Spatial distribution of sodium vapor in the atmosphere of Mercury

Abstract

The density and shape of the sodium exosphere of Mercury can give clues to the interactions of cosmic dust, the solar wind, and the radiation field with the planetary surface and magnetosphere. In past observations, A.E. Potter and T.H. Morgan, (1987, Icarus 71, 472–477) establish that radiation pressure affected the sodium density. In this work, we find, as before, that the disk-averaged sodium column density is correlated with the magnitude of radiation pressure in such a way that the maximum column density occurs at minimum radiation pressure and vice versa. However, superimposed on this orderly variation is a disorderly variation on a time scale less than a day, and we find anomalous distributions of sodium vapor in polar and subsolar regions. These effects may be a result of solar wind-magnetosphere interactions. Observations of north-south and east-west distributions of sodium emission from Mercury were made at three levels of radiation pressure: one corresponding to near-minimum solar radiation pressure, a second to near maximum, and a third at an intermediate radiation pressure. Corresponding north-south and east-west distributions of sodium column abundance were calculated using radiative transfer theory, and were fit to the observations by varying the assumed density and distribution of sodium vapor. The main features of the observed intensity distributions could be adequately represented by simple and reasonable models for the temperature and surface distribution of sodium vapor. For this limited data set, the calculated subsolar abundance of sodium at minimum radiation pressure was about three times larger than that at maximum radiation pressure, which is approximately consistent with previous observations. However, it was found that significant details of the observed distributions could not be fit by a simple model. The observed sodium emission distribution was concentrated in the sunward direction with polar enhancements. Several north-south distributions were anomalous in that there was clearly more sodium at one pole than the other. In addition, the sodium distribution varied substantially on a daily basis. The subsolar column abundance varied from 3.8 × 10 11 atoms/cm 2 on April 3, 1988, to 2.8 × 10 11 atoms/cm 2 on April 6, 1988. It is suggested that these anomalies may be related to the interaction of the photoionized sodium atoms with the magnestosphere. More detailed observations in the future may better define the respective roles of radiation pressure, solar wind-magnetosphere interactions, source processes, and evaporation of surface material.