Dynamics Of Flux Tubes Research Articles

The Dynamics of Flux Tubes in a High- beta Plasma. II. Buoyancy in Stars and Accretion Disks

The Dynamics of Flux Tubes in a High- beta Plasma. II. Buoyancy in Stars and Accretion Disks

The Dynamics of Flux Tubes in a High- beta Plasma. I. A General Description

view Abstract Citations (39) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Dynamics of Flux Tubes in a High- beta Plasma. I. A General Description Vishniac, Ethan T. Abstract We suggest a new model for the structure of a magnetic field embedded in a plasma whose average turbulent and magnetic energy densities are both much less than the gas pressure. This model is based on the popular notion that the magnetic field will tend to separate into individual flux tubes. We point out that interactions between the flux tubes will be dominated by coherent effects stemming from the turbulent wakes created as the fluid streams by the flux tubes. Balancing the attraction caused by shielding effects with turbulent diffusion we find that flux tubes have typical radii comparable to the local Mach number squared times the large-scale eddy length, are arranged in a one-dimensional fractal pattern, have a radius of curvature comparable to the largest scale eddies in the turbulence, and have an internal magnetic pressure comparable to the ambient pressure. When the average magnetic energy density is much less than the turbulent energy density, the radius and curvature scale of the flux tubes will be smaller than these estimates. Allowing for resistivity changes these properties but does not alter the macroscopic properties of the fluid or the large-scale magnetic field. In either case we show that the Sweet-Parker reconnection rate is at least as fast as an eddy turnover time. Realistic stellar plasmas are expected to either be in the ideal limit (e.g., the solar photosphere) or the resistive limit (the bulk of the solar convection zone). Allowing for significant viscosity drastically changes the macroscopic properties of the magnetic field. We find that all current numerical simulations of three-dimensional MHD turbulence are in the viscous regime and are inapplicable to stars or accretion disks. However, these simulations are in good quantitative agreement with our model in the viscous limit. Publication: The Astrophysical Journal Pub Date: June 1995 DOI: 10.1086/175830 Bibcode: 1995ApJ...446..724V Keywords: MAGNETOHYDRODYNAMICS: MHD; PLASMAS; TURBULENCE full text sources ADS |

Sunspot velocity correlations: Are they due to Reynolds stresses or to the Coriolis force on rising flux tubes?

Observations have consistently pointed out that the longitudinal and latitudinal motions of sunspots are correlated. The magnitude of the covariance was found to increase with latitude, and its sign was found to be positive in the N-hemisphere and negative in the S-hemisphere. This correlation was believed to be due to the underlying turbulence where the sunspot flux tubes are anchored, and the covariance had the right sign and magnitude needed to explain the transfer of angular momentum toward the equator through Reynolds stresses. Here we present an alternate explanation for these sunspot velocity correlations: It is believed that the dynamo operates in a thin overshoot layer beneath the base of the convection zone, and the flux tubes generated there produce sunspots at the photosphere. By studying the dynamics of flux tubes emerging from the base of the convection zone to the photosphere, we show that these velocity correlations of sunspots could be merely a consequence of the effect of Coriolis force on rising flux tubes. The effect of the Coriolis force, as demonstrated by even a back-of-the-envelope calculation, is to push the faster rotating spots equatorward and the slower rotating spots poleward, giving rise to a correlation in their longitudinal and latitudinal velocities, which is positive in the N-hemisphere and negative in the S-hemisphere. The increase in the correlation with latitude is due to the increase in magnitude of the Coriolis force. Hence we show that these velocity correlations might have nothing to do with the Reynolds stresses of the underlying turbulence. We present analyses of observations, and show that the covariances of plages are an order of magnitude higher than the sunspot covariances. If plages and sunspots share the same origin, and if their horizontal velocity correlations are wholly due to the effect of Coriolis force on rising flux tubes, then the study of their dynamics suggests that the flux tubes that form plages should have diameters of a couple of thousand km at the base of the convection zone and remain intact until they reach the photosphere, whereas sunspots should be formed by a collection of small flux tubes (each measuring about a hundred km in diameter), that rise through the convection zone as individual elements and coalesce when they emerge through the photosphere.

Dynamics of Flux Tubes in Thick Accretion Disks.

Dynamics of Flux Tubes in Thick Accretion Disks.

Excited states of light hadrons as a probe of QCD

The dynamics of light hadrons being highly sensitive to the confinement force, it is suggested that a careful study of the excitation pattern of light hadrons might be the most effective way to unravel the mechanisms of confinement dynamics. As a case study, it is shown how the recently confirmed doubling of the ρ' (1600) (which in potential models relies on an accidental level degeneracy) finds a simple interpretation in the framework of a string picture for flux-tube dynamics.

Jets in heavy-ion collisions as a probe of QCD flux-tube dynamics at finite temperature.

The properties of hadronic jets in high-energy heavy-ion collisions are discussed and it is found that thermal effects in the QCD fragmentation process can lead to a broader ${p}_{T}$ distribution and an increased strangeness yield. It is speculated that a phase transition in the central region would cause a significant increase of the effective jet temperature. A theoretical estimate of the thermal correction to the quark-pair-production rate in a strong color-electric field is given. It can be expressed as an effective perturbation of the background color-electric field. This result is applicable to QCD-inspired fragmentation models.

Fine-scale structure of solar magnetic fields

Seeing limitations of the earth's atmosphere have prevented us from spatially resolving most of the basic magnetic flux elements on the sun, since their sizes are all well below one sec of arc (excluding sunspots). No space experiment to overcome this limitation has yet been performed, but the first step will be taken with Spacelab 2. Direct mapping of the circular polarization in spectral lines provides us with information on the morphology and evolution of the partially resolved magnetic structures. In reviewing recent results, special attention is payed to the question of flux disappearance, since it is fundamental for understanding the solar cycle, and depends on a knowledge of the fine-scale structures. The strong-field (kG) nature of the photospheric flux was revealed more than a decade ago using polarization recordings in pairs of spectral lines. A breakthrough in the use of spectral information to deduce the properties of the spatially unresolved magnetic fluxtubes has recently been achieved through the conversion of a Fourier transform spectrometer (FTS) into a polarimeter for Zeeman-effect recordings. We first use the FTS data to illustrate the diagnostic contents of the line-ratio technique, and then indicate how a statistical approach with 400 Fe I lines has recently been applied. In particular we discuss the implications of the observed Stokes V asymmetries for fluxtube dynamics. Finally the ongoing search for a small-scale “turbulent” magnetic field of mixed polarities is described. Observational limits derived from direct magnetograms, spectral line broadening, and the Hanle effect are illustrated.

The lattice gauge theory approach to quantum chromodynamics

The author reviews in a pedagogical fashion some of the recent developments in lattice quantum chromodynamics. This review emphasizes explicit examples and illustrations rather than general proofs and analyses. It begins with a discussion of the heavy-quark potential in continuum quantum chromodynamics. Asymptotic freedom and renormalization-group improved perturbation theory are discussed. A simple dielectric model of confinement is considered as an intuitive guide to the vacuum of non-Abelian gauge theories. Next, the Euclidean form of lattice gauge theory is introduced, and an assortment of calculational methods are reviewed. These include high-temperature expansions, duality, Monte Carlo computer simulations, and weak coupling expansions. A $\ensuremath{\Lambda}$- parameter calculation for asymptotically free-spin models is presented. The Hamiltonian formulation of lattice gauge theory is presented and is illustrated in the context of flux tube dynamics. Roughening transitions, Casimir forces, and the restoration of rotational symmetry are discussed. Mechanisms of confinement in lattice theories are illustrated in the two-dimensional electrodynamics of the planar model and the U(1) gauge theory in four dimensions. Generalized actions for SU(2) gauge theories and the relevance of monopoles and strings to crossover phenomena are considered. A brief discussion of the continuity of fields and topological charge in asymptotically free lattice models is presented. The final major topic of this review concerns lattice fermions. The species doubling problem and its relation to chiral symmetry are illustrated. Staggered Euclidean fermion methods are discussed in detail, with an emphasis on species counting, remnants of chiral symmetry, Block spin variables, and the axial anomaly. Numerical methods for including fermions in computer simulations are considered. Jacobi and Gauss-Siedel inversion methods to obtain the fermion propagator in a background gauge field are reviewed. Langevin, pseudofermion Monte Carlo methods, and random walk methods are discussed, as are proposals for including the fermion determinant in computer simulations. Finally, spontaneous chiral symmetry breaking in lattice models is discussed. The review ends with brief remarks about present research directions.

Quark model for baryons based on quantum chromodynamics

SU(3) flux-tube dynamics and asymptotic freedom are incorporated into a semirelativistic quark model of baryons and mesons. Good agreement with the energy levels of spin-averaged multiplets of mesons and baryons is obtained using conventional values for the strong-interaction coupling constant ${\ensuremath{\alpha}}_{s}$, the string tension $\sqrt{\ensuremath{\sigma}}$, and the constituent quark mass $m$. We calculate meson energy levels with angular momentum $L=0, 1, 2, 3, \mathrm{and} 4$ and baryon levels with $L=0, 1, \mathrm{and} 2$ to within 3% of their experimental values. Baryon radial excitations are also considered and the Roper resonance is calculated with \ensuremath{\sim}5% error. The three-body wave equation for the baryons is solved approximately using variational Monte Carlo methods which have been developed previously for nuclear-physics problems.

Sunspots and the physics of magnetic flux tubes. VI - Convective propulsion. VII - Heat flow in a convective downdraft

The effect of negative aerodynamic drag in an ideal fluid subject to convective instability is considered. It is shown that a cylinder moving in such a fluid is propelled forward in its motion by the convective forces and that the characteristic acceleration time is comparable to the onset time of convective motions in the fluid. It is suggested that convective propulsion plays an important role in the dynamics of flux tubes extending through the surface of the sun. The suppression of the upward heat flow in a Boussinesq convective cell with free upper and lower boundaries by a downdraft is then analyzed. Application to the solar convection zone indicates that downdrafts of 1 to 2 km/s at depths of 1000 to 4000 km beneath the visible surface of the sun are sufficient to reduce the upward heat flux to a small fraction of the ambient value.

Current-induced flux motion in type-I superconducting films studied at 100-ns time resolution

Abstract The motion of magnetic flux tubes in the current-induced resistive state of superconducting Pb films several microns thick has been studied with magnetooptical flux detection in conjunction with a high-speed stroboscope, permitting the time-resolved observation of rapid flux-tube dynamics. A computer model of flux-tube motion has been developed which, when suitably modified, is in reasonable agreement with the experimental results. By measuring the flux-tube velocity directly, it has been determined that the phenomenological expression for the viscous drag coefficient is inadequate and can account for only about 20% of the observed drag force. A recalculation of the viscous drag including quasiparticle diffusion and relaxation effects is offered as an explanation of this disparity.