248 research outputs found
Origin of spatial variations of scattering polarization in the wings of the Ca {\sc i} 4227 \AA line
Polarization that is produced by coherent scattering can be modified by
magnetic fields via the Hanle effect. According to standard theory the Hanle
effect should only be operating in the Doppler core of spectral lines but not
in the wings. In contrast, our observations of the scattering polarization in
the Ca {\sc i} 4227 \AA line reveals the existence of spatial variations of the
scattering polarization throughout the far line wings. This raises the question
whether the observed spatial variations in wing polarization have a magnetic or
non-magnetic origin. A magnetic origin may be possible if elastic collisions
are able to cause sufficient frequency redistribution to make the Hanle effect
effective in the wings without causing excessive collisional depolarization, as
suggested by recent theories for partial frequency redistribution with coherent
scattering in magnetic fields. To model the wing polarization we apply an
extended version of the technique based on the "last scattering approximation".
This model is highly successful in reproducing the observed Stokes
polarization (linear polarization parallel to the nearest solar limb),
including the location of the wing polarization maxima and the minima around
the Doppler core, but it fails to reproduce the observed spatial variations of
the wing polarization in terms of magnetic field effects with frequency
redistribution. This null result points in the direction of a non-magnetic
origin in terms of local inhomogeneities (varying collisional depolarization,
radiation-field anisotropies, and deviations from a plane-parallel atmospheric
stratification).Comment: Accepted in May 2009 for publication in The Astrophysical Journa
Measuring the Hidden Aspects of Solar Magnetism
2008 marks the 100th anniversary of the discovery of astrophysical magnetic
fields, when George Ellery Hale recorded the Zeeman splitting of spectral lines
in sunspots. With the introduction of Babcock's photoelectric magnetograph it
soon became clear that the Sun's magnetic field outside sunspots is extremely
structured. The field strengths that were measured were found to get larger
when the spatial resolution was improved. It was therefore necessary to come up
with methods to go beyond the spatial resolution limit and diagnose the
intrinsic magnetic-field properties without dependence on the quality of the
telescope used. The line-ratio technique that was developed in the early 1970s
revealed a picture where most flux that we see in magnetograms originates in
highly bundled, kG fields with a tiny volume filling factor. This led to
interpretations in terms of discrete, strong-field magnetic flux tubes embedded
in a rather field-free medium, and a whole industry of flux tube models at
increasing levels of sophistication. This magnetic-field paradigm has now been
shattered with the advent of high-precision imaging polarimeters that allow us
to apply the so-called "Second Solar Spectrum" to diagnose aspects of solar
magnetism that have been hidden to Zeeman diagnostics. It is found that the
bulk of the photospheric volume is seething with intermediately strong, tangled
fields. In the new paradigm the field behaves like a fractal with a high degree
of self-similarity, spanning about 8 orders of magnitude in scale size, down to
scales of order 10 m.Comment: 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
Comparison of solar photospheric bright points between SUNRISE observations and MHD simulations
Bright points (BPs) in the solar photosphere are radiative signatures of
magnetic elements described by slender flux tubes located in the darker
intergranular lanes. They contribute to the ultraviolet (UV) flux variations
over the solar cycle and hence may influence the Earth's climate. Here we
combine high-resolution UV and spectro-polarimetric observations of BPs by the
SUNRISE observatory with 3D radiation MHD simulations. Full spectral line
syntheses are performed with the MHD data and a careful degradation is applied
to take into account all relevant instrumental effects of the observations. It
is demonstrated that the MHD simulations reproduce the measured distributions
of intensity at multiple wavelengths, line-of-sight velocity, spectral line
width, and polarization degree rather well. Furthermore, the properties of
observed BPs are compared with synthetic ones. These match also relatively
well, except that the observations display a tail of large and strongly
polarized BPs not found in the simulations. The higher spatial resolution of
the simulations has a significant effect, leading to smaller and more numerous
BPs. The observation that most BPs are weakly polarized is explained mainly by
the spatial degradation, the stray light contamination, and the temperature
sensitivity of the Fe I line at 5250.2 \AA{}. The Stokes asymmetries of the
BPs increase with the distance to their center in both observations and
simulations, consistent with the classical picture of a production of the
asymmetry in the canopy. This is the first time that this has been found also
in the internetwork. Almost vertical kilo-Gauss fields are found for 98 % of
the synthetic BPs. At the continuum formation height, the simulated BPs are on
average 190 K hotter than the mean quiet Sun, their mean BP field strength is
1750 G, supporting the flux-tube paradigm to describe BPs.Comment: Accepted for publication in Astronomy & Astrophysics on May 30 201
On the Sensitivity of Partial Redistribution Scattering Polarization Profiles to Various Atmospheric Parameters
This paper presents a detailed study of the scattering polarization profiles
formed under partial frequency redistribution (PRD) in two thermal models of
the solar atmosphere. Particular attention is given to understanding the
influence of several atmospheric parameters on the emergent fractional linear
polarization profiles. The shapes of these profiles are interpreted in
terms of the anisotropy of the radiation field, which in turn depends on the
source function gradient that sets the angular variation of the specific
intensity. We define a suitable frequency integrated anisotropy factor for PRD
that can be directly related to the emergent linear polarization. We show that
complete frequency redistribution is a good approximation to model weak
resonance lines. We also show that the emergent linear polarization profiles
can be very sensitive to the thermal structure of the solar atmosphere and, in
particular, to spatial variations of the damping parameter.Comment: 45 pages, 16 figures, accepted for publication in the Astrophysical
Journal (2010
First high-resolution images of the Sun in the 2796 \AA{} Mg II k line
We present the first high-resolution solar images in the Mg II k 2796 \AA{}
line. The images, taken through a 4.8 \AA{} broad interference filter, were
obtained during the second science flight of SUNRISE in June 2013 by the SuFI
instrument. The Mg II k images display structures that look qualitatively very
similar to images taken in the core of Ca II H. The Mg II images exhibit
reversed granulation (or shock waves) in the internetwork regions of the quiet
Sun, at intensity contrasts that are similar to those found in Ca II H. Very
prominent in Mg II are bright points, both in the quiet Sun and in plage
regions, particularly near disk center. These are much brighter than at other
wavelengths sampled at similar resolution. Furthermore, Mg II k images also
show fibril structures associated with plage regions. Again, the fibrils are
similar to those seen in Ca II H images, but tend to be more pronounced,
particularly in weak plage.Comment: Accepted for publication in The Astrophysical Journal Letter
Surface waves in solar granulation observed with {\sc Sunrise}
Solar oscillations are expected to be excited by turbulent flows in the
intergranular lanes near the solar surface. Time series recorded by the IMaX
instrument aboard the {\sc Sunrise} observatory reveal solar oscillations at
high resolution, which allow studying the properties of oscillations with short
wavelengths. We analyze two times series with synchronous recordings of Doppler
velocity and continuum intensity images with durations of 32\thinspace min and
23\thinspace min, resp., recorded close to the disk center of the Sun to study
the propagation and excitation of solar acoustic oscillations. In the Doppler
velocity data, both the standing acoustic waves and the short-lived,
high-degree running waves are visible. The standing waves are visible as
temporary enhancements of the amplitudes of the large-scale velocity field due
to the stochastic superposition of the acoustic waves. We focus on the
high-degree small-scale waves by suitable filtering in the Fourier domain.
Investigating the propagation and excitation of - and -modes with wave
numbers \thinspace 1/Mm we find that also exploding granules
contribute to the excitation of solar -modes in addition to the contribution
of intergranular lanes.Comment: 12 pages, 4 figures, to appear in a special volume on Sunrise in
Astrophysical Journal Letter
Detection of vortex tubes in solar granulation from observations with Sunrise
We have investigated a time series of continuum intensity maps and
corresponding Dopplergrams of granulation in a very quiet solar region at the
disk center, recorded with the Imaging Magnetograph eXperiment (IMaX) on board
the balloon-borne solar observatory Sunrise. We find that granules frequently
show substructure in the form of lanes composed of a leading bright rim and a
trailing dark edge, which move together from the boundary of a granule into the
granule itself. We find strikingly similar events in synthesized intensity maps
from an ab initio numerical simulation of solar surface convection. From cross
sections through the computational domain of the simulation, we conclude that
these `granular lanes' are the visible signature of (horizontally oriented)
vortex tubes. The characteristic optical appearance of vortex tubes at the
solar surface is explained. We propose that the observed vortex tubes may
represent only the large-scale end of a hierarchy of vortex tubes existing near
the solar surface.Comment: Astrophysical Journal Letters: Sunrise Special Issue, reveived 2010
June 16; accepted 2010 August
Transverse component of the magnetic field in the solar photosphere observed by Sunrise
We present the first observations of the transverse component of photospheric
magnetic field acquired by the imaging magnetograph Sunrise/IMaX. Using an
automated detection method, we obtain statistical properties of 4536 features
with significant linear polarization signal. Their rate of occurrence is 1-2
orders of magnitude larger than values reported by previous studies. We show
that these features have no characteristic size or lifetime. They appear
preferentially at granule boundaries with most of them being caught in downflow
lanes at some point in their evolution. Only a small percentage are entirely
and constantly embedded in upflows (16%) or downflows (8%).Comment: Accepted for the Sunrise Special Issue of ApJ
Fully resolved quiet-Sun magnetic flux tube observed with the Sunrise IMaX instrument
Until today, the small size of magnetic elements in quiet Sun areas has
required the application of indirect methods, such as the line-ratio technique
or multi-component inversions, to infer their physical properties. A consistent
match to the observed Stokes profiles could only be obtained by introducing a
magnetic filling factor that specifies the fraction of the observed pixel
filled with magnetic field. Here, we investigate the properties of a small
magnetic patch in the quiet Sun observed with the IMaX magnetograph on board
the balloon-borne telescope Sunrise with unprecedented spatial resolution and
low instrumental stray light. We apply an inversion technique based on the
numerical solution of the radiative transfer equation to retrieve the
temperature stratification and the field strength in the magnetic patch. The
observations can be well reproduced with a one-component, fully magnetized
atmosphere with a field strength exceeding 1 kG and a significantly enhanced
temperature in the mid- to upper photosphere with respect to its surroundings,
consistent with semi-empirical flux tube models for plage regions. We therefore
conclude that, within the framework of a simple atmospheric model, the IMaX
measurements resolve the observed quiet-Sun flux tube.Comment: Accepted for publication in The Astrophysical Journal Letters on Aug
11 201
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