4,349 research outputs found
Measurement of macroscopic plasma parameters with a radio experiment: Interpretation of the quasi-thermal noise spectrum observed in the solar wind
The ISEE-3 SBH radio receiver has provided the first systematic observations of the quasi-thermal (plasma waves) noise in the solar wind plasma. The theoretical interpretation of that noise involves the particle distribution function so that electric noise measurements with long antennas provide a fast and independent method of measuring plasma parameters: densities and temperatures of a two component (core and halo) electron distribution function have been obtained in that way. The polarization of that noise is frequency dependent and sensitive to the drift velocity of the electron population. Below the plasma frequency, there is evidence of a weak noise spectrum with spectral index -1 which is not yet accounted for by the theory. The theoretical treatment of the noise associated with the low energy (thermal) proton population shows that the moving electrical antenna radiates in the surrounding plasma by Carenkov emission which becomes predominant at the low frequencies, below about 0.1 F sub P
Sparse square roots.
We show that it can be decided in polynomial time whether a graph of maximum degree 6 has a square root; if a square root exists, then our algorithm finds one with minimum number of edges. We also show that it is FPT to decide whether a connected n-vertex graph has a square root with at most n − 1 + k edges when this problem is parameterized by k. Finally, we give an exact exponential time algorithm for the problem of finding a square root with maximum number of edges
Dark Matter directional detection: comparison of the track direction determination
Several directional techniques have been proposed for a directional detection
of Dark matter, among others anisotropic crystal detectors, nuclear emulsion
plates, and low-pressure gaseous TPCs. The key point is to get access to the
initial direction of the nucleus recoiling due to the elastic scattering by a
WIMP. In this article, we aim at estimating, for each method, how the
information of the recoil track initial direction is preserved in different
detector materials. We use the SRIM simulation code to emulate the motion of
the first recoiling nucleus in each material. We propose the use of a new
observable, D, to quantify the preservation of the initial direction of the
recoiling nucleus in the detector. We show that in an emulsion mix and an
anisotropic crystal, the initial direction is lost very early, while in a
typical TPC gas mix, the direction is well preserved.Comment: 9 pages, 5 figure
Probe of Lorentz Invariance Violation effects and determination of the distance of PG 1553+113
The high frequency peaked BL Lac object PG 1553+113 underwent a flaring event
in 2012. The High Energy Stereoscopic System (H.E.S.S.) observed this source
for two consecutive nights at very high energies (VHE, 100~GeV). The data
show an increase of a factor of three of the flux with respect to archival
measurements with the same instrument and hints of intra-night variability. The
data set has been used to put constraints on possible Lorentz invariance
violation (LIV), manifesting itself as an energy dependence of the velocity of
light in vacuum, and to set limits on the energy scale at which Quantum Gravity
effects causing LIV may arise. With a new method to combine H.E.S.S. and Fermi
large area telescope data, the previously poorly known redshift of PG 1555+113
has been determined to be close to the value derived from optical measurements.Comment: 2014 Fermi Symposium proceedings - eConf C14102.
Constraints on Lorentz Invariance Violation from Fermi-Large Area Telescope Observations of Gamma-Ray Bursts
We analyze the MeV/GeV emission from four bright Gamma-Ray Bursts (GRBs)
observed by the Fermi-Large Area Telescope to produce robust, stringent
constraints on a dependence of the speed of light in vacuo on the photon energy
(vacuum dispersion), a form of Lorentz invariance violation (LIV) allowed by
some Quantum Gravity (QG) theories. First, we use three different and
complementary techniques to constrain the total degree of dispersion observed
in the data. Additionally, using a maximally conservative set of assumptions on
possible source-intrinsic spectral-evolution effects, we constrain any vacuum
dispersion solely attributed to LIV. We then derive limits on the "QG energy
scale" (the energy scale that LIV-inducing QG effects become important, E_QG)
and the coefficients of the Standard Model Extension. For the subluminal case
(where high energy photons propagate more slowly than lower energy photons) and
without taking into account any source-intrinsic dispersion, our most stringent
limits (at 95% CL) are obtained from GRB090510 and are E_{QG,1}>7.6 times the
Planck energy (E_Pl) and E_{QG,2}>1.3 x 10^11 GeV for linear and quadratic
leading order LIV-induced vacuum dispersion, respectively. These limits improve
the latest constraints by Fermi and H.E.S.S. by a factor of ~2. Our results
disfavor any class of models requiring E_{QG,1} \lesssim E_Pl.Comment: Accepted for publication by Physical Review
Stability of an oscillating tip in Non-Contact Atomic Force Microscopy: theoretical and numerical investigations
This paper is a theoretical and a numerical investigation of the stability of
a tip-cantilever system used in Non-Contact Atomic Force Microscopy (NC-AFM)
when it oscillates close to a surface. No additional dissipative force is
considered. The theoretical approach is based on a variationnal method
exploiting a coarse grained operation that gives the temporal dependence of the
nonlinear coupled equations of motion in amplitude and phase of the oscillator.
Stability criterions for the resonance peak are deduced and predict a stable
behavior of the oscillator in the vicinity of the resonance. The numerical
approach is based on results obtained with a virtual NC-AFM developped in our
group. The effect of the size of the stable domain in phase is investigated.
These results are in particularly good agreement with the theoretical
predictions. Also they show the influence of the phase shifter in the feedback
loop and the way it can affect the damping signal
Ciclo biologico de Battus polydamas (Linnaeus) (Lepidoptera : Papilionidade), en la Amazonia de Peru
Insectos de la "una de gato" (Uncaria guianenesis y U. tomentosa : Rubiaceae), planta medical de la Amazonia peruana
Two species of #Rubiaceae$ known as "una de gato", vernaculate name, are medicinal plants very used in the Peruvian Amazonia. Its excessive exploitation in the natural environment has made its cultivation necessary. Different species of phytophagous insects have been observed on "una de gato" in an experimental plantation in Iquitos, Peru. (Résumé d'auteur
Trophic relationships between palms and bruchids (Coleoptera: Bruchidae: Pachymerini) in Peruvian Amazonia
Qualitative and quantitative host plant data are provided for palm bruchids in a part of Amazonia from where previously few or no data were available. The host range and geographical distribution of several species are extended. The role of fruit structure on susceptibility to bruchid infestation and fruit maturity on oviposition site selection is discussed. The importance of mesologic conditions, particularly in areas subjected to flooding, on population dynamics of palm seed beetles is stressed. A specialist parasitoid of Caryoborus serripes, which is a specific predator of Astrocaryum nuts, is recorded for the first time; a trichogrammatid parasitic on the eggs of the same bruchid is also identified
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