6.7 Summary: The violent pre-main sequence Sun
Both the T Tauri and the protostellar Sun were extremely magnetically active, as far as we can tell from
observations of contemporaneous objects at these stages. There is no definitive evidence for a turn-on of
magnetic activity (for a contrasting view, see Linsky et al., 2007), but the stellar environment (gas and
dust disks and envelopes) makes observations challenging. On the other hand, perhaps the most
interesting aspect of magnetic activity in this phase is indeed its influence on the environment
itself. High X-ray and UV fluxes produced both by magnetic activity and magnetically funneled
accretion flows onto the star heat and ionize the circumstellar disk, thus controlling mechanisms as
diverse as gas-disk photoevaporation, accretion through the magnetorotational instability, and
chemical networks across the disk. Strong flaring and observations of non-thermal gyrosynchrotron
emission in T Tauri and protostars further indicate the presence of strongly elevated particle
fluxes in the pre-main sequence Sun’s environment. The impact on solid-state matter forming in
the circumstellar disk may be visible to the present day, in the form of daughter products of
short-lived radioactive isotopes formed by proton impact. The study of the young Sun’s environment
under the aspect of magnetic activity of the central star is still in its early stage and rapidly
developing.
