Subsonic and sunward-orientated lunar wake observed by ARTEMIS in the geomagnetotail

Abstract

On January 8th 2012, the Moon and its two orbiters (ARTEMIS P1 and P2) are located in the terrestrial magnetosphere. During 0716 UT to 0720 UT, P2 is located tailward of the Moon and observes earthward subsonic ion flow, while P1 is located earthward of the Moon but does not detect earthward flow. Comparing with the measurements of P2, the ion differential energy flux observed by P1 is much sparser. The ion pitch angle distribution as well as azimuth ion flux spectra at P1 demonstrates that there is obvious plasma refilling process operating mainly along the magnetic field line and in the moonward direction earthward of the Moon. From the ion flow velocity distribution function of P1, we can detect that both the earthward and tailward fluxes can exist but there is a predominance of tailward fluxes at P1 which is in accordance with both the measurement of ion flow velocity at P1 and the theoretical prediction of wake formation. We interpret this special region detected by P1 as the subsonic, sunward-orientated lunar wake interior to the magnetosphere. In this study, we will illustrate, for the first time, the formation and characteristics of this special ‘wake’ region which is caused by the interaction between the earthward subsonic ion flow in the magnetosphere and the Moon, unlike the one formed in the solar wind. Comparisons between our observations and previous simulation results are also presented. The origin of the earthward ion flow in the magnetotail at lunar distances is discussed as well.