EVOLUTION OF THE SOLAR IONIZING FLUX

A young magnetically active Sun, with enhanced ionizing radiations and an elevated solar wind, might have contributed to erosion of the prim ordial atmosphere of Mars (which is particularly vulnerable to dissoci ative recombination and sputtering by solar wind pickup ions). Spacecr aft and ground-based observations of solar-type dwarfs in young galact ic clusters have yielded a unified view of the early evolution and sub sequent systematic decline of magnetic activity with age. Rotational b raking by the coronal wind ultimately quenches the spin-catalyzed ''dy namo'' at the heart of stellar magnetism. Decay of the 10(6)-10(7) K c orona is much faster than the 10(4) K chromosphere, but XUV emissions of both can be predicted reliably, and photoionization of key planetar y atmospheric constituents can be modeled. The early Martian atmospher e (age similar to 1 Gyr) probably was subjected to photoionization rat es similar to 5 times contemporary peak values (sunspot maximum), poss ibly more if the ages of galactic clusters have been underestimated.