TY  - JOUR
A1  - Kuzanyan, Kirill M.
A1  - Pipin, Valerij V.
A1  - Seehafer, Norbert
T1  - The alpha effect and the observed twist and current helicity of solar magnetic fields
N2  - 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
Y1  - 2006
UR  - http://www.springerlink.com/content/100339
U6  - https://doi.org/10.1007/s11207-006-1636-6
SN  - 0038-0938
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  -