Conformal maps in periodic dynamo flows and in stretch-twist suppression on Riemannian manifolds

Examples of conformal dynamo maps have been presented earlier [Phys Plasmas 14(2007)] where fast dynamos in twisted magnetic flux tubes in Riemannian manifolds were obtained. This paper shows that conformal maps, under the Floquet condition, leads to coincidence between exponential stretching or Lyapunov exponent, conformal factor of fast dynamos. Unfolding conformal dynamo maps can be obtained in Riemann-flat manifolds since here, Riemann curvature plays the role of folding. Previously, Oseledts [Geophys Astrophys Fluid Dyn 73 (1993)] has shown that the number of twisted and untwisting orbits in a two torus on a compact Riemannian manifold induces a growth of fast dynamo action. In this paper, the stretching of conformal thin magnetic flux tubes is constrained to vanish, in order to obtain the conformal factor for non-stretching non-dynamos. Since thin flux tube can be considered as a twisted or untwisting two-torus map, it is shown that the untwisting, weakly torsion, and nonstretching conformal torus map cannot support a fast dynamo action, a marginal dynamo being obtained. This is an example of an anti-fast dynamo theorem besides the ones given by Vishik and Klapper and Young [Comm Math Phys 173(1996)] in ideally high conductive flow. From the Riemann curvature tensor it is shown that new conformal non-dynamo, is actually singular as one approaches the magnetic flux tube axis. Thus conformal map suppresses the stretching directions and twist, leading to the absence of fast dynamo action while Riemann-flat unfolding manifolds favors non-fast dynamos.Key-words:anti-dynamo theorems, conformal stretching.PACS numbers:2.40.Hw:differential geometries. 91.25.Cw-dynamo theories.