680 research outputs found
Primaire biliaire cirrhose met sclerodermie en hypothyreoidie
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Validation of six years of SCIAMACHY carbon monoxide observations using MOZAIC CO profile measurements
This paper presents a validation study of SCanning Imaging Absorption SpectroMeter for Atmospheric CartograpHY (SCIAMACHY) carbon monoxide (CO) total column measurements from the Iterative Maximum Likelihood Method (IMLM) algorithm using vertically integrated profile aircraft measurements obtained within the MOZAIC project for the six year time period of 2003-2008. Overall we find a good agreement between SCIAMACHY and airborne measurements for both mean values also on a year-to-year basis as well as seasonal variations. Several locations show large biases that are attributed to local effects like orography and proximity of large emission sources. Differences were detected for individual years: 2003, 2004 and 2006 have larger biases than 2005, 2007 and 2008, which appear to be related to SCIAMACHY instrumental issues but require more research. Results from this study are consistent with, and complementary to, findings from a previous validation study using ground-based measurements (de Laat et al., 2010b). According to this study, the SCIAMACHY data, if individual measurements are of sufficient quality-good signal-to-noise, can be used to determine the spatial distribution and seasonal cycles of CO total columns over clean areas. Biases found over areas with strong emissions (Africa, China) could be explained by low sensitivity of the instrument in the boundary layer and users are recommended to avoid using the SCIAMACHY data while trying to quantify CO burden and/or retrieve CO emissions in such areas. © 2012 Author(s)
Evidence for Steady Heating: Observations of an Active Region Core with Hinode and TRACE
Previous observations have not been able to exclude the possibility that high
temperature active region loops are actually composed of many small scale
threads that are in various stages of heating and cooling and only appear to be
in equilibrium. With new observations from the EUV Imaging Spectrometer (EIS)
and X-ray Telescope (XRT) on \textit{Hinode} we have the ability to investigate
the properties of high temperature coronal plasma in extraordinary detail. We
examine the emission in the core of an active region and find three independent
lines of evidence for steady heating. We find that the emission observed in XRT
is generally steady for hours, with a fluctuation level of approximately 15% in
an individual pixel. Short-lived impulsive heating events are observed, but
they appear to be unrelated to the steady emission that dominates the active
region. Furthermore, we find no evidence for warm emission that is spatially
correlated with the hot emission, as would be expected if the high temperature
loops are the result of impulsive heating. Finally, we also find that
intensities in the "moss", the footpoints of high temperature loops, are
consistent with steady heating models provided that we account for the local
expansion of the loop from the base of the transition region to the corona. In
combination, these results provide strong evidence that the heating in the core
of an active region is effectively steady, that is, the time between heating
events is short relative to the relevant radiative and conductive cooling
times.Comment: Minor changes based on the final report from the referee; Movies are
available from the first autho
Ejection of cool plasma into the hot corona
We investigate the processes that lead to the formation, ejection and fall of
a confined plasma ejection that was observed in a numerical experiment of the
solar corona. By quantifying physical parameters such as mass, velocity, and
orientation of the plasma ejection relative to the magnetic field, we provide a
description of the nature of this particular phenomenon. The time-dependent
three-dimensional magnetohydrodynamic (3D MHD) equations are solved in a box
extending from the chromosphere to the lower corona. The plasma is heated by
currents that are induced through field line braiding as a consequence of
photospheric motions. Spectra of optically thin emission lines in the extreme
ultraviolet range are synthesized, and magnetic field lines are traced over
time. Following strong heating just above the chromosphere, the pressure
rapidly increases, leading to a hydrodynamic explosion above the upper
chromosphere in the low transition region. The explosion drives the plasma,
which needs to follow the magnetic field lines. The ejection is then moving
more or less ballistically along the loop-like field lines and eventually drops
down onto the surface of the Sun. The speed of the ejection is in the range of
the sound speed, well below the Alfven velocity. The plasma ejection is
basically a hydrodynamic phenomenon, whereas the rise of the heating rate is of
magnetic nature. The granular motions in the photosphere lead (by chance) to a
strong braiding of the magnetic field lines at the location of the explosion
that in turn is causing strong currents which are dissipated. Future studies
need to determine if this process is a ubiquitous phenomenon on the Sun on
small scales. Data from the Atmospheric Imaging Assembly on the Solar Dynamics
Observatory (AIA/SDO) might provide the relevant information.Comment: 12 pages, 10 figure
A Generating Function for all Semi-Magic Squares and the Volume of the Birkhoff Polytope
We present a multivariate generating function for all n x n nonnegative
integral matrices with all row and column sums equal to a positive integer t,
the so called semi-magic squares. As a consequence we obtain formulas for all
coefficients of the Ehrhart polynomial of the polytope B_n of n x n
doubly-stochastic matrices, also known as the Birkhoff polytope. In particular
we derive formulas for the volumes of B_n and any of its faces.Comment: 24 pages, 1 figure. To appear in Journal of Algebraic Combinatoric
A polynomial-time algorithm for optimizing over N-fold 4-block decomposable integer programs
In this paper we generalize N-fold integer programs and two-stage integer
programs with N scenarios to N-fold 4-block decomposable integer programs. We
show that for fixed blocks but variable N, these integer programs are
polynomial-time solvable for any linear objective. Moreover, we present a
polynomial-time computable optimality certificate for the case of fixed blocks,
variable N and any convex separable objective function. We conclude with two
sample applications, stochastic integer programs with second-order dominance
constraints and stochastic integer multi-commodity flows, which (for fixed
blocks) can be solved in polynomial time in the number of scenarios and
commodities and in the binary encoding length of the input data. In the proof
of our main theorem we combine several non-trivial constructions from the
theory of Graver bases. We are confident that our approach paves the way for
further extensions
Characteristics and Evolution of the Magnetic field and Chromospheric Emission in an Active Region Core Observed by Hinode
We describe the characteristics and evolution of the magnetic field and
chromospheric emission in an active region core observed by the Solar Optical
Telescope on Hinode. Consistent with previous studies, we find that the moss is
unipolar, the spatial distribution of magnetic flux evolves slowly, and the
magnetic field is only moderately inclined. We show that the field line
inclination and horizontal component are coherent, and that the magnetic field
is mostly sheared in the inter-moss regions where the highest magnetic flux
variability is seen. Using extrapolations from SP magnetograms we show that the
magnetic connectivity in the moss is different than in the quiet Sun because
most of the magnetic field extends to significant coronal heights. The magnetic
flux, field vector, and chromospheric emission in the moss also appear highly
dynamic, but actually show only small scale variations in magnitude on
time-scales longer than the cooling times for hydrodynamic loops computed from
our extrapolations, suggesting high-frequency (continuous) heating events. Some
evidence is found for flux (Ca 2 intensity) changes on the order of 100--200 G
(DN) on time-scales of 20--30 mins that could be taken as indicative of
low-frequency heating. We find, however, that only a small fraction (10%) of
our simulated loops would be expected to cool on these time-scales, and we find
no clear evidence that the flux changes consistently produce intensity changes
in the chromosphere. The magnetic flux and chromospheric intensity in most
individual SOT pixels in the moss vary by less than ~ 20% and ~ 10%,
respectively, on loop cooling time-scales. In view of the high energy
requirements of the chromosphere, we suggest that these variations could be
sufficient for the heating of `warm' EUV loops, but that the high basal levels
may be more important for powering the hot core loops rooted in the moss.Comment: Accepted by ApJ, 16 pages, 20 figures. Abridged abstract (original is
in PDF file). Figures 1 & 2 are reduced resolution to meet size limit
Effects of heathland management on seedling recruitment of common juniper (Juniperus communis)
Background and aims: Common juniper (Juniperus communis L.) is one of the most widespread woody species on the planet. Over recent decades, however, common juniper populations are decreasing in size and number in different regions. Lack of recruitment, caused by extremely low seed viability and the absence of suitable microsites for recruitment, is the key reason for this decline. For successful germination, the seeds need gaps in the existing vegetation and a soil with a relatively high base saturation. The aim of this study was therefore to assess how management actions such as sod cutting, rotavation and liming (alone or in various combinations) influence soil characteristics, seed germination and seedling survival of common juniper.
Methods: We installed a sowing experiment across 104 1-m2 plots in four different sites in Belgium and the Netherlands using treatments with different combinations of fencing, sod cutting, rotavation, litter addition and liming. We determined how these treatments affected soil characteristics and how they influenced seed germination and seedling survival.
Key results and conclusions: Across the whole experiment, germination rates of juniper seeds were very low (almost always < 1%). Our results confirm that bare ground promotes the germination of juniper seeds. Secondly, higher silt and lutum (clay) proportions in the soil and higher soil organic matter content seemed to have a positive impact on recruitment, possibly due to drought reduction. Management actions that negatively affect those soil characteristics, such as deep sod cutting, should thus be avoided in heathlands on sandy soils. Our results reveal a complex relationship between seedling recruitment success, soil conditions and management of common juniper populations. Overall, combinations of fencing, (superficial) sod cutting and liming or rotavation were most successful
On mesogranulation, network formation and supergranulation
We present arguments which show that in all likelihood mesogranulation is not
a true scale of solar convection but the combination of the effects of both
highly energetic granules, which give birth to strong positive divergences
(SPDs) among which we find exploders, and averaging effects of data processing.
The important role played by SPDs in horizontal velocity fields appears in the
spectra of these fields where the scale 4 Mm is most energetic; we
illustrate the effect of averaging with a one-dimensional toy model which shows
how two independent non-moving (but evolving) structures can be transformed
into a single moving structure when time and space resolution are degraded.
The role of SPDs in the formation of the photospheric network is shown by
computing the advection of floating corks by the granular flow. The coincidence
of the network bright points distribution and that of the corks is remarkable.
We conclude with the possibility that supergranulation is not a proper scale of
convection but the result of a large-scale instability of the granular flow,
which manifests itself through a correlation of the flows generated by SPDs.Comment: 10 pages, 11 figures, to appear in Astronomy and Astrophysic
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