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Magnetic helicity in stellar dynamos: new numerical experiments
Author(s) -
Brandenburg A.,
Dobler W.,
Subramanian K.
Publication year - 2002
Publication title -
astronomische nachrichten
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.394
H-Index - 63
eISSN - 1521-3994
pISSN - 0004-6337
DOI - 10.1002/1521-3994(200207)323:2<99::aid-asna99>3.0.co;2-b
Subject(s) - dynamo , helicity , magnetic helicity , physics , solar dynamo , magnetic field , saturation (graph theory) , dynamo theory , equipartition theorem , field strength , magnetohydrodynamics , magnetic energy , computational physics , mechanics , quantum electrodynamics , classical mechanics , quantum mechanics , magnetization , mathematics , combinatorics
The theory of large scale dynamos is reviewed with particular emphasis on the magnetic helicity constraint in the presence of closed and open boundaries. In the presence of closed or periodic boundaries, helical dynamos respond to the helicity constraint by developing small scale separation in the kinematic regime, and by showing long time scales in the nonlinear regime where the scale separation has grown to the maximum possible value. A resistively limited evolution towards saturation is also found at intermediate scales before the largest scale of the system is reached. Larger aspect ratios can give rise to different structures of the mean field which are obtained at early times, but the final saturation field strength is still decreasing with decreasing resistivity. In the presence of shear, cyclic magnetic fields are found whose period is increasing with decreasing resistivity, but the saturation energy of the mean field is in strong super‐equipartition with the turbulent energy. It is shown that artificially induced losses of small scale field of opposite sign of magnetic helicity as the large scale field can, at least in principle, accelerate the production of large scale (poloidal) field. Based on mean field models with an outer potential field boundary condition in spherical geometry, we verify that the sign of the magnetic helicity flux from the large scale field agrees with the sign of α . For solar parameters, typical magnetic helicity fluxes lie around 10 47 Mx 2 per cycle.