In order to estimate the dynamic structure of VLF ionospheric exit point, we conducted multipointground-based observation of natural VLF emissions at three unmanned sites: West Ongul (69°01' S, 39°30' E), Skallen (69°40' S, 39°24' E), and H100 (69°18' S, 41°19' E) around Japanese Syowa station, Antarctica,during a whole year of 2006. In this observation, we developed three sets of unmanned autonomous observation systems for natural VLF emissions. Each observation system consists of two crossed vertical loop antennas to pick up North-South (NS) and East-West (EW) magnetic components, a multi-channel analyzer, and a data logger. The intensity and polarization of NS and EW magnetic components are obtained in 4 spaced frequency (0.5, 1.0, 2.0, and 6.0 kHz) channels by the multi-channel analyzer. The VLF emissions observed at the three sites exhibit the interesting difference in wave intensity as well as polarization which would give us an important information on the locations of their ionospheric exit point. To find the distinct exit point, at first we have theoretically calculated the spatial distributions of wave intensity and polarization on the Earth for VLF whistler mode waves coming down from the magnetized ionosphere, by using the full-wave analysis. Then, we have compared the calculated results with the observed data, to evaluate the possible locations of the ionospheric exit point for the auroral hiss events.As an example, the direction of the estimated ionospheric exit point for the auroral hiss event at 31 March 2006 was found to be consistent with a bright aurora region. However, in this case, the estimated ionospheric exit point was located a few hundred kilometers equatorward of the associated aurora. This would suggest that the ray paths for the auroral hiss could be different from the directions of the geomagnetic field lines for auroral precipitation.
Localization of VLF ionospheric exit point by comparison of multipoint ground-based observation with full-wave analysis
概要
