Excitation of Plasma Resonances by a Small Pulsed Dipole

Abstract

The space and time dependence of decaying resonant oscillations excited in a collisionless plasma by an infinitesimally small pulsed dipole is determined in absence and in presence of an external magnetic field. Oscillations near the plasma frequency, the hybrid frequencies, and harmonics of the electron cyclotron frequency are treated. At a fixed time and at a short distance from the source, the field of the oscillations is nearly uniform. At longer distances the amplitude of the oscillations is proportional to some power of the distance, which in the presence of an external magnetic field is greater than unity and varies for different resonances. The phase of the oscillations also varies with the distance, and this probably explains the complicated interference effects observed with large satellite-borne antennas in the ionosphere. At very long distances the amplitude of the oscillations decreases after reaching a maximum. At the point of excitation the amplitude decreases as some inverse power of the time; at other fixed positions the amplitude at first grows then decreases as a higher inverse power of the time, and eventually the region of uniform decaying field near the point of excitation expands to include the position considered.