Solar activity is produced by a variety of complex interactions between its ionized plasma and the powerful magnetic field that it creates. Studying it is important both for purely scientific as well as practical reasons. The Sun is the only star whose surface we can resolve in detail, we can even measure its magnetic field in a more or less routine manner. These observations show the magneto-hydrodynamical interactions between the plasma and the magnetic field under conditions that cannot be reproduced in Earth laboratories, or even in numerical simulations.
In Astrophysics, the Sun is a universal reference, a Rosetta stone that helps us interpret the studies we conduct on other stars. The knowledge, but also the uncertainties, that we attain when we observe the Sun, immediately propagates through all of Astrophysics.
An unsettling example of this is the debate that has been sparked with the so-called solar oxygen crisis, after a series of recent works with claims that the solar metallicity needs to be revised downwards.
From a more practical point of view, there is a growing societal interest in understand solar activity, space weather and the impact that its violent phenomena that we observe may have on our technological society.
A good outreach program is particularly important in this subject, where it is necessary to maintain a delicate balance between raising awareness about the real risks that we are facing, and at the same time fighting the alarmist, even apocalyptic, messages that are often seen in the media. And yet solar storms are not the only practical interest of solar observations. The development of future nuclear fusion reactors, one of the most promising options for clean, safe and abundant energy supply, will require the use of magnetic confinement of plasmas at high temperatures. This is because no material exists that can resist the temperatures needed for nuclear fusion.
The Sun provides us with an exceptional laboratory of Physics where we can routinely observed fundamental processes of interaction between plasma and strong magnetic fields.
At the IAC there is a longstanding tradition of solar research and observations. In particular, magnetism has been the core of a series of previous projects that have supported developments in instrumentation, theory of polarized radiative transfer (polarimetry is extremely useful to diagnose magnetic fields), spectro-polarimetric observations and inversion techniques. The previously funded projects are: AYA2001-1649, AYA2004-05792, AYA2007-63881 and AYA2010-18029. Their legacy has left a group that is a pioneer in this subject and numerous scientific advances (see http://www.iac.es/proyecto/magnetism). This new project aims at seizing the opportunities that are presented to us with the new generation of large solar telescopes such as the European Solar Telescope (EST), whose design has been led by our group, the Solar-C space mission, which is expected to succeed the currently operational Hinode, the NASA-JAXA rocket CLASP in which our group is a co-PI, or the Daniel K. Inouye Solar Telescope (formerly known as ATST).
Magnetic fields pervade all astrophysical plasmas and govern most of the variability in the Universe at intermediate time scales. They are present in stars across the whole Hertzsprung-Russell diagram, in galaxies, and even perhaps in the intergalactic medium. Polarized light provides the most reliable source of information at our disposal for the