The galactic dynamo effect due to Parker-shearing instability of magnetic flux tubes

-field electron transport, cross-field electric field, dynamo effect, localized electrostatic waves, pressure gradient 1.

, violently fluctuating Reynolds stress associated with a reduced magnetic activity. The resulting (dynamo-) α-effect is negative in the upper disk plane and positive in the lower disk plane, it is small and highly noisy. Key words: Magnetohydrodynamics (MHD) – instabilities – turbulence – magnetic fields 1.

. Dynamo theory is reviewed with particular emphasis on recent developments. There now seems to be a strong case for dynamo effects that are driven by the magnetic field itself. This is linked to recent interpretations of the observed stellar cycle periods which suggest that the ratio of cycle

We show that an α effect is driven by the cosmic-ray (CR) Bell instability exciting left–right asymmetric turbulence. Alfvén waves of a preferred polarization have maximally helical motion, because the transverse motion of each mode is parallel to its curl. We show how large-scale Alfvén modes

of the dynamo effect within the simulations and discuss global meanfield models based on the adopted turbulence coefficients. The results are compared to global simulations of the magneto-rotational instability.

We derive analytically the vorticity generated downstream of a two-dimensional rippled hydro-magnetic shock neglecting fluid viscosity and resistivity. The growth of the turbulent component of the downstream magnetic field is driven by the vortical eddies motion. We determine an analytic time-evolution of the magnetic field amplification at shocks, so far described only numerically, until saturation occurs due to seed-field reaction to field lines whirling. The explicit expression of the ampli-fication growth rate and of the non-linear field back-reaction in terms of the parameters of shock and interstellar density fluctuations is derived from MHD jump conditions at rippled shocks. A magnetic field saturation up to the order of milligauss and a short-time variability in the X-ray observations of supernova remnants can be obtained by using reasonable parameters for the interstellar turbulence. Subject headings: Physical Data and Processes: turbulence; ISM: cosmic rays, magnetic fields 1.

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Abstract. Convectively driven dynamos with rotation generating magnetic fields on scales large compared with the scale of the turbulent eddies are being reviewed. It is argued that such fields can be understood as the result of an α effect. Simulations in Cartesian domains show that such large

of cross-helicity leads to a mean-field dynamo effect that is known as the Yoshizawa effect. Direct numerical simulations of such flows demonstrate the presence of magnetic fields on scales larger than the scale of the flow. Contrary to earlier expectations, the Yoshizawa effect is found to be proportional