Although the Sun is our closest star by many orders of magnitude and despite having sunspot records stretching back to ancient China, our knowledge of the Sun's magnetic field is far from complete. Indeed, even now, after decades of study, the most obvious manifestations of magnetic fields in the... Show moreAlthough the Sun is our closest star by many orders of magnitude and despite having sunspot records stretching back to ancient China, our knowledge of the Sun's magnetic field is far from complete. Indeed, even now, after decades of study, the most obvious manifestations of magnetic fields in the Sun (e.g. sunspots, flares and the corona) are scarcely understood at all. These failures in spite of intense effort suggest that to improve our grasp of magnetic fields in stars and of astrophysical dynamos in general, we must broaden our base of examples beyond the Sun; we must study stars with a variety of ages, masses, rotation rates, and other properties, so we can test models against as broad a range of circumstances as possible. Although optical interferometry continues to make great strides (e.g. Monnier et al. 2007; Zhao et al. 2008), the tiny angular sizes of most stars will make direct imaging of stellar surface features very difficult. This means that we will have to rely on indirect methods to obtain information about the surfaces of cool stars and their environment. Over the next decade, this array of techniques will be supplemented by rapidly maturing new capabilities such as gyrochronology, asteroseismology and precision photometry from space, which will transform our understanding of the temporal variability of stars and stellar systems. In the next sections we will outline some of the key science questions in this area along with the techniques that could be used to bring new insights to these questions. Show less
