Retrieval of sodium number density profiles in the mesosphere and lower thermosphere from SCIAMACHY limbemission measurements

[摘要] An algorithm has been developed for the retrieval of sodium atom (Na)number density on a latitude and altitude grid from SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) limb measurements of the Na resonance fluorescence.The results are obtained between 50 and 150 km altitude andthe resulting global seasonal variations of Na are analyzed.The retrieval approach is adapted from that used for the retrievalof magnesium atom (Mg) and magnesium ion (Mg⁺) number density profiles recently reported by Langowski et al. (2014).Monthly mean values of Na are presented as a function of altitude and latitude.This data set was retrieved from the 4 years of spectroscopic limb data of the SCIAMACHY mesosphere and lower thermosphere (MLT)measurement mode (mid-2008 to early 2012).

The Na layer has a nearly constant peak altitude of 90–93 km for all latitudesand seasons, and has a full width at half maximum of5–15 km. Small butsignificant seasonal variations in Na are identified for latitudes lessthan 40°, where the maximum Na number densities are3000–4000 atoms cm⁻³. At middle to high latitudes a clear seasonalvariation with a winter maximum of up to 6000 atoms cm⁻³ is observed.The high latitudes, which are only measured in the summer hemisphere, have lowernumber densities, with peak densities being approximately 1000 Na atoms cm⁻³. The full width at half maximum of the peak varies strongly at high latitudes and is 5 kmnear the polar summer mesopause, while it exceeds 10 km at lower latitudes.In summer the Na atom concentration at high latitudesand at altitudes below 88 km is significantly smaller than that at middle latitudes.The results are compared with other observations and models and there is overall a good agreementwith these.

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