Norbert Seehafer

Preprocessing of solar vector magnetograms for force-free magnetic field extrapolation

Context. Reliable measurements of the solar magnetic field are restricted to the phoptosphere. As an alternative to measurements, the field in the higher layers of the atmosphere is calculated from the measured photospheric field, mostly under the assumption that it is force-free. However, the magnetic field in the photosphere is not force-free. Moreover, most methods for the extrapolation of the photospheric magnetic field into the higher layers prescribe the magnetic vector on the whole boundary of the considered volume, which overdetermines the force-free field. Finally, the extrapolation methods are very sensitive to small-scale noise in the magnetograph data, which, however, if sufficienly resolved numerically, should affect the solution only in a thin boundary layer close to the photosphere. Aims. A new method for the preprocessing of solar photospheric vector magnetograms has been developed that, by improving their compatibility with the condition of force-freeness and removing small-scale noise, makes them more suitable for extrapolations into three-dimensional nonlinear force-free magnetic fields in the chromosphere and corona. Methods. A functional of the photospheric field values is minimized whereby the total magnetic force and the total magnetic torque on the considered volume above the photosphere, as well as a quantity measuring the degree of small-scale noise in the photospheric boundary data, are simultaneously made small. For the minimization, the method of simulated annealing is used and the smoothing of noisy magnetograph data is attained by windowed median averaging. Results. The method was applied to a magnetogram derived from a known nonlinear force-free test field to which an artificial noise had been added. The algorithm recovered all main structures of the magnetogram and removed small-scale noise. The main test was to extrapolate from the noisy photospheric vector magnetogram before and after the preprocessing. The preprocessing was found to significantly improve the agreement of the extrapolated with the exact field.

Stellar dynamos with

Context. The standard dynamo model for the solar and stellar magnetic fields is based on the

{\Omega} \times J

alphaOmega

effect

mechanism, namely, an interplay between differential rotation (the Ω xa0effect) and a mean electromotive force generated by helical turbulent convection flows (the α xa0effect). There are, however, a number of problems with the α xa0effect and

An advective solar-type dynamo without the α effect

alphaOmega

Helicity transport in a simulated coronal mass ejection

dynamo models. Two of them are that, in the case of the Sun, the obtained cycle periods are too short and the magnetic activity is not sufficiently concentrated at low latitudes. Aims. We explore the role of turbulent induction effects that may appear in addition to the α xa0effect. The additional effects result from the combined action of rotation and an inhomogeneity of the large-scale magnetic field. The best known of them is the

Heliospheric Links Explorer (HELIX)

vec{Omega}timesvec{J}

Helical kink instability in the confined solar eruption on 2002 May 27: Helical kink instability in the confined solar eruption on 2002 May 27

effect. We also include anisotropic diffusion and a new dynamo term that is of third order in the rotation vector

Nichtlineare Dynamik in der Physik: Forschungsbeispiele und Forschungstrends

vec{Omega}

The Solar Activity Cycle is Weakly Synchronized with the Solar Inertial Motion

. Methods. We studied axisymmetric mean-field dynamo models containing differential rotation, the α xa0effect, and the additional turbulent induction effects. The model calculations were carried out using the rotation profile of the Sun as obtained from helioseismic measurements and radial profiles of other quantities according to a standard model of the solar interior. In addition, we consider a dynamo model for a full sphere that is based solely on the joint induction effects of rotation and an inhomogeneity of the large-scale magnetic field, without differential rotation and the α xa0effect (a