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.
Figure 25: Spectrograms registered by Demeter on February 18, 2006 during 2 minutes.
Top: electric component in the HF range up to 3 MHz.
Middle: the same component in the VLF range up to 20 kHz.
Bottom: a magnetic component up to 1.5 kHz. The parameters below the spectrograms indicate the universal time (UT), the local time (LT), the geographic latitude and longitude, and the McIlwain parameter L. The VLF spectrograms are dominated by emissions called ‘whistlers’. Around 20:51:50 UT one can see an HF pulse in the top panel.
Figure 26: Same presentation as in Figure 25 but the data are now registered during 4 minutes on January 31, 2006 when the satellite is in the survey mode. An HF pulse is seen at the time of an intense whistler around 03 :27 :20 UT.
Figure 27: Top: Location of the HF events.
Bottom: Map obtained with the ionospheric model IRI2001 which shows the critical frequency of the F layer during night time at the local time of Demeter. A comparison between the two maps indicates that the HF waves emitted by the lightning strokes are reflected all around the magnetic equator because the critical frequency of the ionosphere is too large.
Figure 28: Top: Spectrogram of an electric component in the VLF range between 0 and 20 kHz registered on September 23, 2005 between 03:16:00 and 03:23:00 UT. The two vertical white lines show the location of a burst mode. The parameters below the spectrogram indicate the universal time (UT), the local time (LT), the geographic latitudes and longitudes, and the McIlwain parameter L. - Middle: Intensity in kA and lightning occurrence for the same time interval as the top panel. - Bottom: The red stars indicate the geographic positions of the lightning strokes which are shown in the middle panel. The black line is the trace of the Demeter orbit. The time interval is the same as for the two other panels, i.e., the map has the same latitudinal limits as the top panel.
Figure 29: Modelling of the event shown in Figure 28. We consider that the source of the wave emission is a vertical dipole because the lightning strokes are concentrated at a single place. The interferences are produced by the different propagation modes which are excited in the Earth-ionosphere waveguide. The x axis represents the distance from the source.
Figure 30: Example of particle precipitation induced by a lightning stroke. The top panel shows a spectrogram between 0 and 20 kHz obtained with an electric component. The vertical trace around 20:03:50 UT is the electromagnetic mark of a lightning stroke which occurred in the atmosphere. At the same time one can see on the two bottom panels which display the IDP data an increase of the particle flux up to 200 keV.
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.
Figure 31: Global map showing the emissions in the frequency range 18-25 kHz. The more intense emissions correspond to the locations of ground-based VLF transmitters.
Figure 32: Global map showing the emissions in the frequency range 2000-2500 kHz. The more intense emissions correspond to the locations of ground-based VLF transmitters but they are produced by the HF part of the atmospheric lightning strokes (see text).
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).
Figure 33: Whistler intensity registered onboard Demeter as function of the distance (color) and lightning stroke intensity. On the left for the day and on the right for the night. The intensity is much lower during day time because the ionospheric density is much larger. This intensity is proportional to the lightning current and inversely proportional to the distance between the satellite and the lightning stroke.