Abstract
Low-mass stars show evidence of vigorous magnetic activity in the form of large flares and coronal mass ejections. Such space weather events may have important ramifications for the habitability and observational fingerprints of exoplanetary atmospheres. Here, employing a suite of three-dimensional coupled chemistry-climate models (CCMs), we explore effects of time-dependent stellar activity on rocky planet atmospheres orbiting G-, K-, and M-dwarfs. We use observed data from the MUSCLES survey and the Transiting Exoplanet Satellite Survey and test a range of rotation periods, magnetic field strengths, and flare frequency assumptions. We find that recurring flares can drive K- and M-dwarf planet atmospheres into chemical equilibria that substantially deviate from their pre-flare regimes, whereas G-dwarf planet atmospheres quickly return to their baseline states. Interestingly, simulated O2-poor and O2-rich atmospheres experiencing flares produce similar mesospheric nitric oxide abundances, suggesting that stellar flares can highlight otherwise undetectable chemical species. Applying a radiative transfer model to our CCM results, we find that flare-driven transmission features of bio-indicating species, such as nitrogen dioxide, nitrous oxide, and nitric acid, show particular promise for detection by future instruments.