154 research outputs found
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&
Magnetostatic penumbra models with field-free gaps
We present numerical 2D magnetostatic models for sunspot penumbrae consisting of radially aligned field-free gaps in a potential magnetic field, as proposed by Spruit and Scharmer (2006). The shape of the gaps and the field configurations around them are computed consistently from the condition of magnetostatic pressure balance between the gap and the magnetic field. The results show that field-free gaps in the {\it inner} penumbra are cusp-shaped and bounded by a magnetic field inclined by about from the vertical. Here, the magnetic component has a Wilson depression on the order 200--300 km relative to the top of the field-free gap; the gaps should thus appear as noticeably elevated features. This structure explains the large variations in field strength in the inner penumbra inferred from magnetograms and two-component inversions, and the varying appearance of the inner penumbra with viewing angle. In the {\it outer} penumbra, on the other hand, the gaps are flat-topped with a horizontal magnetic field above the middle of the gap. The magnetic field has large inclination variations horizontally, but only small fluctuations in field strength, in agreement with observations
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
Small-scale convection signatures associated with strong plage solar magnetic field
In this work, we study and quantify properties of strong-field small-scale
convection and compare observed properties with those predicted by numerical
simulations. We analyze spectropolarimetric 630.25 nm data from a unipolar
ephemeral region near sun center. We use line-of-sight velocities and magnetic
field measurements obtained with Milne-Eddington inversion techniques along
with measured continuum intensities and Stokes V amplitude asymmetry at a
spatial resolution of 0.15 arcseconds to establish statistical relations
between the measured quantities. We also study these properties for different
types of distinct magnetic features, such as micropores, bright points,
ribbons, flowers and strings. We present the first direct observations of a
small-scale granular magneto-convection pattern within extended regions of
strong (more than 600 G average) magnetic field. Along the boundaries of the
flux concentrations we see mostly downflows and asymmetric Stokes V profiles,
consistent with synthetic line profiles calculated from MHD simulations. We
note the frequent occurrence of bright downflows along these boundaries. In the
interior of the flux concentrations, we observe an up/down flow pattern that we
identify as small-scale magnetoconvection, appearing similar to that of
field-free granulation but with scales 4 times smaller. Measured RMS velocities
are 70% of those of nearby field-free granulation, even though the average
radiative flux is not reduced. The interiors of these flux concentrations are
dominated by upflows.Comment: Accepted for publication in Astronomy and Astrophysic
CRISP Spectropolarimetric Imaging of Penumbral Fine Structure
We discuss penumbral fine structure in a small part of a pore, observed with
the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope (SST),
close to its diffraction limit of 0.16 arcsec. Milne-Eddington inversions
applied to these Stokes data reveal large variations of field strength and
inclination angle over dark-cored penumbral intrusions and a dark-cored light
bridge. The mid-outer part of this penumbra structure shows 0.3 arcsec wide
spines, separated by 1.6 arcsec (1200 km) and associated with 30 deg
inclination variations. Between these spines, there are no small-scale magnetic
structures that easily can be be identified with individual flux tubes. A
structure with nearly 10 deg more vertical and weaker magnetic field is seen
midways between two spines. This structure is co-spatial with the brightest
penumbral filament, possibly indicating the location of a convective upflow
from below.Comment: Accepted for publication in ApJL 17 Oct 2008. One Figure adde
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