Statistical studies of electron density around lunar wake boundary derived from WFC observation onboard KAGUYA

The waveform capture (WFC) [1,2] onboard KAGUYA measured two components of electric wave signals detected by the two orthogonal 30 m tip-to-tip antennas from 100Hz to 1MHz during the mission period of KAGUYA from November, 2007 to June 2009. By taking advantage of a moon orbiter, the WFC was expected to measure plasma waves related to solar wind-moon interaction, mini-magnetospheres caused by magnetic anomaly on the lunar surface, and radio emissions to be observed from the moon. Because the moon is basically non-magnetized, the solar wind particles directly hit the lunar surface and a plasma cavity called the "lunar wake" is created behind the moon. We investigated electron density profile around the terminator of the moon from the local plasma frequency obtained by WFC. Because our measurement is a direct method measuring the local plasma frequency, we expect absolute density can be derived. KAGUYA experienced encounters with the lunar wake every 2 hours at an altitude of ~100km in the nominal mission, we first analyzed electron density statistically when KAGUYA was located in the solar wind comparing with the data from WIND. Using these observation data, we constructed an electron density model around the lunar wake boundary region. We also report several interesting feature in the profile such as asymmetric structure depending on the direction of interplanetary magnetic field (IMF). KAGUYA was descended to the 50 km altitude and was descended again down to 10-30km in lower altitude (perilune). Electron density in the lower altitude region is also studied using the data obtained in the extended mission. We found electron density slightly increases in the lower altitude region. [1] Y. Kasahara et al., Earth, Planets and Space, 60, 341-351, 2008. [2] T. Ono et al., Space Science Review, doi:10.1007/s11214-010-9673-8, 2010.