Suppression of Cross-Field Transport of a Passive Scalar in Two-Dimensional Magnetohydrodynamic Turbulence

The theory of passive scalar transport in two dimensional turbulent fluids is generalized to the case of 2D MHD. Invariance of the cross correlation of scalar concentration and magnetic potential produces a novel contribution to the concentration flux. This pinch effect is proportional to the mean potential gradient, and is shown to drastically reduce transport of the passive scalar across the mean magnetic field when . Transport parallel to the mean magnetic field is unchanged. Implications for models of transport in turbulent magnetofluids are discussed. PAC NOS. 47.25.Jn, 47.65.+aComment: uuencoded compressed postscript fil

Small-scale-field Dynamo

Generation of magnetic field energy, without mean field generation, is studied. Isotropic mirror-symmetric turbulence of a conducting fluid amplifies the energy of small-scale magnetic perturbations if the magnetic Reynolds number is high, and the dimensionality of space d satisfies 2.103 < d <8.765. The result does not depend on the model of turbulence, incompressibility and isotropy being the only requirements.Comment: 11 pages Plain TeX, no figures, Accepted by Phys. Rev. Let

Turbulence and Particle Heating in Advection-Dominated Accretion Flows

We extend and reconcile recent work on turbulence and particle heating in advection-dominated accretion flows. For approximately equipartition magnetic fields, the turbulence primarily heats the electrons. For weaker magnetic fields, the protons are primarily heated. The division between electron and proton heating occurs between $\beta \sim 5$ and $\beta \sim 100$ (where $\beta$ is the ratio of the gas to the magnetic pressure), depending on unknown details of how Alfv\'en waves are converted into whistlers on scales of the proton Larmor radius. We also discuss the possibility that magnetic reconnection could be a significant source of electron heating.Comment: 17 pages (Latex), incl. 2 Figures; submitted to Ap

Radiative Efficiency of Collisionless Accretion

Radiative efficiency of a slowly accreting black hole is estimated using a two-temperature model of accretion. The radiative efficiency depends on the magnetic field strength near the Schwarzschild radius. For weak magnetic fields (magnetic energy=equipartition/1000), the low efficiency 0.0001 assumed in some theoretical models might be achieved. For stronger fields, a significant fraction of viscous heat is dissipated by electrons and radiated away resulting in a larger efficiency. At equipartition magnetic fields, we estimate efficiency = of order 10%.Comment: 12 pages, Latex, Submitted to Ap