Cool-star magnetic fields are in the center of interest for a large number of reasons. Their influence on star-formation, solar and stellar activity, habitability of planets, magnetic dynamos, stellar structure, angular momentum evolution, and many other topics, put magnetic fields in the focus of vastly different research areas. Our vision of magnetic fields in different astronomical contexts, however, is motivated by only a very limited number of empirical facts, but influenced by an almost unlimited number of assumptions. An arsenal of different methods is available to search for magnetic fields in cool stars, but usually the different methods provide insights into very different aspects of magnetism.
Together with observations of stellar activity and rotation, direct magnetic fields measurements and reconstruction of field geometries provide an empirical picture of stellar dynamos. With observations of young stars on one side, and detailed information about the solar magnetic field on the other, we have boundary conditions that allow an investigation of an evolution of stellar magnetism. Our observations are crossing the physically important boundaries between partially and fully convective stars, between saturated activity and unsaturated stars, and between stars and brown dwarfs.
Magnetic fields in cool stars are just on the edge of detectability for our methods. The interpretation of results from the various methods opens a parameter space that certainly contains deep information about the fields, their geometry, and the underlying physical principles. Unfortunately, it is not always clear how to interpret our observational results, and how measurements from different analyses can be compared. Therefore, it is not only important to improve the general observational picture by collecting more observational material, but it is perhaps of even greater importance to understand the assumptions in and limitations of the methodology we are applying. Technology and computer power is improving rapidly and will at some point make more and more details available to direct observation. Nevertheless, the design of future instrumentation is driven by expectations and interpretation of earlier results. Given the fact that still a large fraction of the magnetic universe is unknown, we should be prepared to find a richness of magnetic phenomena that does not yet exist in our imagination of stellar magnetic fields – and we must ensure that our observations can find it.
Living Rev. Solar Phys. 8, (2012), 1
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