Lunar photoelectron layer

Abstract

A layer of photoelectrons exists just above the sunlit lunar surface, owing to the solar photon flux. To aid physical understanding, two simple models of this layer are examined, and for these the net solar wind electron flux is neglected altogether. The general theory of a photoelectron layer (PEL) is developed and applied to obtain a third, reasonably accurate, model of the lunar PEL. The potential, electric field, and electron density are found as functions of height above the lunar surface for various positions on the sunlit hemisphere. This structure is obtained by using the best estimates available to date of the photoemissive properties of lunar soil. It is pointed out that there is a dense photoelectron ‘blanket’ confined to the first few centimeters above the moon's surface. The possibility of photoelectric ‘hot spots’ is discussed, as is also the possibility that the entire lunar surface may be photoelectrically hot. The consequences of the PEL are considered, namely, its role in the moon-solar wind interaction, in ion acceleration, and in surface dust transport. It is predicted that the NASA electric field detector would measure a field of about 6 v/m at its intended measuring height of 10 cm above the lunar surface and for all positions on the sunlit hemisphere from the subsolar point out to 60° from the moon-sun line.