Coronal Mass Ejections from the Young Sun: Deceleration-Powered Solar Energetic Particles as a Self-Extinguishing Forcing of the Early Earth

At about 3.9 Ga (stellar age about 0.6 Gyr), the young Sun was far more magnetically active than today. It launched about 10 squared large, fast coronal mass ejections (CMEs) per year into a denser, more magnetized wind. We follow these eruptions to Earth's atmosphere with a semi-analytic model chain. We find that a CME launched at v 0 about3--4 km,s-1 dissipates ΔE k about6 as it decelerates, an amount set only by its launch and final speeds. Diffusive shock acceleration converts about 10% of this energy into solar energetic particles (SEPs), whose deposition depth depends mainly on a spectral break energy E b. We show that the stronger field moves the Alfvén surface, inside which the magnetic field controls the wind, from about 10--25, to about 40--60,, so CME shocks deliver over ten times the particle fluence of flares. Because the forcing is powered by deceleration, it follows the measured wind history (M proportional to t-2.3) and declines as about t-3. It was about 50--100 times stronger at 3.9 Ga than at 2 Ga and several hundred times today's level. This decline, strongest when the Sun was faintest, is what greenhouse solutions of the faint young Sun (FYS) problem usually impose by hand. The magnitude is small: SEP-driven N 2O adds at most a few to a 20--40 greenhouse requirement, and only if a fractal organic haze extends its lifetime. Counting energy deposited below a survival level near 1 hPa, we find the particles fix about 4 -- 4, which could supply prebiotic feedstock to surface pools. The largest uncertainties are E b, the event rate, the survival depth, and the N 2O lifetime.