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

Abstract

Abstract. 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 and the resulting global seasonal variations of Na are analyzed. The retrieval approach is adapted from that used for the retrieval of 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 latitudes and seasons, and has a full width at half maximum of 5-15 km. Small but significant seasonal variations in Na are identified for latitudes less than 40 • , where the maximum Na number densities are 3000-4000 atoms cm −3 . At middle to high latitudes a clear seasonal variation 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 lower number 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 km near the polar summer mesopause, while it exceeds 10 km at lower latitudes. In summer the Na atom concentration at high latitudes and 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 agreement with these.

Comparison of global datasets of sodium densities in the mesosphere and lower thermosphere from GOMOS, SCIAMACHY and OSIRIS measurements and WACCM model simulations from 2008 to 2012

“…The retrieval grid uses 40 latitude bins between 82 • N and 82 • S for the descending part of the orbit, which corresponds to a latitudinal sampling of roughly 4.1 • latitude. More details on the retrieval of the SCIAMACHY dayglow Na dataset are described by Langowski et al (2016). The statistical error of the vertical profiles is roughly 10 % at the peak altitude and is similar for the VCD.…”

“…With respect to the differences this means that the difference of the mean to the two individual density fields is half as large as the difference between the two individual density fields. Errors for the centroid altitude and FWHM are not provided by Langowski et al (2016) but are estimated to be less than 1 km. One systematic error source when determining the centroid altitude is an error in the determination of the tangent altitude of the measurements used.…”

“…The statistical error of the vertical profiles is roughly 10 % at the peak altitude and is similar for the VCD. However, as Na is retrieved independently from both D lines, both individual results can be compared, which was done in Langowski et al (2016). For most latitudes and months the relative differences between the Na D 1 and Na D 2 -line results are within ±10 %.…”

“…However, the focus of this study shall be on the comparison of the datasets, so that the instruments and techniques, which are well documented by Fussen et al (2010), Gumbel et al (2007) and Langowski et al (2016) are only briefly described. In addition, information on the errors of the different datasets is provided, which is useful when comparing different datasets.…”

“…These spaceborne spectrometers typically were on board satellites with Sun-synchronous and polar orbits and a maximum scanned latitude of up to 82 • that retrieved densities for Mg and/or Mg + (see, e.g., Joiner and Aikin, 1996;Correira et al, 2008;Scharringhausen et al, 2008;Langowski et al, 2015), K (see, e.g., Dawkins et al, 2014) and Na (see, e.g., Fussen et al, 2004;Casadio et al, 2007;Fan et al, 2007;Gumbel et al, 2007;Fussen et al, 2010;Hedin and Gumbel, 2011;Langowski et al, 2016). Along with the measurements, global atmospheric models for the metal layers in the MLT have been developed for Na (Marsh et al, 2013a), Fe (Feng et al, 2013), Mg (Langowski et al, 2015), K Dawkins et al, 2015), and Si atoms, molecules and ions.…”