Theory of VLF Doppler signatures and their relation to magnetospheric density structure
Journal Article
·
· J. Geophys. Res.; (United States)
The published spectrograms of signals from ground-based VLF transmitters observed aboard satellites show a wide variation from sample to sample. In general, the observed signals are Doppler shifted by the motion of the satellite, but in some published samples there is no observed Doppler shift. A ray tracing study of differing VLF Dopper signatures observed by the Ogo 4, FR 1, and Isis 2 VLF experiments has shown that each signature can be reproduced by a distinct class of electron density gradient models. Large positive and negative Doppler shifts (approx.100 Hz) are reproduced by a strong decreasing electron density gradient which interacts with the magnetic field curvature gradient between Lapprox.2 and Lapprox.3. Presence of a strong signal with no Doppler shift is shown to be a result of trapping of upgoing rays by steep density drop-offs. Merging of the spectral components in the Doppler shifted signals is interpreted in terms of scattering by small-scale field-aligned density irregularities. These results allow VLF Doppler observations to be used as a new diagnostic tool in determining magnetospheric density structure below Lapprox.3 and in estimating the effective coverage of the lower ionosphere by the ground-based VLF transmitter in the excitation hemisphere. (AIP)
- Research Organization:
- Space Science Laboratory, Aerospace Corporation, Los Angeles, California 90009
- OSTI ID:
- 7175593
- Journal Information:
- J. Geophys. Res.; (United States), Journal Name: J. Geophys. Res.; (United States) Vol. 81:19; ISSN JGREA
- Country of Publication:
- United States
- Language:
- English
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