Detection of m/q = 2 pickup ions in the plasma environment of the Moon: The trace of exospheric H2 +

The Solar Wind Ion Detectors (SWIDs) on the Chang' E-1 spacecraft, while orbiting the Moon, occasionally observed two continuous flux peaks with energies not exceeding 8 and 4 times that of the prevailing solar wind proton energy. These form parallel curves (PCs) with an energy ratio of 2 in the energy-time spectrogram. The fluxes of the two curves are comparable, around 10(-5) similar to 10(-4) of the solar wind flux. The pitch angle distribution of PC particles is concentrated around 90 degrees. The velocity space distribution of PC particles shows distinct double-ring feature, suggesting the existence of a pickup ion species with m/q = 2. Pickup ions from local interstellar medium, the inner sources and the lunar exosphere are investigated. We conclude that this observation may be the first in situ evidence for H-2(+) ions in the lunar exosphere, thus providing new insights on the evolution and fate of solar wind hydrogen in the solar system. Citation: Wang, X.-D., et al. (2011), Detection of m/q = 2 pickup ions in the plasma environment of the Moon: The trace of exospheric H-2(+), Geophys. Res. Lett., 38, L14204, doi: 10.1029/2011GL047488.