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During the last decade, several limb sounding satellites havemeasured the global sodium (Na) number densities in the mesosphere and lowerthermosphere (MLT). Datasets are now available from Global OzoneMonitoring by Occultation of Stars (GOMOS), the SCanning Imaging AbsorptionspectroMeter for Atmospheric CHartography (SCIAMACHY) (both on Envisat) andthe Optical Spectrograph and InfraRed Imager System (OSIRIS) (on Odin).Furthermore, global model simulations of the Na layer in the MLT simulated bythe Whole Atmosphere Community Climate Model, including the Na species(WACCM-Na), are available. In this paper, we compare these global datasets. The observed and simulated monthly averages of Na vertical column densities agree reasonably well with each other. They show a clear seasonal cyclewith a summer minimum most pronounced at the poles. They also show signs of a semi-annual oscillation in the equatorial region. The vertical columndensities vary from 0. 5  ×  109 to 7  ×  109 cm−2 near the poles and from 3  ×  109to 4  ×  109 cm−2 at the Equator.The phase of the seasonal cycle and semi-annual oscillation shows small differences between the Na amounts retrieved from different instruments.The full width at half maximum of the profiles is 10 to 16 km for most latitudes, but significantly smaller in the polar summer. The centroidaltitudes of the measured sodium profiles range from 89 to 95 km, whereas the model shows on average 2 to 4 km lower centroid altitudes.This may be explained by the mesopause being 3 km lower in the WACCM simulations than in measurements.Despite this global 2–4 km shift, the model captures well the latitudinal and temporal variations.The variation of the WACCM dataset during the year at different latitudesis similar to the one of the measurements. Furthermore, the differences between the measured profiles with different instruments and thereforedifferent local times (LTs) are also present in the model-simulated profiles.This capturing of latitudinal and temporal variations is also found for the vertical column densities and profile widths

atmospheric measurement techniques

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