Transient response of the non-uniform equatorial ionosphere to compressional MHD waves

By assuming a realistic distribution of the height-integrated Cowling conductivity along the noon meridian, the transient response of the non-uniform equatorial ionosphere to plane compressional MHD waves is examined numerically. The direct incidence of an SC-type disturbance (or DL-field) on the ionosphere produces no preliminary reverse impulse (PRI) changes around the dayside equator, supporting the view that the equatorial PRI is caused by an extension of the ionospheric current responsible for the high-latitude PRI. The rise of the ground DL-field at the dip equator lags behind those in the off-equatorial region. This is a new finding which has not yet been reported observationally. When a magnetic pulsation is incident on the dayside ionosphere, the phase of the ground magnetic perturbation at the dip equator also lags behind those in the off-equatorial region. The phase lag at the dip equator obtained numerically agrees approximately with those observed for daytime Pi2 pulsations. However, the amplitude of the ground magnetic perturbation obtained numerically is depressed at the dip equator. Such a depression is contrary to the observational fact that the pulsation amplitude is enhanced at the dayside equator. Since there exists a sharp increase of the ionospheric zonal current at the dayside equator, the Pi2 pulsations around the dayside equator may also be due to the ionospheric current of polar origin.