728 research outputs found
Modelling magnetic flux emergence in the solar convection zone
[Abridged] Bipolar magnetic regions are formed when loops of magnetic flux
emerge at the solar photosphere. Our aim is to investigate the flux emergence
process in a simulation of granular convection. In particular we aim to
determine the circumstances under which magnetic buoyancy enhances the flux
emergence rate (which is otherwise driven solely by the convective upflows). We
use three-dimensional numerical simulations, solving the equations of
compressible magnetohydrodynamics in a horizontally-periodic Cartesian domain.
A horizontal magnetic flux tube is inserted into fully developed hydrodynamic
convection. We systematically vary the initial field strength, the tube
thickness, the initial entropy distribution along the tube axis and the
magnetic Reynolds number. Focusing upon the low magnetic Prandtl number regime
(Pm<1) at moderate magnetic Reynolds number, we find that the flux tube is
always susceptible to convective disruption to some extent. However, stronger
flux tubes tend to maintain their structure more effectively than weaker ones.
Magnetic buoyancy does enhance the flux emergence rates in the strongest
initial field cases, and this enhancement becomes more pronounced when we
increase the width of the flux tube. This is also the case at higher magnetic
Reynolds numbers, although the flux emergence rates are generally lower in
these less dissipative simulations because the convective disruption of the
flux tube is much more effective in these cases. These simulations seem to be
relatively insensitive to the precise choice of initial conditions: for a given
flow, the evolution of the flux tube is determined primarily by the initial
magnetic field distribution and the magnetic Reynolds number.Comment: 12 pages, 15 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysic
A physical approach to modelling large-scale galactic magnetic fields
A convenient representation of the structure of the large-scale galactic
magnetic field is required for the interpretation of polarization data in the
sub-mm and radio ranges, in both the Milky Way and external galaxies. We
develop a simple and flexible approach to construct parametrised models of the
large-scale magnetic field of the Milky Way and other disc galaxies, based on
physically justifiable models of magnetic field structure. The resulting models
are designed to be optimised against available observational data.
Representations for the large-scale magnetic fields in the flared disc and
spherical halo of a disc galaxy were obtained in the form of series expansions
whose coefficients can be calculated from observable or theoretically known
galactic properties. The functional basis for the expansions is derived as
eigenfunctions of the mean-field dynamo equation or of the vectorial magnetic
diffusion equation. The solutions presented are axially symmetric but the
approach can be extended straightforwardly to non-axisymmetric cases. The
magnetic fields are solenoidal by construction, can be helical, and are
parametrised in terms of observable properties of the host object, such as the
rotation curve and the shape of the gaseous disc. The magnetic field in the
disc can have a prescribed number of field reversals at any specified radii.
Both the disc and halo magnetic fields can separately have either dipolar or
quadrupolar symmetry. The model is implemented as a publicly available software
package GalMag which allows, in particular, the computation of the synchrotron
emission and Faraday rotation produced by the model's magnetic field. The model
can be used in interpretations of observations of magnetic fields in the Milky
Way and other spiral galaxies, in particular as a prior in Bayesian analyses.
(Abridged.)Comment: 20 pages, 14 figures. Accepted for publication in A&
Localised plumes in three-dimensional compressible magnetoconvection
Within the umbrae of sunspots, convection is generally inhibited by the
presence of strong vertical magnetic fields. However, convection is not
completely suppressed in these regions: bright features, known as umbral dots,
are probably associated with weak, isolated convective plumes. Motivated by
observations of umbral dots, we carry out numerical simulations of
three-dimensional, compressible magnetoconvection. By following solution
branches into the subcritical parameter regime (a region of parameter space in
which the static solution is linearly stable to convective perturbations), we
find that it is possible to generate a solution which is characterised by a
single, isolated convective plume. This solution is analogous to the steady
magnetohydrodynamic convectons that have previously been found in
two-dimensional calculations. These results can be related, in a qualitative
sense, to observations of umbral dots.Comment: submitted to MNRA
Interactions between magnetohydrodynamic shear instabilities and convective flows in the solar interior
Motivated by the interface model for the solar dynamo, this paper explores
the complex magnetohydrodynamic interactions between convective flows and
shear-driven instabilities. Initially, we consider the dynamics of a forced
shear flow across a convectively-stable polytropic layer, in the presence of a
vertical magnetic field. When the imposed magnetic field is weak, the dynamics
are dominated by a shear flow (Kelvin-Helmholtz type) instability. For stronger
fields, a magnetic buoyancy instability is preferred. If this stably stratified
shear layer lies below a convectively unstable region, these two regions can
interact. Once again, when the imposed field is very weak, the dynamical
effects of the magnetic field are negligible and the interactions between the
shear layer and the convective layer are relatively minor. However, if the
magnetic field is strong enough to favour magnetic buoyancy instabilities in
the shear layer, extended magnetic flux concentrations form and rise into the
convective layer. These magnetic structures have a highly disruptive effect
upon the convective motions in the upper layer.Comment: 11 pages, 10 figures, accepted for publication in MNRA
Reversible metallisation of soft UV patterned substrates
Soft UV (365 nm) patterning of ortho-nitrobenzyl functionalized thiol-on-gold self-assembled monolayers (SAMs) using acid catalysis, produces surfaces which can be used for the selective electro-deposition of copper. Exploiting the difference in the reduction peak potential between the photolysed and the masked regions of the SAM allows copper to be deposited selectively on those areas that have been exposed to the light. The copper can be removed by raising the electrode potential. The process is fully reversible so that depositing a pattern of copper, and removing it again is something that can be repeated many times. The copper deposited on the photolysed regions, like copper deposited on bare gold, forms a film of copper oxide, and so it is presumably formed on top of the SAM. Preliminary results for two-photon photocleavage show that it is also possible to implement patterning with sub-wavelength features
Convective intensification of magnetic fields in the quiet Sun
Kilogauss-strength magnetic fields are often observed in intergranular lanes at the photosphere in the quiet Sun. Such fields are stronger than the equipartition field B_e, corresponding to a magnetic energy density that matches the kinetic energy density of photospheric convection, and comparable with the field B_p that exerts a magnetic pressure equal to the ambient gas pressure. We present an idealised numerical model of three-dimensional compressible magnetoconvection at the photosphere, for a range of values of the magnetic Reynolds number. In the absence of a magnetic field, the convection is highly supercritical and is characterised by a pattern of vigorous, time-dependent, “granular” motions. When a weak magnetic field is imposed upon the convection, magnetic flux is swept into the convective downflows where it forms localised concentrations. Unless this process is significantly inhibited by magnetic diffusion, the resulting fields are often much greater than B_e, and the high magnetic pressure in these flux elements leads to their being partially evacuated. Some of these flux elements contain ultra-intense magnetic fields that are significantly greater than B_p. Such fields are contained by a combination of the thermal pressure of the gas and the dynamic pressure of the convective motion, and they are constantly evolving. These ultra-intense fields develop owing to nonlinear interactions between magnetic fields and convection; they cannot be explained in terms of “convective collapse” within a thin flux tube that remains in overall pressure equilibrium with its surroundings
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