone 2 type before the substorm expansion phase onset T0 decreases. As the geomagnetic activity rises before a substorm, the probability of the beginning of the field-aligned currents before T0 increases. These results indicate that substorms in which the field-aligned currents of zone 2 type the T0 onset are absent, can be probably developed in the distant part of the magnetotail.