5,747 research outputs found
What drives the dust activity of comet 67P/Churyumov-Gerasimenko?
We use the gravitational instability formation scenario of cometesimals to
derive the aggregate size that can be released by the gas pressure from the
nucleus of comet 67P/Churyumov-Gerasimenko for different heliocentric distances
and different volatile ices. To derive the ejected aggregate sizes, we
developed a model based on the assumption that the entire heat absorbed by the
surface is consumed by the sublimation process of one volatile species. The
calculations were performed for the three most prominent volatile materials in
comets, namely, H_20 ice, CO_2 ice, and CO ice. We find that the size range of
the dust aggregates able to escape from the nucleus into space widens when the
comet approaches the Sun and narrows with increasing heliocentric distance,
because the tensile strength of the aggregates decreases with increasing
aggregate size. The activity of CO ice in comparison to H_20 ice is capable to
detach aggregates smaller by approximately one order of magnitude from the
surface. As a result of the higher sublimation rate of CO ice, larger
aggregates are additionally able to escape from the gravity field of the
nucleus. Our model can explain the large grains (ranging from 2 cm to 1 m in
radius) in the inner coma of comet 67P/Churyumov-Gerasimenko that have been
observed by the OSIRIS camera at heliocentric distances between 3.4 AU and 3.7
AU. Furthermore, the model predicts the release of decimeter-sized aggregates
(trail particles) close to the heliocentric distance at which the gas-driven
dust activity vanishes. However, the gas-driven dust activity cannot explain
the presence of particles smaller than ~1 mm in the coma because the high
tensile strength required to detach these particles from the surface cannot be
provided by evaporation of volatile ices. These smaller particles can be
produced for instance by spin-up and centrifugal mass loss of ejected larger
aggregates
Summer sudden Na number density enhancements measured with the ALOMAR Weber Na Lidar
We present summer Na-densities and atmospheric temperatures measured 80 to 110 km above the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR). The Weber Na Lidar is part of ALOMAR, located at 69&deg; N in Norway, 150 km north of the Arctic Circle. The sun does not set here during the summer months, and measurements require a narrowband Faraday Anomalous Dispersion Optical Filter (FADOF). <br><br> We discuss an observed sudden enhancement in the Na number density around 22:00 UT on 1 to 2 June 2006. We compare this observation with previous summer measurements and find a frequent appearance of Na number density enhancements near local midnight. We describe the time of appearance, the altitude distribution, the duration and the strength of these enhancements and compare them to winter observations. We investigate possible formation mechanisms and, as others before, we find a strong link between these Na number density enhancements and sporadic E layers
Optimized design of universal two-qubit gates
We construct optimized implementations of the CNOT and other universal
two-qubit gates that, unlike many of the previously proposed protocols, are
carried out in a single step. The new protocols require tunable inter-qubit
couplings but, in return, show a significant improvements in the quality of
gate operations. Our optimization procedure can be further extended to the
combinations of elementary two-qubit as well as irreducible many-qubit gates.Comment: 6 pages, 2 figure
The Bonn University lidar at the Esrange: technical description and capabilities for atmospheric research
International audienceThe Bonn University operates a Rayleigh/Mie/Raman backscatter lidar at the Esrange near the Swedish city of Kiruna, north of the Arctic circle. The lidar system covers the atmosphere from about 4 km to 100 km altitude and it is capable of measuring aerosols in the tropo-, strato-, and mesosphere, as well as of determining temperature profiles in the aerosol-free part of the atmosphere (i.e. above 30-km altitude). Density tuned fixed-spacer etalons provide daylight capability and thus increased sensitivity to noctilucent clouds during polar summer. Polarisation measurements allow liquid and solid phase discrimination for aerosol and cloud particles in the tropo- and stratosphere. The derived temperature profiles can be used for the detection and analysis of atmospheric gravity waves. Although several lidar experiments are situated in polar latitudes, a comprehensive instrument which covers the troposphere, stratosphere, and mesosphere, is daylight capable, and observes temperature profiles, as well as aerosols, is exceptional. In this article a technical description, in particular, of the optical configuration of this experiment is given, as well as an overview of achievable geophysical parameters. The potential for geophysical analyses is shown. Keywords. Atmospheric composition and structure (Instruments and techniques; Aerosols and particles; Pressure, density and temperature) ? Meteorology and atmospheric dynamics (Middle atmosphere dynamics
2+1 flavor simulations of QCD with improved staggered quarks
The MILC collaboration has been performing realistic simulations of full QCD
with 2+1 flavors of improved staggered quarks. Our simulations allow for
controlled continuum and chiral extrapolations. I present results for the light
pseudoscalar sector: masses and decay constants, quark masses and
Gasser-Leutwyler low-energy constants. In addition I will present some results
for heavy-light mesons, decay constants and semileptonic form factors, obtained
in collaboration with the HPQCD and Fermilab lattice collaborations. Such
calculations will help in the extraction of CKM matrix elements from
experimental measurements.Comment: To appear in the proceedings of QNP06, IVth International Conference
on Quarks and Nuclear Physics, Madrid, June 200
Parasiten als Ursache von Urtikaria: Helminthen und Protozooen als Auslöser der Nesselsucht?
Zusammenfassung: Die Urtikaria ist eine der häufigsten Erkrankungen im dermatologischen Alltag. Die Beschwerden sind für die Betroffenen oft sehr einschränkend. Es gibt deutliche pathophysiologische und epidemiologische Hinweise, dass Helminthen und Protozoen zwar seltene, aber behandelbare Auslöser von akuter und chronischer Urtikaria sein können. Oftmals fehlt allerdings das Bewusstsein, dass Parasitosen auch in der industrialisierten Gesellschaft, bedingt durch einen steilen Anstieg von Migration und weltweiter Reisetätigkeit, mit zunehmender Prävalenz vorkommen. Der vorliegende Beitrag stellt die häufigsten parasitären Ursachen von Urtikaria vor und erläutert wichtige Aspekte von Anamnese, Klinik, Diagnostik und Therapi
A many-fermion generalization of the Caldeira-Leggett model
We analyze a model system of fermions in a harmonic oscillator potential
under the influence of a dissipative environment: The fermions are subject to a
fluctuating force deriving from a bath of harmonic oscillators. This represents
an extension of the well-known Caldeira-Leggett model to the case of many
fermions. Using the method of bosonization, we calculate one- and two-particle
Green's functions of the fermions. We discuss the relaxation of a single extra
particle added above the Fermi sea, considering also dephasing of a particle
added in a coherent superposition of states. The consequences of the separation
of center-of-mass and relative motion, the Pauli principle, and the
bath-induced effective interaction are discussed. Finally, we extend our
analysis to a more generic coupling between system and bath, that results in
complete thermalization of the system.Comment: v3: fixed pdf problem; v2: added exact formula (Eq. 42) for Green's
function and discussion of equilibrium density matrix (new Fig. 2); 10
figures, 21 pages, see quant-ph/0305098 for brief version of some of these
result
Simultaneous lidar observations of temperatures and waves in the polar middle atmosphere on the east and west side of the Scandinavian mountains: a case study on 19/20 January 2003
Atmospheric gravity waves have been the subject of intense research for several decades because of their extensive effects on the atmospheric circulation and the temperature structure. The U. Bonn lidar at the Esrange and the ALOMAR RMR lidar at the Andøya Rocket Range are located in northern Scandinavia 250 km apart on the east and west side of the Scandinavian mountain ridge. During January and February 2003 both lidar systems conducted measurements and retrieved atmospheric temperatures. On 19/20 January 2003 simultaneous measurements for more than 7 h were possible. Although during most of the campaign time the atmosphere was not transparent for the propagation of orographically induced gravity waves, they were nevertheless observed at both lidar stations with considerable amplitudes during these simultaneous measurements. And while the source of the observed waves cannot be determined unambiguously, the observations show many characteristics of orographically excited gravity waves. The wave patterns at ALOMAR show a random distribution with time whereas at the Esrange a persistency in the wave patterns is observable. This persistency can also be found in the distribution of the most powerful vertical wavelengths. The mode values are both at about 5 km vertical wavelength, however the distributions are quite different, narrow at the Esrange with values from λ<i><sub>z</sub></i>=2–6 km and broad at ALOMAR, covering λ<i><sub>z</sub></i>=1–12 km vertical wavelength. In particular the difference between the observations at ALOMAR and at the Esrange can be understood by different orographic conditions while the propagation conditions were quite similar. At both stations the waves deposit energy in the atmosphere with increasing altitude, which leads to a decrease of the observed gravity wave potential energy density with altitude. The meteorological situation during these measurements was different from common winter situations. The ground winds were mostly northerlies, changed in the upper troposphere and lower stratosphere to westerlies and returned to northerlies in the middle stratosphere
Simultaneous lidar observations of temperatures and waves in the polar middle atmosphere on both sides of the Scandinavian mountains: a case study on 19/20 January 2003
International audienceAtmospheric gravity waves have been the subject of intense research for several decades because of their extensive effects on the atmospheric circulation and the temperature structure. The U. Bonn lidar at the Esrange and the ALOMAR RMR lidar at the Andøya Rocket Range are located in northern Scandinavia 250 km apart on either side of the Scandinavian mountain ridge. During January and February 2003 both lidar systems conducted measurements and retrieved atmospheric temperatures. On 19/20 January 2003 simultaneous measurements for more than 7 h were possible. Although during most of the campaign time the atmosphere was not transparent for the propagation of orographically induced gravity waves, they could propagate and were observed at both lidar stations during these simultaneous measurements. The wave patterns at ALOMAR show a random distribution with time whereas at the Esrange a persistency in the wave patterns is observable. This persistency can also be found in the distribution of the most powerful vertical wavelengths. The mode values are both at about 5 km vertical wavelength, however the distributions are quite different, narrow at the Esrange containing values from ?z=2?6 km and broad at ALOMAR, covering ?z=1?12 km vertical wavelength. At both stations the waves deposit energy in the atmosphere with increasing altitude, which leads to a decrease of the observed gravity wave potential energy density with altitude. These measurements show unambigiously orographically induced gravity waves on both sides of the mountains as well as a clear difference of the characteristics of these waves, which might be caused by different excitation and propagation conditions on either side of the Scandinavian mountain ridge
- …