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° 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.&nbsp;Bonn&nbsp;lidar at the Esrange and the ALOMAR RMR lidar at the And&#248;ya 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&nbsp;2003 both lidar systems conducted measurements and retrieved atmospheric temperatures. On 19/20 January&nbsp;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 &lambda;<i>_z</i>=2&ndash;6 km and broad at ALOMAR, covering &lambda;<i>_z</i>=1&ndash;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