969 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
Effectiveness of palm and bamboo geotextiles in reducing concentrated flow erosion
Resumen del trabajo presentado al IV International Symposium on Gully Erosion, celebrado en la Universidad Pública de Navarra del 17 al 19 de septiembre de 2007.Rills and gullies, caused by concentrated flow erosion,
represent an important sediment source in many environments
(Poesen et al., 2003). Studies indicate that vegetation can be
very effective in controlling gully development. However, the
establishment of a vegetation cover can be delayed or
obstructed by the development of rills and gullies due to
concentrated flow erosion. Before the vegetation has reached
a critical cover and root density to significantly reduce
concentrated flow erosion, a period of high erosion risk
occurs. Hann and Morgan (2006) indicate that applying
geotextiles on the soil surface is the most efficient method to
control erosion until a critical vegetation cover has been
established. Preliminary investigations suggest palm-mat
geotextiles could be an effective and cheap soil conservation
method, with enormous global potential. However, very little
is known about the effectiveness of (palm) geotextiles in
reducing concentrated flow erosion. Almost no data are
available on the impacts of palm geotextiles on the hydraulic,
hydrologic and erosion characteristics of concentrated flow for
a range of environmental conditions. Therefore, the objectives
of this study are (i) to assess the effectiveness of two palm-mat
and one bamboo geotextile in increasing the hydraulic
roughness of the soil surface under concentrated overland flow
and in reducing soil erosion rates by concentrated flow on an
erodible soil type and for a range of flow shear stresses; and
(ii) to investigate which is the most appropriate hydraulic
variable (e.g. shear stress, unit length shear force or stream
power) to predict the net soil detachment by concentrated flow.This research is part of the BORASSUS Project: ‘The
environmental and socio-economic contribution of palm-leaf geotextiles to
sustainable development and soil conservation’ (contract number: INCOCT-
2005-510745), funded by the European Commission (EC), Specific
Targeted Research Projects (FP6-STREPs) for Developing Countries
(INCO-DEV) Programme
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
Brightness, distribution, and evolution of sunspot umbral dots
We present a 106-minute TiO (705.7nm) time series of high spatial and
temporal resolution that contains thousands of umbral dots (UDs) in a mature
sunspot in the active region NOAA 10667 at =0.95. The data were acquired
with the 1-m Swedish Solar Telescope on La Palma. With the help of a multilevel
tracking (MLT) algorithm the sizes, brightnesses, and trajectories of 12836
umbral dots were found and analyzed. The MLT allows UDs with very low contrast
to be reliably identified. Inside the umbra we determine a UD filling factor of
11%. The histogram of UD lifetimes is monotonic, i.e. a UD does not have a
typical lifetime. Three quarters of the UDs lived for less than 150s and showed
no or little motion. The histogram of the UD diameters exhibits a maximum at
225km, i.e. most of the UDs are spatially resolved. UDs display a typical
horizontal velocity of 420m/s and a typical peak intensity of 51% of the mean
intensity of the quiet photosphere, making them on average 20% brighter than
the local umbral background. Almost all mobile UDs (large birth-death distance)
were born close to the umbra-penumbra boundary, move towards the umbral center,
and are brighter than average. Notably bright and mobile UDs were also observed
along a prominent UD chain, both ends of which are located at the
umbra-penumbra boundary. Their motion started primarily at either of the ends
of the chain, continued along the chain, and ended near the chain's center. We
observed the splitting and merging of UDs and the temporal succession of both.
For the first time the evolution of brightness, size, and horizontal speed of a
typical UD could be determined in a statistically significant way. Considerable
differences between the evolution of central and peripheral UDs are found,
which point to a difference in origin
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
Acoustic Power Absorption and its Relation with Vector Magnetic Field of a Sunspot
The distribution of acoustic power over sunspots shows an enhanced absorption
near the umbra--penumbra boundary. Earlier studies revealed that the region of
enhanced absorption coincides with the region of strongest transverse potential
field. The aim of this paper is to (i) utilize the high-resolution vector
magnetograms derived using Hinode SOT/SP observations and study the
relationship between the vector magnetic field and power absorption and (ii)
study the variation of power absorption in sunspot penumbrae due to the
presence of spine-like radial structures. It is found that (i) both potential
and observed transverse fields peak at a similar radial distance from the
center of the sunspot, and (ii) the magnitude of the transverse field, derived
from Hinode observations, is much larger than the potential transverse field
derived from SOHO/MDI longitudinal field observations. In the penumbra, the
radial structures called spines (intra-spines) have stronger (weaker) field
strength and are more vertical (horizontal). The absorption of acoustic power
in the spine and intra-spine shows different behaviour with the absorption
being larger in the spine as compared to the intra-spine.Comment: 18 pages, 7 figures, In Press Solar Physics, Topical Issue on
Helio-and-Astroseismolog
Two-dimensional solar spectropolarimetry with the KIS/IAA Visible Imaging Polarimeter
Spectropolarimetry at high spatial and spectral resolution is a basic tool to
characterize the magnetic properties of the solar atmosphere. We introduce the
KIS/IAA Visible Imaging Polarimeter (VIP), a new post-focus instrument that
upgrades the TESOS spectrometer at the German VTT into a full vector
polarimeter. VIP is a collaboration between the KIS and the IAA. We describe
the optical setup of VIP, the data acquisition procedure, and the calibration
of the spectropolarimetric measurements. We show examples of data taken between
2005 and 2008 to illustrate the potential of the instrument. VIP is capable of
measuring the four Stokes profiles of spectral lines in the range from 420 to
700 nm with a spatial resolution better than 0.5". Lines can be sampled at 40
wavelength positions in 60 s, achieving a noise level of about 2 x 10E-3 with
exposure times of 300 ms and pixel sizes of 0.17" x 0.17" (2 x 2 binning). The
polarization modulation is stable over periods of a few days, ensuring high
polarimetric accuracy. The excellent spectral resolution of TESOS allows the
use of sophisticated data analysis techniques such as Stokes inversions. One of
the first scientific results of VIP presented here is that the ribbon-like
magnetic structures of the network are associated with a distinct pattern of
net circular polarization away from disk center. VIP performs
spectropolarimetric measurements of solar magnetic fields at a spatial
resolution that is only slightly worse than that of the Hinode
spectropolarimeter, while providing a 2D field field of view and the
possibility to observe up to four spectral regions sequentially with high
cadence. VIP can be used as a stand-alone instrument or in combination with
other spectropolarimeters and imaging systems of the VTT for extended
wavelength coverage.Comment: 10 pages, 8 figures, accepted by Astronomy and Astrophysics v2:
figures updated with improved qualit
- …