Neutral solar wind generated by lunar exospheric dust at the terminator

Abstract

We calculate the flux of neutral solar wind observed on the lunar surface at the terminator due to solar wind protons penetrating exospheric dust grains with (1) radii greater than 0.1 μm and (2) radii greater than 0.01 μm. For grains with radii larger than 0.1 μm, the ratio of the neutral solar wind flux produced by exospheric dust to the incident ionized solar wind flux is estimated to be ∼10−4–10−3 for solar wind speeds in excess of 800 km s−1 but much lower (<10−5) at average to slow solar wind speeds. However, when the smaller grain sizes are considered, this ratio is estimated to be ≥10−5 at all speeds and at speeds in excess of 700 km s−1 reaches ∼10−3. These neutral solar wind fluxes are easily measurable with current low‐energy neutral atom instrumentation. Observations of neutral solar wind from the surface of the Moon would provide independent information on the distribution of very small dust grains in the lunar exosphere that would complement and constrain optical measurements at ultraviolet and visible wavelengths.