Night side lunar surface potential in the Earth's magnetosphere

In the Earth's magnetotail, Japanese Moon orbiter Kaguya repeatedly encountered the plasmoid or plasma sheet. The encounters were characterized by the low energy ion signatures including lobe cold ions, cold ion acceleration in the plasma sheet-lobe boundaries, and hot plasma sheet ions or fast flowing ions associated with plasmoids. Different from the previous observations made in the magnetotail by the GEOTAIL spacecraft, the ions were affected by the existence of the Moon. On the dayside of the Moon, tailward flowing cold ions and their acceleration were observed. However, on the night side, tailward flowing cold ions could not be observed since the Moon blocked them. In stead, ion acceleration by the spacecraft potential and the electron beam accelerated by the potential difference between lunar surface and spacecraft were simultaneously observed. These electron and ion data enabled us to determine the night side lunar surface potential and spacecraft potential only from the observed data for the first time.