Solar and stellar magnetism
Pascal Petit
Magnetic fields are involved in many aspects of stellar physics. They are key parameters in physical processes like accretion, mass-loss, diffusion, convection, rotation and can therefore deeply influence stellar evolution. If many issues related to stellar magnetism remain open today, our understanding of the origin and impact of magnetic fields in solar-type stars have benefited from important progresses during the last decade, thanks to combined breakthroughs in observing techniques and numerical simulations. In this course, I review some of the most striking advances achieved in this domain for the Sun itself and, in a broader context, for solar analogues.
I will detail our present observational knowledge of solar activity, which has been monitored for almost four centuries and is now investigated down to spatial scales of order of 150~km on the solar surface. I will also discuss how secular fluctuations of solar activity may affect the Earth's climate.
The complex and variable magnetic field of the Sun is thought to originate from a series of MHD mechanisms called dynamo processes. I will review the current status of solar dynamo theories, which can now be directly confronted to helioseismological observations unveiling the internal velocity field of the Sun. I will also detail the translation of dynamo models to the stellar case, and the so-called solar-stellar connexion.
In this last part, I will review some of the most significant effects of magnetic fields on the physics and evolution of solar-type stars. I will especially emphasize the role of stellar magnetism in early phases of stellar evolution, with magnetospheric accretion and magnetically-driven stellar spin-down.