805 research outputs found
In-flight calibration of STEREO-B/WAVES antenna system
The STEREO/WAVES (SWAVES) experiment on board the two STEREO spacecraft
(Solar Terrestrial Relations Observatory) launched on 25 October 2006 is
dedicated to the measurement of the radio spectrum at frequencies between a few
kilohertz and 16 MHz. The SWAVES antenna system consists of 6 m long orthogonal
monopoles designed to measure the electric component of the radio waves. With
this configuration direction finding of radio sources and polarimetry (analysis
of the polarization state) of incident radio waves is possible. For the
evaluation of the SWAVES data the receiving properties of the antennas,
distorted by the radiation coupling with the spacecraft body and other onboard
devices, have to be known accurately. In the present context, these properties
are described by the antenna effective length vectors. We present the results
of an in-flight calibration of the SWAVES antennas using the observations of
the nonthermal terrestrial auroral kilometric radiation (AKR) during STEREO
roll maneuvers in an early stage of the mission. A least squares method
combined with a genetic algorithm was applied to find the effective length
vectors of the STEREO Behind (STEREO-B)/WAVES antennas in a quasi-static
frequency range () which fit best to the model
and observed AKR intensity profiles. The obtained results confirm the former
SWAVES antenna analysis by rheometry and numerical simulations. A final set of
antenna parameters is recommended as a basis for evaluations of the SWAVES
data
The random phase approximation applied to ice
Standard density functionals without van der Waals interactions yield an
unsatisfactory description of ice phases, specifically, high density phases
occurring under pressure are too unstable compared to the common low density
phase I observed at ambient conditions. Although the description is
improved by using functionals that include van der Waals interactions, the
errors in relative volumes remain sizable. Here we assess the random phase
approximation (RPA) for the correlation energy and compare our results to
experimental data as well as diffusion Monte Carlo data for ice. The RPA yields
a very balanced description for all considered phases, approaching the accuracy
of diffusion Monte Carlo in relative energies and volumes. This opens a route
towards a concise description of molecular water phases on surfaces and in
cavities
Historical Lassa Fever Reports and 30-year Clinical Update
Five cases of Lassa fever have been imported from West Africa to the United States since 1969. We report symptoms of the patient with the second imported case and the symptoms and long-term follow-up on the patient with the third case. Vertigo in this patient has persisted for 30 years
Modified screen-printed carbon electrodes application for protein tumor markers determination
Screen-printed carbon electrodes were modified with gold nanoparticles bound with DNA-aptamers by two different methods. Aptamers can selectively bind protein tumor markers from the blood plasma. The electrodes were tested. Signals obtained via squire-wavy voltammetry from modified electrodes covered with blood plasma of the healthy donors and donors with lung cancer can be distinguished
A switch from α‐helical to β‐strand conformation during co‐translational protein folding
Cellular proteins begin to fold as they emerge from the ribosome.The folding landscape of nascent chains is not only shaped by theiramino acid sequence but also by the interactions with the ribo-some. Here, we combine biophysical methods with cryo-EM struc-ture determination to show that folding of aβ-barrel proteinbegins with formation of a dynamicα-helix inside the ribosome. Asthe growing peptide reaches the end of the tunnel, the N-terminalpart of the nascent chain refolds to aβ-hairpin structure thatremains dynamic until its release from the ribosome. Contactswith the ribosome and structure of the peptidyl transferase centerdepend on nascent chain conformation. These results indicate thatproteins may start out asα-helices inside the tunnel and switchinto their native folds only as they emerge from the ribosome.Moreover, the correlation of nascent chain conformations withreorientation of key residues of the ribosomal peptidyl-transferasecenter suggest that protein folding could modulate ribosome activity
Confronting the trans-Planckian question of inflationary cosmology with dissipative effects
We provide a class of QFTs which exhibit dissipation above a threshold
energy, thereby breaking Lorentz invariance. Unitarity is preserved by coupling
the fields to additional degrees of freedom (heavy fields) which introduce the
rest frame. Using the Equivalence Principle, we define these theories in
arbitrary curved spacetime. We then confront the trans-Planckian question of
inflationary cosmology. When dissipation increases with the energy, the quantum
field describing adiabatic perturbations is completely damped at the onset of
inflation. However it still exists as a composite operator made with the
additional fields. And when these are in their ground state, the standard power
spectrum obtains if the threshold energy is much larger that the Hubble
parameter. In fact, as the energy redshifts below the threshold, the composite
operator behaves as if it were a free field endowed with standard vacuum
fluctuations. The relationship between our models and the Brane World scenarios
studied by Libanov and Rubakov displaying similar effects is discussed. The
signatures of dissipation will be studied in a forthcoming paper.Comment: 30 pages, 1 Figure, to appear in CQ
Signatures of trans-Planckian dispersion in inflationary spectra
The primordial spectra are calculated using dispersion relations which
deviate from the relativistic one above a certain energy scale Lambda. We
determine the properties of the leading modifications with respect to the
standard spectra when Lambda is much greater than H, the Hubble scale during
inflation. To be generic, we parameterize the lowest order deviation from the
relativistic law by alpha, the power of P/Lambda where P is the proper
momentum. When working in the asymptotic vacuum, the leading modification
scales as (H/Lambda)^alpha for all alpha, except for a discrete set where the
power is higher. Moreover, this modification is robust against introducing
higher order terms in the dispersion relation. We then algebraically deduce the
modifications of scalar and tensor power spectra in slow roll inflation from
modifications calculated in de Sitter space. The modifications do not exhibit
oscillations unless the dispersion relation induces some non-adiabaticity near
a given scale. Finally, we explore the much less studied regime where H and
Lambda are comparable. Our results indicate that the project of reconstructing
the inflaton potential cannot be pursued without making some hypothesis about
the dispersion relation of the fluctuation modes.Comment: Version accepted for publication in PRD. 15 pages, 10 figures.
Comment on observational consequences adde
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