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. A reservoir of nitric oxide (NO) in the lower thermosphere efficiently coolsthe atmosphere after periods of enhanced geomagnetic activity. Transport fromthis reservoir to the stratosphere within the winter polar vortex allows NOto deplete ozone levels and thereby affect the middle atmospheric heatbudget. As more climate models resolve the mesosphere and lower thermosphere(MLT) region, the need for an improved representation of NO-related processesincreases. This work presents a detailed comparison of NO in the AntarcticMLT region between observations made by the Solar Occultation for IceExperiment (SOFIE) instrument on-board the Aeronomy of Ice in the Mesosphere(AIM) satellite and simulations performed by the Whole Atmosphere CommunityClimate Model with Specified Dynamics (SD-WACCM). We investigate 8 years ofSOFIE observations, covering the period 2007–2015, and focus on the SouthernHemisphere (SH), rather than on dynamical variability in the NorthernHemisphere (NH) or a specific geomagnetic perturbed event. The morphology ofthe simulated NO is in agreement with observations though the long-term meanis too high and the short-term variability is too low in the thermosphere.Number densities are more similar during winter, though the altitude of peakNO density, which reaches between 102 and 106 km in WACCM and between 98 and104 km in SOFIE, is most separated during winter. Using multiple linearregression (MLR) and superposed epoch analysis (SEA) methods, we investigatehow well the NO production and transport are represented in the model. Theimpact of geomagnetic activity is shown to drive NO variations in the lowerthermosphere similarly across both datasets. The dynamical transport from thelower thermosphere into the mesosphere during polar winter is found to agreevery well with a descent rate of about 2.2 km day −1 in the 80–110 kmregion in both datasets. The downward-transported NO fluxes are, however, toolow in WACCM, which is likely due to medium energy electrons (MEE) andD-region ion chemistry that are not represented in the model.

Production and transport mechanisms of NO in the polar upper mesosphere and lower thermosphere in observations and models