1,245 research outputs found
Galactic and Accretion Disk Dynamos
Dynamos in astrophysical disks are usually explained in terms of the standard
alpha-omega mean field dynamo model where the local helicity generates a radial
field component from an azimuthal field. The subsequent shearing of the radial
field gives rise to exponentially growing dynamo modes. There are several
problems with this model. The exponentiation time for the galactic dynamo is
hard to calculate, but is probably uncomfortably long. Moreover, numerical
simulations of magnetic fields in shearing flows indicate that the presence of
a dynamo does not depend on a non-zero average helicity. However, these
difficulties can be overcome by including a fluctuating helicity driven by
hydrodynamic or magnetic instabilities. Unlike traditional disk dynamo models,
this `incoherent' dynamo does not depend on the presence of systematic fluid
helicity or any kind of vertical symmetry breaking. It will depend on geometry,
in the sense that the dynamo growth rate becomes smaller for very thin disks,
in agreement with constraints taken from the study of X-ray novae. In this
picture the galactic dynamo will operate efficiently, but the resulting field
will have a radial coherence length which is a fraction of the galactic radius.Comment: 16 pages, in Proceedings of the Chapman Conference on Magnetic
Helicit
Microbial ecology of extreme environments: Antarctic dry valley yeasts and growth in substrate-limited habitats
The success of the Antarctic Dry Valley yeasts presumeably results from adaptations to multiple stresses, to low temperatures and substrate-limitation as well as prolonged resting periods enforced by low water availability. Previous investigations have suggested that the crucial stress is substrate limitation. Specific adaptations may be pinpointed by comparing the physiology of the Cryptococcus vishniacii complex, the yeasts of the Tyrol Valley, with their congeners from other habitats. Progress was made in methods of isolation and definition of ecological niches, in the design of experiments in competition for limited substrate, and in establishing the relationships of the Cryptococcus vishniacii complex with other yeasts. In the course of investigating relationships, a new method for 25SrRNA homology was developed. For the first time it appears that 25SrRNA homology may reflect parallel or convergent evolution
The Saturation Limit of the Magnetorotational Instability
Simulations of the magnetorotational instability (MRI) in a homogeneous
shearing box have shown that the asymptotic strength of the magnetic field
declines steeply with increasing resolution. Here I model the MRI driven dynamo
as a large scale dynamo driven by the vertical magnetic helicity flux. This
growth is balanced by large scale mixing driven by a secondary instability. The
saturated magnetic energy density depends almost linearly on the vertical
height of the typical eddies. The MRI can drive eddies with arbitrarily large
vertical wavenumber, so the eddy thickness is either set by diffusive effects,
by the magnetic tension of a large scale vertical field component, or by
magnetic buoyancy effects. In homogeneous, zero magnetic flux, simulations only
the first effect applies and the saturated limit of the dynamo is determined by
explicit or numerical diffusion. The exact result depends on the numerical
details, but is consistent with previous work, including the claim that the
saturated field energy scales as the gas pressure to the one quarter power
(which we interpret as an artifact of numerical dissipation). The magnetic
energy density in a homogeneous shearing box will tend to zero as the
resolution of the simulation increases, but this has no consequences for the
dynamo or for angular momentum transport in real accretion disks. The claim
that the saturated state depends on the magnetic Prandtl number may also be an
artifact of simulations in which microphysical transport coefficients set the
MRI eddy thickness. Finally, the efficiency of the MRI dynamo is a function of
the ratio of the Alfv\'en velocity to the product of the pressure scale height
and the local shear. As this approaches unity from below the dynamo reaches
maximum efficiency.Comment: Accepted by The Astrophysical Journa
Magnetorotational turbulence transports angular momentum in stratified disks with low magnetic Prandtl number but magnetic Reynolds number above a critical value
The magnetorotational instability (MRI) may dominate outward transport of
angular momentum in accretion disks, allowing material to fall onto the central
object. Previous work has established that the MRI can drive a mean-field
dynamo, possibly leading to a self-sustaining accretion system. Recently,
however, simulations of the scaling of the angular momentum transport parameter
\alphaSS with the magnetic Prandtl number \Prandtl have cast doubt on the
ability of the MRI to transport astrophysically relevant amounts of angular
momentum in real disk systems. Here, we use simulations including explicit
physical viscosity and resistivity to show that when vertical stratification is
included, mean field dynamo action operates, driving the system to a
configuration in which the magnetic field is not fully helical. This relaxes
the constraints on the generated field provided by magnetic helicity
conservation, allowing the generation of a mean field on timescales independent
of the resistivity. Our models demonstrate the existence of a critical magnetic
Reynolds number \Rmagc, below which transport becomes strongly
\Prandtl-dependent and chaotic, but above which the transport is steady and
\Prandtl-independent. Prior simulations showing \Prandtl-dependence had
\Rmag < \Rmagc. We conjecture that this steady regime is possible because the
mean field dynamo is not helicity-limited and thus does not depend on the
details of the helicity ejection process. Scaling to realistic astrophysical
parameters suggests that disks around both protostars and stellar mass black
holes have \Rmag >> \Rmagc. Thus, we suggest that the strong \Prandtl
dependence seen in recent simulations does not occur in real systems.Comment: 17 pages, 9 figures. as accepted to Ap
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