Dynamics of lunar dust

Lunar horizon glow observed at the terminator has been shown to be caused by dust particles originating from the surface. These dust grains are electrically charged by the moon's interaction with the local plasma environment and the photoemission of electrons due to solar ultraviolet and X-rays. The charged dust grains can be accelerated by the surface electric field upwards through a narrow sheath region. This surface electric field results from the large-scale interaction of the moon with the interplanetary plasma and solar radiation. Previous models by Criswell and others assume that the dust grains then settle at heights where the electrostatic and gravitational forces acting on them balance, such that they are levitated. These models only explain observations of dust concentrations at a meter or so from the surface, whereas light scattering by dust has been observed at altitudes up to 3 km. Therefore, we have developed a ``fountain'' model in which the dust grains are accelerated through the sheath region, follow ballistic trajectories, and then return to the lunar surface. Above the sheath region, the charge on the dust grains is dictated by their individual interactions with the plasma environment and electron photoemission, such that the dust grains now form part of a dusty plasma. We present some simple models of the dust grain ``fountain'' density distribution with altitude, and compare the model predictions with observations.