Fast Zonal Field Dynamo in Collisionless Kinetic Alfven Wave Turbulence

The possibility of fast dynamo action by collisionless kinetic Alfven Wave turbulence is demonstrated. The irreversibility necessary to lock in the generated field is provided by electron Landau damping, so the induced electric field does not vanish with resistivity. Mechanisms for self-regulation of the system and the relation of these results to the theory of alpha quenching are discussed. The dynamo-generated fields have symmetry like to that of zonal flows, and thus are termed zonal fields

Convection-Dominated Accretion Flows

Non-radiating, advection-dominated, accretion flows are convectively unstable. We calculate the two-dimensional (r-theta) structure of such flows assuming that (1) convection transports angular momentum inwards, opposite to normal viscosity and (2) viscous transport by other mechanisms (e.g., magnetic fields) is weak (alpha << 1). Under such conditions convection dominates the dynamics of the accretion flow and leads to a steady state structure that is marginally stable to convection. We show that the marginally stable flow has a constant temperature and rotational velocity on spherical shells, a net flux of energy from small to large radii, zero net accretion rate, and a radial density profile proportional to r^{-1/2}, flatter than the r^{-3/2} profile characteristic of spherical accretion flows. This solution accurately describes the full two-dimensional structure of recent axisymmetric numerical simulations of advection-dominated accretion flows.Comment: final version accepted by ApJ; discussion expanded, references adde

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

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

An algorithm for solving the pulsar equation

We present an algorithm of finding numerical solutions of pulsar equation. The problem of finding the solutions was reduced to finding expansion coefficients of the source term of the equation in a base of orthogo- nal functions defined on the unit interval by minimizing a multi-variable mismatch function defined on the light cylinder. We applied the algorithm to Scharlemann & Wagoner boundary conditions by which a smooth solu- tion is reconstructed that by construction passes success- fully the Gruzinov's test of the source function exponent.Comment: 4 pages, 4 figures, accepted for publication in ApSS (a shortened version of the previous one

Graviton Mass or Cosmological Constant?

To describe a massive graviton in 4D Minkowski space-time one introduces a quadratic term in the Lagrangian. This term, however, can lead to a readjustment or instability of the background instead of describing a massive graviton on flat space. We show that for all local Lorentz-invariant mass terms Minkowski space is unstable. We start with the Pauli-Fierz (PF) term that is the only local mass term with no ghosts in the linearized approximation. We show that nonlinear completions of the PF Lagrangian give rise to instability of Minkowski space. We continue with the mass terms that are not of a PF type. Although these models are known to have ghosts in the linearized approximations, nonlinear interactions can lead to background change due to which the ghosts are eliminated. In the latter case, however, the graviton perturbations on the new background are not massive. We argue that a consistent theory of a massive graviton on flat space can be formulated in theories with extra dimensions. They require an infinite number of fields or non-local description from a 4D point of view.Comment: 16 pages; references and comments adde

The pulsar synchrotron: coherent radio emission

We propose a simple physical picture for the generation of coherent radio emission in the axisymmetric pulsar magnetosphere that is quite different from the canonical paradigm of radio emission coming from the magnetic polar caps. In this first paper we consider only the axisymmetric case of an aligned rotator. Our picture capitalizes on an important element of the MHD representation of the magnetosphere, namely the separatrix between the corotating closed-line region (the `dead zone') and the open field lines that originate in the polar caps. Along the separatrix flows the return current that corresponds to the main magnetospheric electric current emanating from the polar caps. Across the separatrix, both the toroidal and poloidal components of the magnetic field change discontinuously. The poloidal component discontinuity requires the presence of a significant annular electric current which has up to now been unaccounted for. We estimate the position and thickness of this annular current at the tip of the closed line region, and show that it consists of electrons (positrons) corotating with Lorentz factors on the order of 10^5, emitting incoherent synchrotron radiation that peaks in the hard X-rays. These particles stay in the region of highest annular current close to the equator for a path-length of the order of one meter. We propose that, at wavelengths comparable to that path-length, the particles emit coherent radiation, with radiated power proportional to N^2, where N is the population of particles in the above path-length. We calculate the total radio power in this wavelength regime and its scaling with pulsar period and stellar magnetic field and show that it is consistent with estimates of radio luminosity based on observations.Comment: Monthly Notices Letters, in pres

Force-free magnetosphere of an aligned rotator with differential rotation of open magnetic field lines

Here we briefly report on results of self-consistent numerical modeling of a differentially rotating force-free magnetosphere of an aligned rotator. We show that differential rotation of the open field line zone is significant for adjusting of the global structure of the magnetosphere to the current density flowing through the polar cap cascades. We argue that for most pulsars stationary cascades in the polar cap can not support stationary force-free configurations of the magnetosphere.Comment: 5 pages, 4 figures. Presented at the conference "Isolated Neutron Stars: from the Interior to the Surface", London, April 24-28, 2006; to appear in Astrophysics and Space Science. Significantly revised version, a mistake found by ourselfs in the numerical code was corrected, all presented results are obtained with the correct version of the cod

Ten per cent polarized optical emission from GRB 090102

The nature of the jets and the role of magnetic fields in gamma-ray bursts (GRB) remains unclear. In a baryon-dominated jet only weak, tangled fields generated in situ through shocks would be present. In an alternative model, jets are threaded with large scale magnetic fields that originate at the central engine and which accelerate and collimate the jets. The way to distinguish between the models is to measure the degree of polarization in early-time emission, however previous claims of gamma-ray polarization have been controversial. Here we report that the early optical emission from GRB 090102 was polarized at the level of P=10+/-1%, indicating the presence of large-scale fields originating in the expanding fireball. If the degree of polarization and its position angle were variable on timescales shorter than our 60-s exposure, then the peak polarization may have been larger than 10 per cent.Comment: 16 pages, 4 figures. Published in Nature (2009), Vol. 462, p767-76

Nonlinear Turbulent Magnetic Diffusion and Mean-Field Dynamo

The nonlinear coefficients defining the mean electromotive force (i.e., the nonlinear turbulent magnetic diffusion, the nonlinear effective velocity, the nonlinear kappa-tensor, etc.) are calculated for an anisotropic turbulence. A particular case of an anisotropic background turbulence (i.e., the turbulence with zero mean magnetic field) with one preferential direction is considered. It is shown that the toroidal and poloidal magnetic fields have different nonlinear turbulent magnetic diffusion coefficients. It is demonstrated that even for a homogeneous turbulence there is a nonlinear effective velocity which exhibits diamagnetic or paramagnetic properties depending on anisotropy of turbulence and level of magnetic fluctuations in the background turbulence. Analysis shows that an anisotropy of turbulence strongly affects the nonlinear mean electromotive force. Two types of nonlinearities (algebraic and dynamic) are also discussed. The algebraic nonlinearity implies a nonlinear dependence of the mean electromotive force on the mean magnetic field. The dynamic nonlinearity is determined by a differential equation for the magnetic part of the alpha-effect. It is shown that for the alpha-Omega axisymmetric dynamo the algebraic nonlinearity alone cannot saturate the dynamo generated mean magnetic field while the combined effect of the algebraic and dynamic nonlinearities limits the mean magnetic field growth. Astrophysical applications of the obtained results are discussed.Comment: 15 pages, REVTEX