Grand challenges in the physics of the sun and sun-like stars

1. INTRODUCTION " If the Sun had no magnetic field, it would be as uninteresting as most astronomers think it is. " This statement is attributed to R. B. Leighton (Moore and Rabin, 1985). Personally I think the Sun is of enormous interest in all respects, magnetic and non-magnetic; nonetheless, the grand challenges I have selected for this article do indeed pertain to the Sun's magnetic field. The study of stellar structure and evolution is one of the main building blocks of astrophysics, and the Sun has an importance both as the star that is most amenable to detailed study and as the star that has by far the biggest impact on the Earth and near-Earth environment through its radiative and particulate outputs. Over the past decades, studies of stars and of the Sun have become somewhat separate. But in recent years, the rapid advances in asteroseismology, as well as the quest to better understand solar and stellar dynamos, have emphasized once again the synergy between studies of the stars and the Sun. In this article I have selected two " grand challenges " both for their crucial importance and because I think that these two problems are tractable to significant progress in the next decade. They are (i) understanding how solar and stellar dynamos generate magnetic field, and (ii) improving the predictability of geo-effective space weather. How does the Sun generate its periodically reversing large-scale magnetic field? How do other solar-like stars generate their magnetic fields, and what are the similarities and differences between stellar activity cycles and that of the Sun? What can be learned about the solar dynamo by studying other stars? One of the most evident manifestations of solar magnetism is the number of sunspots, which waxes and wanes with an approximately 11-year quasi-periodic cycle. Once the polarity flip between 11-year cycles is taken into account, this becomes an approximately 22-year cycle. The Sun's large-scale ambient field, which is predominantly dipolar, has a similar 22-year cycle. Sunspots occur where concentrations of magnetic flux poke out through the Sun's surface, inhibiting the convection and causing that portion of the surface to be cooler (and hence darker) than its surroundings. Sunspots often occur in identifiable bipolar pairs, roughly oriented along lines of constant latitude but with the leading spot typically closer to the equator. The polarity of the leading spot is oppositely signed in the two hemispheres, and …