76 research outputs found
Enhanced Joule Heating in Umbral Dots
We present a study of magnetic profiles of umbral dots (UDs) and its
consequences on the Joule heating mechanisms. Hamedivafa (2003) studied Joule
heating using vertical component of magnetic field. In this paper UDs magnetic
profile has been investigated including the new azimuthal component of magnetic
field which might explain the relatively larger enhancement of Joule heating
causing more brightness near circumference of UD.Comment: 8 pages, 1 figure, accepted in Solar Physic
Comparison of the thin flux tube approximation with 3D MHD simulations
The structure and dynamics of small vertical photospheric magnetic flux
concentrations has been often treated in the framework of an approximation
based upon a low-order truncation of the Taylor expansions of all quantities in
the horizontal direction, together with the assumption of instantaneous total
pressure balance at the boundary to the non-magnetic external medium. Formally,
such an approximation is justified if the diameter of the structure (a flux
tube or a flux sheet) is small compared to all other relevant length scales
(scale height, radius of curvature, wavelength, etc.). The advent of realistic
3D radiative MHD simulations opens the possibility of checking the consistency
of the approximation with the properties of the flux concentrations that form
in the course of a simulation.
We carry out a comparative analysis between the thin flux tube/sheet models
and flux concentrations formed in a 3D radiation-MHD simulation. We compare the
distribution of the vertical and horizontal components of the magnetic field in
a 3D MHD simulation with the field distribution in the case of the thin flux
tube/sheet approximation. We also consider the total (gas plus magnetic)
pressure in the MHD simulation box. Flux concentrations with
super-equipartition fields are reasonably well reproduced by the second-order
thin flux tube/sheet approximation. The differences between approximation and
simulation are due to the asymmetry and the dynamics of the simulated
structures
Oscillatory Modes of a Prominence-PCTR-Corona Slab Model
Oscillations of magnetic structures in the solar corona have often been
interpreted in terms of magnetohydrodynamic waves. We study the adiabatic
magnetoacoustic modes of a prominence plasma slab with a uniform longitudinal
magnetic field, surrounded by a prominence-corona transition region (PCTR) and
a coronal medium. Considering linear small-amplitude oscillations, the
dispersion relation for the magnetoacoustic slow and fast modes is deduced
assuming evanescent-like perturbations in the coronal medium. In the system
without PCTR, a classification of the oscillatory modes according to the
polarisation of their eigenfunctions is made in order to distinguish modes with
fast-like or slow-like properties. Internal and external slow modes are
governed by the prominence and coronal properties respectively, and fast modes
are mostly dominated by prominence conditions for the observed wavelengths. In
addition, the inclusion of an isothermal PCTR does not substantially influence
the mode frequencies, but new solutions (PCTR slow modes) are present.Comment: Accepted for publication in Solar Physic
Relationships between magnetic foot points and G-band bright structures
Magnetic elements are thought to be described by flux tube models, and are
well reproduced by MHD simulations. However, these simulations are only
partially constrained by observations. We observationally investigate the
relationship between G-band bright points and magnetic structures to clarify
conditions, which make magnetic structures bright in G-band. The G-band
filtergrams together with magnetograms and dopplergrams were taken for a plage
region covered by abnormal granules as well as ubiquitous G-band bright points,
using the Swedish 1-m Solar Telescope (SST) under very good seeing conditions.
High magnetic flux density regions are not necessarily associated with G-band
bright points. We refer to the observed extended areas with high magnetic flux
density as magnetic islands to separate them from magnetic elements. We
discover that G-band bright points tend to be located near the boundary of such
magnetic islands. The concentration of G-band bright points decreases with
inward distance from the boundary of the magnetic islands. Moreover, G-band
bright points are preferentially located where magnetic flux density is higher,
given the same distance from the boundary. There are some bright points located
far inside the magnetic islands. Such bright points have higher minimum
magnetic flux density at the larger inward distance from the boundary.
Convective velocity is apparently reduced for such high magnetic flux density
regions regardless of whether they are populated by G-band bright points or
not. The magnetic islands are surrounded by downflows.These results suggest
that high magnetic flux density, as well as efficient heat transport from the
sides or beneath, are required to make magnetic elements bright in G-band.Comment: 9 pages, 14 figures, accepted for publication in A&
SUMER observations of the inverse Evershed effect in the transition region above a sunspot
Aims. We analyse SUMER spectral scans of a large sunspot within active region
NOAA 10923, obtained on 14-15 November 2006, to determine the morphology and
dynamics of the sunspot atmosphere at different heights/temperatures. Methods:
The data analysed here consist of spectroheliograms in the continuum around
142.0 nm and in the Si iv 140.2 nm, O iii 70.3 nm, N iv 76.5 nm, and O iv 79.0
nm spectral lines. Gaussian-fitting of the observed profiles provides
line-of-sight velocity and Doppler-width maps. Results: The data show an
asymmetric downflow pattern compatible with the presence of the inverse
Evershed flow in a region within roughly twice the penumbral radius at
transition-region temperatures up to 0.18 MK. The motions, highly inhomogeneous
on small scales, seem to occur in a collar of radially directed filamentary
structures, with an average width less than the 1 Mm spatial resolution of
SUMER and characterised by different plasma speeds. Assuming that the flows are
directed along the field lines, we deduce that such field lines are inclined by
10 deg to 25 deg with respect to the solar surface.Comment: 5 pages, 3 figures. Text height reduced for better layout. No changes
in conten
Photometric properties of resolved and unresolved magnetic elements
We investigate the photometric signature of magnetic flux tubes in the solar
photosphere. We developed two dimensional, static numerical models of isolated
and clustered magnetic flux tubes. We investigated the emergent intensity
profiles at different lines-of-sight for various spatial resolutions and
opacity models. We found that both geometric and photometric properties of
bright magnetic features are determined not only by the physical properties of
the tube and its surroundings, but also by the particularities of the
observations, including the line/continuum formation height, the spatial
resolution and the image analyses techniques applied. We show that some
observational results presented in the literature can be interpreted by
considering bright magnetic features to be clusters of smaller elements, rather
than a monolithic flux tube.Comment: 12 page
Discovery of inward moving magnetic enhancements in sunspot penumbrae
Sunspot penumbrae show a fine structure in continuum intensity that displays
considerable dynamics. The magnetic field, in contrast, although also highly
structured, has appeared to be relatively static. Here we report the discovery
of inward moving magnetic enhancements in the penumbrae of two regular sunspots
based on time series of SOHO/MDI magnetograms. Local enhancements of the LOS
component of the magnetic field in the inner part of the penumbral region move
inward to the umbra-penumbra boundary with a radial speed of about 0.3 km
s. These local inward-moving enhancements of the LOS component of the
magnetic fields appear to be relatively common. They are associated with dark
structures and tend to display downflows relatively to the penumbral
background. Possible explanations are discussed.Comment: 4 pages, 4 figures, submitted to ApJ Letter
Stokes diagnostics of simulated solar magneto-convection
We present results of synthetic spectro-polarimetric diagnostics of radiative
MHD simulations of solar surface convection with magnetic fields. Stokes
profiles of Zeeman-sensitive lines of neutral iron in the visible and infrared
spectral ranges emerging from the simulated atmosphere have been calculated in
order to study their relation to the relevant physical quantities and compare
with observational results. We have analyzed the dependence of the Stokes-I
line strength and width as well as of the Stokes-V signal and asymmetries on
the magnetic field strength. Furthermore, we have evaluated the correspondence
between the actual velocities in the simulation with values determined from the
Stokes-I (Doppler shift of the centre of gravity) and Stokes-V profiles
(zero-crossing shift). We confirm that the line weakening in strong magnetic
fields results from a higher temperature (at equal optical depth) in the
magnetic flux concentrations. We also confirm that considerable Stokes-V
asymmetries originate in the peripheral parts of strong magnetic flux
concentrations, where the line of sight cuts through the magnetopause of the
expanding flux concentration into the surrounding convective donwflow.Comment: Astronomy & Astrophysics, in pres
A retrospective of the GREGOR solar telescope in scientific literature
In this review, we look back upon the literature, which had the GREGOR solar
telescope project as its subject including science cases, telescope subsystems,
and post-focus instruments. The articles date back to the year 2000, when the
initial concepts for a new solar telescope on Tenerife were first presented at
scientific meetings. This comprehensive bibliography contains literature until
the year 2012, i.e., the final stages of commissioning and science
verification. Taking stock of the various publications in peer-reviewed
journals and conference proceedings also provides the "historical" context for
the reference articles in this special issue of Astronomische
Nachrichten/Astronomical Notes.Comment: 6 pages, 2 color figures, this is the pre-peer reviewed version of
Denker et al. 2012, Astron. Nachr. 333, 81
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