The young Sun: activity and radiation

Magnetic activity in the young Sun

The Sun's magnetic activity has steadily declined throughout its main-sequence lifetime. This is an immediate consequence of the declining dynamo as a star spins down by losing angular momentum through its magnetized wind. Along with the decline in magnetic activity, solar radiation ultimately induced by the magnetic fields declined as well, and hence the short-wavelength radiative input into planetary atmospheres diminished with time. (By contrast, solar radiation at visible wavelengths increased with time, as discussed below.) Similarly, the magnetically guided solar wind and high-energy particle fluxes were very likely to be different in the young solar environment compared to present-day conditions. A closer understanding of the magnetic behaviour of the young Sun is therefore pivotal for further modelling of young planetary atmospheres, their chemistry, heating and erosion.

Magnetic activity expresses itself in a variety of features, including dark photospheric, magnetic spots, photospheric faculae and chromospheric plage producing optical and ultraviolet excess radiation, and – most dramatically – magnetically confined coronae containing million-degree plasma that emits extreme-ultraviolet and X-ray emission. Occasional magnetic instabilities (flares) and shocks both in the corona and in interplanetary space accelerate particles to energies much beyond 1 MeV; related electromagnetic radiation (e.g. from collisions) is emitted in the hard X-ray and gamma-ray range (Lin et al., 2002).