Excitation of plasma resonances by a small pulsed dipole in a weakly inhomogeneous plasma

The field of a small pulsed dipole in a slightly non-uniform plasma is calculated with the aid of WKB solutions and stationary phase approximations. McAfee's explanation of the quasi-periodic amplitude fluctuation of decaying oscillations excited near the plasma frequency by topside sounders, in terms of a beat between two waves following different group paths, is thus confirmed. The amplitude is shown to be enhanced by the non-uniformity of the plasma; the decay with time follows a t−1/2 law in contrast to the t−5/2 decay in a uniform plasma. The treatment is restricted to a horizontally stratified ionosphere in which the satellite travels parallel to a horizontal magnetic field.