VLF pulse propagation in the magnetosphere

Pulse distortion is important in controlled experiments on very low frequency (VLF) wave particle interaction (WPI) processes in the magnetosphere. Whistler-mode (WM) propagation of RF pulses through a homogeneous magnetoplasma as well as through a duct in the magnetosphere has been investigated by using the fast Fourier transform technique. This technique can be applied in both homogeneous and slowly varying media. As far as we know this is the first time that the distortion of a VLF pulse propagating in the magnetosphere has been calculated. In a homogeneous medium at frequencies below
f_{H}/4
(
f_{H}
is the electron gyrofrequency) the high frequency components arrive prior to the main body of the pulse while the low frequency components lag behind. This sequence is reversed when the carrier frequency exceeds
f_{H}/4
. The distortion increases as the frequency departs from
f_{H}/4
. In the magnetosphere it is found that the frequency of minimum distortion is the "nose" frequency
f_{N}
, as expected. When the carrier frequency is below
f_{N}
the high frequency components of a pulse always arrive first at the equator and the distortions increase as the carrier frequency departs from
f_{N}
. Above
f_{N}
, on the other hand, there is always a location along the path where the pulse distortion is minimum. It is also found that the stretching of the front end of a pulse is large enough (
\sim30
ms for a pulse at 4 kHz traveling through a duct at
4 R_{E}
) to require compensatory pre- and postprocessing of signals in certain wave-injection experiments. An equalizer to compensate for the phase distortion introduced by the medium has been designed. Computer simulation results show that when the preprocessed signals arrive at the interaction region they have the prescribed waveforms. By the same principle the propagation distortion developed between the interaction region and the receiving site can also be removed.