Nighttime O(1D) distributions in the mesopause region derived from SABER data

In this study, the new source of O(1D) in the mesopause region due to the process OH(ν ≥ 5)+O(3P)→ OH(0≤ ν ≤ ν−5)+O(1D) is applied to SABER data to estimate the nighttime O(1D) distributions for the years 2003– 2005. It is found that O(1D) evolutions in these years are very similar to each other. Depending on the month, monthly averaged O(1D) distributions show two to four maxima with values up to 340 cm−3 which are localized in height (at ∼ 92– 96 km) and latitude (at ∼ 20–40 and ∼ 60–80 S, N). Annually averaged distributions in 2003–2005 have one weak maximum at ∼ 93 km and ∼ 65 S with values of 150– 160 cm−3 and three pronounced maxima (with values up to 230 cm−3) at ∼ 95 km and ∼ 35 S, at ∼ 94 km and ∼ 40 N and at∼ 93 km and∼ 65–75 N, correspondingly. In general, there is slightly more O(1D) in the Northern Hemisphere than in the Southern Hemisphere. The obtained results are a useful data set for subsequent estimation of nighttime O(1D) influence on the chemistry of the mesopause region.


Introduction
Daytime O( 1 D) is considered to be one of the important chemical minor species of the stratosphere, mesosphere and thermosphere, as it plays a significant role in the chemistry and the radiative and thermal balance of this region (Brasseur and Solomon, 2005). First of all, formed by photolysis of O 2 and O 3 , O( 1 D) is a mediator involved in the transformation of absorbed solar radiation energy into the heating of this region and, in particular, excitation of N 2 (ν) and CO 2 (ν) (Harris and Adams, 1983;Panka et al., 2017). Also, O( 1 D) atoms participate in the reactions of destruction of long-lived green-house gases (Baasandorj et al., 2012), CH 4 oxidation, and HO x and NO x production. Moreover, the red line emission from O( 1 D) atoms is one of the most important airglow phenomena which are used as a diagnostic of the ionosphere, for example, to monitor the electron density and neutral winds in the F region (Shepherd et al., 2019). Therefore, many papers and experimental campaigns are devoted to measurements of features of O 3 photolysis to O( 1 D) (Taniguchi et al., 2003;Hofzumahaus et al., 2004).
Until recently, it was believed that the above-mentioned processes stopped at night as a continuous source of O( 1 D) is absent, while the lifetime of the component is extremely short (less than 1 s). In principle, O( 1 D) can be generated in sprite halos but for a short duration of 1 ms (Hiraki et al., 2004). Recently, Sharma et al. (2015) and Kalogerakis et al. (2016), based on laboratory experiments, proposed that O( 1 D) could be produced in the mesopause region via the process OH(ν ≥ 5) + O( 3 P) → OH(0 ≤ ν ≤ ν −5) + O( 1 D), which is multiquantum quenching of high excited states of OH by collisions with atomic oxygen in the ground state.
Last year, Kalogerakis (2019) showed that a new model of O 2 A-band that takes this process into account describes well (qualitatively and quantitatively) the results of early nighttime rocket measurements of volume emission rate profiles of this airglow. Thus, he proved that the process OH(ν ≥ 5) + O( 3 P) → OH(0 ≤ ν ≤ ν − 5) + O( 1 D) really took place in the nighttime mesopause, and the produced O( 1 D) distributions can be evaluated from available data.
In this study, the new source of O( 1 D) in the mesopause region is applied to SABER data to estimate the O( 1 D) nighttime distributions for the years 2003-2005.

O( 1 D) derivation from SABER data
All processes used for O( 1 D) determination are summarized in Table 1. Here, we apply the new OH(v) model of Fytterer et al. (2019). Their "best-fit model" includes all commonly used production and loss processes of OH(v) (see Table 1), but some parameters of the model, in particular, branching ratios of quenching OH(v) + O 2 and rate coefficients of OH(ν ≥ 5) + O( 3 P) → OH(0 ≤ ν ≤ ν −5) + O( 1 D), were adjusted with the use of volume emission rate profiles at four different wavelengths measured by SABER and SCIA-MACHY.
Due to low values of chemical lifetimes (less than 1 s), O( 1 D) can be considered in chemical equilibrium: To determine O( 3 P) and P OH , we use the known (e.g., Mlynczak et al., 2013Mlynczak et al., , 2018 approach for O( 3 P) derivation from the simultaneous SABER measurements of total volume emission rate of (9-7) and (8-6) OH transitions (VER 2 µm ), O 3 (9.6 µm), and temperature (T ). The approach employs the chemical equilibrium condition for nighttime ozone. As a result, it is done with the use of the following system of equations: (3) Thus, we derive the local values of O( 3 P), P OH , and OH(v = 5-9) from SABER data with the use of Eqs. (2)-(3) and apply sets of data (T , M, OH(v = 5-9), and O( 3 P)) to retrieve the local concentrations of O( 1 D) with the use of Eq. (1).
The systematic uncertainty of retrieved data is defined by uncertainties in VER 2 µm , O 3 , T measurements, and the rates of chemical and physical processes included in the OH(v) model. We reproduced the analysis presented in Fytterer et al. (2019) (see Sect. 3.4) and took into account the uncertainties of measured data and rate constants which are shown in Table 2. The third column of the table demonstrates the uncertainties' individual impact at derived O( 1 D) local concentration. It can be noted that the most critical for O( 1 D) are the uncertainties in T , rates of Reactions (2)-(3), Einstein coefficients for the v = 8-9 states, and VER 2 µm . The total systematic O( 1 D) uncertainty was obtained by calculating the root-sum square of all individual uncertainties. It was found to vary in the range of (37%-52%) depending on the pressure level. Due to averaging, the random error of data presented below is negligible.

O( 1 D) nighttime distributions
We use version 2.0 of the SABER data product (Level2A) for the simultaneously measured VER 2 µm , O 3 , and T profiles within the 0.01-0.0001 hPa pressure (p) interval (approximately 80-105 km in 2003-2005). We take only nighttime data when the solar zenith angle is greater than 95 • . The range of latitudes covered by the satellite trajectory in a month was divided into 20 bins of ∼ (5.5-8) • each; 1500-3000 single profiles of O( 1 D) concentration fall into 1 bin during a month of SABER observations. For each bin we calculate monthly averaged zonal mean <O( 1 D)> distributions (hereafter, the angle brackets are used to denote timely and spatially averaged values). For annually averaged distributions, we use 40 bins of ∼ 4 • each.

Discussion and conclusion
According to various early papers (Nicolet, 1959;Ghosh and Gupta, 1970;Shimazaki and Laird, 1970;Harris and Adams, 1983), daytime O( 1 D) concentrations at 90-100 km varied in the range of (10 2 -10 3 ) cm −3 . Brasseur and Solomon (2005) published the table (see Table A   the chemistry and thermal balance of the mesopause region. The analysis of this impact should be carried out with the use of a global 3D chemical transport model of the mesospherelower thermosphere. Additionally, it may indicate measurable characteristics of this region that could indirectly confirm the results obtained in this article. In principle, direct evidence of O( 1 D) layer existence in the nighttime mesopause can be established by in situ measurements of O( 1 D) airglow at 630 nm, which can be carried out, for example, as a part of a future WADIS rocket sounding mission Grygalashvyly et al., 2019). More detailed analysis is out of this short article's scope.
Author contributions. Both authors contributed equally to this paper.
Competing interests. The authors declare that they have no conflict of interest.