The boundary of nighttime ozone chemical equilibrium as an indicator of local chemistry disturbances in the polar mesopause region

Abstract. The nighttime ozone chemical equilibrium (NOCE) is a key assumption widely employed for various applications in the mesopause region, such as retrieving hard-to-measure characteristics from observations. Previously, the criterion for determining the NOCE boundary was used to analyze its long-term evolution from SABER/TIMED data, primarily at low and middle latitudes. This work focuses on polar latitudes. We demonstrate that the NOCE criterion clearly indicates the main features of the transition zone separating deep and weak O and H diurnal photochemical oscillations. During the polar night, this zone degenerates into a step in their profiles, and the criterion identifies the altitude of this step. Next, we analyze the evolution of the NOCE boundary using SABER/TIMED data in conjunction with MERRA-2 data for the winter–spring period of 2002–2025. We demonstrate that its most pronounced variability is observed at northern latitudes during and after strong SSWs with an elevated stratopause. Prior to or immediately during such warmings, the daily mean boundary can rise to ~87 km. Immediately after, it rapidly descends to 72–74 km and can remain at this altitude for an extended period. Model studies of the winter of 2009 show that the boundary closely follows the variability of the medians of the vertical distributions of O and the volume emission rates of OH*, O*, and O₂*. Thus, we can assume that the NOCE boundary can be used to monitor significant disturbances in both the O_x–HO_x components and the airglows generated by physicochemical processes involving them.