Propagation of the preliminary reverse impulse of sudden commencements to low latitudes

It has been thought that the preliminary reverse impulse (PRI) of the sudde n commencements (SC) phenomena occurs simultaneously on the ground at diffe rent locations. A popular explanation is that the PRI propagates through th e Earth-ionosphere waveguide at the 35 ground magnetometer stations during the SC event on September 24, 1998, and found clear differences in the arri val time of PRI. We calculated the MHD wave propagation time from the locat ion of the first compression of the magnetosphere to the low-latitude groun d stations and found good agreement with the observed PRI arrival times, Ou r calculation also indicates that the wavefront is seriously distorted by t he inhomogeneity of the magnetosphere and the small difference in PRI arriv al time between high-latitude and low-latitude observations cannot be an in dicator of a super-Alfvenic propagation. We also found implications that hi gh-latitude PRIs can be induced by the vortex of ionospheric currents at ne arby latitudes, and the motion of the current vortex can affect the arrival time of high-latitude PRIs.