Horizon-glow and the motion of lunar dust

Surveyor-7 photographed a bright glow along the western lunar horizon one hour after local sunset. This horizon-glow (HG) must result from the forward scattering of sunlight by electrically charged dust grains [a (grain radius) = 5 × 10−4 cm] which are electrostatically levitated 3 to 30 cm above rocks or surface irregularities located in the lunar terminator zone. The levitation condition is E(V cm−1) ≥ 270 × [a(µ)]1/2.Column densities the order of 50 grains cm−2 are produced. The electrostatic field is generated in the following manner. High energy (500–1500 eV) photoelectrons are ejected from directly illuminated surfaces in the terminator zone. A positive monopole charge is produced which forces the return of subsequently ejected photoelectrons to the vicinity of the illuminated surface. Computer modeling indicates that 1% to 5% of the returned flux will accrete on adjacent completely dark areas. A stable, multipole charge distribution is generated between the light and dark areas. The associated intense multipole electric field (E ≳ 103 V cm−1) can levitate micron size soil grains located in the charged regions. Approximately 10−2 gr cm−2 yr−1 of surface material are ‘churned’ by this process. The photoelectron work function of the lunar material, flux level of solar X-rays (λ ~ 25 A), and the attenuation of solar ultraviolet photons by multiple scattering (as controlled by local surface geometry and reflectivity) are the physically significant factors.