Relation between the formation of preliminary and main SI impulses during a sharp compression of the magnetosphere by solar wind

Specific features and development of Pi and Pm impulses at latitudes from the equator to the polar cap are studied. It is shown that formation of Pi and Pm impulses at high latitudes (Ф'=70°) is related to simultaneous intensifications of the westward and eastward currents on the dayside of the magnetosphere with a consequent shift of the eastward current to the pole. It leads to an increase of duration of Pi impulse with the growth of latitude. Inversion of Pi and Pm signs relative to the noon meridian takes place only at Ф'≈36-60°. At these latitudes the duration of Pi impulse also increases with growth of latitude, but the increase velocity is one order of magnitude larger than at Ф'=63-83°. The current system of SI impulse and possible reasons of Pi and Pm formation at different latitudes are discussed.