34,675 research outputs found
The Power of Low Frequencies: Faraday Tomography in the sub-GHz regime
Faraday tomography, the study of the distribution of extended polarized
emission by strength of Faraday rotation, is a powerful tool for studying
magnetic fields in the interstellar medium of our Galaxy and nearby galaxies.
The strong frequency dependence of Faraday rotation results in very different
observational strengths and limitations for different frequency regimes. I
discuss the role these effects take in Faraday tomography below 1 GHz,
emphasizing the 100-200 MHz band observed by the Low Frequency Array and the
Murchison Widefield Array. With that theoretical context, I review recent
Faraday tomography results in this frequency regime, and discuss expectations
for future observations.Comment: 12 pages, 4 figures. Accepted for publication in Galaxies as part of
the special issue "The Power of Faraday Tomography
Helioseismology of Sunspots: Confronting Observations with Three-Dimensional MHD Simulations of Wave Propagation
The propagation of solar waves through the sunspot of AR 9787 is observed
using temporal cross-correlations of SOHO/MDI Dopplergrams. We then use
three-dimensional MHD numerical simulations to compute the propagation of wave
packets through self-similar magneto-hydrostatic sunspot models. The
simulations are set up in such a way as to allow a comparison with observed
cross-covariances (except in the immediate vicinity of the sunspot). We find
that the simulation and the f-mode observations are in good agreement when the
model sunspot has a peak field strength of 3 kG at the photosphere, less so for
lower field strengths. Constraining the sunspot model with helioseismology is
only possible because the direct effect of the magnetic field on the waves has
been fully taken into account. Our work shows that the full-waveform modeling
of sunspots is feasible.Comment: 21 pages, Accepted in Solar Physic
SLiM: a code for the simulation of wave propagation through an inhomogeneous, magnetised solar atmosphere
In this paper we describe the semi-spectral linear MHD (SLiM) code which we
have written to follow the interaction of linear waves through an inhomogeneous
three-dimensional solar atmosphere. The background model allows almost
arbitrary perturbations of density, temperature, sound speed as well as
magnetic and velocity fields. We give details of several of the tests we have
used to check the code. The code will be useful in understanding the
helioseismic signatures of various solar features, including sunspots.Comment: 6 pages, 7 figure
Convectively stabilised background solar models for local helioseismology
In local helioseismology numerical simulations of wave propagation are useful
to model the interaction of solar waves with perturbations to a background
solar model. However, the solution to the equations of motions include
convective modes that can swamp the waves we are interested in. For this
reason, we choose to first stabilise the background solar model against
convection by altering the vertical pressure gradient. Here we compare the
eigenmodes of our convectively stabilised model with a standard solar model
(Model S) and find a good agreement.Comment: 3 pages, 3 figures, HELAS NA3, The Acoustic Solar Cycle, Birmingham,
6-8 January 200
SLiM: a code for the simulation of wave propagation through an inhomogeneous, magnetised solar atmosphere
In this paper we describe the semi-spectral linear MHD (SLiM) code which we
have written to follow the interaction of linear waves through an inhomogeneous
three-dimensional solar atmosphere. The background model allows almost
arbitrary perturbations of density, temperature, sound speed as well as
magnetic and velocity fields. We give details of several of the tests we have
used to check the code. The code will be useful in understanding the
helioseismic signatures of various solar features, including sunspots.Comment: 6 pages, 7 figure
Physical conditions in the primitive solar nebula
Physical conditions for model of primitive solar nebul
Bridge number, Heegaard genus and non-integral Dehn surgery
We show there exists a linear function w: N->N with the following property.
Let K be a hyperbolic knot in a hyperbolic 3-manifold M admitting a
non-longitudinal S^3 surgery. If K is put into thin position with respect to a
strongly irreducible, genus g Heegaard splitting of M then K intersects a thick
level at most 2w(g) times. Typically, this shows that the bridge number of K
with respect to this Heegaard splitting is at most w(g), and the tunnel number
of K is at most w(g) + g-1.Comment: 76 page, 48 figures; referee comments incorporated and typos fixed;
accepted at TAM
The mechanics of a chain or ring of spherical magnets
Strong magnets, such as neodymium-iron-boron magnets, are increasingly being
manufactured as spheres. Because of their dipolar characters, these spheres can
easily be arranged into long chains that exhibit mechanical properties
reminiscent of elastic strings or rods. While simple formulations exist for the
energy of a deformed elastic rod, it is not clear whether or not they are also
appropriate for a chain of spherical magnets. In this paper, we use
discrete-to-continuum asymptotic analysis to derive a continuum model for the
energy of a deformed chain of magnets based on the magnetostatic interactions
between individual spheres. We find that the mechanical properties of a chain
of magnets differ significantly from those of an elastic rod: while both
magnetic chains and elastic rods support bending by change of local curvature,
nonlocal interaction terms also appear in the energy formulation for a magnetic
chain. This continuum model for the energy of a chain of magnets is used to
analyse small deformations of a circular ring of magnets and hence obtain
theoretical predictions for the vibrational modes of a circular ring of
magnets. Surprisingly, despite the contribution of nonlocal energy terms, we
find that the vibrations of a circular ring of magnets are governed by the same
equation that governs the vibrations of a circular elastic ring
- …