@article{KliemSeehafer2022,
  author    = {Kliem, Bernhard and Seehafer, Norbert},
  title     = {Helicity shedding by flux rope ejection},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {659},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/202142422},
  pages     = {9},
  year      = {2022},
  abstract  = {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.},
  language  = {en}
}
@article{HassaninKliemSeehaferetal.2022,
  author    = {Hassanin, Alshaimaa and Kliem, Bernhard and Seehafer, Norbert and T{\"o}r{\"o}k, Tibor},
  title     = {A model of homologous confined and ejective eruptions involving kink instability and flux cancellation},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  volume    = {929},
  journal   = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  number    = {2},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {2041-8205},
  doi       = {10.3847/2041-8213/ac64a9},
  pages     = {7},
  year      = {2022},
  abstract  = {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.},
  language  = {en}
}
@article{HassaninKliemSeehafer2016,
  author    = {Hassanin, Alshaimaa and Kliem, Bernhard and Seehafer, Norbert},
  title     = {Helical kink instability in the confined solar eruption on 2002 May 27},
  series = {Astronomische Nachrichten = Astronomical notes},
  volume    = {337},
  journal   = {Astronomische Nachrichten = Astronomical notes},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {0004-6337},
  doi       = {10.1002/asna.201612446},
  pages     = {1082 -- 1089},
  year      = {2016},
  language  = {en}
}
@article{FeudelTuckermanGellertetal.2015,
  author    = {Feudel, Fred and Tuckerman, L. S. and Gellert, Marcus and Seehafer, Norbert},
  title     = {Bifurcations of rotating waves in rotating spherical shell convection},
  series = {Physical Review E},
  volume    = {92},
  journal   = {Physical Review E},
  number    = {5},
  publisher = {American Physical Society},
  address   = {Woodbury},
  issn      = {1539-3755},
  doi       = {10.1103/PhysRevE.92.053015},
  year      = {2015},
  abstract  = {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.},
  language  = {en}
}
@article{FeudelTuckermanGellertetal.2015,
  author    = {Feudel, Fred and Tuckerman, L. S. and Gellert, M. and Seehafer, Norbert},
  title     = {Bifurcations of rotating waves in rotating spherical shell convection},
  series = {Physical review : E, Statistical, nonlinear and soft matter physics},
  volume    = {92},
  journal   = {Physical review : E, Statistical, nonlinear and soft matter physics},
  number    = {5},
  publisher = {American Physical Society},
  address   = {College Park},
  issn      = {1539-3755},
  doi       = {10.1103/PhysRevE.92.053015},
  pages     = {7},
  year      = {2015},
  abstract  = {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.},
  language  = {en}
}
@article{FeudelSeehaferTuckerman2013,
  author    = {Feudel, Fred and Seehafer, Norbert and Tuckerman, Laurette S.},
  title     = {Multistability in rotating spherical shell convection},
  issn      = {1539-3755},
  year      = {2013},
  language  = {en}
}
@article{MirandaRempelChianetal.2013,
  author    = {Miranda, Rodrigo A. and Rempel, Erico L. and Chian, Abraham C.-L. and Seehafer, Norbert and Toledo, Benjamin A.},
  title     = {Lagrangian coherent structures at the onset of hyperchaos in the two-dimensional Navier-Stokes equations},
  year      = {2013},
  language  = {en}
}
@article{MirandaRempelChianetal.2013,
  author    = {Miranda, Rodrigo A. and Rempel, Erico L. and Chian, Abraham C.-L. and Seehafer, Norbert and Toledo, Benjamin A. and Munoz, Pablo R.},
  title     = {Lagrangian coherent structures at the onset of hyperchaos in the two-dimensional Navier-Stokes equations},
  series = {Chaos : an interdisciplinary journal of nonlinear science},
  volume    = {23},
  journal   = {Chaos : an interdisciplinary journal of nonlinear science},
  number    = {3},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {1054-1500},
  doi       = {10.1063/1.4811297},
  pages     = {13},
  year      = {2013},
  abstract  = {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.},
  language  = {en}
}
@article{FeudelSeehaferTuckermanetal.2013,
  author    = {Feudel, Fred and Seehafer, Norbert and Tuckerman, Laurette S. and Gellert, Marcus},
  title     = {Multistability in rotating spherical shell convection},
  series = {Physical review : E, Statistical, nonlinear and soft matter physics},
  volume    = {87},
  journal   = {Physical review : E, Statistical, nonlinear and soft matter physics},
  number    = {2},
  publisher = {American Physical Society},
  address   = {College Park},
  issn      = {1539-3755},
  doi       = {10.1103/PhysRevE.87.023021},
  pages     = {8},
  year      = {2013},
  abstract  = {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.},
  language  = {en}
}
@article{FuhrmannSeehaferValorietal.2011,
  author    = {Fuhrmann, Marcel and Seehafer, Norbert and Valori, Gherardo and Wiegelmann, Thomas},
  title     = {A comparison of preprocessing methods for solar force-free magnetic field extrapolation},
  issn      = {0004-6361},
  year      = {2011},
  language  = {en}
}