1,967 research outputs found
Potential of the TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor for the monitoring of terrestrial chlorophyll fluorescence
Global monitoring of sun-induced chlorophyll fluorescence (SIF) is improving our knowledge about the photosynthetic functioning of terrestrial ecosystems. The feasibility of SIF retrievals from spaceborne atmospheric spectrometers has been demonstrated by a number of studies in the last years. In this work, we investigate the potential of the upcoming TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor satellite mission for SIF retrieval. TROPOMI will sample the 675–775 nm spectral window with a spectral resolution of 0.5 nm and a pixel size of 7 km × 7 km. We use an extensive set of simulated TROPOMI data in order to assess the uncertainty of single SIF retrievals and subsequent spatio-temporal composites. Our results illustrate the enormous improvement in SIF monitoring achievable with TROPOMI with respect to comparable spectrometers currently in-flight, such as the Global Ozone Monitoring Experiment-2 (GOME-2) instrument. We find that TROPOMI can reduce global uncertainties in SIF mapping by more than a factor of 2 with respect to GOME-2, which comes together with an approximately 5-fold improvement in spatial sampling. Finally, we discuss the potential of TROPOMI to map other important vegetation parameters at a global scale with moderate spatial resolution and short revisit time. Those include leaf photosynthetic pigments and proxies for canopy structure, which will complement SIF retrievals for a self-contained description of vegetation condition and functioning
On General Axial Gauges for QCD
General Axial Gauges within a perturbative approach to QCD are plagued by
'spurious' propagator singularities. Their regularisation has to face major
conceptual and technical problems. We show that this obstacle is naturally
absent within a Wilsonian or 'Exact' Renormalisation Group approach and explain
why this is so. The axial gauge turns out to be a fixed point under the flow,
and the universal 1-loop running of the gauge coupling is computed.Comment: 4 pages, latex, talk presented by DFL at QCD'98, Montpellier, July
2-8, 1998; to be published in Nucl. Phys. B (Proc. Suppl.
A Large Atom Number Metastable Helium Bose-Einstein Condensate
We have produced a Bose-Einstein condensate of metastable helium (4He*)
containing over 1.5x10^7 atoms, which is a factor of 25 higher than previously
achieved. The improved starting conditions for evaporative cooling are obtained
by applying one-dimensional Doppler cooling inside a magnetic trap. The same
technique is successfully used to cool the spin-polarized fermionic isotope
(3He*), for which thermalizing collisions are highly suppressed. Our detection
techniques include absorption imaging, time-of-flight measurements on a
microchannel plate detector and ion counting to monitor the formation and decay
of the condensate.Comment: 4 pages, 3 figures (changed content
Dzyaloshinsky-Moriya interaction in vesignieite: A route to freezing in a quantum kagome antiferromagnet
We report an electron spin resonance investigation of the geometrically
frustrated spin-1/2 kagome antiferromagnet vesignieite,
BaCuVO(OH). Analysis of the line widths and line shifts
indicates the dominance of in-plane Dzyaloshinsky-Moriya anisotropy that is
proposed to suppress strongly quantum spin fluctuations and thus to promote
long-range ordering rather than a spin-liquid state. We also evidence an
enhanced spin-phonon contribution that might originate from a lattice
instability and discuss the origin of a low-temperature mismatch between
intrinsic and bulk susceptibility in terms of local inhomogeneity
Rotationally induced Penning ionization of ultracold photoassociated helium dimers
We have studied photoassociation of metastable \tripS helium atoms near the
\tripS-\tripP asymptote by both ion detection in a magneto-optical trap and
trap-loss measurements in a magnetic trap. A detailed comparison between the
results of the two experiments gives insight into the mechanism of the Penning
ionization process. We have identified four series of resonances corresponding
to vibrational molecular levels belonging to different rotational states in two
potentials. The corresponding spin states become quasi-purely quintet at small
interatomic distance, and Penning ionization is inhibited by spin conservation
rules. Only a weak rotational coupling is responsible for the contamination by
singlet spin states leading to a detectable ion signal. However, for one of
these series Bose statistics does not enable the rotational coupling and the
series detected through trap-loss does not give rise to sufficient ionization
for detection.Comment: 7 pages, 4 figures, submitted to EuroPhysics Letter
Quantifying Vegetation Biophysical Variables from Imaging Spectroscopy Data: A Review on Retrieval Methods
An unprecedented spectroscopic data stream will soon become available with forthcoming Earth-observing satellite missions equipped with imaging spectroradiometers. This data stream will open up a vast array of opportunities to quantify a diversity of biochemical and structural vegetation properties. The processing requirements for such large data streams require reliable retrieval techniques enabling the spatiotemporally explicit quantification of biophysical variables. With the aim of preparing for this new era of Earth observation, this review summarizes the state-of-the-art retrieval methods that have been applied in experimental imaging spectroscopy studies inferring all kinds of vegetation biophysical variables. Identified retrieval methods are categorized into: (1) parametric regression, including vegetation indices, shape indices and spectral transformations; (2) nonparametric regression, including linear and nonlinear machine learning regression algorithms; (3) physically based, including inversion of radiative transfer models (RTMs) using numerical optimization and look-up table approaches; and (4) hybrid regression methods, which combine RTM simulations with machine learning regression methods. For each of these categories, an overview of widely applied methods with application to mapping vegetation properties is given. In view of processing imaging spectroscopy data, a critical aspect involves the challenge of dealing with spectral multicollinearity. The ability to provide robust estimates, retrieval uncertainties and acceptable retrieval processing speed are other important aspects in view of operational processing. Recommendations towards new-generation spectroscopy-based processing chains for operational production of biophysical variables are given
On the Role of Penning Ionization in Photoassociation Spectroscopy
We study the role of Penning ionization on the photoassociation spectra of
He(^3S)-He(^3S). The experimental setup is discussed and experimental results
for different intensities of the probe laser are shown. For modelling the
experimental results we consider coupled-channel calculations of the crossing
of the ground state with the excited state at the Condon point. The
coupled-channel calculations are first applied to model systems, where we
consider two coupled channels without ionization, two coupled channels with
ionization, and three coupled channels, for which only one of the excited
states is ionizing. Finally, coupled-channel calculations are applied to
photoassociation of He(^3S)-He(^3S) and good agreement is obtained between the
model and the experimental results.Comment: 14 pages, 18 figures, submitted to the special issue on Cold
Molecules of J. Phys.
Dzyaloshinsky-Moriya Anisotropy in the Spin-1/2 Kagom\'e Compound ZnCu(OH)Cl
We report the determination of the Dzyaloshinsky-Moriya interaction, the
dominant magnetic anisotropy term in the \kagome spin-1/2 compound {\herbert}.
Based on the analysis of the high-temperature electron spin resonance (ESR)
spectra, we find its main component K to be perpendicular to the
\kagome planes. Through the temperature dependent ESR line-width we observe a
building up of nearest-neighbor spin-spin correlations below 150 K.Comment: 4 pages, 3 figures, minor modification
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