Study of Dispersion of Lightning Whistlers Observed by Akebono Satellite in the Earth's Plasmasphere

Abstract

SUMMARYWhen the Akebono (EXOS-D) satellite passed through the plasmasphere, a series of lightning whistlers was observed by its analog wideband receiver (WBA). Recently, we developed an intelligent algorithm to detect lightning whistlers from WBA data. In this study, we analyzed two typical events representing the clear dispersion characteristics of lightning whistlers along the trajectory of Akebono. The event on March 20, 1991 was observed at latitudes ranging from 47.83 • (47,83 • N) to −11.09 • (11.09 • S) and altitudes between ∼2232 and ∼7537 km. The other event on July 12, 1989 was observed at latitudes from 34.94 • (34.94 • N) and −41.89 • (41.89 • S) and altitudes ∼1420-∼7911 km. These events show systematic trends; hence, we can easily determine whether the wave packets of lightning whistlers originated from lightning strikes in the northern or the southern hemispheres. Finally, we approximated the path lengths of these lightning whistlers from the source to the observation points along the Akebono trajectory. In the calculations, we assumed the dipole model as a geomagnetic field and two types of simple electron density profiles in which the electron density is inversely proportional to the cube of the geocentric distance. By scrutinizing the dipole model we propose some models of dispersion characteristic that proportional to the electron density. It was demonstrated that the dispersion D theoretically agrees with observed dispersion trend. While our current estimation is simple, it shows that the difference between our estimation and observation data is mainly due to the electron density profile. Furthermore, the dispersion analysis of lightning whistlers is a useful technique for reconstructing the electron density profile in the Earth's plasmasphere.

Automatic Detection of Lightning Whistlers Observed by the Plasma Wave Experiment Onboard the Arase Satellite Using the OpenCV Library

“…where D is a constant value called dispersion of lightning whistler; t is the arrival time at a frequency f ; and t 0 is the time when the lightning strikes. Bayupati [15] used the simple principle of Eckersley's law on waveform data from AKEBONO to detect lightning whistlers. The conventional technique for the automatic detection of lightning whistlers is to identify straight lines in a diagram in which the dynamic power spectra are converted into a diagram with t on the horizontal axis and 1/ √ f on the vertical axis.…”

“…Bayupati et al [15] analyzed the dispersion of lightning whistlers observed by the AKEBONO satellite along its trajectory and discussed the relationship between propagation times of lightning whistlers and the electron density profile along their propagation path. Their work showed that analyzing the dispersion trends of lightning whistlers is a powerful method to determine the global electron density profile in the plasmasphere.…”

“…Due to the limitation of the altitude coverage of AKEBONO; however, the spatial distribution of lightning whistlers in higher altitude regions has not been thoroughly examined. In addition, note that the detection method introduced by Bayupati et al [15] and Oike et al [16] can only cover the most popular type of whistlers because they only took into account a spectral shape that has the ideal dispersion of frequency and time for a lightning whistler. However, there are various types of lightning whistlers.…”

“…In previous studies, the propagation characteristics of lightning whistlers have been intensively studied and, accordingly, lightning whistlers are recognized as important phenomena for monitoring the geospace environment. For example, they could be used as a powerful tool to remotely sense the electron density profile, if we can identify the source location of lightning flashes using the World Wide Lightning Location Network (WWLLN) and then determine the propagation time of the lightning whistlers from the source point to the observation point as a function of their frequency, because the propagation time can theoretically be derived as a function of the electron density along the propagation path [15]. The continuous measurement of lightning whistlers using spacecraft is very helpful for monitoring the spatial and temporal variation of the geospace electron density profile.…”

“…Whistlers are prominent in VLF electromagnetic energy bursts produced by frequent lightning 126 discharges. Figure 1 shows a spectrogram, or the dynamic power spectra, of a whistler observed by 127 the AKEBONO satellite [15]. Dynamic power spectra or spectrograms are visual representations of f conversion technique (image source [15]).…”