296 research outputs found
Hydrodynamic simulations of shell convection in stellar cores
Shell convection driven by nuclear burning in a stellar core is a common
hydrodynamic event in the evolution of many types of stars. We encounter and
simulate this convection (i) in the helium core of a low-mass red giant during
core helium flash leading to a dredge-down of protons across an entropy
barrier, (ii) in a carbon-oxygen core of an intermediate-mass star during core
carbon flash, and (iii) in the oxygen and carbon burning shell above the
silicon-sulfur rich core of a massive star prior to supernova explosion. Our
results, which were obtained with the hydrodynamics code HERAKLES, suggest that
both entropy gradients and entropy barriers are less important for stellar
structure than commonly assumed. Our simulations further reveal a new dynamic
mixing process operating below the base of shell convection zones.Comment: 8 pages, 3 figures .. submitted to a proceedings of conference about
"Red Giants as Probes of the Structure and Evolution of the Milky Way" which
has taken place between 15-17 November 2010 in Rom
Simulation of the Formation of a Solar Active Region
We present a radiative magnetohydrodynamics simulation of the formation of an
Active Region on the solar surface. The simulation models the rise of a buoyant
magnetic flux bundle from a depth of 7.5 Mm in the convection zone up into the
solar photosphere. The rise of the magnetic plasma in the convection zone is
accompanied by predominantly horizontal expansion. Such an expansion leads to a
scaling relation between the plasma density and the magnetic field strength
such that . The emergence of magnetic flux into the
photosphere appears as a complex magnetic pattern, which results from the
interaction of the rising magnetic field with the turbulent convective flows.
Small-scale magnetic elements at the surface first appear, followed by their
gradual coalescence into larger magnetic concentrations, which eventually
results in the formation of a pair of opposite polarity spots. Although the
mean flow pattern in the vicinity of the developing spots is directed radially
outward, correlations between the magnetic field and velocity field
fluctuations allow the spots to accumulate flux. Such correlations result from
the Lorentz-force driven, counter-streaming motion of opposite-polarity
fragments. The formation of the simulated Active Region is accompanied by
transient light bridges between umbrae and umbral dots. Together with recent
sunspot modeling, this work highlights the common magnetoconvective origin of
umbral dots, light bridges and penumbral filaments.Comment: Accepted for publication in Ap
Downward pumping of magnetic flux as the cause of filamentary structures in sunspot penumbrae
The structure of a sunspot is determined by the local interaction between magnetic fields and convection near the Sun's surface. The dark central umbra is surrounded by a filamentary penumbra, whose complicated fine structure has only recently been revealed by high-resolution observations. The penumbral magnetic field has an intricate and unexpected interlocking-comb structure and some field lines, with associated outflows of gas, dive back down below the solar surface at the outer edge of the spot. These field lines might be expected to float quickly back to the surface because of magnetic buoyancy, but they remain submerged. Here we show that the field lines are kept submerged outside the spot by turbulent, compressible convection, which is dominated by strong, coherent, descending plumes. Moreover, this downward pumping of magnetic flux explains the origin of the interlocking-comb structure of the penumbral magnetic field, and the behaviour of other magnetic features near the sunspot
Analysis of the shearing instability in nonlinear convection and magnetoconvection
Numerical experiments on two-dimensional convection with or without a vertical magnetic field reveal a bewildering variety of periodic and aperiodic oscillations. Steady rolls can develop a shearing instability, in which rolls turning over in one direction grow at the expense of rolls turning over in the other, resulting in a net shear across the layer. As the temperature difference across the fluid is increased, two-dimensional pulsating waves occur, in which the direction of shear alternates. We analyse the nonlinear dynamics of this behaviour by first constructing appropriate low-order sets of ordinary differential equations, which show the same behaviour, and then analysing the global bifurcations that lead to these oscillations by constructing one-dimensional return maps. We compare the behaviour of the partial differential equations, the models and the maps in systematic two-parameter studies of both the magnetic and the non-magnetic cases, emphasising how the symmetries of periodic solutions change as a result of global bifurcations. Much of the interesting behaviour is associated with a discontinuous change in the leading direction of a fixed point at a global bifurcation; this change occurs when the magnetic field is introduced
Gravity Waves in the Sun
We present numerical simulations of penetrative convection and gravity wave
excitation in the Sun. Gravity waves are self-consistently generated by a
convective zone overlying a radiative interior. We produce power spectra for
gravity waves in the radiative region as well as estimates for the energy flux
of gravity waves below the convection zone. We calculate a peak energy flux in
waves below the convection zone to be three orders of magnitude smaller than
previous estimates for m=1. The simulations show that the linear dispersion
relation is a good approximation only deep below the convective-radiative
boundary. Both low frequency propagating gravity waves as well as higher
frequency standing modes are generated; although we find that convection does
not continually drive the standing g-mode frequencies.Comment: 22 pages, 14 figures, submitted to MNRA
Magnetoconvection and dynamo coefficients: Dependence of the alpha-effect on rotation and magnetic field
We present numerical simulations of three-dimensional compressible
magnetoconvection in a rotating rectangular box that represents a section of
the solar convection zone. The box contains a convectively unstable layer,
surrounded by stably stratified layers with overshooting convection. The
magnetic Reynolds number, Rm, is chosen subcritical, thus excluding spontaneous
growth of the magnetic field through dynamo action, and the magnetic energy is
maintained by introducing a constant magnetic field into the box, once
convection has attained a statistically stationary state. Under the influence
of the Coriolis force, the advection of the magnetic field results in a
non-vanishing contribution to the mean electric field, given by uxb. From this
electric field, we calculate the alpha-effect, separately for the stably and
the unstably stratified layers, by averaging over time and over suitably
defined volumes. From the variation of alpha we derive an error estimate, and
the dependence of alpha on rotation and magnetic field strength is studied.
Evidence is found for rotational quenching of the vertical alpha-effect, and
for a monotonic increase of the horizontal alpha-effect with increasing
rotation. For Rm~30, our results for both vertical and horizontal alpha-effect
are consistent with magnetic quenching by a factor 1/[1+Rm(B_0/B_eq)^2]. The
signs of the small-scale current helicity and of the vertical component of
alpha are found to be opposite to those for isotropic turbulence.Comment: 14 pages, 11 figures; to appear in Astronomy & Astrophysics
(accepted
Astrophysical turbulence modeling
The role of turbulence in various astrophysical settings is reviewed. Among
the differences to laboratory and atmospheric turbulence we highlight the
ubiquitous presence of magnetic fields that are generally produced and
maintained by dynamo action. The extreme temperature and density contrasts and
stratifications are emphasized in connection with turbulence in the
interstellar medium and in stars with outer convection zones, respectively. In
many cases turbulence plays an essential role in facilitating enhanced
transport of mass, momentum, energy, and magnetic fields in terms of the
corresponding coarse-grained mean fields. Those transport properties are
usually strongly modified by anisotropies and often completely new effects
emerge in such a description that have no correspondence in terms of the
original (non coarse-grained) fields.Comment: 88 pages, 26 figures, published in Reports on Progress in Physic
Theoretical Models of Sunspot Structure and Dynamics
Recent progress in theoretical modeling of a sunspot is reviewed. The
observed properties of umbral dots are well reproduced by realistic simulations
of magnetoconvection in a vertical, monolithic magnetic field. To understand
the penumbra, it is useful to distinguish between the inner penumbra, dominated
by bright filaments containing slender dark cores, and the outer penumbra, made
up of dark and bright filaments of comparable width with corresponding magnetic
fields differing in inclination by some 30 degrees and strong Evershed flows in
the dark filaments along nearly horizontal or downward-plunging magnetic
fields. The role of magnetic flux pumping in submerging magnetic flux in the
outer penumbra is examined through numerical experiments, and different
geometric models of the penumbral magnetic field are discussed in the light of
high-resolution observations. Recent, realistic numerical MHD simulations of an
entire sunspot have succeeded in reproducing the salient features of the
convective pattern in the umbra and the inner penumbra. The siphon-flow
mechanism still provides the best explanation of the Evershed flow,
particularly in the outer penumbra where it often consists of cool, supersonic
downflows.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
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