October 17, 2014

Survey of the height of the ionosphere using the VLF waves recorded by Demeter

A recent study (Toledo-Redondo, S., M. Parrot, and A. Salinas (2012), Variation of the first cut-off frequency of the Earth-ionosphere waveguide observed by DEMETER, J. Geophys. Res., 117, A04321, doi:10.1029/2011JA017400) shows a new remote sensing method which allows to check during night the low part of the ionosphere, a not well know area because it is too high for the balloons and to low for the satellites. This method uses the cutoff frequency of the VLF whistler waves which are observed by Demeter. These waves due to the thunderstorm activity propagate in the Earth-ionosphere waveguide but they can also escape and then be observed by a satellite. In such a case they display a cutoff frequency which is inversely proportional to the height of the Earth-ionosphere waveguide (Figure 1). Demeter had a survey mode to onboard calculate the electric field spectrum in the VLF range (0 - 20 kHz) all around the Earth with a time resolution of ~ 2 s and a frequency resolution of ~ 20 Hz. The cutoff frequency was automatically determined from the spectral intensities and the height of the ionosphere has been studied as function of the seasons and of the years in the low part of the solar cycle. It has been shown that the height of the ionosphere was not the same above the oceans and above the land (Figure 2).

Figure 1: Spectrogram of the electric field in the VLF range between 0 and 20 kHz along a half-orbit. The vertical elements are whistler waves due to lightning strokes. The cutoff frequency around 1.7 kHz is clearly seen.

Figure 2: Global maps (summer in the top panel, winter in the bottom panel) showing a seasonal variation of the height of the ionosphere and a difference between the oceans and the land. The data in the areas at high latitudes are not significant because the natural noise is too high and masks the cutoff frequency.