was the estimate by @Feigelson.etal:02 that energetic proton fluxes accelerated as a result of flaring in T Tauri stars should be of the order of $10^5$ times those of the contemporary Sun. They noted that such fluxes were consistent with proton spallation being responsible for the observed meteoritic abundances of several important short-lived radioactive isotopes. In support of this, evidence has been found using the [*Herschel*]{} Space Observatory of enhanced ionization of a protoplanetary disk due to energetic protons of at least GeV energies [@Ceccarelli.etal:14], although the inferred particle flux is challengingly high at 4–5 orders of magnitude above the @Feigelson.etal:02 estimate.

Using the proton flux estimate of @Feigelson.etal:02, TD09 found that stellar energetic protons have 40 times the ionizing power of coronal X-rays, from the innermost regions of the disk out to much larger distances (e.g., $\sim 10^4$ stellar radii or $100$ AU) and are the strongest ionization process for the protoplanetary disk they modelled. The TD09 study has potentially important implications. While X-ray fluxes can be directly observed from T Tauri stars and readily included in models of protoplanetary disks [e.g., @Ercolano.etal:09; @Mohanty.etal:13], EPs cannot be observed directly. If EPs dominate the ionization of the outer disk layers, 