Thermospheric nitric oxide NO during solar minimum modulated by O/O₂ ratio and thermospheric transport and mixing

The formation of NO by geomagnetic activity and EUV photoionization in the upper mesosphere and lower thermosphere and its subsequent impact on ozone contributes to the natural forcing of the climate system, and is recommended to be included in chemistry-climate model experiments since CMIP6. We compare NO concentrations simulated by five high-top chemistry-climate models – WACCM-X, EMAC, HAMMONIA, WACCM-D and KASIMA – in the mesosphere and thermosphere with satellite observations during a period of low geomagnetic and solar forcing from January to December 2010. While qualitatively the latitudinal and temporal variability of NO is captured by most models, we find disagreements of several orders of magnitude in high-latitude winter. Possible reasons are explored using snapshots at 12 UT on January 9, 2010. Two processes interacting with each other are identified as likely sources of these discrepancies, quenching of N(2D) by atomic oxygen in the mid-thermosphere, and meridional transport and mixing from the mid-thermosphere to the lower thermosphere. In the mid-thermosphere, the amount of atomic oxygen available from dissociation of molecular oxygen balances N(4S) and N(2D) via quenching of N(2D). N(4S) can then be transported or mixed into the lower thermosphere, where it efficiently reduces the lifetime of NO, leading to lower values of NO there. In high-latitude winter, meridional downward-poleward transport of N(4S) from the low-and midlatitude mid-thermosphere into the high-latitude lower thermosphere modulates the NO lifetime. This transport is affected by gravity waves, and therefore depends on the models gravity wave drag schemes and resolved gravity wave spectra.