Upper limit of helium-4 in the sunlit lunar exosphere during magnetotail passage under low solar wind condition: Result from CHACE aboard MIP in Chandrayaan-1

The outcome of efforts to detect He-4 in the sunlit lunar exosphere as evinced by the CHACE mass spectrometer aboard the Moon Impact Probe in Chandrayaan-1 is reported. The in situ observations by CHACE were carried out in the lunar dayside, covering a broad range of lunar latitudes, when the Moon was on the verge of exiting the Earth's magnetotail. A combination of daytime He depletion and decrease during magnetotail passage of the Moon, along with low flux of alpha particles in the solar wind at the time of CHACE observations present a case when the He abundance in the Moon had hit one of its lowest values. CHACE, thus had the opportunity to explore the lunar exosphere in an extreme combination of the factors that control the lunar He abundance. Based on the observations and instrument sensitivity, an upper limit of  ∼  8.0  ×  102 cm−3 for the surface density of lunar He-4 in the sunlit hemisphere is proposed. This result is expected to provide realistic constraints to the lunar He exosphere models under similar extreme conditions.