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Lyapunov spectra instability of chaotic dynamo Ricci flows in twisted magnetic flux tubes  [PDF]
Garcia de Andrade
Physics , 2008,
Abstract: Previously Casetti, Clementi and Pettini [\textbf{Phys.Rev.E \textbf{54},6,(1996)}] have investigated the Lyapunov spectrum of Hamiltonian flows for several Hamiltonian systems by making use of the Riemannian geometry. Basically the Lyapunov stability analysis was substituted by the Ricci sectional curvature analysis. In this report we apply Pettini's geometrical framework to determine the potential energy of a twisted magnetic flux tube, from its curved Riemannian geometry. Actually the Lyapunov exponents, are connected to a Riemann metric tensor, of the twisted magnetic flux tubes (MFTs). The Hamiltonian flow inside the tube is actually given by Perelman Ricci flows constraints in twisted magnetic flux tubes, where the Lyapunov eigenvalue spectra for the Ricci tensor associated with the Ricci flow equation in MFTs leads to a finite-time Lyapunov exponential stretching along the toroidal direction of the tube and a contraction along the radial direction of the tube. The Jacobi equation for the MFTs is shown to have a constant sectional Ricci curvature which allows us to compute the Jacobi-Levi-Civita (JLC) geodesic deviation for the spread of lines on the tube manifold and chaotic action through the greatest of its Lyapunov exponents. By analyzing the spectra of the twisted MFT, it is shown that the greater exponent is positive and proportional to the random radial flow of the tube, which allows the onset of chaos is guaranted. The randomness in the twisted flow reminds a discussed here is similar of a recent work by Shukurov, Stepanov, and Sokoloff on dynamo action on Moebius flow [\textbf{Phys Rev E 78 (2008)}]. The dynamo action in twisted flux tubes discussed here may also serve as model for dynamo experiments in laboratory.
Spectrum of kinematic fast dynamo operator in Ricci flows  [PDF]
Garcia de Andrade
Physics , 2009,
Abstract: Spectrum of kinematic fast dynamo operators in Ricci compressible flows in Einstein 2-manifolds is investigated. A similar expression, to the one obtained by Chicone, Latushkin and Montgomery-Smith (Comm Math Phys (1995)) is given, for the fast dynamo operator. The operator eigenvalue is obtained in a highly conducting media, in terms of linear and nonlinear orders of Ricci scalar. Eigenvalue spectra shows that there is a relation between the Ricci scalar and expansion of the flow. Spatial 3-Einstein manifold section of Friedmann-Robertson-Walker (FRW) is obtained in the limit of ideal plasma. If the trace of the Ricci curvature tensor is negative, a contraction of the inflationary phase of the universe takes place, and the dynamo action takes place. When the universe expands a decaying magnetic field or non-dynamo is obtained. As in Latushkin and Vishik (Comm Math Phys (2003)) the Lyapunov exponents in kinematic dynamos is also investigated. Since positive curvature scalar are preserved under Ricci flow, it is shown that fast dynamos are preserved under this same flow.
Cosmic dynamo analogue and decay of magnetic fields in 3D Ricci flows  [PDF]
Garcia de Andrade
Physics , 2009,
Abstract: Magnetic curvature effects, investigated by Barrow and Tsagas (BT) [Phys Rev D \textbf{77},(2008)],as a mechanism for magnetic field decay in open Friedmann universes (${\Lambda}<0$), are applied to dynamo geometric Ricci flows in 3D curved substrate in laboratory. By simple derivation, a covariant three-dimensional magnetic self-induced equation, presence of these curvature effects, indicates that de Sitter cosmological constant (${\Lambda}\ge{0}$), leads to enhancement in the fast kinematic dynamo action which adds to stretching of plasma flows. From the magnetic growth rate, the strong shear case, anti-de Sitter case (${\Lambda}<0$) BT magnetic decaying fields are possible while for weak shear, fast dynamos are possible. The self-induced equation in Ricci flows is similar to the equation derived by BT in $(3+1)$-spacetime continuum. Lyapunov-de Sitter metric is obtained from Ricci flow eigenvalue problem. In de Sitter analogue there is a decay rate of ${\gamma}\approx{-{\Lambda}}\approx{-10^{-35}s^{-2}}$ from corresponding cosmological constant ${\Lambda}$, showing that, even in the dynamo case, magnetic field growth is slower than de Sitter inflation, which strongly supports to BT result.
Ricci dynamo stretch-shear plasma flows and magnetic energy bounds  [PDF]
Garcia de Andrade
Physics , 2009,
Abstract: Geometrical tools, used in Einstein's general relativity (GR), are applied to dynamo theory, in order to obtain fast dynamo action bounds to magnetic energy, from Killing symmetries in Ricci flows. Magnetic field is shown to be the shear flow tensor eigendirection, in the case of marginal dynamos. Killing symmetries of the Riemann metric, bounded by Einstein space, allows us to reduce the computations. Techniques used are similar to those strain decomposition of the flow in Sobolev space, recently used by Nu\~nez [JMP \textbf{43} (2002)] to place bounds in the magnetic energy in the case of hydromagnetic dynamos with plasma resistivity. Contrary to Nu\~nez case, we assume that the dynamos are kinematic, and the velocity flow gradient is decomposed into expansion, shear and twist. The effective twist vanishes by considering that the frame vorticity coincides with Ricci rotation coefficients. Eigenvalues are here Lyapunov exponents. In analogy to GR, where curvature plays the role of gravity, here Ricci curvature seems to play the role of diffusion.
On shrinking Gradient Ricci Soliton With Nonnegative Sectional Curvature  [PDF]
Mingliang Cai
Mathematics , 2013,
Abstract: In this paper, we prove some rigidity theorems for shrinking gradient Ricci solitons with nonnegative sectional curvature.
Bounding sectional curvature along a K?hler-Ricci flow  [PDF]
Wei-Dong Ruan,Yuguang Zhang,Zhenlei Zhang
Mathematics , 2007,
Abstract: If a normalized K\"{a}hler-Ricci flow $g(t),t\in[0,\infty),$ on a compact K\"{a}hler $n$-manifold, $n\geq 3$, of positive first Chern class satisfies $g(t)\in 2\pi c_{1}(M)$ and has $L^{n}$ curvature operator uniformly bounded, then the curvature operator will also uniformly bounded along the flow. Consequently the flow will converge along a subsequence to a K\"{a}hler-Ricci soliton.
Riemann curvature-stretching coupling in dynamo torus laboratory and in UHF twisted plasma loops  [PDF]
Garcia de Andrade
Physics , 2009,
Abstract: A plasma loop twisted Riemannian model is applied to torus dynamos twisted flows it leading to a slow dynamo such as in Moebius strip dynamo, recently considered by Shukurov, Stepanov and Sokoloff [Phys. Rev. \textbf{E 78},025301,(2008)] to modelling Perm dynamo torus in liquid sodium. Since diffusion and advection (stretching), are competing effects for dynamo action, plasma resistivity term is shown to be proportional to loops Riemann curvature (folding). Shukurov et al, also showed that based on Ponomarenko dynamo, a broader torus channel produces a better dynamo. These results agree with Schekochihin et al [Phys Rev \textbf{E} (2002)] where random filamentary magnetic fields are strengthen by curvature. Analysis of spectrum of chaotic fast dynamos, shows that Riemann curvature acts as a damping, since growth magnetic field rate is inversely proportional to Riemann curvature. Comparison with general relativistic MHD dynamo equation, shows that the Ricci tensor, which is a contraction of the Riemann tensor also appears in the diffusion term. Curvature of plasma loop is ${{R^{1}}_{212}}|_{\textbf{Plasma}}\approx{5.6{\times}10^{-19}m^{-2}}$, while for Perm torus is certainly higher. Thus slow dynamos are favoured in dynamo laboratories rather than in plasma loops. It is shown that the curvature-stretching flux rope dynamo coupling energy, coincides with the minimum twist energy ${\epsilon}_{\textbf{twist}}\approx{10^{30}TeV}$ stored in flux ropes. Torus flux tubes around black-holes remain in the order of $2MeV$ and GBR are around $10^{52}TeV$. Since the ${R^{1}}_{212}$ is negative, inflexionary flux tubes fast dynamos may be responsible for this CME mechanism in UHF plasma loops.
Positive Complex Sectional Curvature, Ricci Flow and the Differential Sphere Theorem  [PDF]
Lei Ni,Jon Wolfson
Mathematics , 2007,
Abstract: The paper provides a different proof of the result of Brendle-Schoen on the differential sphere theorem. It is shown directly that the invariant cone of curvature operators with positive (or non-negative) complex sectional curvature is preserved by the Ricci flow. This implies, by a result of B\"ohm-Wilking, that the normalized Ricci flow deforms such a metric to a metric of constant positive curvature. Using earlier work of Yau and Zheng it can be shown that a metric with strictly (pointwise) 1/4-pinched sectional curvature has positive complex sectional curvature. This gives a direct proof of Brendle-Schoen's recent differential sphere theorem, bypassing any discussion of positive isotropic curvature.
The role of curvature and stretching in dynamo plasmas  [PDF]
Garcia de Andrade
Physics , 2008,
Abstract: Vishik's antidynamo theorem is applied to non-stretched twisted magnetic flux tube in Riemannian space. Marginal or slow dynamos along curved (folded), torsioned (twisted) and non-stretching flux tubes plasma flows are obtained}. Riemannian curvature of twisted magnetic flux tube is computed in terms of the Frenet curvature in the thin tube limit. It is shown that, for non-stretched filaments fast dynamo action in diffusive case cannot be obtained, in agreement with Vishik's argument, that fast dynamo cannot be obtained in non-stretched flows. \textbf{In this case a non-uniform stretching slow dynamo is obtained}.\textbf{An example is given which generalizes plasma dynamo laminar flows, recently presented by Wang et al [Phys Plasmas (2002)], in the case of low magnetic Reynolds number $Re_{m}\ge{210}$. Curved and twisting Riemannian heliotrons, where non-dynamo modes are found even when stretching is presented, shows that the simple presence of stretching is not enough for the existence of dynamo action. Folding is equivalent to Riemann curvature and can be used to cancell magnetic fields, not enhancing the dynamo action. In this case non-dynamo modes are found for certain values of torsion or Frenet curvature (folding) in the spirit of anti-dynamo theorem. It is shown that curvature and stretching are fundamental for the existence of fast dynamos in plasmas.
Geodesic dynamo chaotic flows and non-Anosov maps in twisted magnetic flux tubes  [PDF]
Garcia de Andrade
Physics , 2008,
Abstract: Recently Tang and Boozer [{\textbf{Phys. Plasmas (2000)}}], have investigated the anisotropies in magnetic field dynamo evolution, from local Lyapunov exponents, giving rise to a metric tensor, in the Alfven twist in magnetic flux tubes (MFTs). Thiffeault and Boozer [\textbf{Chaos}(2001)] have investigated the how the vanishing of Riemann curvature constrained the Lyapunov exponential stretching of chaotic flows. In this paper, Tang-Boozer-Thiffeault differential geometric framework is used to investigate effects of twisted magnetic flux tube filled with helical chaotic flows on the Riemann curvature tensor. When Frenet torsion is positive, the Riemann curvature is unstable, while the negative torsion induces an stability when time $t\to{\infty}$. This enhances the dynamo action inside the MFTs. The Riemann metric, depends on the radial random flows along the poloidal and toroidal directions. The Anosov flows has been applied by Arnold, Zeldovich, Ruzmaikin and Sokoloff [\textbf{JETP (1982)}] to build a uniformly stretched dynamo flow solution, based on Arnold's Cat Map. It is easy to show that when the random radial flow vanishes, the magnetic field vanishes, since the exponential Lyapunov stretches vanishes. This is an example of the application of the Vishik's anti-fast dynamo theorem in the magnetic flux tubes. Geodesic flows of both Arnold and twisted MFT dynamos are investigated. It is shown that a constant random radial flow can be obtained from the geodesic equation. Throughout the paper one assumes, the reasonable plasma astrophysical hypothesis of the weak torsion. Pseudo-Anosov dynamo flows and maps have also been addressed by Gilbert [\textbf{Proc Roy Soc A London (1993)}
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