Determination of Source Hemisphere of Auroral Kilometric Radiation (AKR) observed by KAGUYA

The auroral kilometric radiation (AKR) were observed with a plasma wave receiver called WFC (waveform capture) [1] onboard the lunar orbiter KAGUYA during the whole mission period (2007-2009). It is well-known that the AKR is dominated by right-handed mode wave with small contribution of left-handed one. Investigation of observation conditions of each mode wave is still an interesting subject associated with AKR generation and propagation characteristics. A problem in the AKR polarization analysis is that the relation between the source polarization and the observed polarization depends on the source hemisphere. Because the WFC measured only two orthogonal electric field components, wave normal vectors of the AKR cannot be determined from the measurements. A simple way to determine the source hemisphere is to examine whether AKR is observed when the polar region of one hemisphere is seen and that of the other is shadowed by the Moon. However, its usable conditions are limited. In this study, we used a new technique to determine the source hemisphere of the observed AKR. Because the KAGUYA orbits around the moon in two-hour periods, the log-ratio of the two measured electric fields of the AKR has a clear sinusoidal time variation with the two-hour periodicity. The phase of the variation depends on the source location. For example, the times when the log-ratio becomes zero are several minutes different in two cases: the AKR sources exist in the north and south polar regions, respectively. Since the time resolution of the WFC measurements were 0.5 - 8 seconds, it is not difficult to determine the phase of the sinusoidal variations. Using the technique, the source hemispheres of the AKR were determined even when the AKR from the both hemispheres had been simultaneously observed. The statistical characteristic of the AKR polarization will be discussed. [1] Y. Kasahara, Y. Goto, K. Hashimoto, T. Imachi, A. Kumamoto, T. Ono, and H. Matsumoto, Plasma Wave Observation Using Waveform Capture in the Lunar Radar Sounder on board the SELENE Spacecraft, Earth, Planets and Space, 60, 341-351, 2008.