Estimation of Lunar Surface Permittivity for Low Frequency from Polarizations of Natural Waves Observed by SELENE (Kaguya)

Abstract:

Lunar surface permittivity is an important parameter for understanding the Moon and has been observed by many radar experiments from spacecraft since 1960s. In these observations, frequencies of radio transmissions are generally high (HF to UHF band) because of instrumental limitations. One of the interesting topics about the lunar surface permittivity is its frequency response in lower bands. It is known that frequency response of permittivity in the low bands can be a good indicator of the moisture content. In theory, there is a considerable difference between the permittivity of dry soil and ice especially below 100 kHz.

The lunar orbiter SELENE (Kaguya) carried out radar observations of the lunar subsurface structures and passive observations of natural waves with the Lunar Radar Sounder instrument (LRS). In the passive observations, an electromagnetic wave from the Earth’s auroral region that is called auroral kilometric radiation (AKR) was frequently observed in low frequency (LF) band. An interesting feature of AKR observation from SELENE is that the observed AKR consists of not only waves reached directly from the Earth's auroral region but also waves reflected on the lunar surface. Since such reflections influence polarizations of the observed AKR, we can estimate the lunar surface permittivity for low frequency waves from the comparison between observed polarizations and those theoretically calculated for various parameter sets of the lunar surface. As a result, the observed polarizations are well reconstructed when the permittivity of the lunar surface is assumed to be around 4 to 8 in average. In the presentation, we show the regional distributions of the permittivity that is derived from detailed data analysis and compare them with permittivity map for high frequencies (HF band) that are derived from radar observations from SELENE.