Spatial distribution of sodium on Mercury

We observed the Mercury sodium exosphere during the period 1997–2003, collecting images of planetary sodium emission covering the full range of true anomaly angles with only a few small gaps. The distribution of sodium emission over the surface was generally non-uniform and changeable. When the dawn terminator was in view, the terminator was generally brighter than the limb, as expected for evaporation of condensed sodium at the dawn terminator. Also, the excess emission reached its largest values when radiation acceleration reached one or the other of its two maxima, as expected for the effect of radiation acceleration on sodium distribution. When the dusk terminator was in view, the limb was generally brighter than the terminator. The difference was larger than would be expected for a uniform sodium exosphere, suggesting that there is a deficit of sodium near the dusk terminator. There was no apparent effect of radiation acceleration on the ratio, which might be the result of a very large deficit of sodium near the dusk terminator. For the northern and southern hemispheres, excess sodium was observed about a third of the time in one or the other hemisphere, appearing at random intervals of true anomaly and longitude. The random nature of these occurrences suggests an external cause, one not correlated with any characteristic of the planetary orbit or planetary geochemistry. We suggest that the northern or southern excess sodium events are the result of solar weather, whereby solar particles are precipitated to the surface at high latitudes, and produce localized sources of sodium. IMF configurations for which solar particles can precipitate to high latitudes on the surface occur about 30% of the time, in general agreement with the observed frequency of north or south excess emission. Near periods of maximum radiation acceleration, some images displayed two peaks of sodium emission, one peak at high northern latitudes, the other at high southern latitudes. One possible cause could be the accumulation of sodium near the terminator, pushed there by radiation acceleration.