Occurrence characteristics of the preliminary impulse of geomagnetic sudden commencement detected at middle and low latitudes

The occurrence frequency of preliminary impulse (PI) of geomagnetic sudden commencements was statistically studied using the geomagnetic records of H-component at Fredericksburg (FRD; 49.0 degrees geomagnetic latitude) and San Juan (SJG; 29.2 degrees). In the daytime, positive PI predominantly occurs before noon and negative PI in the afternoon. These features, which are the same as those of high-latitude PI, indicate that high-latitude ionospheric currents responsible for PI extends toward middle and low latitudes. Almost all Pls detected in the nighttime are negative at FRD though the occurrence rate is much smaller than in the daytime. The diurnal variation of the occurrence rates of PI shows a drastic seasonal change at SJG, which is probably caused by a seasonal variation of the ionosphere. No correlation between the occurrence rates and the solar activity was found at either observatories. Ionospheric currents which are driven by a pair of field-aligned currents were numerically calculated on the assumption of the stationary state, and the geomagnetic field variation caused by the three-dimensional currents is simulated. Although the symmetric distribution of the field-aligned currents with respect to the noon-midnight meridian has been assumed, the calculated diurnal variation of magnetic field exhibits a severe deflection from the symmetry in middle and low latitudes.