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

The nighttime ozone chemical equilibrium (NOCE) is a key assumption widely employed in various applications within the mesopause region, such as retrieving difficult-to-measure characteristics from observational data. Previously, the NOCE boundary criterion was used to analyze its long-term evolution based on SABER/TIMED data, primarily at low and mid-latitudes. This study extends this focus to polar latitudes. We demonstrate that the NOCE criterion clearly delineates the main features of the transition zone that vertically separates regions of strong and weak diurnal photochemical oscillations of O and H. During the polar night, this transition zone degenerates into a sharp step in the vertical profiles of these species, and the criterion accurately identifies the altitude of this step. Furthermore, we analyze the evolution of the NOCE boundary using SABER/TIMED observations in conjunction with MERRA-2 reanalysis data for the winter–spring periods from 2002 to 2025. We show that the most pronounced variability of the NOCE boundary occurs at northern polar latitudes during and after strong Sudden Stratospheric Warmings accompanied by stratopause elevation events. Prior to or during the onset of such warmings, the daily mean altitude of the boundary can rise to ∼86 km. Following these events, it rapidly descends to 72–74 km, where it can remain for an extended period. Model studies of the 2009 winter show that the NOCE boundary closely tracks the variability of the median vertical distributions of O, as well as the volume emission rates of OH( v =1–9), O( 1 S), and O 2 (b1Σg+).