279 research outputs found
Twist, Writhe & Helicity in the inner penumbra of a sunspot
The aim of this work is the determination of the twist, writhe, and self
magnetic helicity of penumbral filaments located in an inner Sunspot penumbra.
To this extent, we inverted data taken with the spectropolarimeter (SP) aboard
Hinode with the SIR (Stokes Inversion based on Response function) code. For the
construction of a 3D geometrical model we applied a genetic algorithm
minimizing the divergence of the magnetic field vector and the net
magnetohydrodynamic force, consequently a force-free solution would be reached
if possible. We estimated two proxies to the magnetic helicity frequently used
in literature: the force-free parameter and the current helicity term. We show
that both proxies are only qualitative indicators of the local twist as the
magnetic field in the area under study significantly departures from a
force-free configuration. The local twist shows significant values only at the
borders of bright penumbral filaments with opposite signs on each side. These
locations are precisely correlated to large electric currents. The average
twist (and writhe) of penumbral structures is very small. The spines (dark
filaments in the background) show a nearly zero writhe. The writhe per unit
length of the intraspines diminishes with increasing length of the tube axes.
Thus, the axes of tubes related to intraspines are less wrung when the tubes
are more horizontal. As the writhe of the spines is very small, we can conclude
that the writhe reaches only significant values when the tube includes the
border of a intraspine.Comment: 7 pages, 4 figures; Astrophysical Journal, in pres
Liquidus Tracking: Controlled Rate Vitrification for the Cryopreservation of Larger Volumes and Tissues
BACKGROUND: Vitrification of cells or tissue at controlled cooling rates suitable for larger volumes, and with reduced cryoprotectant toxicity. OBJECTIVE: To set out the current understanding of the LiquidusTracking (LT) vitrification technique, and to discuss the challenges and benefits of translating the method into laboratory protocols more generally applicable to meet requirements of large volume and 3-D cryo-banking in the era of regenerative medicine. METHODS: By adding small amounts of cryoprotectants at each step and subsequently cooling the sample just above its freezing point before further increasing CPA concentration, cryoprotectant toxicity is minimized. RESULT: CPA toxicity can be reduced by lowering the temperature. Different manual approaches to LT were evaluated and further improved. CONCLUSIONS: Manual liquidus tracking is complicated and exhibits potential high variability. Nevertheless, this approach offers the possibility of testing several conditions simultaneously and could be used to pre-test conditions prior to automatic LT development
Three dimensional structure of penumbral filaments from Hinode observations
We analyse spectropolarimetric observations of the penumbra of the NOAA AR
10953 at high spatial resolution (0.3"). The full Stokes profiles of the Fe I
lines at 630.1 nm and 630.2 nm have been obtained with the Solar Optical
Telescope (SOT) on board the Hinode satellite. The data have been inverted by
means of the SIR code, deriving the stratifications of temperature, line of
sight velocity, and the components of the magnetic field vector in optical
depth. In order to evaluate the gas pressure and to obtain an adequate
geometrical height scale, the motion equation has been integrated for each
pixel taking into account the terms of the Lorentz force. To establish the
boundary condition, a genetic algorithm has been applied. The final resulting
magnetic field has a divergence compatible with 0 inside its uncertainties.
First analyses of the correlation of the Wilson depression with velocity,
temperature, magnetic field strength, and field inclination strongly support
the uncombed penumbral model proposed by Solanki & Montavon (1993).Comment: 6 pages, 3 figures, conference proceeding: SEA meeting 2008,
Santander, Spai
Liquidus Tracking: Large scale preservation of encapsulated 3-D cell cultures using a vitrification machine
Currently, cryo-banking of multicellular structures such as organoids, especially in large volumes at clinical scale >1 L, remains elusive for reasons such as insufficient dehydration and cryoprotectant additive (CPA(1)) penetration, slow cooling and warming rates and devitrification processes. Here we introduce the concept of Liquidus Tracking (LT) using a semi-automated process for liquid volumes of up to 450 ml including 130 ml of alginate encapsulated liver cells (AELC) that archived controlled and reversible vitrification with minimized toxicity. First a CPA solution with optimal properties for LT was developed by employing different small scale test systems. Combining sugars such as glucose and raffinose with Me2SO improved post-exposure (at +0.5 °C) viabilities from 6% ±3.6 for Me2SO alone up to 58% ±6.1 and 65% ±14.2 respectively (p < 0.01). Other permeating CPAs (e.g. ethylene glycol, propylene glycol, methanol) were investigated as partial replacements for Me2SO. A mixture of Me2SO, ethylene glycol and glucose (ratio 4:2:1- termed LTdeg) supported glass-forming tendencies with appropriate low viscosities and toxicities required for LT. When running the full LT process, using Me2SO alone, no viable cells were recovered; using LTdeg, viable recoveries were improved to 40% ±8 (p<0.001%). Further refinements of improved mixing technique further improved recovery after LT. Recoveries of specific liver cell functions such as synthesis of albumin and alpha-fetoprotein (AFP) were retained in post thaw cultures. In summary: By developing a low-toxicity CPA solution of low viscosity (LTdeg) suitable for LT and by improving the stirring system, post-warming viability of AELC of up to 90% and a AFP secretion of 89% were reached. Results show that it may be possible to develop LT as a suitable cryogenic preservation process for different cell therapy products at large scale
How Digital Are the Digital Humanities? An Analysis of Two Scholarly Blogging Platforms
In this paper we compare two academic networking platforms, HASTAC and Hypotheses, to show the distinct ways in which they serve specific communities in the Digital Humanities (DH) in different national and disciplinary contexts. After providing background information on both platforms, we apply co-word analysis and topic modeling to show thematic similarities and differences between the two sites, focusing particularly on how they frame DH as a new paradigm in humanities research. We encounter a much higher ratio of posts using humanities-related terms compared to their digital counterparts, suggesting a one-way dependency of digital humanities-related terms on the corresponding unprefixed labels. The results also show that the terms digital archive, digital literacy, and digital pedagogy are relatively independent from the respective unprefixed terms, and that digital publishing, digital libraries, and digital media show considerable cross-pollination between the specialization and the general noun. The topic modeling reproduces these findings and reveals further differences between the two platforms. Our findings also indicate local differences in how the emerging field of DH is conceptualized and show dynamic topical shifts inside these respective contexts
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
The Structure and Dynamics of the Upper Chromosphere and Lower Transition Region as Revealed by the Subarcsecond VAULT Observations
The Very high Angular resolution ULtraviolet Telescope (VAULT) is a sounding
rocket payload built to study the crucial interface between the solar
chromosphere and the corona by observing the strongest line in the solar
spectrum, the Ly-a line at 1216 {\AA}. In two flights, VAULT succeeded in
obtaining the first ever sub-arcsecond (0.5") images of this region with high
sensitivity and cadence. Detailed analyses of those observations have
contributed significantly to new ideas about the nature of the transition
region. Here, we present a broad overview of the Ly-a atmosphere as revealed by
the VAULT observations, and bring together past results and new analyses from
the second VAULT flight to create a synthesis of our current knowledge of the
high-resolution Ly-a Sun. We hope that this work will serve as a good reference
for the design of upcoming Ly-a telescopes and observing plans.Comment: 28 pages, 11 figure
Time series of high resolution photospheric spectra in a quiet region of the Sun. I. Analysis of global and spatial variations of line parameters
A 50 min time series of one-dimensional slit-spectrograms, taken in quiet sun
at disk center, observed at the German Vacuum Tower Telescope (Observatorio del
Teide), was used to study the global and spatial variations of different line
parameters. In order to determine the vertical structure of the photosphere two
lines with well separated formation heights have been considered. The data have
been filtered of p-modes to isolate the pure convective phenomenon. From our
studies of global correlation coefficients and coherence and phase shift
analyzes between the several line parameters, the following results can be
reported. The convective velocity pattern preserves structures larger than 1.0"
up to the highest layers of the photosphere (~ 435 km). However, at these
layers, in the intensity pattern only structures larger than 2.0" are still
connected with those at the continuum level although showing inverted
brightness contrast. This confirms an inversion of temperature that we have
found at a height of ~140 km. A possible evidence of gravity waves superimposed
to the convective motions is derived from the phase shift analysis. We
interpret the behavior of the full width at half maximum and the equivalent
width as a function of the distance to the granular borders, as a consequence
of enhanced turbulence and/or strong velocity gradients in the intergranular
lanes.Comment: 16 pages, 15 figures, 5 tables; Astronomy & Astrophysics, Volume 408,
p.363-378, 200
Spicule emission profiles observed in \ion{He}{i} 10830 \AA
Off-the-limb observations with high spatial and spectral resolution will help
us understand the physical properties of spicules in the solar chromosphere
Spectropolarimetric observations of spicules in the \ion{He}{i} 10830 \AA\
multiplet were obtained with the Tenerife Infrared Polarimeter on the German
Vacuum Tower Telescope at the Observatorio del Teide (Tenerife, Spain). The
analysis shows the variation of the off-limb emission profiles as a function of
the distance to the visible solar limb. The ratio between the intensities of
the blue and the red components of this triplet is an observational signature of the optical thickness along the light
path, which is related to the intensity of the coronal irradiation. We present
observations of the intensity profiles of spicules above a quiet Sun region.
The observable as a function of the distance to the visible limb is
also given. We have compared our observational results to the intensity ratio
obtained from detailed radiative transfer calculations in semi-empirical models
of the solar atmosphere assuming spherical geometry. The agreement is purely
qualitative. We argue that future models of the solar chromosphere and
transition region should account for the observational constraints presented
here.Comment: letter accepted in A&A. 5 figure
Supersonic Evershed flow outside Sunspots
We report on the discovery of mostly horizontal field channels just outside
sunspot penumbrae (in the so-called `moat' region) that are seen to sustain
supersonic flows (line-of-sight component of 6 km s{-1}). The spectral
signature of these supersonic flows corresponds to circular polarization
profiles with an additional, satellite, third lobe of the same sign as the
parent sunspot' Stokes V blue lobe, for both downflows and upflows. This is
consistent with an outward directed flow that we interpret as the continuation
of the magnetized Evershed flow outside sunspots at supersonic speeds. In
Stokes Q and U, a clear signature of a transverse field connecting the two flow
streams is observed. Such an easily detectable spectral signature should allow
for a clear identification of these horizontal field channels in other
spectropolarimetric sunspot data. For the spot analyzed in this paper, a total
of 5 channels with this spectral signature have been unambiguously found
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