Measurements of solar wind helium in the lunar atmosphere

Measurements of lunar atmospheric helium during 1973 from the Apollo 17 lunar surface mass spectrometer are presented. The average helium abundance is shown to be about 70% of the theoretical model value, suggesting that the solar wind helium flux in 1973, during sunspot minimum, was substantially less than the expected average flux. Large amplitude transients in the helium data indicate rapid response of the lunar atmosphere to changes in solar wind. The atmospheric helium abundance is shown to be correlated with the geomagnetic index Kp.