No-one who was lucky enough to have seen the Sun's corona during the recent eclipse could forget the prominences, tinged with red, that clung to the edge of the dark disc, or the streamers that extended outwards into space. These features are all caused by magnetic fields generated deep inside the Sun. However, the origin of these fields poses a major problem for classical astrophysics. Although the physical laws that describe the fields are known, the governing equations cannot be fully solved. A combination of new developments - both observational and theoretical - does, however, offer hope of rapid progress in the new millennium.
High-resolution observations of the Sun made from the ground and from space, and ranging from the visible spectrum to X-rays, are now revealing the fine structure of the magnetic field. These observations show features on all scales - from large active regions containing sunspots that are 50 000 km in diameter, down to slender loops that can only just be resolved by the world's most powerful telescopes.
In the December issue of Physics World magazine, Steven Tobias and Nigel Weiss from the Department of Applied Mathematics and Theoretical Physics, University of Cambridge, UK describes our current understanding of the sun's magnetic fields.