Ionospheric perturbations linked to the thunderstorms
With the selection of the micro-satellite Taranis as future mission where the French ionospheric community is engaged, the data recorded by DEMETER have been examined with another view. Demeter registers the electromagnetic emissions induced by the atmospheric lightning strokes in all frequency ranges. Papers show the HF signature of powerful lightning strokes (see Figures 25 and 26) which was not revealed until now. The Figure 27 shows the areas where these HF events have been observed and gives an explanation about their spatial occurrence. Nobody also has detected the V emissions which are observed when Demeter is just above isolated and very intense thunderstorms (see Figure 28). The Figure 29 shows a modelling of these V-shaped emissions.
Finally the simultaneous comparison of VLF spectrograms where the signature of atmospheric lightning strokes is seen, with the data of the particle detector IDP allowed to reveal the precipitation of the particles which are in the radiation belts by the waves emitted by the lightning stroke which are propagated along the magnetic field lines from one hemisphere to another (Inan et al., 2007). An example of these particle perturbations is shown in Figure 30.
The figures 31 and 32 show the combined effects of the VLF transmitters and of the whistler waves due to atmospheric lightning strokes which are propagated in the Earth-ionosphere waveguide. The global map of the emissions between 18 and 25 kHz shows the locations of the most powerful transmitters. These transmitters heat the ionosphere and induce perturbations of the ionospheric density. The waves which are propagated in the Earth-ionospheric waveguide can then cross the ionosphere more easily (they are less attenuated) and can be observed above by a satellite. The figure 32 which concerns the frequency range 2 MHz shows the waves due to thunderstorms which were able to cross the ionosphere at the location of the transmitters.
A study was also performed by Fisher et al. (2010) on the whistler intensities registered onboard Demeter as function of the lightning stroke intensities and of their distances from the satellite (see Figure 33).