DEMETER Observations of EM Emissions Related to Thunderstorms
Résumé
The paper is related to specific emissions at frequency < 3 MHz observed by the low altitude satellite DEMETER in relation with the thunderstorm activity. At its altitude (similar to 700 km), the phenomena observed on the E-field and B-field spectrograms recorded by the satellite are mainly dominated by whistlers. Particular observations performed by DEMETER are reported. It concerns multiple hop whistlers and interaction between whistlers and lower hybrid noise. Two new phenomena discovered by the satellite are discussed. First, V-shaped emissions up to 20 kHz are observed at mid-latitude during night time. They are centered at the locations of intense thunderstorm activity. By comparison with VLF saucers previously observed by other satellites in the auroral zones it is hypothesized that the source region is located below the satellite and that the triggering mechanism is due to energetic electrons accelerated during sprite events. Second, emissions at frequency similar to 2 MHz are observed at the time of intense whistlers. These emissions are produced in the lower ionosphere in probable relation with Transient Luminous Events (TLEs).