Chemical equilibria and characteristic times in the mesopause region during SSW events.

Atomic oxygen is a critical species in the mesosphere and lower thermosphere, governing the chemistry, airglow, and energy budget (taking part in exothermic chemical processes and microwave cooling processes). It participates in chemical reactions in that region. Hence, it is involved in the coupling between dynamics, chemistry and energetics. However, to date no satellite mission has measured atomic oxygen directly. It and related photochemically active species (atomic hydrogen, hydroxyl and hydroperoxyl) are deduced through indirect methods from airglow observations. Such techniques are based on the assumption of ozone photochemical equilibrium. In time of Sudden Stratospheric Warmings (SSWs) strong dynamical perturbations of the mesopause chemical system occur. With 3D modelling we find that ozone strongly deviates from photochemical equilibrium in the mesopause region during SSW events and nighttime conditions. The lower boundary of ozone equilibrium jumps up to a height of 90 km, implying that traditional techniques for retrieving atomic oxygen, atomic hydrogen, and chemical heat from airglow observations cannot be applied at times of SSWs below 90 km under nighttime conditions. We discuss and explain our results in terms of characteristic times. Additionally, to better understand the behavior of exothermic chemical heat, we calculate odd-hydrogens photochemical equilibria and characteristic times, which are involved into exothermic chemical reactions.