Abstract
AbstractReliable measurements of the solar magnetic field are restricted to the level of the photosphere. For about half a century attempts have been made to calculate the field in the layers above the photosphere, i.e. in the chromosphere and in the corona, from the measured photospheric field. The procedure is known as magnetic field extrapolation. In the superphotospheric parts of active regions the magnetic field is approximately force‐free, i.e. electric currents are aligned with the magnetic field. The practical application to solar active regions has been largely confined to constant‐α or linear force‐free fields, with a spatially constant ratio, α, between the electric current and the magnetic field. We review results obtained from extrapolations with constant‐α force‐free fields, in particular on magnetic topologies favourable for flares and on magnetic and current helicities. Presently, different methods are being developed to calculate non‐constant‐α or nonlinear force‐free fields from photospheric vector magnetograms. We also briefly discuss these methods and present a comparison of a linear and a nonlinear force‐free magnetic field extrapolation applied to the same photospheric boundary data. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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