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

At $\sim$3.9~Ga (stellar age $\sim$0.6~Gyr), the young Sun was far more magnetically active than today. It launched $\sim$${10}^2$ large, fast coronal mass ejections (CMEs) per year into a denser, more magnetized wind. We follow these eruptions to Earth’s atmosphere with a semi-analytic model chain. We find that a CME launched at $v_0\sim 3\text{–}4\text{e}3\text{unit}km{s}^{-1}$ dissipates $\mathrm{\Delta }{E}_k\sim 6\text{e}25\text{unit}J$ as it decelerates, an amount set only by its launch and final speeds. Diffusive shock acceleration converts $\sim$10% of this energy into solar energetic particles (SEPs), whose deposition depth depends mainly on a spectral break energy $E_b$. We show that the stronger field moves the Alfvén surface, inside which the magnetic field controls the wind, from $\sim$10–25$\text{Rsun}$ to $\sim$40–60$\text{Rsun}$, so CME shocks deliver over ten times the particle fluence of flares. Because the forcing is powered by deceleration, it follows the measured wind history ($\dot{M}\propto t^{-2.3}$) and declines as $\sim t^{-3}$. It was $\sim$50–100 times stronger at 3.9~Ga than at 2~Ga and several hundred times today’s level. This decline, strongest when the Sun was faintest, is what greenhouse solutions of the faint young Sun (FYS) problem usually impose by hand. The magnitude is small: SEP-driven ${\mathrm{N}}_2\mathrm{O}$ adds at most a few $\text{unit}W{m}^{-2}$ to a 20–40$\text{unit}W{m}^{-2}$ greenhouse requirement, and only if a fractal organic haze extends its lifetime. Counting energy deposited below a survival level near 1~hPa, we find the particles fix $\sim$$4\text{e}10$–$4\text{e}11\text{unit}molNy{r}^{-1}$, which could supply prebiotic feedstock to surface pools. The largest uncertainties are $E_b$, the event rate, the survival depth, and the ${\mathrm{N}}_2\mathrm{O}$ lifetime.