Estimation Techniques of Dispersion Curves of Lightning Whistlers Propagating in the Geospace

It is well known that electron density profile in the geospace changes day by day and thus remote sensing techniques using electromagnetic waves are useful for obtaining the global electron density profile in the earth's plasmasphere with high time resolution. As whistler mode wave originated from lightning discharge has a unique spectrum characterized by so-called "dispersion curve", an electron density profile in the plasmasphere can be estimated by solving the inverse problem from the trend of dispersion curve obtained along an trajectory of a satellite. In the present paper, we introduce estimation techniques of dispersion curves of lightning whistlers propagating in the plasmasphere and present some features of dispersions of lightning whistlers observed by the Akebono satellite. We developed an automatic detection method of lightning whistlers from the wide-band spectrum data obtained by the VLF instrument onboard Akebono. First, a smoothing in the frequency space and a simple noise filtering are carried out. Secondly, a template matching method is adopted in order to achieve a rapid and simple detection of lightning whistler. We also propose an additional method in order to quantify the dispersion curves for non-ducted whistlers, because the non-ducted whistlers are, in general, not characterized by a simple dispersion parameter. The developed method was applied to the VLF wide-band spectrum, whose maximum frequency is 20 kHz and the time and frequency resolution are 20msec and 50Hz, respectively. The results are satisfactory enough to analyze the trend of dispersion curve along the trajectory of the satellite. For example, lightning whistlers originated from northern hemisphere were continuously observed and the estimated dispersion of the whistlers became larger as the satellite moved from northern to southern hemisphere. Computation time for the analysis was practical enough for the real-time application. In other words, our proposed method can be applied to a onboard software for VLF wave receiver so as to detect and estimate important properties of lightning whistlers automatically.