Chromospheric Semi-empirical Model of Proxima Centauri at Activity Cycle Extremes: Reconstructing the Radiative Environment of Proxima b

Stellar activity plays a fundamental role in determining the radiation environment and habitability of orbiting planets. Variations in stellar output, ranging from transient flares to long-term magnetic cycles, directly influence atmospheric chemistry, circulation, and potential biosignatures. Among cool stars, M dwarfs are especially active, showing strong magnetic fields and intense chromospheric emission in lines such as Ca II H&K (393 nm), Na I D1&D2 (580 nm), and the Balmer series (Hα, Hβ, Hϵ). Proxima Centauri (M5.5Ve) is a key benchmark for understanding magnetic activity in active M dwarfs and its impact on the habitability of terrestrial exoplanets. While its reported ~7-year activity cycle is quite well-documented, the physical changes in its atmosphere remain poorly constrained.

In this work, we present the semi-empirical modeling of Proxima Centauri’s atmosphere at the minimum and maximum of its activity cycle to investigate the evolution of its chromospheric thermal structure. Our analysis is based on flux-calibrated visible spectra from the HKα Project at CASLEO Observatory. We find that the integrated flux of Ca II K and Hβ show variations of 114% and 58% respectively between the cycle extremes. Using the SSRPM (Solar Stellar Radiative Physical Modelling) library of codes, we construct self-consistent non-LTE atmospheric models for both stages by fitting these diagnostic lines. The ultimate goal of this research is to reconstruct the full spectral irradiance at the orbit of Proxima b to provide physically-grounded stellar forcing for future planetary climate and photochemical models.