966 research outputs found
Is the solar spectrum latitude dependent? An investigation with SST/TRIPPEL
Context: In studies of the solar spectrum relative to spectra of solar twin
stars, it has been found that the chemical composition of the Sun seems to
depart systematically from those of the twins. One possible explanation is that
the effect is due to the special aspect angle of the Sun when observed from
Earth, as compared with the aspect angles of the twins. Thus, a latitude
dependence of the solar spectrum, even with the heliocentric angle constant,
could lead to effects of the type observed.
Aim: We explore a possible variation in the strength of certain spectral
lines, used in the comparisons between the composition of the Sun and the
twins, at loci on the solar disk with different latitudes but at constant
heliocentric angle.
Methods: We use the TRIPPEL spectrograph at the Swedish 1-m Solar Telescope
on La Palma to record spectra in five spectral regions in order to compare
different locations on the solar disk at a heliocentric angle of 45 deg.
Equivalent widths and other parameters are measured for fifteen different lines
representing nine atomic species.
Results: The relative variations in equivalent widths at the equator and at
solar latitude 45 deg are found to be less than 1.5 % for all spectral lines
studied. Translated to elemental abundances as they would be measured from a
terrestrial and a hypothetical pole-on observer, the difference is estimated to
be within 0.005 dex in all cases.
Conclusion: It is very unlikely that latitude effects could cause the
reported abundance difference between the Sun and the solar twins. The accuracy
obtainable in measurements of small differences in spectral line strengths
between different solar disk positions is very high.Comment: 9 pages, 10 figures, accepted by Astronomy & Astrophysic
The quiet Sun's magnetic flux estimated from CaIIH bright inter-granular G-band structures
We determine the number density and area contribution of small-scale
inter-granular calcium-II bright G-band structures in images of the quiet Sun
as tracers of kilo-Gauss magnetic flux-concentrations.
In a 149" x 117" G-band image of the disk center at the activity minimum,
7593 small inter-granular structures ['IGS']were segmented with the
`multiple-level tracking' pattern recognition algorithm ['MLT_4']. The
scatter-plot of the continuum versus the G-band brightness shows the known
magnetic and non-magnetic branches. These branches are largely disentangled by
applying an intrinsic Ca-II excess criterion. The thus obtained 2995 structures
contain 1152 G-band bright points ['BP'] and 1843 G-band faint points ['FP'].
They show a tendency of increasing size with decreasing G-band excess, as
expected from the `hot wall' picture. Their Ca-H and G-band brightness are
slightly related, resembling the known relation of Ca-II and magnetic field
strength. The magnetic flux density of each individual BP and FP is estimated
from their G-band brightness according to MHD-model calculations.
The entity of BP and FP covers the total field-of-view ['FOV'] with a number
density of 0.32/Mm^2 and a total area contribution of 2.0%. Their individual
calibrations yield a mean flux density of 20 Mx/cm^2 in the entire FOV and 13
Mx/cm^2 for inter-network regions
Decreased infarct size after focal cerebral ischemia in mice chronically infected with Toxoplasma gondii
To determine whether Toxoplasma gondii infection could modify biological phenomena associated with brain ischemia, we investigated the effect of permanent middle cerebral artery occlusion (MCAO) on neuronal survival, inflammation and redox state in chronically infected mice. Infected animals showed a 40% to 50% decrease of infarct size compared with non-infected littermates 1, 4 and 14 days after MCAO. The resistance of infected mice may be associated with increased basal levels of anti-inflammatory cytokines and/or a marked reduction of the MCAO-related brain induction of two pro-inflammatory cytokines, tumor necrosis factor-alpha and interferon-gamma (IFNÎł). In addition, potential anti-inflammatory/neuroprotective factors such as nerve growth factor, suppressor of cytokine signaling-3, superoxide dismutase activity, uncoupling protein-2 and glutathione (GSH) were upregulated in the brain of infected mice. Consistent with a role of GSH in central cytokine regulation, GSH depletion by diethyl maleate inhibited Toxoplasma gondii lesion resistance by increasing the proinflammatory cytokine IFNÎł brain levels. Overall, these findings indicate that chronic toxoplasmosis decisively influences both the inflammatory molecular events and outcome of cerebral ischemia
Tracking magnetic bright point motions through the solar atmosphere
High-cadence, multiwavelength observations and simulations are employed for the analysis of solar photospheric magnetic bright points (MBPs) in the quiet Sun. The observations were obtained with the Rapid Oscillations in the Solar Atmosphere (ROSA) imager and the Interferometric Bidimensional Spectrometer at the Dunn Solar Telescope. Our analysis reveals that photospheric MBPs have an average transverse velocity of approximately 1 km sâ1, whereas their chromospheric counterparts have a slightly higher average velocity of 1.4 km sâ1. Additionally, chromospheric MBPs were found to be around 63 per cent larger than the equivalent photospheric MBPs. These velocity values were compared with the output of numerical simulations generated using the MURAM code. The simulated results were similar, but slightly elevated, when compared to the observed data. An average velocity of 1.3 km sâ1 was found in the simulated G-band images and an average of 1.8 km sâ1 seen in the velocity domain at a height of 500 km above the continuum formation layer. Delays in the change of velocities were also analysed. Average delays of âŒ4 s between layers of the simulated data set were established and values of âŒ29 s observed between G-band and Ca II K ROSA observations. The delays in the simulations are likely to be the result of oblique granular shock waves, whereas those found in the observations are possibly the result of a semi-rigid flux tube
Theoretical Models of Sunspot Structure and Dynamics
Recent progress in theoretical modeling of a sunspot is reviewed. The
observed properties of umbral dots are well reproduced by realistic simulations
of magnetoconvection in a vertical, monolithic magnetic field. To understand
the penumbra, it is useful to distinguish between the inner penumbra, dominated
by bright filaments containing slender dark cores, and the outer penumbra, made
up of dark and bright filaments of comparable width with corresponding magnetic
fields differing in inclination by some 30 degrees and strong Evershed flows in
the dark filaments along nearly horizontal or downward-plunging magnetic
fields. The role of magnetic flux pumping in submerging magnetic flux in the
outer penumbra is examined through numerical experiments, and different
geometric models of the penumbral magnetic field are discussed in the light of
high-resolution observations. Recent, realistic numerical MHD simulations of an
entire sunspot have succeeded in reproducing the salient features of the
convective pattern in the umbra and the inner penumbra. The siphon-flow
mechanism still provides the best explanation of the Evershed flow,
particularly in the outer penumbra where it often consists of cool, supersonic
downflows.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
Models and Observations of Sunspot Penumbrae
The mysteries of sunspot penumbrae have been under an intense scrutiny for
the past 10 years. During this time, some models have been proposed and
refuted, while the surviving ones had to be modified, adapted and evolved to
explain the ever-increasing array of observational constraints. In this
contribution I will review two of the present models, emphasizing their
contributions to this field, but also pinpointing some of their inadequacies to
explain a number of recent observations at very high spatial resolution. To
help explaining these new observations I propose some modifications to each of
them. These modifications bring those two seemingly opposite models closer
together into a general picture that agrees well with recent 3D
magneto-hydrodynamic simulations.Comment: 9 pages, 1 color figure. Review talk to appear in the proceedings of
the International Workshop of 2008 Solar Total Eclipse: Solar Magnetism,
Corona and Space Weather--Chinese Space Solar Telescope Scienc
Proceedings, Pot Chrysanthemum School, 1971
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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
Fine structure, magnetic field and heating of sunspot penumbrae
We interpret penumbral filaments as due to convection in field-free, radially
aligned gaps just below the visible surface of the penumbra, intruding into a
nearly potential field above. This solves the classical discrepancy between the
large heat flux and the low vertical velocities observed in the penumbra. The
presence of the gaps causes strong small-scale fluctuations in inclination,
azimuth angle and field strength, but without strong forces acting on the gas.
The field is nearly horizontal in a region around the cusp-shaped top of the
gap, thereby providing an environment for Evershed flows. We identify this
region with the recently discovered dark penumbral cores. Its darkness has the
same cause as the dark lanes in umbral light-bridges, reproduced in numerical
simulations by Nordlund and Stein (2005). We predict that the large vertical
and horizontal gradients of the magnetic field inclination and azimuth in the
potential field model will produce the net circular polarization seen in
observations. The model also explains the significant elevation of bright
filaments above their surroundings. It predicts that dark areas in the penumbra
are of two different kinds: dark filament cores containing the most inclined
(horizontal) fields, and regions between bright filaments, containing the least
inclined field lines.Comment: submitted to A&
High Resolution Observations using Adaptive Optics: Achievements and Future Needs
Over the last few years, several interesting observations were obtained with
the help of solar Adaptive Optics (AO). In this paper, few observations made
using the solar AO are enlightened and briefly discussed. A list of
disadvantages with the current AO system are presented. With telescopes larger
than 1.5m are expected during the next decade, there is a need to develop the
existing AO technologies for large aperture telescopes. Some aspects of this
development are highlighted. Finally, the recent AO developments in India are
also presented
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