TY - JOUR A1 - Feudel, Fred A1 - Gellert, Marcus A1 - Rüdiger, Sten A1 - Witt, Annette A1 - Seehafer, Norbert T1 - Dynamo effect in a driven helical flow Y1 - 2003 UR - http://link.aps.org/abstract/PRE/v68/e046302 ER - TY - JOUR A1 - Demircan, Ayhan A1 - Seehafer, Norbert T1 - Dynamo in asymmetric square convection Y1 - 2002 SN - 0309-1929 ER - TY - JOUR A1 - Demircan, Ayhan A1 - Seehafer, Norbert T1 - Dynamos in rotating and nonrotating convection in the form of asymmetric squares N2 - We study the dynamo properties of asymmetric square patterns in Boussinesq Rayleigh-B'enard convection in a plane horizontal layer. Cases without rotation and with weak rotation about a vertical axis are considered. There exist different types of solutions distinguished by their symmetry, among them such with flows possessing a net helicity and being capable of kinematic dynamo action in the presence as well as in the absence of rotation. In the nonrotating case these flows are, however, always only kinematic, not nonlinear dynamos. Nonlinearly the back-reaction of the magnetic field then forces the solution into the basin of attraction of a roll pattern incapable of dynamo action. But with rotation added parameter regions are found where the Coriolis force counteracts the Lorentz force in such a way that the asymmetric squares are also nonlinear dynamos. Y1 - 2001 ER - TY - JOUR A1 - Seehafer, Norbert T1 - Filaments and the solar dynamo Y1 - 1998 SN - 1-88673-370-8 ER - TY - JOUR A1 - Feudel, Fred A1 - Seehafer, Norbert A1 - Schmidtmann, Olaf T1 - Fluid helicity and dynamo bifurcations Y1 - 1995 ER - TY - JOUR A1 - Seehafer, Norbert A1 - Demircan, Ayhan A1 - Feudel, Fred T1 - Fluid helicity and dynamo effect N2 - Using the incompressible magnetohydrodynamic equations, we have numerically studied the dynamo effect in electrically conducting fluids. The necessary energy input into the system was modeled either by an explicit forcing term in the Navier-Stokes equation or fully selfconsistently by thermal convection in a fluid layer heated from below. If the fluid motion is capable of dynamo action, the dynamo effect appears in the form of a phase transition or bifurcation at some critical strength of the forcing. Both the dynamo bifurcation and subsequent bifurcations that occur when the strength of the forcing is further raised were studied, including the transition to chaotic states. Special attention was paid to the helicity of the flow as well as to the symmetries of the system and symmetry breaking in the bifurcations. The magnetic field tends to be accumulated in special regions of the flow, notably in the vicinity of stagnation points or near the boundaries of convection cells. Y1 - 2001 ER - TY - JOUR A1 - Seehafer, Norbert A1 - Fuhrmann, M. A1 - Valori, Gherardo A1 - Kliem, Bernhard T1 - Force-free magnetic fields in the solar atmosphere Y1 - 2007 UR - http://www3.interscience.wiley.com/cgi-bin/abstract/117872628/ABSTRACT ER - TY - JOUR A1 - Hassanin, Alshaimaa A1 - Kliem, Bernhard A1 - Seehafer, Norbert T1 - Helical kink instability in the confined solar eruption on 2002 May 27 JF - Astronomische Nachrichten = Astronomical notes KW - instabilities KW - magnetohydrodynamics (MHD) KW - Sun: corona KW - Sun: coronal mass ejections (CMEs) KW - Sun: flares Y1 - 2016 U6 - https://doi.org/10.1002/asna.201612446 SN - 0004-6337 SN - 1521-3994 VL - 337 SP - 1082 EP - 1089 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Seehafer, Norbert A1 - Gellert, Marcus A1 - Kuzanyan, Kirill M. A1 - Pipin, V. V. T1 - Helicity and the solar dynamo Y1 - 2003 ER - TY - JOUR A1 - Kliem, Bernhard A1 - Seehafer, Norbert T1 - Helicity shedding by flux rope ejection JF - Astronomy and astrophysics : an international weekly journal N2 - We quantitatively address the conjecture that magnetic helicity must be shed from the Sun by eruptions launching coronal mass ejections in order to limit its accumulation in each hemisphere. By varying the ratio of guide and strapping field and the flux rope twist in a parametric simulation study of flux rope ejection from approximately marginally stable force-free equilibria, different ratios of self- and mutual helicity are set and the onset of the torus or helical kink instability is obtained. The helicity shed is found to vary over a broad range from a minor to a major part of the initial helicity, with self helicity being largely or completely shed and mutual helicity, which makes up the larger part of the initial helicity, being shed only partly. Torus-unstable configurations with subcritical twist and without a guide field shed up to about two-thirds of the initial helicity, while a highly twisted, kink-unstable configuration sheds only about one-quarter. The parametric study also yields stable force-free flux rope equilibria up to a total flux-normalized helicity of 0.25, with a ratio of self- to total helicity of 0.32 and a ratio of flux rope to external poloidal flux of 0.94. These results numerically demonstrate the conjecture of helicity shedding by coronal mass ejections and provide a first account of its parametric dependence. Both self- and mutual helicity are shed significantly; this reduces the total initial helicity by a fraction of ∼0.4--0.65 for typical source region parameters. KW - instabilities KW - magnetic fields KW - magnetohydrodynamics (MHD) KW - Sun KW - corona KW - coronal mass ejections (CMEs) KW - flares Y1 - 2022 U6 - https://doi.org/10.1051/0004-6361/202142422 SN - 0004-6361 SN - 1432-0746 VL - 659 PB - EDP Sciences CY - Les Ulis 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 - Rust, David M. A1 - Crooker, N. U. A1 - Gold, R. E. A1 - Golub, Leon A1 - Hundhausen, A. J. A1 - Lanzerotti, L. J. A1 - Lazarus, A. J. A1 - Seehafer, Norbert A1 - Zanetti, L. J. T1 - Heliospheric lins explorer (HELIX) Y1 - 1996 ER - TY - JOUR A1 - Demircan, Ayhan A1 - Scheel, S. A1 - Seehafer, Norbert T1 - Heteroclinic behavior in rotating Rayleigh-Benard convection N2 - We investigate numerically the appearance of heteroclinic behavior in a three-dimensional, buoyancy-driven fluid layer with stress-free top and bottom boundaries, a square horizontal periodicity with a small aspect ratio, and rotation at low to moderate rates about a vertical axis. The Prandtl number is 6.8. If the rotation is not too slow, the skewed-varicose instability leads from stationary rolls to a stationary mixed-mode solution, which in turn loses stability to a heteroclinic cycle formed by unstable roll states and connections between them. The unstable eigenvectors of these roll states are also of the skewed-varicose or mixed-mode type and in some parameter regions skewed-varicose like shearing oscillations as well as square patterns are involved in the cycle. Always present weak noise leads to irregular horizontal translations of the convection pattern and makes the dynamics chaotic, which is verified by calculating Lyapunov exponents. In the nonrotating case the primary rolls lose, depending on the aspect ratio, stability to traveling waves or a stationary square pattern. We also study the symmetries of the solutions at the intermittent fixed points in the heteroclinic cycle. Y1 - 2000 UR - http://link.springer.de/link/service/journals/10051/bibs/0013004/00130765.htm ER - TY - JOUR A1 - Demircan, Ayhan A1 - Seehafer, Norbert T1 - Heteroclinic behavior in rotating Rayleigh-Bénard convection N2 - We investigate numerically the appearance of heteroclinic behavior in a three-dimensional, buoyancy-driven, rotating fluid layer. Periodic boundary conditions in the horizontal directions and stress-free boundary conditions at the top and bottom are assumed. Y1 - 2000 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 - Miranda, Rodrigo A. A1 - Rempel, Erico L. A1 - Chian, Abraham C.-L. A1 - Seehafer, Norbert A1 - Toledo, Benjamin A. T1 - Lagrangian coherent structures at the onset of hyperchaos in the two-dimensional Navier-Stokes equations Y1 - 2013 UR - http://dx.doi.org/10.1063/1.4811297 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 - Donner, Reik Volker A1 - Seehafer, Norbert A1 - Sanjuan, Miguel Angel Fernandez A1 - Feudel, Fred T1 - Low-dimensional dynamo modelling and symmetry-breaking bifurcations JF - Physica. D, Nonlinear phenomena N2 - Motivated by the successful Karlsruhe dynamo experiment, a relatively low-dimensional dynamo model is proposed. It is based on a strong truncation of the magnetohydrodynamic (MHD) equations with an external forcing of the Roberts type and the requirement that the model system satisfies the symmetries of the full MHD system, so that the first symmetry-breaking bifurcations can be captured. The backbone of the Roberts dynamo is formed by the Roberts flow, a helical mean magnetic field and another part of the magnetic field coupled to these two by triadic mode interactions. A minimum truncation model (MTM) containing only these energetically dominating primary mode triads is fully equivalent to the widely used first-order smoothing approximation. However, it is shown that this approach works only in the limit of small wave numbers of the excited magnetic field or small magnetic Reynolds numbers ($Rm ll 1$). To obtain dynamo action under more general conditions, secondary mode Y1 - 2006 UR - http://www.sciencedirect.com/science/journal/01672789 U6 - https://doi.org/10.1016/j.physd.2006.08.022 SN - 0167-2789 VL - 223 IS - 2 SP - 151 EP - 162 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Brown, M. R. A1 - Canfield, R. C. A1 - Field, G. A1 - Kulsrud, R. A1 - Pevtsov, A. A. A1 - Rosner, R. A1 - Seehafer, Norbert T1 - Magnetic helicity in space and laboratory plasmas: Editorial summary Y1 - 1999 ER - TY - JOUR A1 - Feudel, Fred A1 - Seehafer, Norbert A1 - Tuckerman, Laurette S. T1 - Multistability in rotating spherical shell convection Y1 - 2013 UR - http://link.aps.org/doi/10.1103/PhysRevE.87.023021 (9.9.2013) SN - 1539-3755 ER -