Mikhail V. Belikovich

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+).

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.

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.

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.

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.

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 Ox–HOx components and the airglows generated by physicochemical processes involving them.

​​​​​​​At the altitudes of the mesosphere–lower thermosphere, OH and HO₂ play a significant role in many physicochemical processes. Thus, monitoring their spatiotemporal evolution, together with other chemically active trace gases, is one of the most important problems for this atmosphere region, in which direct measurements are difficult. This paper studies the nighttime OH and HO₂ chemical equilibria using the 3D chemical transport modeling within the general approach, which includes the identification of the main sources and sinks in the equilibrium space–time areas and the derivation of analytical criteria for equilibrium validity. The presented analysis shows that there are extended areas where nighttime HO₂ and OH are close to their local equilibrium concentrations, determined mainly by the reaction between HO_x and O_x components among themselves and with H₂O₂, N, NO, NO₂, and CO. In the upper mesosphere–lower thermosphere, the equilibrium expressions can be shortened so that they include the HO_x–O_x chemistry only. These expressions describe the HO₂ and OH equilibria from the top down to some boundaries, the altitude positions of which vary in the interval between 72–73 and 85 km and depend essentially on season and latitude. The developed analytical criteria reproduce the main features of these boundaries well almost everywhere. Due to weak sensitivity to uncertainties of reaction rates and other parameters, the criteria can be regarded as a robust instrument for HO₂ and OH equilibrium validation. The obtained results allow us to extend previously proposed methods for the retrieval of poorly measured components from measurement data and to develop new approaches.

At the altitudes of the mesosphere – lower thermosphere, OH and HO₂ play a significant role in many physicochemical processes. Thus, monitoring of their spatiotemporal evolution together with other chemically active trace gases is one of the most important problems for this atmosphere region, in which direct measurements are difficult. The paper studies the nighttime OH and HO₂ chemical equilibria using the 3D chemical transport modeling within the general approach including the extraction of the main sources and sinks in the equilibrium space-time areas and derivation of analytical criteria for equilibrium validity. The presented analysis shows that there are extended areas, where nighttime HO₂ and OH are close to their local equilibrium concentrations determined mainly by the reaction between HO_x – O_x components themselves and with H₂O₂, N, NO, NO₂, and CO. In the upper mesosphere – lower thermosphere, the equilibrium expressions can be shortened, including the HO_x – O_x chemistry only. These conditions describes the HO₂ and OH equilibrium from the top to the some lower borders, the altitude position of which vary in the interval between 72–73 and 85 km and depends essentially on season and latitude. The developed analytical criteria almost everywhere well reproduce the main features of these borders. The obtained results allow to extend the abilities of previously proposed methods for the retrieval of poorly measured components from measurement data and to develop new approaches.

At the altitudes of the mesosphere – lower thermosphere, OH and HO₂ play a significant role in many physicochemical processes. Thus, monitoring of their spatiotemporal evolution together with other chemically active trace gases is one of the most important problems for this atmosphere region, in which direct measurements are difficult. The paper studies the nighttime OH and HO₂ chemical equilibria using the 3D chemical transport modeling within the general approach including the extraction of the main sources and sinks in the equilibrium space-time areas and derivation of analytical criteria for equilibrium validity. The presented analysis shows that there are extended areas, where nighttime HO₂ and OH are close to their local equilibrium concentrations determined mainly by the reaction between HO_x – O_x components themselves and with H₂O₂, N, NO, NO₂, and CO. In the upper mesosphere – lower thermosphere, the equilibrium expressions can be shortened, including the HO_x – O_x chemistry only. These conditions describes the HO₂ and OH equilibrium from the top to the some lower borders, the altitude position of which vary in the interval between 72–73 and 85 km and depends essentially on season and latitude. The developed analytical criteria almost everywhere well reproduce the main features of these borders. The obtained results allow to extend the abilities of previously proposed methods for the retrieval of poorly measured components from measurement data and to develop new approaches.

At the altitudes of the mesosphere – lower thermosphere, OH and HO₂ play a significant role in many physicochemical processes. Thus, monitoring of their spatiotemporal evolution together with other chemically active trace gases is one of the most important problems for this atmosphere region, in which direct measurements are difficult. The paper studies the nighttime OH and HO₂ chemical equilibria using the 3D chemical transport modeling within the general approach including the extraction of the main sources and sinks in the equilibrium space-time areas and derivation of analytical criteria for equilibrium validity. The presented analysis shows that there are extended areas, where nighttime HO₂ and OH are close to their local equilibrium concentrations determined mainly by the reaction between HO_x – O_x components themselves and with H₂O₂, N, NO, NO₂, and CO. In the upper mesosphere – lower thermosphere, the equilibrium expressions can be shortened, including the HO_x – O_x chemistry only. These conditions describes the HO₂ and OH equilibrium from the top to the some lower borders, the altitude position of which vary in the interval between 72–73 and 85 km and depends essentially on season and latitude. The developed analytical criteria almost everywhere well reproduce the main features of these borders. The obtained results allow to extend the abilities of previously proposed methods for the retrieval of poorly measured components from measurement data and to develop new approaches.