Vertical Extent of the Circumlunar Plasma Estimated by Natural Plasma Wave Observations

From radio occultation experiments performed by the Soviet Luna spacecraft in the 1970s, remarkable enhancements of integrated electron content along ray paths traversed near the lunar sunrise terminator were detected. Altitude profiles estimated from the measurements show 500-1000 /cc peak densities at altitudes of 5-10 km and the densities smoothly decreased with scale heights of 10-30 km both upward and downward. This high-density layer was interpreted to be the lunar ionosphere. The lunar atmosphere is extremely tenuous, and plasma produced by photoionization is considered to be less dense than solar wind. The measurements taken by the Luna have been, therefore, viewed with skepticism over the past three decades. Since observation data on the circumlunar plasma were very scarce, the existence of the lunar ionosphere had been neither experimentally proved nor disproved. The Japanese Kaguya project provided the opportunity to determine the vertical extent of the circumlunar plasma by a newly-developed method using natural plasma wave observations in addition to the conventional radio occultation experiment. In this new method, like topside sounding of the earth's ionosphere, an electron density profile above the lunar surface is estimated by reflection altitudes of auroral kilometric radiations (AKR) propagating from the earth. On dynamic spectrograms of the observed AKR, interference patterns appear because the AKR is reflected on the lunar surface or lunar ionosphere and superposed on the directly arrived AKR. The reflection altitudes of the AKR can be estimated from such interference patterns. Although the radio occultation technique is profoundly influenced by the earth's ionosphere, the new method can sensitively detect plasma layers regardless of the conditions of the earth's ionosphere. We applied the developed method to more than twenty interference patterns observed near the terminator regions and confirmed that the AKRs were reflected very near the lunar surface whether or not reflection points were located at a magnetic anomaly where a strong remnant magnetic field existed. The result denotes that the existence of a steady-state ionosphere like that proposed based on the Luna measurements was disproved.