728 research outputs found
Measuring the Wilson depression of sunspots using the divergence-free condition of the magnetic field vector
Context: The Wilson depression is the difference in geometric height of unit
continuum optical depth between the sunspot umbra and the quiet Sun. Measuring
the Wilson depression is important for understanding the geometry of sunspots.
Current methods suffer from systematic effects or need to make assumptions on
the geometry of the magnetic field. This leads to large systematic
uncertainties of the derived Wilson depressions.
Aims: We aim at developing a robust method for deriving the Wilson depression
that only requires the information about the magnetic field that is accessible
from spectropolarimetry, and that does not rely on assumptions on the geometry
of sunspots or on their magnetic field.
Methods: Our method is based on minimizing the divergence of the magnetic
field vector derived from spectropolarimetric observations. We focus on large
spatial scales only in order to reduce the number of free parameters.
Results: We test the performance of our method using synthetic Hinode data
derived from two sunspot simulations. We find that the maximum and the umbral
averaged Wilson depression for both spots determined with our method typically
lies within 100 km of the true value obtained from the simulations. In
addition, we apply the method to Hinode observations of a sunspot. The derived
Wilson depression (about 600 km) is consistent with results typically obtained
from the Wilson effect. We also find that the Wilson depression obtained from
using horizontal force balance gives 110 - 180 km smaller Wilson depressions
than both, what we find and what we deduce directly from the simulations. This
suggests that the magnetic pressure and the magnetic curvature force contribute
to the Wilson depression by a similar amount.Comment: 12 pages, 8 figures. Accepted for publication in Astronomy &
Astrophysic
No universal connection between the vertical magnetic field and the umbra-penumbra boundary in sunspots
Context. It has been reported that the boundary between the umbra and the
penumbra of sunspots occurs at a canonical value of the strength of the
vertical magnetic field, independently of the size of the spot. This critical
field strength is interpreted as to be the threshold for the onset of
magnetoconvection.
Aims. Here we investigate the reasons why this criterion, also called the
Jur\v{c}\'ak criterion in the literature, does not always identify the boundary
between umbra and penumbra.
Methods. We perform a statistical analysis of 23 sunspots observed with
Hinode/SOT. We compare the properties of the continuum intensity and the
vertical magnetic field between filaments and spines and how they vary between
spots of different sizes.
Results. We find that the inner boundary of the penumbra is not related to a
universal value of the vertical magnetic field. The properties of spines and
filaments vary between spots of different sizes. Both components are darker in
larger spots and the spines exhibit stronger vertical magnetic field. These
variations of the properties of filaments and spines with spot size are also
the reason for the reported invariance of the averaged vertical magnetic field
at 50% of the mean continuum intensity.
Conclusions. The formation of filaments and the onset of magnetoconvection
are not related to a canonical value of the strength of the vertical magnetic
field. Such a seemingly unique magnetic field strength is rather an effect of
the filling factor of spines and penumbral filaments.Comment: 15 pages, 11 figures. Accepted for publication in Astronomy and
Astrophysic
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
Magnetoacoustic shocks as driver of quiet Sun mottles
We present high spatial and high temporal resolution observations of the
quiet Sun in H-alpha obtained with the Swedish 1-m Solar Telescope on La Palma.
We observe that many mottles, jet-like features in the quiet Sun, display clear
up- and downward motions along their main axis. In addition, many mottles show
vigorous transverse displacements. Unique identification of the mottles
throughout their lifetime is much harder than for their active region
counterpart, dynamic fibrils. This is because many seem to lack a sharply
defined edge at their top, and significant fading often occurs throughout their
lifetime. For those mottles that can be reliably tracked, we find that the
mottle tops often undergo parabolic paths. We find a linear correlation between
the deceleration these mottles undergo and the maximum velocity they reach,
similar to what was found earlier for dynamic fibrils. Combined with an
analysis of oscillatory properties, we conclude that at least part of the quiet
Sun mottles are driven by magnetoacoustic shocks. In addition, the mixed
polarity environment and vigorous dynamics suggest that reconnection may play a
significant role in the formation of some quiet Sun jets.Comment: 12 pages, 4 figures. ApJ Letters, in pres
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
Small heat shock proteins are induced during multiple sclerosis lesion development in white but not grey matter
INTRODUCTION: The important protective role of small heat-shock proteins (HSPs) in regulating cellular survival and migration, counteracting protein aggregation, preventing apoptosis, and regulating inflammation in the central nervous system is now well-recognized. Yet, their role in the neuroinflammatory disorder multiple sclerosis (MS) is largely undocumented. With the exception of alpha B-crystallin (HSPB5), little is known about the roles of small HSPs in disease. RESULTS: Here, we examined the expression of four small HSPs during lesion development in MS, focussing on their cellular distribution, and regional differences between white matter (WM) and grey matter (GM). It is well known that MS lesions in these areas differ markedly in their pathology, with substantially more intense blood-brain barrier damage, leukocyte infiltration and microglial activation typifying WM but not GM lesions. We analysed transcript levels and protein distribution profiles for HSPB1, HSPB6, HSPB8 and HSPB11 in MS lesions at different stages, comparing them with normal-appearing brain tissue from MS patients and non-neurological controls. During active stages of demyelination in WM, and especially the centre of chronic active MS lesions, we found significantly increased expression of HSPB1, HSPB6 and HSPB8, but not HSPB11. When induced, small HSPs were exclusively found in astrocytes but not in oligodendrocytes, microglia or neurons. Surprisingly, while the numbers of astrocytes displaying high expression of small HSPs were markedly increased in actively demyelinating lesions in WM, no such induction was observed in GM lesions. This difference was particularly obvious in leukocortical lesions covering both WM and GM areas. CONCLUSIONS: Since induction of small HSPs in astrocytes is apparently a secondary response to damage, their differential expression between WM and GM likely reflects differences in mediators that accompany demyelination in either WM or GM during MS. Our findings also suggest that during MS, cortical structures fail to benefit from the protective actions of small HSPs. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s40478-015-0267-2) contains supplementary material, which is available to authorized users
Multiplexing genetic and nucleosome positioning codes: A computational approach
Theoretical PhysicsBiological and Soft Matter Physic
Chromospheric Velocities of a C-class Flare
We use high spatial and temporal resolution observations from the Swedish
Solar Telescope to study the chromospheric velocities of a C-class flare
originating from active region NOAA 10969. A time-distance analysis is employed
to estimate directional velocity components in H-alpha and Ca II K image
sequences. Also, imaging spectroscopy has allowed us to determine flare-induced
line-of-sight velocities. A wavelet analysis is used to analyse the periodic
nature of associated flare bursts. Time-distance analysis reveals velocities as
high as 64 km/s along the flare ribbon and 15 km/s perpendicular to it. The
velocities are very similar in both the H-alpha and Ca II K time series.
Line-of-sight H-alpha velocities are red-shifted with values up to 17 km/s. The
high spatial and temporal resolution of the observations have allowed us to
detect velocities significantly higher than those found in earlier studies.
Flare bursts with a periodicity of approximately 60 s are also detected. These
bursts are similar to the quasi-periodic oscillations observed at hard X-ray
and radio wavelength data. Some of the highest velocities detected in the solar
atmosphere are presented. Line-of-sight velocity maps show considerable mixing
of both the magnitude and direction of velocities along the flare path. A
change in direction of the velocities at the flare kernel has also been
detected which may be a signature of chromospheric evaporation.Comment: Accepted for publication in Astronomy and Astrophysics, 5 figure
The effect of prophylactic cranial irradiation (PCI) for young stage III NSCLC patients:Subgroup analyses of the NVALT-11/DLCRG-02 study
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