Lightning-induced chemistry on tidally locked Earth-like exoplanets

The chemical state of an atmosphere greatly affects the climate and habitability of an exoplanet. Correctly interpreting spectrally resolved exoplanet observations therefore requires an understanding of the underlying atmospheric chemistry and physics. We use the Met Office Unified Model (UM), a three-dimensional Global Circulation Model, to study the emergence of lightning and its chemical impact on tidally locked exoplanets. For our calculations, we assume an Earth-like initial atmospheric composition for Proxima Centauri b orbiting in the habitable zone of its M-dwarf star. By coupling the UM and the UKCA global atmospheric composition model, we develop a chemically consistent model to study the atmosphere of this exoplanet. Driven by photochemistry, an ozone layer forms globally, with a dayside column comparable to Earth and a nightside column that is up to five times thicker with localized hotspots over the two cyclonic Rossby gyres. We parameterize lightning flashes as a function of cloud-top height, following Earth sciences, and the resulting production of nitric oxide (NO) from the thermal dissociation of molecular nitrogen. Rapid dayside convection over and around the substellar point results in lightning flash rates of up to six flashes per km per year, peaking in a crescent-like shape westward of the substellar point. The flashes induce a dayside atmosphere that is rich in nitrogen oxides (NO_x=NO+NO₂) below altitudes of 20 km. Dayside-nightside thermal gradients result in strong winds that subsequently advect NO_x towards the nightside. Here, the absence of photochemistry allows NO_x to engage in more complex chemistry. In my presentation, I will discuss the relative importance of atmospheric physics and chemistry in determining ozone and NO_x distributions across the hemispheres and implications for observability.