Simulation of nonducted whistler spectrograms observed aboard the MAGION 4 and 5 satellites

Abstract

New observations of nonducted VLF waves aboard MAGION 4 and 5 are used as a basis for discussion of the magnetospherically reflected (MR) whistlers first observed by the Ogo 1 and 3 satellites. The attention is focused on general properties of MR whistler spectrograms determined only by propagation features in a smooth magnetosphere. We develop a computer program that accumulates the results of the ray tracing calculations for a representative 2D continuum of wave trajectories, which permits to generate in one step a spectrogram for a given observation point and illuminating region. Such a spectrogram keeps no trace of spectral intensity, but in turn, it contains the information on the region of origin, the number of hops, and the direction of group velocity of the wave packets which form the spectrogram. The simulations performed together with the analytical consideration yield understanding of several features of MR whistler spectrograms, such as principal dependence of a spectrogram on the observation point rather than on the illuminating region; the correspondence of the nose frequency to the wave packet propagating along the geomagnetic field ( v g⊥=0); the decrease of the nose frequency with increasing trace number, and others.