Solar radiation acceleration effects on Mercury sodium emission

A set of Mercury sodium emission data collected over a range of true anomaly angles during 1997-2003 was used to analyze the effect of solar radiation acceleration on sodium emissions. The variation of emission intensity with changing Doppler velocities throughout the orbit was minimized by normalizing the intensities to a constant true anomaly angle. The normalized intensities should be independent of orbital position if sodium density is constant. Plots of the normalized intensities against solar radiation acceleration showed very considerable scatter. However, the scatter was not random, but the result of a systematic variation, such that the normalized emission at a particular value of radiation acceleration took one or the other of two values, depending on the value of the true anomaly angle. We propose that this was the result of solar radiation acceleration changing the velocity of the sodium atoms, and consequently changing the solar continuum seen by the atoms. There is a positive feedback loop in the “out” leg of the orbit, such that radiation acceleration increases the solar continuum intensity seen by the atoms, and a negative feedback loop in the “in” leg of the orbit, such radiation acceleration decreases the continuum intensity. The observations could be approximately fit by assuming that sodium atoms are exposed to sunlight for an average of 1700 s. The emission values corrected for this effect showed much less scatter, with a general trend of about 30% to lower values from minimum to maximum radiation acceleration. The corrected emissions were used to calculate average column densities, and the result compared with the predictions of Smyth and Marconi [Smyth, W.H., Marconi, M.L., 1995. Astrophys. J. 441, 839-864] for the variation of column density with true anomaly angle. The comparison suggests that sodium atoms interact weakly with the surface. The effect of radiation acceleration on emission intensities should be taken into account if column densities are to be calculated from emission intensities.