247,620 research outputs found
The effect of the stellar absorption line centre-to-limb variation on exoplanet transmission spectrum observations
Transit spectroscopy is one of the most commonly used techniques for
exoplanet atmosphere characterisation. This technique has been used to detect
ionized and neutral species in exoplanet atmospheres by comparing the observed
stellar lines in and out of transit. The centre-to-limb variation (CLV) of the
stellar lines across the stellar disk is an important effect for transmission
spectroscopy, since it results in a change of stellar line depth when the
planet transits different parts of the stellar disk. We reanalyse the transit
data of HD 189733b taken with the HARPS spectrograph to study the CLV effect
during transit. The transmission light curve of the Na i D line so obtained
shows a clear imprint of the CLV effect. We use a one-dimensional non-LTE
stellar spectral model to simulate the CLV effect. After applying the
correction, the measurement of the Na i absorption in the atmosphere of HD
189733b becomes better determined. We compare the CLV effect of HD 189733b to
that of HD 209458b. The CLV effects are different for these two benchmark
planetary systems and this is attributed to their different stellar effective
temperatures and transit impact parameters. We then explore the general CLV
effect that occurs during exoplanet transits. Normally, a star with a lower
effective temperature exhibits a stronger CLV effect and its CLV feature
extends over a relatively broad wavelength range. The transit impact parameter
(b) describes the transit trajectory on the stellar disk and thus determines
the actual manifestation of the CLV effect. We introduce a b-diagram which
describes the behavior of the CLV effect as the function of different impact
parameters. With improving observational precision, a careful modeling and
correction of the CLV effect is necessary for exoplanet atmosphere
characterisation using transit spectroscopy.Comment: Accepted for publishing on A&
Shedding Light on Photosynthesis: The Impacts of Atmospheric Conditions and Plant Canopy Structure on Ecosystem Carbon Uptake.
The Earth’s climate is influenced by complex interactions of physical, chemical, and biological processes that link terrestrial ecosystems and the atmosphere. One of these interactions involves the use of light in photosynthesis, which allows plants to remove CO2 from the atmosphere and slow the unprecedented rate of climate change the Earth is experiencing. However, modeling future climate remains challenging, in part because of limited knowledge of mechanisms controlling the effects of light on gross ecosystem CO2 uptake (conceptually, photosynthetic activity integrated across all leaves in a plant canopy). Unlike previous studies, this dissertation uses data from atmospheric science, ecosystem ecology, and plant physiology to provide evidence for mechanistic links between physical, biophysical, and ecological controls on the effects of light on processes tied to gross ecosystem CO2 uptake—specifically, ecosystem gross primary production (GPP) and leaf photosynthesis. First, this dissertation empirically demonstrates that the dominant effect of clouds is to reduce total light above canopies. However, optically thin clouds increase scattered, diffuse light, which canopies use more efficiently than they use direct light. This offsets reductions in total light and results in no net change in GPP under thin clouds, while GPP decreases under optically thick clouds because both diffuse and direct light decrease. Second, ground-based measurements indicate that the rate of increase in GPP with diffuse light changes throughout the day. The magnitude of increase depends on how canopies interact with the angle of incoming light to biophysically alter the distribution of light within canopies and thus, the proportions of leaves contributing to GPP. Third, the distribution of species and light within one forest canopy leads to differences in some of the rate-limiting biochemical reactions in leaf photosynthesis. These field-based data indicate which assumptions representing canopies in Earth system models may not have support in situ, and could be contributing to errors in model estimates of future climate. Overall, this dissertation identifies mechanisms through which clouds and plant canopy structure alter land-atmosphere CO2 fluxes and subsequently, Earth’s climate. It also provides an important interdisciplinary framework for testing assumptions about the feedbacks that living organisms form with their environment.PhDEcology and Evolutionary BiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133446/1/chengs_1.pd
Characterization of the radiative impact of aerosols on CO₂ and energy fluxes in the Amazon deforestation arch using artificial neural networks
In vegetation canopies with complex architectures, diffuse solar radiation can enhance carbon assimilation through photosynthesis because isotropic light is able to reach deeper layers of the canopy. Although this effect has been studied in the past decade, the mechanisms and impacts of this enhancement over South America remain poorly understood. Over the Amazon deforestation arch large amounts of aerosols are released into the atmosphere due to biomass burning, which provides an ideal scenario for further investigation of this phenomenon in the presence of canopies with complex architecture. In this paper, the relation of aerosol optical depth and surface fluxes of mass and energy are evaluated over three study sites with artificial neural networks and radiative transfer modeling. Results indicate a significant effect of the aerosol on the flux of carbon dioxide between the vegetation and the atmosphere, as well as on energy exchange, including that surface fluxes are sensitive to second-order radiative impacts of aerosols on temperature, humidity, and friction velocity. CO₂ exchanges increased in the presence of aerosol in up to 55 % in sites with complex canopy architecture. A decrease of approximately 12 % was observed for a site with shorter vegetation. Energy fluxes were negatively impacted by aerosols over all study sites
ESAF: Full Simulation of Space-Based Extensive Air Showers Detectors
Future detection of Extensive Air Showers (EAS) produced by Ultra High Energy
Cosmic Particles (UHECP) by means of space based fluorescence telescopes will
open a new window on the universe and allow cosmic ray and neutrino astronomy
at a level that is virtually impossible for ground based detectors. In this
paper we summarize the results obtained in the context of the EUSO project by
means of a detailed Monte Carlo simulation of all the physical processes
involved in the fluorescence technique, from the Extensive Air Shower
development to the instrument response. Particular emphasis is given to
modeling the light propagation in the atmosphere and the effect of clouds. Main
results on energy threshold and resolution, direction resolution and Xmax
determination are reported. Results are based on EUSO telescope design, but are
also extended to larger and more sensitive detectors.Comment: 38 pages, 48 figures Corrected typos. Changed content. Added figure
Center to limb observations and modeling of the Ca I 4227 A line
The observed center-to-limb variation (CLV) of the scattering polarization in
different lines of the Second Solar Spectrum can be used to constrain the
height variation of various atmospheric parameters, in particular the magnetic
fields via the Hanle effect. Here we attempt to model non-magnetic CLV
observations of the profiles of the Ca I 4227 A line recorded with the
ZIMPOL-3 at IRSOL. For modeling, we use the polarized radiative transfer with
partial frequency redistribution with a number of realistic 1-D model
atmospheres. We find that all the standard FAL model atmospheres, used by us,
fail to simultaneously fit the observed (, ) at all the limb distances
(). However, an attempt is made to find a single model which can provide a
fit at least to the CLV of the observed instead of a simultaneous fit to
the (, ) at all . To this end we construct a new 1-D model by
combining two of the standard models after modifying their temperature
structures in the appropriate height ranges. This new combined model closely
reproduces the observed at all the , but fails to reproduce the
observed rest intensity at different . Hence we find that no single 1-D
model atmosphere succeeds in providing a good representation of the real Sun.
This failure of 1-D models does not however cause an impediment to the magnetic
field diagnostic potential of the Ca I 4227 A line. To demonstrate this we
deduce the field strength at various positions without invoking the use
of radiative transfer.Comment: 20 pages, 10 figures, Accepted for publication in Ap
New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation
(abridged) The heating mechanism at high densities during M dwarf flares is
poorly understood. Spectra of M dwarf flares in the optical and
near-ultraviolet wavelength regimes have revealed three continuum components
during the impulsive phase: 1) an energetically dominant blackbody component
with a color temperature of T 10,000 K in the blue-optical, 2) a smaller
amount of Balmer continuum emission in the near-ultraviolet at lambda 3646
Angstroms and 3) an apparent pseudo-continuum of blended high-order Balmer
lines. These properties are not reproduced by models that employ a typical
"solar-type" flare heating level in nonthermal electrons, and therefore our
understanding of these spectra is limited to a phenomenological interpretation.
We present a new 1D radiative-hydrodynamic model of an M dwarf flare from
precipitating nonthermal electrons with a large energy flux of erg
cm s. The simulation produces bright continuum emission from a
dense, hot chromospheric condensation. For the first time, the observed color
temperature and Balmer jump ratio are produced self-consistently in a
radiative-hydrodynamic flare model. We find that a T 10,000 K
blackbody-like continuum component and a small Balmer jump ratio result from
optically thick Balmer and Paschen recombination radiation, and thus the
properties of the flux spectrum are caused by blue light escaping over a larger
physical depth range compared to red and near-ultraviolet light. To model the
near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer
lines, we include the extra Balmer continuum opacity from Landau-Zener
transitions that result from merged, high order energy levels of hydrogen in a
dense, partially ionized atmosphere. This reveals a new diagnostic of ambient
charge density in the densest regions of the atmosphere that are heated during
dMe and solar flares.Comment: 50 pages, 2 tables, 13 figures. Accepted for publication in the Solar
Physics Topical Issue, "Solar and Stellar Flares". Version 2 (June 22, 2015):
updated to include comments by Guest Editor. The final publication is
available at Springer via http://dx.doi.org/10.1007/s11207-015-0708-
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