WHISTLER EMISSIONS PRODUCED BY MODULATED ELECTRON-BEAMS INJECTED INTOTHE IONOSPHERE

The electric and magnetic fields emitted by semi-infinite, modulated e lectron beams artificially injected into the Earth ionosphere are calc ulated. For whistler mode waves recorded by a remote VLF receiver the spectrum of the signal depends crucially on the time from the beginnin g of the injection. The amplitude of a transient signal from the beam front is controlled by the difference between the modulation frequency and the frequency determined from the stationary phase point in the c orresponding integral (the latter varies with time due to the beam fro nt motion). Far from the beam front, only the forced oscillations due to the modulation of the beam density will be registered. The intrinsi c frequency width of the whistler signal delta f connected with the ra y representation of a wave field determines the optimal frequency band width of the receiver in use. The value of delta f similar to (1/R)(1/ 2) (R is the transverse distance between the beam axis and the receive r) also determines the beam domain, similar to R(1/2) along its axis w hich gives the main contribution to the registered signal (Fresnel zon e). The interference of waves emitted by electrons with different para llel velocities u can reduce the amplitude of the registered wave fiel d significantly. This problem is discussed in connection with the exis tence of a stationary phase point (SPP) over u. It is shown that this SPP condition can be satisfied only for the wave packets emitted in th e near zone of the electron gun. The amplitudes of the coherent and in coherent beam emissions are compared. In the case of a dense plasma (w here the plasma frequency omega(p) is much greater than the electron g yrofrequency omega(c)) the main results are obtained analytically.