On the relative roles of dynamics and chemistry governing the abundance and diurnal variation of low latitude thermospheric nitric oxide

Monday, February 25, 2019

Nitric oxide (NO) has long been recognized as one of the most important trace constituents in the middle and upper atmosphere. This is due to its role in cooling the thermosphere, as a source for NO+ ions in the lower ionosphere, and more generally as an indicator of energy input into the atmosphere.

Graph depicting monthly averaged atomic oxygen profiles
Monthly averaged atomic oxygen profiles from three simulations with the TIME-GCM and two versions of the SABER database. The curve labeled "old sab" is the Version 2 data of Mlynczak et al [2013]; the curve labeled "new sab" is the reprocessed data described by Mlynczak et al., [2018]. For the "Kzz/10" simulation the vertical transport is reduced through decreased diffusion, which reduces the atomic oxygen recombination in the mesosphere and leads to increased atomic oxygen in the lower thermosphere.

There has been much recent work on the role of NO at high latitudes but less so of low latitude nitric oxide, although it has recently been shown that equatorial NO can be used as a diagnostic of non-migrating tides. In this study we focus on equatorial NO, its absolute abundance and its diurnal variability.

The diurnal variation is captured by comparing sunrise SOFIE data with 1100 LT SNOE data in the Nitric Oxide Empirical Model (NOEM). The observed diurnal variation will be compared to results from two Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIME-GCM) simulation, one with tidal climatology and planetary waves defined by reanalysis data at the 30 km lower boundary and the other nudges the neutral wind and temperature to results from the high altitude version of NAVGEM (Navy Global Environmental Model).

Both SOFIE and NOEM yield peak NO abundances of around 4 × 107 cm−3 with the SOFIE profile peak about 10 km lower than in NOEM. This altitude difference appears to be dynamically driven due to the 6-hour phase change of the migrating semi-diurnal tide as it propagates up from the stratosphere to the lower thermosphere. The TIME-GCM nudged by NAVGEM does support the existence of a low altitude NO peak at dawn. All versions of the TIME-GCM overestimate the absolute NO abundance. The study demonstrates the importance of simultaneously comparing simulation results to the SABER atomic oxygen data as well as to empirical models of the E-region ionosphere. Tuning the model to yield better agreement with TIMED atomic (O) data improves the agreement with NO SOFIE/NOEM data, but leads to TIME-GCM underestimation of the electron density as compared with empirical models. This suggests a potential conflict with the requirements of NO and electron density modeling. The study points to the value of a multi-constituent approach (i.e. O, NO, and electron density) towards validating models such as the TIME-GCM and demonstrates the utility of nitric oxide as a useful diagnostic of chemical and dynamical processes at the base of the thermosphere.

David E. Siskind, McArthur Jones Jr., Douglas P. Drob, John P. McCormack, Mark E. Hervig, Daniel R. Marsh, Martin G. Mlynczak, Scott M. Bailey, Astrid Maute, and Nicholas J. Mitchel

Ann. Geophys., 37, 37–48, 2019— https://doi.org/10.5194/angeo-37-37-2019