arXiv subjects

Matthias Rheinhardt

Publications and source records attributed to Matthias Rheinhardt.

21 records · Page 2Linked to original sources

arXiv2011

The fratricide of alpha-Omega dynamos by their alpha-squared siblings

Context. Helically forced magneto-hydrodynamic shearing-sheet turbulence can support different large-scale dynamo modes, although the α - Ω mode is generally expected to dominate because it is the fastest growing. In an α - Ω dynamo, most of the field amplification is produced by the shear. As differential rotation is an ubiquitous source of shear in astrophysics, such dynamos are believed to be the source of most astrophysical large-scale magnetic fields. Aims. We study the stability of oscillatory migratory α - Ω type dynamos in turbulence simulations. Methods. We use shearing-sheet simulations of hydromagnetic turbulence that is helically forced at a wavenumber that is about three times larger than the lowest wavenumber in the domain so that both α - Ω and α2 dynamo action is possible. Results. After initial dominance and saturation, the α - Ω mode is found to be destroyed by an orthogonal α2 mode sustained by the helical turbulence alone. We show that there are at least two processes through which this transition can occur. Conclusions. The fratricide of α - Ω dynamos by its α2 sibling is discussed in the context of grand minima of solar and stellar activity. However, the genesis of α - Ω dynamos from an α2 dynamo has not yet been found.

astro-ph.SR↗

arXiv2010

Alpha effect due to buoyancy instability of a magnetic layer

A strong toroidal field can exist in form of a magnetic layer in the overshoot region below the solar convection zone. This motivates a more detailed study of the magnetic buoyancy instability with rotation. We calculate the alpha effect due to helical motions caused by a disintegrating magnetic layer in a rotating density-stratified system with angular velocity Omega making an angle theta with the vertical. We also study the dependence of the alpha effect on theta and the strength of the initial magnetic field. We carry out three-dimensional hydromagnetic simulations in Cartesian geometry. A turbulent EMF due to the correlations of the small scale velocity and magnetic field is generated. We use the test-field method to calculate the transport coefficients of the inhomogeneous turbulence produced by the layer. We show that the growth rate of the instability and the twist of the magnetic field vary monotonically with the ratio of thermal conductivity to magnetic diffusivity. The resulting alpha effect is inhomogeneous and increases with the strength of the initial magnetic field. It is thus an example of an "anti-quenched" alpha effect. The alpha effect is nonlocal, requiring around 8--16 Fourier modes to reconstruct the actual EMF based on the actual mean field.

astro-ph.SR↗

arXiv2006

Mean--field electrodynamics: Critical analysis of various analytical approaches to the mean electromotive force

There are various analytical approaches to the mean electromotive force $\cal E =< \vec{u}\times \vec{b}>$ crucial in mean--field electrodynamics, with $\vec{u}$ and $\vec{b}$ being velocity and magnetic field fluctuations. In most cases the traditional approach, restricted to the second--order correlation approximation, has been used. Its validity is only guaranteed for a range of conditions, which is narrow in view of many applications, e.g., in astrophysics. With the intention to have a wider range of applicability other approaches have been proposed which make use of the so--called $τ$--approximation, reducing correlations of third order in $\vec{u}$ and $\vec{b}$ to such of second order. After explaining some basic features of the traditional approach a critical analysis of the approaches of that kind is given. It is shown that they lead in some cases to results which are in clear conflict with those of the traditional approach. It is argued that this indicates shortcomings of the $τ$--approaches and poses serious restrictions to their applicability. These shortcomings do not result from the basic assumption of the $τ$--approximation. Instead, they seem to originate in some simplifications made in order to derive $\cal E$ without really solving the equations governing $\vec{u}$ and $\vec{b}$. A starting point for a new approach is described which avoids the conflict.

astro-ph↗