TY - JOUR A1 - Fuhrmann, Marcel A1 - Seehafer, Norbert A1 - Valori, Gherardo T1 - Preprocessing of solar vector magnetograms for force-free magnetic field extrapolation N2 - 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. Y1 - 2007 UR - http://www.aanda.org/ U6 - https://doi.org/10.1051/0004-6361:20078454 SN - 0004-6361 ER - TY - JOUR A1 - Donner, Reik Volker A1 - Feudel, Fred A1 - Seehafer, Norbert A1 - Sanjuan, Miguel Angel Fernandez T1 - Hierarchical modeling of a forced Roberts Dynamo N2 - We investigate the dynamo effect in a flow configuration introduced by G. O. Roberts in 1972. Based on a clear energetic hierarchy of Fourier components on the steady-state dynamo branch, an approximate model of interacting modes is constructed covering all essential features of the complete system but allowing simulations with a minimum amount of computation time. We use this model to study the excitation mechanism of the dynamo, the transition from stationary to time-dependent dynamo solutions and the characteristic properties of the latter ones. Y1 - 2007 UR - http://www.worldscinet.com/ijbc/ijbc.shtml U6 - https://doi.org/10.1142/S021812740701941X SN - 0218-1274 ER - TY - JOUR A1 - Kliem, Bernhard A1 - Rust, S. A1 - Seehafer, Norbert T1 - Helicity transport in a simulated coronal mass ejection JF - Proceedings of the International Astronomical Union Y1 - 2010 U6 - https://doi.org/10.1017/S1743921311006715 SN - 1743-9213 SN - 1743-9221 SP - 125 EP - 128 PB - International Astronomical Union CY - Cambridge ER - TY - JOUR A1 - Fuhrmann, Marcel A1 - Seehafer, Norbert A1 - Valori, Gherardo A1 - Wiegelmann, Thomas T1 - A comparison of preprocessing methods for solar force-free magnetic field extrapolation Y1 - 2011 UR - http://www.aanda.org/index.php?option=com_article&access=standard&Itemid=129&url=/articles/aa/full_html/ 2011/02/aa15453-10/aa15453-10.html SN - 0004-6361 ER - TY - GEN A1 - Kliem, Bernhard A1 - Rust, S. A1 - Seehafer, Norbert T1 - Helicity transport in a simulated coronal mass ejection T2 - Postprints der Universität Potsdam : Mathematisch Naturwissenschaftliche Reihe N2 - It has been suggested that coronal mass ejections (CMEs) remove the magnetic he-licity of their coronal source region from the Sun. Such removal is often regarded to be necessary due to the hemispheric sign preference of the helicity, which inhibits a simple annihilation by reconnection between volumes of opposite chirality. Here we monitor the relative magnetic he-licity contained in the coronal volume of a simulated flux rope CME, as well as the upward flux of relative helicity through horizontal planes in the simulation box. The unstable and erupting flux rope carries away only a minor part of the initial relative helicity; the major part remains in the volume. This is a consequence of the requirement that the current through an expanding loop must decrease if the magnetic energy of the configuration is to decrease as the loop rises, to provide the kinetic energy of the CME. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 569 KW - magnetic fields KW - MHD KW - coronal mass ejections KW - magnetohydrodynamics KW - sun Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-412907 SN - 1866-8372 IS - 569 SP - 125 EP - 128 ER - TY - JOUR A1 - Fuhrmann, Marcel A1 - Seehafer, Norbert A1 - Valori, Gherardo A1 - Wiegelmann, T. T1 - A comparison of preprocessing methods for solar force-free magnetic field extrapolation JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Extrapolations of solar photospheric vector magnetograms into three-dimensional magnetic fields in the chromosphere and corona are usually done under the assumption that the fields are force-free. This condition is violated in the photosphere itself and a thin layer in the lower atmosphere above. The field calculations can be improved by preprocessing the photospheric magnetograms. The intention here is to remove a non-force-free component from the data. Aims. We compare two preprocessing methods presently in use, namely the methods of Wiegelmann et al. (2006, Sol. Phys., 233, 215) and Fuhrmann et al. (2007, A&A, 476, 349). Methods. The two preprocessing methods were applied to a vector magnetogram of the recently observed active region NOAA AR 10 953. We examine the changes in the magnetogram effected by the two preprocessing algorithms. Furthermore, the original magnetogram and the two preprocessed magnetograms were each used as input data for nonlinear force-free field extrapolations by means of two different methods, and we analyze the resulting fields. Results. Both preprocessing methods managed to significantly decrease the magnetic forces and magnetic torques that act through the magnetogram area and that can cause incompatibilities with the assumption of force-freeness in the solution domain. The force and torque decrease is stronger for the Fuhrmann et al. method. Both methods also reduced the amount of small-scale irregularities in the observed photospheric field, which can sharply worsen the quality of the solutions. For the chosen parameter set, the Wiegelmann et al. method led to greater changes in strong-field areas, leaving weak-field areas mostly unchanged, and thus providing an approximation of the magnetic field vector in the chromosphere, while the Fuhrmann et al. method weakly changed the whole magnetogram, thereby better preserving patterns present in the original magnetogram. Both preprocessing methods raised the magnetic energy content of the extrapolated fields to values above the minimum energy, corresponding to the potential field. Also, the fields calculated from the preprocessed magnetograms fulfill the solenoidal condition better than those calculated without preprocessing. KW - Sun: magnetic topology KW - Sun: atmosphere KW - magnetohydrodynamics (MHD) Y1 - 2011 U6 - https://doi.org/10.1051/0004-6361/201015453 SN - 0004-6361 VL - 526 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Miranda, Rodrigo A. A1 - Rempel, Erico L. A1 - Chian, Abraham C.-L. A1 - Seehafer, Norbert A1 - Toledo, Benjamin A. A1 - Munoz, Pablo R. T1 - Lagrangian coherent structures at the onset of hyperchaos in the two-dimensional Navier-Stokes equations JF - Chaos : an interdisciplinary journal of nonlinear science N2 - We study a transition to hyperchaos in the two-dimensional incompressible Navier-Stokes equations with periodic boundary conditions and an external forcing term. Bifurcation diagrams are constructed by varying the Reynolds number, and a transition to hyperchaos (HC) is identified. Before the onset of HC, there is coexistence of two chaotic attractors and a hyperchaotic saddle. After the transition to HC, the two chaotic attractors merge with the hyperchaotic saddle, generating random switching between chaos and hyperchaos, which is responsible for intermittent bursts in the time series of energy and enstrophy. The chaotic mixing properties of the flow are characterized by detecting Lagrangian coherent structures. After the transition to HC, the flow displays complex Lagrangian patterns and an increase in the level of Lagrangian chaoticity during the bursty periods that can be predicted statistically by the hyperchaotic saddle prior to HC transition. Y1 - 2013 U6 - https://doi.org/10.1063/1.4811297 SN - 1054-1500 VL - 23 IS - 3 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Feudel, Fred A1 - Seehafer, Norbert A1 - Tuckerman, Laurette S. A1 - Gellert, Marcus T1 - Multistability in rotating spherical shell convection JF - Physical review : E, Statistical, nonlinear and soft matter physics N2 - The multiplicity of stable convection patterns in a rotating spherical fluid shell heated from the inner boundary and driven by a central gravity field is presented. These solution branches that arise as rotating waves (RWs) are traced for varying Rayleigh number while their symmetry, stability, and bifurcations are studied. At increased Rayleigh numbers all the RWs undergo transitions to modulated rotating waves (MRWs) which are classified by their spatiotemporal symmetry. The generation of a third frequency for some of the MRWs is accompanied by a further loss of symmetry. Eventually a variety of MRWs, three-frequency solutions, and chaotic saddles and attractors control the dynamics for higher Rayleigh numbers. Y1 - 2013 U6 - https://doi.org/10.1103/PhysRevE.87.023021 SN - 1539-3755 VL - 87 IS - 2 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Feudel, Fred A1 - Tuckerman, L. S. A1 - Gellert, M. A1 - Seehafer, Norbert T1 - Bifurcations of rotating waves in rotating spherical shell convection JF - Physical review : E, Statistical, nonlinear and soft matter physics N2 - The dynamics and bifurcations of convective waves in rotating and buoyancy-driven spherical Rayleigh-Benard convection are investigated numerically. The solution branches that arise as rotating waves (RWs) are traced by means of path-following methods, by varying the Rayleigh number as a control parameter for different rotation rates. The dependence of the azimuthal drift frequency of the RWs on the Ekman and Rayleigh numbers is determined and discussed. The influence of the rotation rate on the generation and stability of secondary branches is demonstrated. Multistability is typical in the parameter range considered. Y1 - 2015 U6 - https://doi.org/10.1103/PhysRevE.92.053015 SN - 1539-3755 SN - 1550-2376 VL - 92 IS - 5 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Hassanin, Alshaimaa A1 - Kliem, Bernhard A1 - Seehafer, Norbert A1 - Török, Tibor T1 - A model of homologous confined and ejective eruptions involving kink instability and flux cancellation JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - In this study, we model a sequence of a confined and a full eruption, employing the relaxed end state of the confined eruption of a kink-unstable flux rope as the initial condition for the ejective one. The full eruption, a model of a coronal mass ejection, develops as a result of converging motions imposed at the photospheric boundary, which drive flux cancellation. In this process, parts of the positive and negative external flux converge toward the polarity inversion line, reconnect, and cancel each other. Flux of the same amount as the canceled flux transfers to a flux rope, increasing the free magnetic energy of the coronal field. With sustained flux cancellation and the associated progressive weakening of the magnetic tension of the overlying flux, we find that a flux reduction of approximate to 11% initiates the torus instability of the flux rope, which leads to a full eruption. These results demonstrate that a homologous full eruption, following a confined one, can be driven by flux cancellation. Y1 - 2022 U6 - https://doi.org/10.3847/2041-8213/ac64a9 SN - 2041-8205 SN - 2041-8213 VL - 929 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER -