DISTRIBUTION AND ABUNDANCE OF SODIUM IN MERCURY ATMOSPHERE, 1985-1988

We report spectroscopic measurements of sodium in Mercury's atmosphere for the period July 1985-May 1988. Na abundance varies from <1 x 10(1 0) atoms cm(-2) in equatorial regions in March 1987 to as much as simi lar to 1.5 x 10(12) atoms cm(-2) in May 1988. All observations were ma de using a high-resolution echelle spectrograph on the 61-in. telescop e on Mt. Bigelow, Arizona. Strong high-latitude enhancements are commo n, with the added characteristic that there is usually more at one hem isphere than the other. There is an obvious diurnal correlation, with morning abundances significantly higher than afternoon. We see notable Na emission enhancements when our spectrograph slit is placed over th e radar-bright spots at 340 degrees mercurian longitude and 55 degrees N and 25 degrees S latitude. We do not see any trend in column abunda nce with increasing radiation acceleration, We do not see a significan t correlation of abundance with either solar F10.7 flux or sunspot num ber. Other larger variations occur and must be caused by other factors . (C) 1997 Academic Press.