Abstract:
The ionospheric plasma in midlatitude moves upward/downward during the geomagnetic
sudden commencement causing the HF Doppler frequency changes; SCF (+ ) and ( +) on the dayside and
nightside, respectively, except for the SCF (+ ) in the evening as found by Kikuchi et al. (1985). Although the
preliminary and main frequency deviations (PFD, MFD) of the SCF have been attributed to the dusk-to-dawn
and dawn-to-dusk potential electric fields, there still remain questions if the positive PFD can be caused by the
compressional magnetohydrodynamic (MHD) wave and what causes the evening anomaly of the SCF. With the
HF Doppler sounder, we show that the dayside ionosphere moves upward toward the Sun during the main
impulse (MI) of the SC, when the compressional wave is supposed to push the ionosphere downward. The
motion of the ionosphere is shown to be correlated with the equatorial electrojet, matching the potential electric
field transmitted with the ionospheric currents from the polar ionosphere. We confirmed that the electric field of
the compressional wave is severely suppressed by the conducting ionosphere and reproduced the SC electric
fields using the global MHD simulation in which the potential solver is employed. The model calculations well
reproduced the preliminary impulse and MI electric fields and their evening anomaly. It is suggested that the
electric potential is transmitted from the polar ionosphere to the equator by the zeroth-order transverse
magnetic (TM0) mode waves in the Earth-ionosphere waveguide. The near-instantaneous transmission of the
electric potential leads to instantaneous global response of the incompressible ionosphere.
