22,082 research outputs found
The Solar Wind Energy Flux
The solar-wind energy flux measured near the ecliptic is known to be
independent of the solar-wind speed. Using plasma data from Helios, Ulysses,
and Wind covering a large range of latitudes and time, we show that the
solar-wind energy flux is independent of the solar-wind speed and latitude
within 10%, and that this quantity varies weakly over the solar cycle. In other
words the energy flux appears as a global solar constant. We also show that the
very high speed solar-wind (VSW > 700 km/s) has the same mean energy flux as
the slower wind (VSW < 700 km/s), but with a different histogram. We use this
result to deduce a relation between the solar-wind speed and density, which
formalizes the anti-correlation between these quantities.Comment: 12 pages, 5 figure
Interstellar H2 toward HD 37903
We present an analysis of interstellar H2 toward HD 37903, which is a hot, B
1.5 V star located in the NGC 2023 reflection nebula. Meyer et al. (2001) have
used a rich spectrum of vibrationally excited H2 observed by the HST to
calculate a model of the interstellar cloud toward HD 37903. We extend Mayer's
analysis by including the v"=0 vibrational level observed by the FUSE
satellite.
The T01 temperature should not be interpreted as a rotational temperature,
but rather as a temperature of thermal equilibrium between the ortho and para
H2. The ortho to para H2 ratio is lower for collisionally populated levels than
for the levels populated by fluorescence.
The PDR model of the cloud located in front of HD 37903 points to a gas
temperature Tkin=110-377 K, hydrogen density nH=1874-544 cm^-3 and the
star-cloud distance of 0.45 pc
Nanodust detection near 1 AU from spectral analysis of Cassini/RPWS radio data
Nanodust grains of a few nanometer in size are produced near the Sun by
collisional break-up of larger grains and picked-up by the magnetized solar
wind. They have so far been detected at 1 AU by only the two STEREO spacecraft.
Here we analyze the spectra measured by the radio and plasma wave instrument
onboard Cassini during the cruise phase close to Earth orbit; they exhibit
bursty signatures similar to those observed by the same instrument in
association to nanodust stream impacts on Cassini near Jupiter. The observed
wave level and spectral shape reveal impacts of nanoparticles at about 300
km/s, with an average flux compatible with that observed by the radio and
plasma wave instrument onboard STEREO and with the interplanetary flux models
Enhancement factor distribution around a single SERS Hot-spot and its relation to Single Molecule detection
We provide the theoretical framework to understand the phenomenology and
statistics of single-molecule (SM) signals arising in Surface-Enhanced Raman
Scattering (SERS) under the presence of so-called electromagnetic hot-spots
(HS's). We show that most characteristics of the SM-SERS phenomenon can be
tracked down to the presence of tail-like (power law) distribution of
enhancements and we propose a specific model for it. We analyze, in the light
of this, the phenomenology of SM-SERS and show how the different experimental
manifestations of the effect reported in the literature can be analyzed and
understood under a unified ``universal'' framework with a minimum set of
parameters.Comment: 13 pages, 4 figures, submitted to J. Chem. Phy
Thermal momentum distribution from path integrals with shifted boundary conditions
For a thermal field theory formulated in the grand canonical ensemble, the
distribution of the total momentum is an observable characterizing the thermal
state. We show that its cumulants are related to thermodynamic potentials. In a
relativistic system for instance, the thermal variance of the total momentum is
a direct measure of the enthalpy. We relate the generating function of the
cumulants to the ratio of (a) a partition function expressed as a Matsubara
path integral with shifted boundary conditions in the compact direction, and
(b) the ordinary partition function. In this form the generating function is
well suited for Monte-Carlo evaluation, and the cumulants can be extracted
straightforwardly. We test the method in the SU(3) Yang-Mills theory and obtain
the entropy density at three different temperatures.Comment: 4 pages, 1 figure, minor revisions; version accepted in PR
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CHEMEPASS – Innovative Tools to promote Chemical Engineering Mobilit
The ocean carbon sink – impacts, vulnerabilities and challenges
Carbon dioxide (CO2) is, next to water vapour, considered to be the most important natural greenhouse gas on Earth. Rapidly rising atmospheric CO2 concentrations caused by human actions such as fossil fuel burning, land-use change or cement production over the past 250 years have given cause for concern that changes in Earth’s climate system may progress at a much faster pace and larger extent than during the past 20 000 years. Investigating global carbon cycle pathways and finding suitable adaptation and mitigation strategies has, therefore, become of major concern in many research fields. The oceans have a key role in regulating atmospheric CO2 concentrations and currently take up about 25% of annual anthropogenic carbon emissions to the atmosphere. Questions that yet need to be answered are what the carbon uptake kinetics of the oceans will be in the future and how the increase in oceanic carbon inventory will affect its ecosystems and their services. This requires comprehensive investigations, including high-quality ocean carbon measurements on different spatial and temporal scales, the management of data in sophisticated databases, the application of Earth system models to provide future projections for given emission scenarios as well as a global synthesis and outreach to policy makers. In this paper, the current understanding of the ocean as an important carbon sink is reviewed with respect to these topics. Emphasis is placed on the complex interplay of different physical, chemical and biological processes that yield both positive and negative air–sea flux values for natural and anthropogenic CO2 as well as on increased CO2 (uptake) as the regulating force of the radiative warming of the atmosphere and the gradual acidification of the oceans. Major future ocean carbon challenges in the fields of ocean observations, modelling and process research as well as the relevance of other biogeochemical cycles and greenhouse gases are discussed
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