Asymmetric V shaped streaks recorded on board DEMETER satellite above powerful thunderstorms

We report here observations of both symmetric and asymmetric forms of V-shaped streaks on VLF spectrograms recorded on board DEMETER . 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 via a visual inspection of the VLF spectrograms covering 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. A full wave method is used to compute, at the lower inospheric boundary, the electromagnetic pattern due to a streak of lightning. A small asymmetry is pointed out 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 magnetic field. Finally, a detailed analysis of an asymmetric V-shaped event is performed. The precise localization of the lightnings associated with the V-shaped event is provided by the METEORAGE network.