43 research outputs found
Mesogranulation and small-scale dynamo action in the quiet Sun
Regions of quiet Sun generally exhibit a complex distribution of small-scale
magnetic field structures, which interact with the near-surface turbulent
convective motions. Furthermore, it is probable that some of these magnetic
fields are generated locally by a convective dynamo mechanism. In addition to
the well-known granular and supergranular convective scales, various
observations have indicated that there is an intermediate scale of convection,
known as mesogranulation, with vertical magnetic flux concentrations
accumulating preferentially at mesogranular boundaries. Our aim is to
investigate the small-scale dynamo properties of a convective flow that
exhibits both granulation and mesogranulation, comparing our findings with
solar observations. Adopting an idealised model for a localised region of quiet
Sun, we use numerical simulations of compressible magnetohydrodynamics, in a 3D
Cartesian domain, to investigate the parametric dependence of this system
(focusing particularly upon the effects of varying the aspect ratio and the
Reynolds number). In purely hydrodynamic convection, we find that
mesogranulation is a robust feature of this system provided that the domain is
wide enough to accommodate these large-scale motions. The mesogranular peak in
the kinetic energy spectrum is more pronounced in the higher Reynolds number
simulations. We investigate the dynamo properties of this system in both the
kinematic and the nonlinear regimes and we find that the dynamo is always more
efficient in larger domains, when mesogranulation is present. Furthermore, we
use a filtering technique in Fourier space to demonstrate that it is indeed the
larger scales of motion that are primarily responsible for driving the dynamo.
In the nonlinear regime, the magnetic field distribution compares very
favourably to observations, both in terms of the spatial distribution and the
measured field strengths.Comment: 12 pages, 11 figures, accepted for publication in Astronomy &
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&
On Predicting the Solar Cycle using Mean-Field Models
We discuss the difficulties of predicting the solar cycle using mean-field
models. Here we argue that these difficulties arise owing to the significant
modulation of the solar activity cycle, and that this modulation arises owing
to either stochastic or deterministic processes. We analyse the implications
for predictability in both of these situations by considering two separate
solar dynamo models. The first model represents a stochastically-perturbed flux
transport dynamo. Here even very weak stochastic perturbations can give rise to
significant modulation in the activity cycle. This modulation leads to a loss
of predictability. In the second model, we neglect stochastic effects and
assume that generation of magnetic field in the Sun can be described by a fully
deterministic nonlinear mean-field model -- this is a best case scenario for
prediction. We designate the output from this deterministic model (with
parameters chosen to produce chaotically modulated cycles) as a target
timeseries that subsequent deterministic mean-field models are required to
predict. Long-term prediction is impossible even if a model that is correct in
all details is utilised in the prediction. Furthermore, we show that even
short-term prediction is impossible if there is a small discrepancy in the
input parameters from the fiducial model. This is the case even if the
predicting model has been tuned to reproduce the output of previous cycles.
Given the inherent uncertainties in determining the transport coefficients and
nonlinear responses for mean-field models, we argue that this makes predicting
the solar cycle using the output from such models impossible.Comment: 22 Pages, 5 Figures, Preprint accepted for publication in Ap