Time variation in exospheric sodium density on Mercury

We conducted continuous spectroscopic observations of the Mercury's sodium exosphere with a 188 cm telescope and a high dispersion echelle spectrograph, for 1-6 h in the daytime on December 4, 13, 14, and 15, 2005. To correct the images of the sodium emission blurred by Earth's atmosphere, the observed distribution was deconvolved with the point spread function which was obtained using Hapke's surface reflection model and the observed surface reflection. The average column density of sodium atoms was 1-2×1011atoms/cm2 and significant diurnal changes were not observed. However, the sodium densities at low latitudes and high latitude changed during the observation and the rate of change in density at low latitude was higher than that at high latitude on December 14 and 15. Although the rates of suggested release processes are higher than the observed rate, the suggested release processes cannot explain the rapid change in density at low latitude. This may suggest the effect of transport of neutral atoms and the recycling of ions to the surface dominates the time variation in the spatial distribution of exospheric sodium atoms on Mercury.