Lidar observations of substantial sodium depletion in the summertime Arctic mesosphere

The chemistry and dynamics of the mesospheric sodium layer have been studied extensively with lidar (laser radar) techniques since the late 1960s. Meteoric ablation is generally regarded as the dominant source of all mesospheric alkali metals including sodium. The Na layer is typically confined to the region between 80 and 110 km with a peak near the mesopause at 90 km, where the density ranges from ∼10³ to 10⁴ cm ^-3. The Na column abundance at mid-latitudes in the Northern Hemisphere varies from a summer minimum of ∼ 3x 10⁹ cm ^-2 to a winter maximum of ∼10¹⁰ cm^-2 in December and January¹. The seasonal and geographical variations in Na abundance are now believed to be related to changes in the mesopause temperature that affect the reaction rates of the main chemical loss processes for Na. In addition to chemical activity, the dynamic effects of the tides, gravity waves and the mean winds have a significant influence on the vertical structure of the layer. In June, the University of Illinois (UIUC) sodium lidar system was installed at Nordlysstasjonen, Svalbard (78^° 12' N, 15^°50' E) on Spitsbergen Island and we conducted more than 27 h of observations during the period 10-15 July 1987. The average Na layer column abundance was ∼ 6 x 10⁸ cm^-2, a value that is almost five times lower than typical summertime Na abundances measured at mid-latitudes. Additional observations were conducted during the period 7-11 September 1987 and the measured Na abundances varied between ∼ 5 x l0 ⁹ and 14 x 10⁹ cm^-2.