Fig. 1.
Top three panels show median electron distribution functions [(km/s)−3/cm−3] observed at 4902 times in March–October 1998 when the Lunar Prospector spacecraft was magnetically connected to the dayside lunar surface in the solar wind and the Electron Reflectometer measured a loss cone, in solar wind, Moon, and deHoffman-Teller frames. Positive parallel velocity points downward towards the surface (along the magnetic field) and negative parallel velocity points upward. Given roughly gyrotropic distributions, we only calculate a single perpendicular velocity, but show it with both positive and negative polarity for ease of visualization. The bottom three panels show the same data, but normalized by the angle-averaged energy spectrum of the downward-going electron population. This normalization removes energy dependence, but leaves angular variations, allowing easier identification of electron anisotropies.