Asymmetric V-shaped streaks recorded on board DEMETER satellite above powerful thunderstorms,
Résumé
We report here observations of both symmetric and asymmetric forms of V-shaped streaks on VLF spectrograms recorded on board the DEMETER satellite. Recent investigations have shown that V-shaped streaks are associated with intense and numerous 0+ whistlers generated in the VLF range by active thunderstorms. To understand the origin of the different spectral forms of the V-shaped events, a systematic survey of these phenomena is performed by means of a visual inspection of the VLF spectrograms covering more than 5 years of DEMETER observations. Asymmetric events are more frequently observed over high-latitude regions. The influence of the magnetic field inclination on the observed asymmetry is investigated. First, a full wave method is used to compute, at the lower ionospheric boundary, the electromagnetic pattern that is due to a streak of lightning. This allows us to show a small asymmetry especially for high frequencies (f > 10 kHz). Then, cold plasma dispersive properties are used to determine the characteristics of the waves propagating from the mesosphere to the satellite altitude. We find that, at the plasma cutoff altitude, the propagation is more efficient for waves having wave-normal directions parallel, or quasi parallel, to the Earth's magnetic field direction. Finally, a detailed analysis of an asymmetric V-shaped event is performed. The precise localization of the lightning associated with the V-shaped event is provided by the METEORAGE network. A relationship between the local inclination of the magnetic field above active thunderstorms and the asymmetry of the V-shaped event is pointed out for this particular event.
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