Effect of Fractional Kinetic Helicity on Turbulent Magnetic Dynamo Spectra

Magnetic field amplification in astrophysics ultimately requires an understanding of magnetohydrodynamic turbulence. Kinetic helicity has long been known to be important for large scale field growth in forced MHD turbulence, and has been recently demonstrated numerically to be asymptotically consistent with slow mean field dynamo action in a periodic box. Here we show numerically that the magnetic spectrum at and below the forcing scale is also strongly influenced by kinetic helicity. We identify a critical value, $f_{h,crit}$ above which the magnetic spectrum develops maxima at wavenumber $= 1$ scale {\it and} at the forcing scale, For $f< f_{h,crit}$ the field peaks only at the resistive scale. Kinetic helicity may thus be important not only for generating a large scale field, but also for establishing observed peaks in magnetic spectra at the forcing scale. The turbulent Galactic disk provides an example where both large scale ($>$ supernova forcing scale) fields and small scale ($\le$ forcing scale, with peak at forcing scale) fields are observed. We discuss this, and the potential application to the protogalaxy, but also emphasize the limitations in applying our results to these systems.Comment: version accepted to ApJL, 10 pages, 3 fig

Implications of mean field accretion disc theory for vorticity and magnetic field growth

In addition to the scalar Shakura-Sunyaev $\alpha_{ss}$ turbulent viscosity transport term used in simple analytic accretion disc modeling, a pseudoscalar transport term also arises. The essence of this term can be captured even in simple models for which vertical averaging is interpreted as integration over a half-thickness and one separately studies each hemisphere. The additional term highlights a complementarity between mean field magnetic dynamo theory and accretion disc theory treated as a mean field theory. Such pseudoscalar terms have been studied, and can lead to large scale magnetic field and vorticity growth. Here it is shown that vorticity can grow even in the simplest azimuthal and half-height integrated disc model, for which mean quantities depend only on radius. The simplest vorticity growth solutions seem to have scales and vortex survival times consistent those required for facilitating planet formation. Also it is shown that when the magnetic back-reaction is included to lowest order, the pseudoscalar driving the magnetic field growth and that driving the vorticity growth will behave differently with respect to shearing and non-shearing flows: the former can reverse sign in the two cases, while the latter will have the same sign.Comment: 17 Pages LaTex, revised versio

Star-forming accretion flows and the low luminosity nuclei of giant elliptical galaxies

The luminosities of the centers of nearby elliptical galaxies are very low compared to models of thin disc accretion to their black holes at the Bondi rate, typically a few hundredths to a few tenths of a solar mass per year. This has motivated models of inefficiently-radiated accretion that invoke weak electron-ion thermal coupling, and/or inhibited accretion rates due to convection or outflows. Here we point out that even if such processes are operating, a significant fraction of the accreting gas is prevented from reaching the central black hole because it condenses into stars in a gravitationally unstable disc. Star formation occurs inside the Bondi radius (typically ~100pc in giant ellipticals), but still relatively far from the black hole in terms of Schwarzschild radii. Star formation depletes and heats the gas disc, eventually leading to a marginally stable, but much reduced, accretion flow to the black hole. We predict the presence of cold (~100K), dusty gas discs, containing clustered H-alpha emission and occasional type II supernovae, both resulting from the presence of massive stars. Star formation accounts for several features of the M87 system: a thin disc, traced by H-alpha emission, is observed on scales of about 100pc, with features reminiscent of spiral arms and dust lanes; the star formation rate inferred from the intensity of H-alpha emission is consistent with the Bondi accretion rate of the system. Star formation may therefore help suppress accretion onto the central engines of massive ellipticals. We also discuss some implications for the fueling of the Galactic center and quasars.Comment: 13 pages, accepted to MNRA

Identifying Deficiencies of Standard Accretion Disk Theory: Lessons from a Mean-Field Approach

Turbulent viscosity is frequently used in accretion disk theory to replace the microphysical viscosity in order to accomodate the observational need for in- stabilities in disks that lead to enhanced transport. However, simply replacing the microphysical transport coefficient by a single turbulent transport coeffi- cient hides the fact that the procedure should formally arise as part of a closure in which the hydrodynamic or magnetohydrodynamic equations are averaged, and correlations of turbulent fluctuations are replaced by transport coefficients. Here we show how a mean field approach leads quite naturally two transport coefficients, not one, that govern mass and angular momentum transport. In particular, we highlight that the conventional approach suffers from a seemingly inconsistent neglect of turbulent diffusion in the surface density equation. We constrain these new transport coefficients for specific cases of inward, outward, and zero net mass transport. In addition, we find that one of the new transport terms can lead to oscillations in the mean surface density which then requires a constant or small inverse Rossby number for disks to maintain a monotonic power-law surface density.Comment: 11 page

Simulations of a Magnetic Fluctuation Driven Large Scale Dynamo and Comparison with a Two-scale Model

Models of large scale (magnetohydrodynamic) dynamos (LSD) which couple large scale field growth to total magnetic helicity evolution best predict the saturation of LSDs seen in simulations. For the simplest so called "{\alpha}2" LSDs in periodic boxes, the electromotive force driving LSD growth depends on the difference between the time-integrated kinetic and current helicity associated with fluctuations. When the system is helically kinetically forced (KF), the growth of the large scale helical field is accompanied by growth of small scale magnetic (and current) helicity which ultimately quench the LSD. Here, using both simulations and theory, we study the complementary magnetically forced(MF) case in which the system is forced with an electric field that supplies magnetic helicity. For this MF case, the kinetic helicity becomes the back-reactor that saturates the LSD. Simulations of both MF and KF cases can be approximately modeled with the same equations of magnetic helicity evolution, but with complementary initial conditions. A key difference between KF and MF cases is that the helical large scale field in the MF case grows with the same sign of injected magnetic helicity, whereas the large and small scale magnetic helicities grow with opposite sign for the KF case. The MF case can arise even when the thermal pressure is approximately smaller than the magnetic pressure, and requires only that helical small scale magnetic fluctuations dominate helical velocity fluctuations in LSD driving. We suggest that LSDs in accretion discs and Babcock models of the solar dynamo are actually MF LSDs.Comment: 12 pages, 34 figure