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A.E. Potter (NSO), T.H. Morgan (NASA HQ), R.M. Killen (SWRI)
We used the Main Spectrograph at the McMath-Pierce Solar Telescope at KPNO to measure the profile of sodium emission lines from Mercury with a spectral dispersion of 2.8 mA/pixel. An iodine vapor absorption cell and atmospheric absorption lines were used to define the wavelength scale. At the time of measurement (3 and 4 February 2000), Mercury was east of the Sun, with a phase of 20 degrees, so that most of the planetary surface seen from Earth was illuminated by sunlight. The spectrograph slit was oriented north-south on Mercury for one set of observations (3 February), and east-west for another (4 February). After subtraction of scattered sunlight and Mercury surface reflection backgrounds, the wavelength of the centroid for the emission line was measured as a function of position on the spectrograph slit. We found that the wavelength of the sodium centroid differed from the rest wavelength of sodium (taking into account the relative velocities of Earth and Mercury), and varied with position on the planet. Near the Mercury center, the radial velocity of sodium was in the range 0.3-0.4 km/sec towards the Earth. For the east-west transect, the radial velocity towards the Earth increased near the dawn terminator, and decreased towards the dusk terminator. This suggests that there was a flow of sodium vapor from the dawn to the dusk terminators. For the north-south transect, the radial velocity towards the Earth decreased above both poles, suggesting that radiation pressure was pushing sodium off the planet. This work was supported by the NASA Planetary Astronomy Program