Earth Planets Space, Vol. 63 (No. 1), pp. 47-56, 2011
Yoshitaka Goto1, Takamasa Fujimoto1, Yoshiya Kasahara1, Atsushi Kumamoto2, and Takayuki Ono2
1Kanazawa University, Kanazawa, Japan
2Tohoku University, Sendai, Japan
(Received June 3, 2009; Revised November 10, 2010; Accepted January 14, 2011; Online published February 21, 2011)
The evidence of a lunar ionosphere provided by radio occultation experiments performed by the Soviet spacecraft Luna 19 and 22 has been controversial for the past three decades because the observed large density is difficult to explain theoretically without magnetic shielding from the solar wind. The KAGUYA mission provided an opportunity to investigate the lunar ionosphere with another method. The natural plasma wave receiver (NPW) and waveform capture (WFC) instruments, which are subsystems of the lunar radar sounder (LRS) on board the lunar orbiter KAGUYA, frequently observe auroral kilometric radiation (AKR) propagating from the Earth. The dynamic spectra of the AKR sometimes exhibit a clear interference pattern that is caused by phase differences between direct waves and waves reflected on a lunar surface or a lunar ionosphere if it exists. It was hypothesized that the electron density profiles above the lunar surface could be evaluated by comparing the observed interference pattern with the theoretical interference patterns constructed from the profiles with ray tracing. This method provides a new approach to examining the lunar ionosphere that does not involve the conventional radio occultation technique.
Key words: KAGUYA, lunar ionosphere, auroral kilometric radiation.