V-shaped VLF streaks recorded on DEMETER above powerful thunderstorms

We report the observation of unusual V-shaped streaks on VLF spectra observed on the low-altitude satellite DEMETER during its overcrossing of highly active (i.e., high lightning flash rate) thunderstorms. At similar to 700 km, wave activity observed on the E-field spectrograms at mid-to-low latitudes during nighttime are mainly dominated by upgoing 0+ whistlers and occasional reflected (from the other hemisphere) whistler components. Over a 3 year period, 87 events with V-shaped streaks associated with intense and numerous 0+ whistlers in VLF range have been observed. Using the National Lightning Detection Network for cases above the North American region indicates that these V-shaped streaks exclusively occur when DEMETER passes above intense isolated thunderstorms, with relatively high (tens per second) lightning flash activity. Using a new model of VLF propagation in the Earth-ionosphere waveguide, and transionospheric leakage to the satellite altitude, it is shown that the funnel-like "V" shape of the streaks is a simple consequence of the mapping to high altitude of frequency-dependent position of mode interference nulls (and crests) within the Earth-ionosphere waveguide.