On the inverse cascade of magnetic helicity

We study the inverse cascade of magnetic helicity in conducting fluids by investigating the detailed transfer of helicity between different spherical shells in Fourier space in direct numerical simulations of three-dimensional magnetohydrodynamics (MHD). Two different numerical simulations are used, one where the system is forced with an electromotive force in the induction equation, and one in which the system is forced mechanically with an ABC flow and the magnetic field is solely sustained by a dynamo action. The magnetic helicity cascade at the initial stages of both simulations is observed to be inverse and local (in scale space) in the large scales, and direct and local in the small scales. When saturation is approached most of the helicity is concentrated in the large scales and the cascade is non-local. Helicity is transfered directly from the forced scales to the largest scales. At the same time, a smaller in amplitude direct cascade is observed from the largest scale to small scales.Comment: Submitted to PR

Dynamo quenching due to shear flow

We provide a theory of dynamo (α effect) and momentum transport in three-dimensional magnetohydrodynamics. For the first time, we show that the α effect is reduced by the shear even in the absence of magnetic field. The α effect is further suppressed by magnetic fields well below equipartition (with the large-scale flow) with different scalings depending on the relative strength of shear and magnetic field. The turbulent viscosity is also found to be significantly reduced by shear and magnetic fields, with positive value. These results suggest a crucial effect of shear and magnetic field on dynamo quenching and momentum transport reduction, with important implications for laboratory and astrophysical plasmas, in particular, for the dynamics of the Sun

How astrophysical mean field dynamos can circumvent existing quenching constraints

Mean field dynamo theory is a leading candidate to explain the observed large scale magnetic fields of galaxies and stars. However, controversy arises over the extent of premature quenching by the backreaction of the growing field. We distinguish between rapid mean field dynamo action, which is required by astrophysical systems, and resistively limited action. We show how the flow of magnetic helicity is important for rapid action. Existing numerical and analytic work suggesting that mean field dynamos are prematurely quenched and resistively limited include approximations or boundary conditions which suppress the magnetic helicity flow from the outset. Thus they do not unambiguously reveal whether real astrophysical mean field dynamos are dynamically suppressed when the helicity flow is allowed. An outflow of helicity also implies an outflow of magnetic energy and so active coronae or winds should accompany mean field dynamos. Open boundaries alone may not be sufficient for rapid dynamo action and the additional physics of buoyancy and outflows may be required. Possible simulation approaches to test some of the principles are briefly discussed. Some limitations of the ``Zeldovich relation'' are also addressed.Comment: 19 pages LaTex, invited submission to Physics of Plasmas, APS/DPP Quebec 2000 Meeting Proceeding

Direct democracy and intergenerational conflicts in ageing societies

To evaluate the potential effects of population ageing on the outcomes of direct democracy, we analyze the effect of age on voting decisions in public referendums. In a case study of the Stuttgart 21 referendum on one of the largest infrastructure projects in Germany, we find that support for the project decreased significantly in age. A quan‐ titative review of the relevant literature affirms that similar lifecycle patterns appear to be the norm in referendums on projects that require initial expenditures and pay off in the long run. Population ageing, thus, presents a potential threat to investment‐like reform projects

Large-Scale Magnetic-Field Generation by Randomly Forced Shearing Waves

A rigorous theory for the generation of a large-scale magnetic field by random non-helically forced motions of a conducting fluid combined with a linear shear is presented in the analytically tractable limit of low Rm and weak shear. The dynamo is kinematic and due to fluctuations in the net (volume-averaged) electromotive force. This is a minimal proof-of-concept quasilinear calculation aiming to put the shear dynamo, a new effect recently found in numerical experiments, on a firm theoretical footing. Numerically observed scalings of the wavenumber and growth rate of the fastest growing mode, previously not understood, are derived analytically. The simplicity of the model suggests that shear dynamo action may be a generic property of sheared magnetohydrodynamic turbulence.Comment: Paper substantially rewritten, results changed (relative to v1). Revised versio

Direct Measurement of Effective Magnetic Diffusivity in Turbulent Flow of Liquid Sodium

The first direct measurements of effective magnetic diffusivity in turbulent flow of electro-conductive fluids (the so-called beta-effect) under magnetic Reynolds number Rm >> 1 are reported. The measurements are performed in a nonstationary turbulent flow of liquid sodium, generated in a closed toroidal channel. The peak level of the Reynolds number reached Re \approx 3 10^6, which corresponds to the magnetic Reynolds number Rm \approx 30. The magnetic diffusivity of the liquid metal was determined by measuring the phase shift between the induced and the applied magnetic fields. The maximal deviation of magnetic diffusivity from its basic (laminar) value reaches about 50% .Comment: 5 pages, 6 figuser, accepted in PR

The electromotive force in multi-scale flows at high magnetic Reynolds number

Recent advances in dynamo theory have been made by examining the competition between small and large-scale dynamos at high magnetic Reynolds number Rm. Small-scale dynamos rely on the presence of chaotic stretching whilst the generation of large-scale fields occurs in flows lacking reflectional symmetry via a systematic electromotive force (emf). In this paper we discuss how the statistics of the emf (at high Rm) depend on the properties of the multi-scale velocity that is generating it. In particular, we determine that different scales of flow have different contributions to the statistics of the emf, with smaller-scales contributing to the mean without increasing the variance. Moreover we determine when scales in such a flow act independently in their contribution to the emf. We further examine the role of large-scale shear in modifying the emf. We conjecture that the distribution of the emf, and not simply the mean, largely determines the dominant scale of the magnetic field generated by the flow

Kinematic frames and "active longitudes": does the Sun have a face?

It has recently been claimed that analysis of Greenwich sunspot data over 120 years reveals that sunspot activity clusters around two longitudes separated by 180 degrees (``active longitudes'') with clearly defined differential rotation during activity cycles.In the present work we extend this critical examination of methodology to the actual Greenwich sunspot data and also consider newly proposed methods of analysis claiming to confirm the original identification of active longitudes. Our analysis revealed that values obtained for the parameters of differential rotation are not stable across different methods of analysis proposed to track persistent active longitudes. Also, despite a very thorough search in parameter space, we were unable to reproduce results claiming to reveal the century-persistent active longitudes. We can therefore say that strong and well substantiated evidence for an essential and century-scale persistent nonaxisymmetry in the sunspot distribution does not exist.Comment: 14 pages, 1 table, 21 figures, accepted in A&