Magnetic field line topology in solar active regions

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According to a recent study comparing magnetic fields extrapolated from photospheric measurements with chromospheric and coronal observations, the electric current helicity in active regions possesses a predominant sign in each of the two solar hemispheres. This indicates that the generation of atmospheric electric currents, which are needed as the energy source for flares and coronal heating, is a global phenomenon connected with the rotation of the Sun rather than a consequence of plasma motions within individual active regions uncorrelated between different active regions. Theoretically, the evolution of the atmospheric magnetic field may be understood as the continual distortion of an existing equilibrium by disturbances propagating upward from the photosphere and subsequent fast relaxation to a new, neighboring equilibrium. Here current helicity proves to be an important quantity decisive for whether the (mean) magnetic field can evolve along a stable path with growing free energy. Disruptive disturbances with an explosive release of magnetic energy, in particular flares, are generally thought to be due to current sheet formation and magnetic reconnection. Reconnection may be defined as a discontinuous change of the field line connectivity to the photosphere. Then field lines running into a magnetic null point or such that are tangential to the photosphere should play a particular role. By comparing extrapolated magnetic fields with flare observations examples have been found suggestive of a connection between tangential field lines and flare activity.