Magnetic latitude and local time dependence of the amplitude of geomagnetic sudden commencements

Statistical analysis of the main impulse (MI) amplitude of geomagnetic sudden commencements (SCs) in a region from the middle latitudes to equator has been made using the long-term geomagnetic field data obtained from the Yap (geomagnetic latitude, theta = 0.38 degrees), Guam (theta = 5.22 degrees), Okinawa (theta = 16.54 degrees), Kakioka (theta = 27.18 degrees), Memanbetsu (theta = 35.16 degrees), and St. Paratunka (theta = 45.58 degrees) stations. The magnetic local time (MLT) dependence of SC amplitude in the middle latitudes showed magnetic field variations produced by two-cell ionospheric currents (DP 2-type currents) which are driven by the dawn-to-dusk electric field accompanying a pair of field-aligned currents (FACs). The effect of the DP 2-type currents at least expands to the low latitude (theta = 16.54 degrees). In this region, the DL part of SC produced by the enhanced Chapman-Ferraro currents can be dominant, but the DP part of SC contaminated 7% of the DL one. On the other hand, at the daytime equator between 8: 00 and 16: 00 (MLT), the SC amplitude is considerably enhanced with its peak amplitude of 3.24 ( normalized SYM-H value) around 11: 00 (MLT) due to the Cowling effect. Another interesting point is that the SC amplitude in the nighttime sector was enhanced at all the stations again, and its peak value increases with increasing magnetic latitude. This result suggests that the effect of the FACs associated with the MI phase of SC expands to the equator.