173 research outputs found
Nonlinear Alpha Effect in Dynamo Theory
We extend the standard two-scale theory of the turbulent dynamo coefficient
to include the nonlinear back reaction of the mean field on
the turbulence. We calculate the turbulent emf as a power series in ,
assuming that the base state of the turbulence () is isotropic, and,
for simplicity, that the magnetic diffusivity equals the kinematic viscosity.
The power series converges for all , and for the special case that the
spectrum of the turbulence is sharply peaked in , our result is proportional
to a tabulated function of the magnetic Reynolds number and the ratio
of (in velocity units) to the rms turbulent velocity .
For (linear regime) we recover the results of Steenbeck et al.
(1966) as modified by Pouquet et al. (1976). For , the usual
astrophysical case, starts to decrease at , dropping
like as . Hence for large ,
saturates at , as estimated by Kraichnan (1979), rather than at
, as inferred by Cattaneo and Hughes (1996) from
their numerical simulations at =100. We plan to carry out simulations with
various values of to investigate the discrepency.Comment: 41 pages, 1 Postscript figure, accepted for publication to Ap
Uniqueness of the compactly supported weak solutions of the relativistic Vlasov-Darwin system
We use optimal transportation techniques to show uniqueness of the compactly
supported weak solutions of the relativistic Vlasov-Darwin system. Our proof
extends the method used by Loeper in J. Math. Pures Appl. 86, 68-79 (2006) to
obtain uniqueness results for the Vlasov-Poisson system.Comment: AMS-LaTeX, 21 page
Force-free twisted magnetospheres of neutron stars
The X-ray spectra observed in the persistent emission of magnetars are
evidence for the existence of a magnetosphere. The high-energy part of the
spectra is explained by resonant cyclotron upscattering of soft thermal photons
in a twisted magnetosphere, which has motivated an increasing number of efforts
to improve and generalize existing magnetosphere models. We want to build more
general configurations of twisted, force-free magnetospheres as a first step to
understanding the role played by the magnetic field geometry in the observed
spectra. First we reviewed and extended previous analytical works to assess the
viability and limitations of semi-analytical approaches. Second, we built a
numerical code able to relax an initial configuration of a nonrotating
magnetosphere to a force-free geometry, provided any arbitrary form of the
magnetic field at the star surface. The numerical code is based on a
finite-difference time-domain, divergence-free, and conservative scheme, based
of the magneto-frictional method used in other scenarios. We obtain new
numerical configurations of twisted magnetospheres, with distributions of twist
and currents that differ from previous analytical solutions. The range of
global twist of the new family of solutions is similar to the existing
semi-analytical models (up to some radians), but the achieved geometry may be
quite different. The geometry of twisted, force-free magnetospheres shows a
wider variety of possibilities than previously considered. This has
implications for the observed spectra and opens the possibility of implementing
alternative models in simulations of radiative transfer aiming at providing
spectra to be compared with observations.Comment: 16 pages, 17 figures, A&A accepte
Emission heights of coronal bright points on Fe XII radiance map
We study the emission heights of the coronal bright points (BPs) above the
photosphere in the bipolar magnetic loops that are apparently associated with
them. As BPs are seen in projection against the disk their true emission
heights are unknown. The correlation of the BP locations on the Fe XII radiance
map from EIT with the magnetic field features (in particular neutral lines) was
investigated in detail. The coronal magnetic field was determined by an
extrapolation of the photospheric field to different altitudes above the disk.
It was found that most BPs sit on or near a photospheric neutral line, but that
the emission occurs at a height of about 5 Mm. Some BPs, while being seen in
projection, still seem to coincide with neutral lines, although their emission
takes place at heights of more than 10 Mm. Such coincidences almost disappear
for emissions above 20 Mm. We also projected the upper segments of the 3-D
magnetic field lines above different heights, respectively, on to the x-y
plane. The shape of each BP was compared with the respective field-line segment
nearby. This comparison suggests that most coronal BPs are actually located on
the top of their associated magnetic loops. Finally, we calculated for each
selected BP region the correlation coefficient between the Fe XII intensity
enhancement and the horizontal component of the extrapolated magnetic field
vector at the same x-y position in planes of different heights, respectively.
We found that for almost all the BP regions we studied the correlation
coefficient, with increasing height, increases to a maximal value and then
decreases again. The height corresponding to this maximum was defined as the
correlation height, which for most bright points was found to range below 20
Mm.Comment: 7 pages, 4 figures, 1 tabl
Dynamo generated field emergence through recurrent plasmoid ejections
Magnetic buoyancy is believed to drive the transport of magnetic flux tubes
from the convection zone to the surface of the Sun. The magnetic fields form
twisted loop-like structures in the solar atmosphere. In this paper we use
helical forcing to produce a large-scale dynamo-generated magnetic field, which
rises even without magnetic buoyancy. A two layer system is used as
computational domain where the upper part represents the solar atmosphere.
Here, the evolution of the magnetic field is solved with the stress--and--relax
method. Below this region a magnetic field is produced by a helical forcing
function in the momentum equation, which leads to dynamo action. We find
twisted magnetic fields emerging frequently to the outer layer, forming
arch-like structures. In addition, recurrent plasmoid ejections can be found by
looking at space--time diagrams of the magnetic field. Recent simulations in
spherical coordinates show similar results.Comment: 4 pages, 8 figures, To appear in the proceedings of the IAU273
"Physics of Sun and Star Spots
Flat galaxies with dark matter halos - existence and stability
We consider a model for a flat, disk-like galaxy surrounded by a halo of dark
matter, namely a Vlasov-Poisson type system with two particle species, the
stars which are restricted to the galactic plane and the dark matter particles.
These constituents interact only through the gravitational potential which
stars and dark matter create collectively. Using a variational approach we
prove the existence of steady state solutions and their nonlinear stability
under suitably restricted perturbations.Comment: 39 page
Comparing magnetic field extrapolations with measurements of magnetic loops
We compare magnetic field extrapolations from a photospheric magnetogram with
the observationally inferred structure of magnetic loops in a newly developed
active region. This is the first time that the reconstructed 3D-topology of the
magnetic field is available to test the extrapolations. We compare the
observations with potential fields, linear force-free fields and non-linear
force-free fields. This comparison reveals that a potential field extrapolation
is not suitable for a reconstruction of the magnetic field in this young,
developing active region. The inclusion of field-line-parallel electric
currents, the so called force-free approach, gives much better results.
Furthermore, a non-linear force-free computation reproduces the observations
better than the linear force-free approximation, although no free parameters
are available in the former case.Comment: 5 pages, 3 figure
Helicity at Photospheric and Chromospheric Heights
In the solar atmosphere the twist parameter has the same sign as
magnetic helicity. It has been observed using photospheric vector magnetograms
that negative/positive helicity is dominant in the northern/southern hemisphere
of the Sun. Chromospheric features show dextral/sinistral dominance in the
northern/southern hemisphere and sigmoids observed in X-rays also have a
dominant sense of reverse-S/forward-S in the northern/southern hemisphere. It
is of interest whether individual features have one-to-one correspondence in
terms of helicity at different atmospheric heights. We use UBF \Halpha images
from the Dunn Solar Telescope (DST) and other \Halpha data from Udaipur Solar
Observatory and Big Bear Solar Observatory. Near-simultaneous vector
magnetograms from the DST are used to establish one-to-one correspondence of
helicity at photospheric and chromospheric heights. We plan to extend this
investigation with more data including coronal intensities.Comment: 5 pages, 1 figure, 1 table 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
How to use magnetic field information for coronal loop identification?
The structure of the solar corona is dominated by the magnetic field because
the magnetic pressure is about four orders of magnitude higher than the plasma
pressure. Due to the high conductivity the emitting coronal plasma (visible
e.g. in SOHO/EIT) outlines the magnetic field lines. The gradient of the
emitting plasma structures is significantly lower parallel to the magnetic
field lines than in the perpendicular direction. Consequently information
regarding the coronal magnetic field can be used for the interpretation of
coronal plasma structures. We extrapolate the coronal magnetic field from
photospheric magnetic field measurements into the corona. The extrapolation
method depends on assumptions regarding coronal currents, e.g. potential fields
(current free) or force-free fields (current parallel to magnetic field). As a
next step we project the reconstructed 3D magnetic field lines on an EIT-image
and compare with the emitting plasma structures. Coronal loops are identified
as closed magnetic field lines with a high emissivity in EIT and a small
gradient of the emissivity along the magnetic field.Comment: 14 pages, 3 figure
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