TRANSIENT-RESPONSE OF THE NONUNIFORM 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 no n-uniform equatorial ionosphere to plane compressional MHD waves is ex amined 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 re sponsible for the high-latitude PRI. The rise of the ground DL-field a t the dip equator lags behind those in the off-equatorial region. This is a new finding which has not yet been reported observationally. Whe n a magnetic pulsation is incident on the dayside ionosphere, the phas e of the ground magnetic perturbation at the dip equator also lags beh ind those in the off-equatorial region. The phase lag at the dip equat or obtained numerically agrees approximately with those observed for d aytime Pi2 pulsations. However, the amplitude of the ground magnetic p erturbation 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 sh arp increase of the ionospheric zonal current at the dayside equator, the Pi2 pulsations around the dayside equator may also be due to the i onospheric current of polar origin. (C) 1998 Elsevier Science Ltd. All rights reserved.