Distribution and Abundance of Sodium in Mercury's Atmosphere, 1985–1988

Abstract We report spectroscopic measurements of sodium in Mercury's atmosphere for the period July 1985–May 1988. Na abundance varies from 10 atoms cm −2 in equatorial regions in March 1987 to as much as ∼1.5 × 10 12 atoms cm −2 in May 1988. All observations were made using a high-resolution echelle spectrograph on the 61-in. telescope on Mt. Bigelow, Arizona. Strong high-latitude enhancements are common, with the added characteristic that there is usually more at one hemisphere 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 the radar-bright spots at 340° mercurian longitude and 55°N and 25°S latitude. We do not see any trend in column abundance with increasing radiation acceleration. We do not see a significant correlation of abundance with either solar F10.7 flux or sunspot number. Other larger variations occur and must be caused by other factors.

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