761 research outputs found
Response to "Stray-light correction in 2D spectroscopy" by R. Schlichenmaier and M. Franz
We discuss a recent paper by Schlichenmaier & Franz (SF; 2013, A&A, 555,
A84), in which the claim is made that the penumbral dark downflows detected for
the first time with the Swedish 1-m Solar Telescope (SST) by Scharmer et al.
and Joshi et al. could be produced by overcompensation for straylight. We show
that the analysis of SF is fundamentally flawed, because it ignores the
constraints on the strength of such straylight from 3D convection simulations
and on the spatial extent of the straylight point spread function from the
measured minimum intensity in the sunspot umbra. Furthermore, we show that the
claim made by SF, that the spatial straylight of Hinode is less than 10%, is
false. We conclude that the analysis of SF is of no relevance in relation to
the straylight compensation method applied to the SST data.Comment: Published in A&A. Rewording of a few sentences, one sentence remove
Upflows in the central dark lane of sunspot light bridges
We use high spatial and spectral resolution observations obtained with the
CRisp Imaging SpectroPolarimeter at the Swedish 1-m Solar Telescope to analyze
the velocity profile of granular light bridges in a sunspot. We find upflows
associated with the central dark lanes of the light bridges. From bisectors in
the Fe I 630.15 nm line we find that the magnitude of the upflows varies with
height with the strongest upflows being deeper in the atmosphere. Typical
upflow velocities measured from the 70% bisector are around 500 m/s with peaks
above 1 km/s. The upflows in the central dark lane are surrounded by downflows
of weaker magnitude, sometimes concentrated in patches with enhanced velocities
reaching up to 1.1 km/s. A small spatial offset between the upflows and the
continuum dark lane is interpreted as a line-of-sight effect due to the
elevated nature of the dark lane and the light bridge above the umbral
surroundings. Our observations show that the central dark lane in granular
light bridges is not equivalent to the intergranular lanes of normal
photospheric granulation that host convective downflows. These results support
recent MHD simulations of magneto-convection in sunspot atmospheres.Comment: Accepted for publication in Astrophysical Journal Letter
Striation and convection in penumbral filaments
Observations with the 1-m Swedish Solar Telescope of the flows seen in
penumbral filaments are presented. Time sequences of bright filaments show
overturning motions strikingly similar to those seen along the walls of small
isolated structures in the active regions. The filaments show outward
propagating striations with inclination angles suggesting that they are aligned
with the local magnetic field. We interpret it as the equivalent of the
striations seen in the walls of small isolated magnetic structures. Their
origin is then a corrugation of the boundary between an overturning convective
flow inside the filament and the magnetic field wrapping around it. The outward
propagation is a combination of a pattern motion due to the downflow observed
along the sides of bright filaments, and the Evershed flow. The observed short
wavelength of the striation argues against the existence of a dynamically
significant horizontal field inside the bright filaments. Its intensity
contrast is explained by the same physical effect that causes the dark cores of
filaments, light bridges and `canals'. In this way striation represents an
important clue to the physics of penumbral structure and its relation with
other magnetic structures on the solar surface. We put this in perspective with
results from the recent 3-D radiative hydrodynamic simulations.Comment: Accepted for publication in A&
Evidence for convection in Sunspot penumbrae
We present an analysis of twisting motions in penumbral filaments in sunspots
located at heliocentric angles from to using three time
series of blue continuum images obtained by the Broadband Filter Imager (BFI)
onboard {\it Hinode}. The relations of the twisting motions to the filament
brightness and the position within the filament and within the penumbra,
respectively, are investigated. Only certain portions of the filaments show
twisting motions. In a statistical sense, the part of the twisting portion of a
filament located closest to the umbra is brightest and possesses the fastest
twisting motion, with a mean twisting velocity of 2.1\,km\,s. The middle
and outer sections of the twisting portion of the filament (lying increasingly
further from the umbra), which are less bright, have mean velocities of
1.7\,km\,s and 1.35\,km\,s, respectively. The observed reduction
of brightness and twisting velocity towards the outer section of the filaments
may be due to reducing upflow along the filament's long axis. No significant
variation of twisting velocity as a function of viewing angles was found. The
obtained correlation of brightness and velocity suggests that overturning
convection causes the twisting motions observed in penumbral filament and may
be the source of the energy needed to maintain the brightness of the filaments.Comment: Accepted for publication in ApJL on 13th September 201
SST/CRISP Observations of Convective Flows in a Sunspot Penumbra
Context. Recent discoveries of intensity correlated downflows in the interior
of a sunspot penumbra provide direct evidence for overturning convection,
adding to earlier strong indications of convection from filament dynamics
observed far from solar disk center, and supporting recent simulations of
sunspots.
Aims. Using spectropolarimetric observations obtained at a spatial resolution
approaching 0'.'1 with the Swedish 1-m Solar Telescope (SST) and its
spectropolarimeter CRISP, we investigate whether the convective downflows
recently discovered in the C i line at 538.03 nm can also be detected in the
wings of the Fe i line at 630.15 nm
Methods. We make azimuthal fits of the measured LOS velocities in the core
and wings of the 538 nm and 630 nm lines to disentangle the vertical and
horizontal flows. To investigate how these depend on the continuum intensity,
the azimuthal fits are made separately for each intensity bin. By using
spatially high-pass filtered measurements of the LOS component of the magnetic
field, the flow properties are determined separately for magnetic spines
(relatively strong and vertical field) and inter-spines (weaker and more
horizontal field).
Results. The dark convective downflows discovered recently in the 538.03 nm
line are evident also in the 630.15 nm line, and have similar strength. This
convective signature is the same in spines and inter-spines. However, the
strong radial (Evershed) outflows are found only in the inter-spines.
Conclusions. At the spatial resolution of the present SST/CRISP data, the
small-scale intensity pattern seen in continuum images is strongly related to a
convective up/down flow pattern that exists everywhere in the penumbra. Earlier
failures to detect the dark convective downflows in the interior penumbra can
be explained by inadequate spatial resolution in the observed data.Comment: Revised and expanded by 2.5 pages. Fig. 14 adde
The Validity Issue in Applied General Equilibrium Tax Models
Series: Department of Economics Working Paper Serie
Instrument and data analysis challenges for imaging spectropolarimetry
The next generation of solar telescopes will enable us to resolve the
fundamental scales of the solar atmosphere, i.e., the pressure scale height and
the photon mean free path. High-resolution observations of small-scale
structures with sizes down to 50 km require complex post-focus instruments,
which employ adaptive optics (AO) and benefit from advanced image restoration
techniques. The GREGOR Fabry-Perot Interferometer (GFPI) will serve as an
example of such an instrument to illustrate the challenges that are to be
expected in instrumentation and data analysis with the next generation of solar
telescopes.Comment: 4 pages, 1 figure, accepted for publication in Astronomische
Nachrichten/AN, special issue of the 1st EAST-ATST Workshop: "Science with
large solar telescopes" in Freiburg, Germany, October 14-16, 200
Spectropolarimetry with CRISP at the Swedish 1-m Solar Telescope
CRISP (Crisp Imaging Spectro-polarimeter), the new spectropolarimeter at the
Swedish 1-m Solar Telescope, opens a new perspective in solar polarimetry. With
better spatial resolution (0.13") than Hinode in the Fe I 6302 A line and
similar polarimetric sensitivity reached through postprocessing, CRISP
complements the SP spectropolarimeter onboard Hinode. We present some of the
data which we obtained in our June 2008 campaign and preliminary results from
LTE inversions of a pore containing umbral dots.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
High-order aberration compensation with Multi-frame Blind Deconvolution and Phase Diversity image restoration techniques
Context. For accurately measuring intensities and determining magnetic field
strengths of small-scale solar (magnetic) structure, knowledge of and
compensation for the point spread function is crucial. For images recorded with
the Swedish 1-meter Solar Telescope, restoration with Multi-Frame Blind
Deconvolution and Joint Phase Diverse Speckle methods lead to remarkable
improvements in image quality but granulation contrasts that are too low,
indicating additional stray light. Aims. We propose a method to compensate for
stray light from high-order atmospheric aberrations not included in MFBD and
JPDS processing. Methods. To compensate for uncorrected aberrations, a
reformulation of the image restoration process is proposed that allows the
average effect of hundreds of high-order modes to be compensated for by relying
on Kolmogorov statistics for these modes. The applicability of the method
requires simultaneous measurements of Fried's parameter r0. The method is
tested with simulations as well as real data and extended to include
compensation for conventional stray light. Results. We find that only part of
the reduction of granulation contrast in SST images is due to uncompensated
high-order aberrations. The remainder is still unaccounted for and attributed
to stray light from the atmosphere, the telescope with its re-imaging system
and to various high-altitude seeing effects. Conclusions. We conclude that
statistical compensation of high-order modes is a viable method to reduce the
loss of contrast occurring when a limited number of aberrations is explicitly
compensated for with MFBD and JPDS processing. We show that good such
compensation is possible with only 10 recorded frames. The main limitation of
the method is that already MFBD and JPDS processing introduces high-order
compensation that, if not taken into account, can lead to over-compensation.Comment: in press in Astronomy & Astrophysic
- …