Vertical plasma extent above the lunar surface derived from interference pattern of auroral kilometric radiation

Abstract

The lunar atmosphere is extremely tenuous compared to the Earth's atmosphere and circumlunar plasma produced by photoionization is considered to be less dense than solar wind. Nevertheless, it was reported that 500 to 1000 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> electron densities were observed at altitudes of 5 to 10 km near the sunrise terminator by the Soviet Luna 19 and 22 in the 1970s. The high-density layer was interpreted to be the lunar ionosphere. This result remains controversial, however, because the observed large density is difficult to explain theoretically without magnetic shielding from the solar wind. Observation data on the circumlunar plasma were very scarce and the existence of the lunar ionosphere had been neither experimentally proved nor disproved while many researchers have tried to explain its generation mechanism. The Japanese Kaguya project provided the opportunity to diagnose the vertical extent of the circumlunar plasma through a new approach using interference patterns of natural plasma waves in addition to the conventional radio occultation experiment. Although the radio occultation technique is profoundly influenced by the earth's ionosphere, the newly-developed method can sensitively detect plasma layers regardless of the conditions of the earth's ionosphere. In the present study, using the new method, we estimated the vertical electron densities at the terminator regions and examined whether the high-density layer actually existed or not.