Topside resonances as oblique echoes

The resonance observed by topside sounders at the plasma frequency is explained in terms of slowly propagating waves that reflect obliquely and intercept the satellite. The analysis is an extension of previous work to include the effects of a nonhorizontal magnetic field and propagation out of the magnetic meridian. The returning signal exhibits a characteristic ‘ringing’ or long continuous response. Furthermore, a beating between two waves of slightly different frequencies takes place during the latter part of the response. This accounts for the fringes observed in the resonance.