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

Abstract

Abstract. We use data from two NASA satellites, the Thermosphere Ionosphere Energetics and Dynamics (TIMED) and the Aeronomy of Ice in the Mesosphere (AIM) satellites, in conjunction with model simulations from the thermosphere-ionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM) to elucidate the key dynamical and chemical factors governing the abundance and diurnal variation of lower thermospheric nitric oxide (NO) at near-solar minimum conditions and low latitudes. This analysis was enabled by the recent orbital precession of the AIM satellite which caused the solar occultation pattern measured by the Solar Occultation for Ice Experiment (SOFIE) to migrate down to low and mid-latitudes for specific periods of time. We use a month of NO data collected in January 2017 to compare with two versions of the TIME-GCM; one is driven solely by climatological tides and analysis-derived planetary waves at the lower boundary and is free running at all other altitudes, and the other is constrained by a high-altitude analysis from the Navy Global Environmental Model (NAVGEM) up to the mesopause. We also compare SOFIE data with a NO climatology from the nitric oxide empirical model (NOEM). Both SOFIE and NOEM yield peak NO abundances of around 4×107 cm−3; however, the SOFIE profile peaks about 6–8 km lower than NOEM. We show that this difference is likely a local time effect, with SOFIE being a dawn measurement and NOEM representing late morning and/or near noon. The constrained version of TIME-GCM exhibits a low-altitude dawn peak, while the model that is forced solely at the lower boundary and free running above does not. We attribute this difference to a phase change in the semi-diurnal tide in the NAVGEM-constrained model, causing the descent of high NO mixing ratio air near dawn. This phase difference between the two models arises due to differences in the mesospheric zonal mean zonal winds. Regarding the absolute NO abundance, all versions of the TIME-GCM overestimate this. Tuning the model to yield calculated atomic oxygen in agreement with TIMED data helps but is insufficient. Furthermore, the TIME-GCM underestimates the electron density (Ne) as compared with the International Reference Ionosphere (IRI) empirical model. This suggests a potential conflict with the requirements of NO modeling and Ne modeling, since one solution typically used to increase model Ne is to increase the solar soft X-ray flux, which would, in this case, worsen the NO model–data discrepancy.

Coupling From the Middle Atmosphere to the Exobase: Dynamical Disturbance Effects on Light Chemical Species

“…These studies (and others) reduced model calculated K zz values to account for the increased mixing caused by including realistic wave perturbations in the NCAR TGCMs. As Jones et al (2017) and Siskind et al (2019) demonstrated, reducing the absolute K zz values in TIME‐GCM mainly affects the absolute concentration of MLT and thermospheric species, not their variability. Therefore, leaving the model eddy diffusion coefficient unchanged is assumed to be negligible with respect to the relative changes in thermospheric light species due to a middle atmospheric dynamical event.…”

Validation of SOFIE nitric oxide measurements

“…The SOFIE -MIPAS comparison above ~130 km in the SH (~140 km in the NH) indicates an increasing bias with SOFIE suggesting higher NO. Siskind et al (2019) noted a similar bias from indirect comparisons of SOFIE with the Student Nitric Oxide Explorer (SNOE) results. Note that the number of measurement pairs used in the comparisons is fairly consistent in height for the SH (SOFIE sunset), in both the ACE and MIPAS comparisons (Figures 7c and 8c).…”

“…Time separation is important in the measurement comparisons because NO abundance can have a strong diurnal dependence, with more than 10% per hour changes in ND near local sunrise or sunset, depending on altitude, latitude, and season (e.g., Siskind et al, 2019). This effect can be managed in the comparisons by 1) keeping the measurement separations as small as possible, or…”

“…The Solar Occultation for Ice Experiment (SOFIE) has measured nitric oxide (NO) from the Aeronomy of Ice in the mesosphere (AIM) satellite since May 2007. SOFIE NO measurements have been the topic of numerous science investigations, including studies of thermospherestratosphere coupling (Bailey et al, 2015;Siskind et al, 2015;Hendrickx et al, 2018), effects of the 27-day solar rotation (Hendrickx et al, 2015), and the roles of dynamics and chemistry in diurnal variability (Siskind et al, 2019). SOFIE NO observations have also been used to determine the importance of changes in geomagnetic activity and solar radiation (Hendrickx et al, 2017), and to characterize the response of NO to electron precipitation (Smith-Johnsen et al, 2017;Newnham et al, 2018).…”

Solar Cycle Variability of Nonmigrating Tides in the 5.3 and 15 μm Infrared Cooling of the Thermosphere (100–150 km) from SABER

“…Flynn et al () discuss the seasonal and annual variability of NO flux over 14 years and highlight the contribution from diurnal tides. Siskind et al () have reported sources for the abundance and diurnal variation in NO at solar minimum conditions by simulating the mesospheric zonal winds. Moreover, Mlynczak et al () showed that both NO and CO 2 radiative cooling in the thermosphere have a strong solar cycle dependence.…”