4,147 research outputs found
Kinematic alpha effect in isotropic turbulence simulations
Using numerical simulations at moderate magnetic Reynolds numbers up to 220
it is shown that in the kinematic regime, isotropic helical turbulence leads to
an alpha effect and a turbulent diffusivity whose values are independent of the
magnetic Reynolds number, \Rm, provided \Rm exceeds unity. These turbulent
coefficients are also consistent with expectations from the first order
smoothing approximation. For small values of \Rm, alpha and turbulent
diffusivity are proportional to \Rm. Over finite time intervals meaningful
values of alpha and turbulent diffusivity can be obtained even when there is
small-scale dynamo action that produces strong magnetic fluctuations. This
suggests that small-scale dynamo-generated fields do not make a correlated
contribution to the mean electromotive force.Comment: Accepted for publication in MNRAS Letter
Compensating Faraday depolarization by magnetic helicity in the solar corona
A turbulent dynamo in spherical geometry with an outer corona is simulated to
study the sign of magnetic helicity in the outer parts. In agreement with
earlier studies, the sign in the outer corona is found to be opposite to that
inside the dynamo. Line-of-sight observations of polarized emission are
synthesized to explore the feasibility of using the local reduction of Faraday
depolarization to infer the sign of helicity of magnetic fields in the solar
corona. This approach was previously identified as an observational diagnostic
in the context of galactic magnetic fields. Based on our simulations, we show
that this method can be successful in the solar context if sufficient
statistics is gathered by using averages over ring segments in the corona
separately for the regions north and south of the solar equator.Comment: 5 pages, 3 figures, published in ApJ
Magnetic helicity and cosmological magnetic field
The magnetic helicity has paramount significance in nonlinear saturation of
galactic dynamo. We argue that the magnetic helicity conservation is violated
at the lepton stage in the evolution of early Universe. As a result, a
cosmological magnetic field which can be a seed for the galactic dynamo obtains
from the beginning a substantial magnetic helicity which has to be taken into
account in the magnetic helicity balance at the later stage of galactic dynamo.Comment: 11 pages, no figures; v3: new references and new paragraphs added,
discussion extended, some mistypings correcte
Astrophysical significance of the anisotropic kinetic alpha effect
The generation of large scale flows by the anisotropic kinetic alpha (AKA)
effect is investigated in simulations with a suitable time-dependent space- and
time-periodic anisotropic forcing lacking parity invariance. The forcing
pattern moves relative to the fluid, which leads to a breaking of the Galilean
invariance as required for the AKA effect to exist. The AKA effect is found to
produce a clear large scale flow pattern when the Reynolds number, R, is small
as only a few modes are excited in linear theory. In this case the
non-vanishing components of the AKA tensor are dynamically independent of the
Reynolds number. For larger values of R, many more modes are excited and the
components of the AKA tensor are found to decrease rapidly with increasing
value of R. However, once there is a magnetic field (imposed and of sufficient
strength, or dynamo-generated and saturated) the field begins to suppress the
AKA effect, regardless of the value of R. It is argued that the AKA effect is
unlikely to be astrophysically significant unless the magnetic field is weak
and R is small.Comment: 8 pages, 10 figures, submitted to A&
Identification of gravity waves in hydrodynamical simulations
The excitation of internal gravity waves by an entropy bubble oscillating in
an isothermal atmosphere is investigated using direct two-dimensional numerical
simulations. The oscillation field is measured by a projection of the simulated
velocity field onto the anelastic solutions of the linear eigenvalue problem
for the perturbations. This facilitates a quantitative study of both the
spectrum and the amplitudes of excited g-modes.Comment: 12 pages, 11 figures, Appendices only available onlin
Determination of biaxial creep strength of T-111 tantalum alloy
Biaxial creep strength of T-111 tantalum alloy tubing in high temperature, high vacuum environmen
The inverse cascade and nonlinear alpha-effect in simulations of isotropic helical hydromagnetic turbulence
A numerical model of isotropic homogeneous turbulence with helical forcing is
investigated. The resulting flow, which is essentially the prototype of the
alpha^2 dynamo of mean-field dynamo theory, produces strong dynamo action with
an additional large scale field on the scale of the box (at wavenumber k=1;
forcing is at k=5). This large scale field is nearly force-free and exceeds the
equipartition value. As the magnetic Reynolds number R_m increases, the
saturation field strength and the growth rate of the dynamo increase. However,
the time it takes to built up the large scale field from equipartition to its
final super-equipartition value increases with magnetic Reynolds number. The
large scale field generation can be identified as being due to nonlocal
interactions originating from the forcing scale, which is characteristic of the
alpha-effect. Both alpha and turbulent magnetic diffusivity eta_t are
determined simultaneously using numerical experiments where the mean-field is
modified artificially. Both quantities are quenched in a R_m-dependent fashion.
The evolution of the energy of the mean field matches that predicted by an
alpha^2 dynamo model with similar alpha and eta_t quenchings. For this model an
analytic solution is given which matches the results of the simulations. The
simulations are numerically robust in that the shape of the spectrum at large
scales is unchanged when changing the resolution from 30^3 to 120^3 meshpoints,
or when increasing the magnetic Prandtl number (viscosity/magnetic diffusivity)
from 1 to 100. Increasing the forcing wavenumber to 30 (i.e. increasing the
scale separation) makes the inverse cascade effect more pronounced, although it
remains otherwise qualitatively unchanged.Comment: 21 pages, 26 figures, ApJ (accepted
Nonaxisymmetric stability in the shearing sheet approximation
Aims: To quantify the transient growth of nonaxisymmetric perturbations in
unstratified magnetized and stratified non-magnetized rotating linear shear
flows in the shearing sheet approximation of accretion disc flows. Method: The
Rayleigh quotient in modal approaches for the linearized equations (with
time-dependent wavenumber) and the amplitudes from direct shearing sheet
simulations using a finite difference code are compared. Results: Both
approaches agree in their predicted growth behavior. The magneto-rotational
instability for axisymmetric and non-axisymmetric perturbations is shown to
have the same dependence of the (instantaneous) growth rate on the wavenumber
along the magnetic field, but in the nonaxisymmetric case the growth is only
transient. However, a meaningful dependence of the Rayleigh quotient on the
radial wavenumber is obtained. While in the magnetized case the total
amplification factor can be several orders of magnitude, it is only of order
ten or less in the nonmagnetic case. Stratification is shown to have a
stabilizing effect. In the present case of shearing-periodic boundaries the
(local) strato-rotational instability seems to be absent.Comment: 8 pages, 7 figures, A&A (in press
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