Active regions, sunspots and their magnetic fields.

Surface magnetism is the progenitor of active regions, sunspots, and all related phenomena. This cause and effect is reversible so that, using well-established empirical laws, the presence and morphology of photospheric magnetic fields can be deduced from active-region light emission structure. In the (simplifying) case of sunspots, MHD and thermodynamic theory find some success in the interpretation of the interaction of magnetic fields and solar plasma. Coronal magnetic fields also appear to be predictable by extrapolation techniques starting from the photospheric conditions. Alternatively, surface magnetism can be observed "directly" by means of the spectroscopic Zeeman effect and Stokes polarimetry. Eventually these empirical, theoretical and direct-measurement techniques must converge to identical results as we better understand the physics of active regions.