Corotating Interaction Regions (CIRs): evolution over a solar lifetime

Abstract: Corotating Interaction Regions (CIRs) are persistent structures in stellar winds that arise from the interaction between fast and slow wind streams. They are known to generate shocks and high-energy particles, potentially influencing the erosion of planetary atmospheres and space weather conditions. Although extensively studied for the present-day Sun, their evolution over a star's lifetime and implications for planetary environments remain less explored. We model the evolution of CIRs around a solar-mass star using a rotational evolution framework and assess how their location, shape, and particle distributions vary with stellar age. We find that CIRs form closer to the star during early spin-up phases and migrate outward during spin down, with the minimum CIR radius inversely related to rotation rate. We show that the distribution of high-energy particles produced in CIR shocks varies significantly with age. During the Hadean period, when Earth's atmosphere evolved significantly, CIRs may have generated a number of energetic particles that is (10^3) to (10^7) times greater than for the present-day Sun. The frequency and strength of CIR-planet interactions also peak during early rapid rotation phases. Furthermore, we demonstrate from this preliminary study that, for stars with mass less than 1.4 Msun, whilst CIRs can form within the habitable zone, their shocks always form beyond it. This suggests that energetic particle impacts may rain inward from more distant regions, as with the present-day Sun. These findings have implications for habitability and the evolution of atmospheres since these particles can alter the chemistry and escape rate of atmospheres.