A study on the generation mechanisms of the lunar originating ions

The Moon is known to have very thin atmosphere called Surface Bounded Exosphere (SBE), which consists of heavy neutral atoms and ions. Although the generation / transportation processes of the ions originating from the Moon have long been discussed, they have not been sufficiently understood yet. At present, it is supposed that ions are generated by multiple processes including ionization of neutral exosphere by solar wind, processes at the lunar surface such as photon-stimulated / thermal desorption, photon / charged-particle / chemical sputtering, and meteoric impact. The generated ions are accelerated by surface potential / convection electric field in the solar wind, and then released to space. We intend to estimate the amount of ions generated by photon-stimulated desorption (PSD) and ions generated by solar wind sputtering quantitatively, using data obtained by ion energy mass spectrometer MAP-PACE IMA on Kaguya, which made detailed observation at nearly 100km altitude. By using IMA, it is possible to know species and directions of the incident ions originating from the Moon. The Moon has no global intrinsic magnetic field but some localized magnetic fields called 'magnetic anomalies'. Previous studies revealed that solar wind ions were reflected to some extent and the detection of scattered solar wind ions at the lunar surface decreased above magnetic anomalies. These phenomena imply a possibility that the solar wind ions cannot reach the lunar surface in magnetic anomalies, which may result in the absence of the solar wind sputtering. On the other hand, there is another possibility that the scattered ions are trapped by magnetic anomalies though there exists the solar wind sputtering. We analyzed IMA data obtained above magnetic anomalies in order to know whether solar wind sputtering generated ions in magnetic anomalies or not. We found that it was quite unlikely that solar wind sputtering occurred in the magnetic anomalies. It was also revealed that the ions were generated by PSD in magnetic anomalies and transported to nearly 100km altitude without being trapped by magnetic anomalies. This result suggests the possibility to estimate the amount of ions generated by photon-stimulated desorption (PSD) and ions generated by solar wind sputtering by comparing the data obtained above magnetic anomalies and above the area without magnetic anomalies.