Understanding the Capability of Future Direct-imaging Observations to Quantify Atmospheric Chemical Effects of Stellar Proton Events

Models developed for Earth are often applied in exoplanet contexts. Validation in extraterrestrial settings can provide an important test of model realism and increase our confidence in model predictions. NASA's upcoming space-based IROUV telescope will provide unprecedented opportunities to perform such tests. Here, we use the Planetary Spectrum Generator to simulate IROUV reflected-light spectroscopic observations of flare-driven photochemical changes produced by the Whole Atmosphere Community Climate Model, part of the Community Earth System Model framework. We find that NO₂ is the most observable gas to target, and integrating the signal for two days following the flare and comparing to a baseline of preflare data would achieve the highest signal-to-noise ratio. The NO₂ response is much larger for K-star tidally locked planets than G-star rapidly rotating planets and does not depend strongly on O₂ level. The NO₂ response should be observable for planets within 3–4 pc independent of the phase angle since the amount of reflected light is larger at smaller phases, but the NO₂ concentration is low near the substellar point. This work outlines a methodology for validating and ground-truthing atmospheric chemistry models developed for Earth that could be useful for the numerical exploration of exoplanets.