157 research outputs found
The Relation between the Radial Temperature Profile in the Chromosphere and the Solar Spectrum at Centimeter, Millimeter, Sub-millimeter, and Infrared Wavelengths
Solar observations from millimeter to ultraviolet wavelengths show that there
is a temperature minimum between photosphere and chromosphere. Analysis based
on semi-empirical models locate this point at about 500 km over the
photosphere. The consistency of these models has been tested by means of
millimeter to infrared observations.
In the present work, we show that variations of the theoretical radial
temperature profile near the temperature minimum impacts the brightness
temperature at centimeter, submillimeter, and infrared wavelengths, but the
millimeter wavelength emission remains unchanged. We found a region between 500
and 1000 km over the photosphere that remains hidden to observations at the
frequencies under study in this work.Comment: Accepted in Solar Physic
High-performance thermal emitters based on laser engineered metal surfaces
Effective thermal management is of paramount importance for all high-temperature
systems operating under vacuum. Cooling of such systems relies mainly on radiative heat transfer
requiring high spectral emissivity of surfaces, which is strongly affected by the surface condition.
Pulsed laser structuring of stainless steel in air resulted in the spectral hemispherical emissivity
values exceeding 0.95 in the 2.5–15 µm spectral region. The effects of surface oxidation and
topography on spectral emissivity as well as high temperature stability of the surface structures
were examined. High performance stability of the laser textured surfaces was confirmed after
thermal aging studies at 320°C for 96 hour
Nothofagus pumilio forest affected by recent tephra deposition in northern patagonia: II-shifts in diversity and structure of rhizosphere fungal communities
Volcanic eruptions have a significant effect on ecosystems, including soil and its fungal communities. Due to the eruption of the Puyehue-Cordón Caulle in 2011, thick tephra deposits accumulated in several Nothofagus pumilio forests in Northern Patagonia. The tephra deposition affected plant establishment and development during the post-eruption period. In this work we analyzed rhizosphere fungal communities associated with N. pumilio seedlings at three study sites sampled three years after the eruption, two with a thick tephra layer (50 cm) and a third site with no tephra deposition. Denaturing gradient gel electrophoresis (DGGE) was performed for the different fungal communities (Ascomycetes, Basidiomycetes and Total fungi). We found clear shifts in rhizosphere fungal communities, especially for Basidiomycetes and Total fungi. Site-related characteristics, such as type of substrate, light intensity and the existence of understory and previous mycelial networks, seemed to be the main factors determining the structure of these fungal communities. Given their sensitivity to different types of environmental change, these microbial communities could be used as bioindicators. Specifically, Basidiomycetes and total fungi seem to be better bioindicators of environmental changes than ascomycetes. The results shown here contribute to the understanding of how fungal communities respond to disturbance, an important issue in the evaluation of how ecosystem processes might be affected in areas commonly subjected to geological risks, such as volcanism. This information is also relevant to the planning and successful implementation of restoration activities based on seedling optimization by rhizosphere fungi.Fil: Fernández, Natalia V.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto Andino Patagónico de Tecnologías Biológicas y Geoambientales. Universidad Nacional del Comahue. Instituto Andino Patagónico de Tecnologías Biológicas y Geoambientales.; Argentina; Argentina. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche; ArgentinaFil: Fontenla, Sonia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto Andino Patagónico de Tecnologías Biológicas y Geoambientales. Universidad Nacional del Comahue. Instituto Andino Patagónico de Tecnologías Biológicas y Geoambientales.; Argentina; ArgentinaFil: Moguilevsky, Denise. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto Andino Patagónico de Tecnologías Biológicas y Geoambientales. Universidad Nacional del Comahue. Instituto Andino Patagónico de Tecnologías Biológicas y Geoambientales.; Argentina; Argentina. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche; ArgentinaFil: Meie, Sebastián. Instituto de Investigaciones Agropecuarias; Chile. Universidad de La Frontera; ChileFil: Rilling, Joaquín. Universidad de La Frontera; ChileFil: Cornejo, Pablo Sebastian. Universidad de La Frontera; Chil
Hanle effect in the solar Ba II D2 line: a diagnostic tool for chromospheric weak magnetic fields
The physics of the solar chromosphere depends in a crucial way on its
magnetic structure. However there are presently very few direct magnetic field
diagnostics available for this region. Here we investigate the diagnostic
potential of the Hanle effect on the Ba II D2 line resonance polarization for
the determination of weak chromospheric turbulent magnetic fields......Comment: In press in astronomy and astrophysic
Magnetohydrodynamic waves in solar partially ionized plasmas: two-fluid approach
We derive the dynamics of magnetohydrodynamic waves in two-fluid partially
ionized plasmas and to compare the results with those obtained under
single-fluid description. Two-fluid magnetohydrodynamic equations are used,
where ion-electron plasma and neutral particles are considered as separate
fluids. Dispersion relations of linear magnetohydrodynamic waves are derived
for simplest case of homogeneous medium. Frequencies and damping rates of waves
are obtained for different parameters of background plasma. We found that two-
and single-fluid descriptions give similar results for low frequency waves.
However, the dynamics of MHD waves in two-fluid approach is significantly
changed when the wave frequency becomes comparable or higher than ion-neutral
collision frequency. Alfven and fast magneto-acoustic waves attain their
maximum damping rate at particular frequencies (for example, the peak frequency
equals 2.5 ion-neutral collision frequency for 50 % of neutral Hydrogen) in
wave spectrum. The damping rates are reduced for higher frequency waves. The
new mode of slow magneto-acoustic wave appears for higher frequency branch,
which is connected to neutral hydrogen fluid. The single-fluid approach
perfectly deals with slow processes in partially ionized plasmas, but fails for
time-scales smaller than ion-neutral collision time. Therefore, two-fluid
approximation should be used for the description of relatively fast processes.
Some results of single-fluid description, for example the damping of
high-frequency Alfven waves in the solar chromosphere due to ion-neutral
collisions, should be revised in future.Comment: 8 pages, 7 figures, accepted in A&
Properties of sunspots in cycle 23: I. Dependence of brightness on sunspot size and cycle phase
In this paper we investigate the dependence of umbral core brightness, as
well as the mean umbral and penumbral brightness on the phase of the solar
cycle and on the size of the sunspot. Albregtsen & Maltby (1978) reported an
increase in umbral core brightness from the early to the late phase of solar
cycle from the analysis of 13 sunspots which cover solar cycles 20 and 21. Here
we revisit this topic by analysing continuum images of more than 160 sunspots
observed by the MDI instrument on board the SOHO spacecraft for the period
between 1998 March to 2004 March, i.e. a sizable part of solar cycle 23. The
advantage of this data set is its homogeneity, with no seeing fluctuations. A
careful stray light correction, which is validated using the Mercury transit of
7th May, 2003, is carried out before the umbral and penumbral intensities are
determined. The influence of the Zeeman splitting of the nearby NiI spectral
line on the measured 'continuum' intensity is also taken into account. We did
not observe any significant variation in umbral core, mean umbral and mean
penumbral intensities with solar cycle, which is in contrast to earlier
findings for the umbral core intensity. We do find a strong and clear
dependence of the umbral brightness on sunspot size, however. The penumbral
brightness also displays a weak dependence. The brightness-radius relationship
has numerous implications, some of which, such as those for the energy
transport in umbrae, are pointed out.Comment: 16 pages, 21 postscript figures, accepted for publication in A&
NLTE modeling of Stokes vector center-to-limb variations in the CN violet system
The solar surface magnetic field is connected with and even controls most of
the solar activity phenomena. Zeeman effect diagnostics allow for measuring
only a small fraction of the fractal-like structured magnetic field. The
remaining hidden magnetic fields can only be accessed with the Hanle effect.
Molecular lines are very convenient for applying the Hanle effect diagnostics
thanks to the broad range of magnetic sensitivities in a narrow spectral
region. With the UV version of the Zurich Imaging Polarimeter ZIMPOL II
installed at the 45 cm telescope of the Istituto Ricerche Solari Locarno
(IRSOL), we simultaneously observed intensity and linear polarization
center-to-limb variations in two spectral regions containing the (0,0) and
(1,1) bandheads of the CN B 2 {\Sigma} - X 2 {\Sigma} system. Here we present
an analysis of these observations. We have implemented coherent scattering in
molecular lines into a NLTE radiative transfer code. A two-step approach was
used. First, we separately solved the statistical equilibrium equations and
compute opacities and intensity while neglecting polariza- tion. Then we used
these quantities as input for calculating scattering polarization and the Hanle
effect. We have found that it is impossible to fit the intensity and
polarization simultaneously at different limb angles in the frame- work of
standard 1D modeling. The atmosphere models that provide correct intensity
center-to-limb variations fail to fit linear polar- ization center-to-limb
variations due to lacking radiation field anisotropy. We had to increase the
anisotropy by means of a specially introduced free parameter. This allows us to
successfully interpret our observations. We discuss possible reasons for
underestimating the anisotropy in the 1D modeling.Comment: 15 pages, 10 figures, accepted for publication in
Astronomy&Astrophysic
Quiet-Sun imaging asymmetries in NaI D1 compared with other strong Fraunhofer lines
Imaging spectroscopy of the solar atmosphere using the NaI D1 line yields
marked asymmetry between the blue and red line wings: sampling a quiet-Sun area
in the blue wing displays reversed granulation, whereas sampling in the red
wing displays normal granulation. The MgI b2 line of comparable strength does
not show this asymmetry, nor does the stronger CaII 8542 line. We demonstrate
the phenomenon with near-simultaneous spectral images in NaI D1, MgI b2, and
CaII 8542 from the Swedish 1-m Solar Telescope. We then explain it with
line-formation insights from classical 1D modeling and with a 3D
magnetohydrodynamical simulation combined with NLTE spectral line synthesis
that permits detailed comparison with the observations in a common format. The
cause of the imaging asymmetry is the combination of correlations between
intensity and Dopplershift modulation in granular overshoot and the sensitivity
to these of the steep profile flanks of the NaI D1 line. The MgI b2 line has
similar core formation but much wider wings due to larger opacity buildup and
damping in the photosphere. Both lines obtain marked core asymmetry from
photospheric shocks in or near strong magnetic concentrations, less from
higher-up internetwork shocks that produce similar asymmetry in the spatially
averaged CaII 8542 profile.Comment: Accepted by Astron & Astrophys. In each in-text citation the year
links to the corresponding ADS abstract pag
Bright fibrils in Ca II K
Context: Except for the Ca II resonance lines, fibrils are ubiquitously
present in most high-resolution observations of chromospheric lines. Aims: We
show that fibrils are also a prevailing feature in Ca II K, provided the
spatial-resolution is sufficiently high. Methods: We present high spatial
resolution observations of an active region in the Ca I} K line from the
Swedish Solar Telescope. Through a comparison between photospheric intensity
and magnetic field data, we study the connection between bright chromospheric
fibrils and photospheric structures. Additionally, using Fourier analysis we
study how the fibrils are linked to the observed dynamics. Results: We find
that very narrow, bright fibrils are a prevailing feature over large portions
of the observed field. We also find a clear connection between the fibril
footpoints and photospheric magnetic features. We show that the fibrils play
two distinct roles in the observed dynamics: depending on their location they
can act as a canopy suppressing oscillations or they can channel low-frequency
oscillations into the chromosphere. Conclusions: The Ca II K fibrils share many
characteristics with fibrils observed in other chromospheric lines, but some
features, such as the very small widths, are unique to these observations.Comment: To be published in A&A. High resolution version can be downloaded
from: http://www.mps.mpg.de/homes/pietarila/fibrils.pd
A new approach to long-term reconstruction of the solar irradiance leads to large historical solar forcing
The variable Sun is the most likely candidate for natural forcing of past
climate change on time scales of 50 to 1000 years. Evidence for this
understanding is that the terrestrial climate correlates positively with solar
activity. During the past 10,000 years, the Sun has experienced substantial
variations in activity and there have been numerous attempts to reconstruct
solar irradiance. While there is general agreement on how solar forcing varied
during the last several hundred years --- all reconstructions are proportional
to the solar activity --- there is scientific controversy on the magnitude of
solar forcing. We present a reconstruction of the Total and Spectral Solar
Irradiance covering 130 nm--10 m from 1610 to the present with annual
resolution and for the Holocene with 22-year resolution. We assume that the
minimum state of the quiet Sun in time corresponds to the observed quietest
area on the present Sun. Then we use available long-term proxies of the solar
activity, which are Be isotope concentrations in ice cores and 22-year
smoothed neutron monitor data, to interpolate between the present quiet Sun and
the minimum state of the quiet Sun. This determines the long-term trend in the
solar variability which is then superposed with the 11-year activity cycle
calculated from the sunspot number. The time-dependent solar spectral
irradiance from about 7000 BC to the present is then derived using a
state-of-the-art radiation code. We derive a total and spectral solar
irradiance that was substantially lower during the Maunder minimum than
observed today. The difference is remarkably larger than other estimations
published in the recent literature. The magnitude of the solar UV variability,
which indirectly affects climate is also found to exceed previous estimates. We
discuss in details the assumptions which leaded us to this conclusion.Comment: 9 pages, 5 figures, accepted for publication in
Astronomy&Astrophysic
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