@article{ZienickeSeehaferLietal.2003, author = {Zienicke, Egbert and Seehafer, Norbert and Li, B.-W. and Schumacher, J{\"o}rg and Politano, H. and Thess, H.}, title = {Voltage-driven instability of electrically conducting fluids}, year = {2003}, language = {en} } @article{SchumacherKliemSeehafer2000, author = {Schumacher, J{\"o}rg and Kliem, Bernhard and Seehafer, Norbert}, title = {Three-dimensional spontaneous magnetic reconnection in neutral current sheets}, year = {2000}, language = {en} } @article{Seehafer1995, author = {Seehafer, Norbert}, title = {The turbulent electromotive force in the high-conductivity limit}, year = {1995}, language = {en} } @article{PalusKurthsSchwarzetal.2007, author = {Palus, Milan and Kurths, J{\"u}rgen and Schwarz, Udo and Seehafer, Norbert and Novotna, Dagmar and Charvatova, Ivanka}, title = {The solar activity cycle is weakly synchronized with the solar inertial motion}, doi = {10.1016/j.physleta.2007.01.039}, year = {2007}, abstract = {We study possible interrelations between the 300-year record of the yearly sunspot numbers and the solar inertial motion (SIM) using the recently developed technique of synchronization analysis. Phase synchronization of the sunspot cycle and the SIM is found and statistically confirmed in three epochs (1734-1790, 1855-1875 and 1907-1960) of the whole period 1700-2000. These results give quantitative support to the hypothesis that there is a weak interaction between the solar activity and the SIM.}, language = {en} } @article{KuzanyanPipinSeehafer2006, author = {Kuzanyan, Kirill M. and Pipin, Valerij V. and Seehafer, Norbert}, title = {The alpha effect and the observed twist and current helicity of solar magnetic fields}, issn = {0038-0938}, doi = {10.1007/s11207-006-1636-6}, year = {2006}, abstract = {We present a straightforward comparison of model calculations for the alpha-effect, helicities, and magnetic field line twist in the solar convection zone with magnetic field observations at atmospheric levels. The model calculations are carried out in a mixing-length approximation for the turbulence with a profile of the solar internal rotation rate obtained from helioseismic inversions. The magnetic field data consist of photospheric vector magnetograms of 422 active regions for which spatially-averaged values of the force-free twist parameter and of the current helicity density are calculated, which are then used to determine latitudinal profiles of these quantities. The comparison of the model calculations with the observations suggests that the observed twist and helicity are generated in the bulk of the convection zone, rather than in a layer close to the bottom. This supports two-layer dynamo models where the large-scale toroidal field is generated by differential rotation in a thin layer at the bottom while the alpha-effect is operating in the bulk of the convection zone. Our previous observational finding was that the moduli of the twist factor and of the current helicity density increase rather steeply from zero at the equator towards higher latitudes and attain a certain saturation at about 12 - 15 degrees. In our dynamo model with algebraic nonlinearity, the increase continues, however, to higher latitudes and is more gradual. This could be due to the neglect of the coupling between small-scale and large-scale current and magnetic helicities and of the latitudinal drift of the activity belts in the model}, language = {en} } @article{SeehaferGalantiFeudeletal.1996, author = {Seehafer, Norbert and Galanti, B. and Feudel, Fred and R{\"u}diger, Sten}, title = {Symmetry breaking bifurcations for the magnetohydrodynamic equations with helical forcing}, year = {1996}, language = {en} } @article{PipinSeehafer2009, author = {Pipin, Valerij V. and Seehafer, Norbert}, title = {Stellar dynamos with Omega x J effect}, issn = {0004-6361}, doi = {10.1051/0004-6361:200810766}, year = {2009}, abstract = {Context. The standard dynamo model for the solar and stellar magnetic fields is based on the \$alphaOmega\$ mechanism, namely, an interplay between differential rotation (the \$Omega\$ effect) and a mean electromotive force generated by helical turbulent convection flows (the \$alpha\$ effect). There are, however, a number of problems with the \$alpha\$ effect and \$alphaOmega\$ 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 \$alpha\$ effect. 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 \$vec{Omega} imesvec{J}\$ effect. We also include anisotropic diffusion and a new dynamo term that is of third order in the rotation vector \$vec{Omega}\$. Methods. We studied axisymmetric mean-field dynamo models containing differential rotation, the \$alpha\$ effect, 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 \$alpha\$ effect (a \$delta^{2}\$ dynamo model). This kind of dynamo model may be relevant for fully convective stars. Results. With respect to the solar dynamo, the inclusion of the additional turbulent induction effects increases the period of the dynamo and brings the large-scale toroidal field closer to the equator, thus improving the agreement of the models with the observations. For the \$delta^{2}\$ dynamo working in a full sphere, we find dynamo modes that are steady if the effect of anisotropic diffusion is not included. The inclusion of anisotropic diffusion yields a magnetic field oscillating with a period close to the turbulent magnetic diffusion time.}, language = {en} } @article{SeehaferSchumacher1997, author = {Seehafer, Norbert and Schumacher, J{\"o}rg}, title = {Squire's theorem for the magnetohydrodynamic sheet pinch}, year = {1997}, language = {en} } @article{SeehaferSchumacher1998, author = {Seehafer, Norbert and Schumacher, J{\"o}rg}, title = {Resistivity profile and instability of the plane sheet pinch}, year = {1998}, language = {en} } @article{Seehafer1994, author = {Seehafer, Norbert}, title = {Relaxation to equilibrium and inverse energy cascades in solar active regions}, isbn = {1-563-47099-3}, issn = {0079-6050}, year = {1994}, language = {en} } @article{FuhrmannSeehaferValori2007, author = {Fuhrmann, Marcel and Seehafer, Norbert and Valori, Gherardo}, title = {Preprocessing of solar vector magnetograms for force-free magnetic field extrapolation}, issn = {0004-6361}, doi = {10.1051/0004-6361:20078454}, year = {2007}, abstract = {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.}, language = {en} } @article{SeehaferSchumacher2000, author = {Seehafer, Norbert and Schumacher, J{\"o}rg}, title = {Patterns in an electrically driven conducting fluid layer}, year = {2000}, abstract = {The equilibrium states of electrically conducting fluids or plasmas have been a subject of intense study for a long time, motivated in particular by the interest in controlled thermonuclear fusion, as well as that in space and astrophysical phenomena such as plasma loops in the solar corona. If high temperatures prohibit solid walls, a conducting fluid can be held together by the action of an electric current passing through it with the pressure gradients being balanced by the Lorentz force. The resultant configuration is known as a pinch. In this paper we report on studies of the pinch in the geometry of a plane sheet.}, language = {en} } @article{FeudelSeehafer1995, author = {Feudel, Fred and Seehafer, Norbert}, title = {On the bifurcation phenomena in truncations of the 2D Navier-Stokes equations}, year = {1995}, language = {en} } @article{KuzanyanPipinSeehafer2005, author = {Kuzanyan, Kirill M. and Pipin, V. V. and Seehafer, Norbert}, title = {On the alpha effect and current helicity of solar magnetic fields}, isbn = {92-9092-911-1}, year = {2005}, language = {en} } @article{DemircanSeehafer2001, author = {Demircan, Ayhan and Seehafer, Norbert}, title = {Nonlinear square patterns in Rayleigh-Benard convection}, year = {2001}, abstract = {We numerically investigate nonlinear asymmetric square patterns in a horizontal convection layer with up-down reflection symmetry. As a novel feature we find the patterns to appear via the skewed varicose instability of rolls. The time-independent nonlinear state is generated by two unstable checkerboard (symmetric square) patterns and their nonlinear interaction. As the bouyancy forces increase the interacting modes give rise to bifurcations leading to a periodic alternation between a nonequilateral hexagonal pattern and the square pattern or to different kinds of standing oscillations.}, language = {en} } @article{SchmidtmannFeudelSeehafer1997, author = {Schmidtmann, Olaf and Feudel, Fred and Seehafer, Norbert}, title = {Nonlinear Galerkin methods for the 3D magnetohydrodynamic equations}, year = {1997}, language = {en} } @article{SchmidtmannFeudelSeehafer1998, author = {Schmidtmann, Olaf and Feudel, Fred and Seehafer, Norbert}, title = {Nonlinear Galerkin methods based on the concept of determining modes for the magnetohydrodynamic equations}, year = {1998}, language = {en} } @article{SeehaferFeudelSchmidtmann1996, author = {Seehafer, Norbert and Feudel, Fred and Schmidtmann, Olaf}, title = {Nonlinear dynamo with ABC forcing}, year = {1996}, language = {en} } @article{KurthsSeehaferSpahn1999, author = {Kurths, J{\"u}rgen and Seehafer, Norbert and Spahn, Frank}, title = {Nichtlineare Dynamik in der Physik : Forschungsbeispiele und Forschungstrends}, isbn = {3-540-65329- 5}, year = {1999}, language = {de} } @article{Seehafer1996, author = {Seehafer, Norbert}, title = {Nature of the alpha effect in magnetohydrodynamics}, year = {1996}, language = {en} }