3,455 research outputs found
Large-scale circulation departures related to wet episodes in northeast Brazil
Large scale circulation features are presented as related to wet spells over northeast Brazil (Nordeste) during the rainy season (March and April) of 1979. The rainy season season is devided into dry and wet periods, the FGGE and geostationary satellite data was averaged and mean and departure fields of basic variables and cloudiness were studied. Analysis of seasonal mean circulation features show: lowest sea level easterlies beneath upper level westerlies; weak meridional winds; high relative humidity over the Amazon basin and relatively dry conditions over the South Atlantic Ocean. A fluctuation was found in the large scale circulation features on time scales of a few weeks or so over Nordeste and the South Atlantic sector. Even the subtropical High SLP's have large departures during wet episodes, implying a short period oscillation in the Southern Hemisphere Hadley circulation
Application of the equipartition theorem to the thermal excitation of quartz tuning forks
The deflection signal of a thermally excited force sensor of an atomic force
microscope can be analyzed to gain important information about the detector
noise and about the validity of the equipartion theorem of thermodynamics.
Here, we measured the temperature dependence of the thermal amplitude of a
tuning fork and compared it to the expected values based on the equipartition
theorem. In doing so, we prove the validity of these assumptions in the
temperature range from 140K to 300K. Furthermore, the application of the
equipartition theorem to quartz tuning forks at liquid helium temperatures is
discussed.Comment: 8 pages, 3 figures, published in Applied Physics Letter
Accretion by Isolated Neutron Stars
Accretion of interstellar material by an isolated neutron star is discussed.
The point I address here is the interaction between the accretion flow and the
stellar magnetosphere. I show that the interchange instabilities of the
magnetospheric boundary under the conditions of interest are basically
suppressed. The entry of the material into the magnetosphere is governed by
diffusion. Due to this reason the persistent accretion luminosity of isolated
neutron stars is limited to < 4E+26 erg/s. These objects can also appear as
X-ray bursters with the burst durations of about 30 minutes and repetition time
of about 1E+5 yr. This indicates that the number of the accreting isolated
neutron stars which could be observed with recent and modern X-ray missions is
a few orders of magnitude smaller than that previously estimated.Comment: 3 pages, to appear in Astrophysics and Space Science, in the
proceedings of "Isolated Neutron Stars: from the Interior to the Surface",
edited by D. Page, R. Turolla and S. Zan
Mechanoelectric Response of Single-Crystal Rubrene from Ab Initio Molecular Dynamics
A robust understanding of the mechanoelectric response of organic semiconductors is crucial for the development of materials for flexible electronics. In particular, the prospect of using external mechanical strain to induce a controlled modulation in the charge mobility of the material is appealing. Here we develop an accurate computational protocol for the prediction of the mechanical strain dependence of charge mobility. Ab initio molecular dynamics simulations with a van der Waals density functional are carried out to quantify the off-diagonal electronic disorder in the system as a function of strain by the explicit calculation of the thermal distributions of electronic coupling matrix elements. The approach is applied to a representative molecular organic semiconductor, single-crystal rubrene. We find that charge mobility along the high-mobility direction a⃗ increases with compressive strain, as one might expect. However, the increase is larger when compressive strain is applied in the perpendicular direction than in the parallel direction with respect to a⃗, in agreement with experimental reports. We show that this seemingly counterintuitive result is a consequence of a significantly greater suppression of electronic coupling fluctuations in the range of 50-150 cm-1, when strain is applied in the perpendicular direction. Thus our study highlights the importance of considering off-diagonal electron-phonon coupling in understanding the mechanoelectric response of organic semiconducting crystals. The computational approach developed here is well suited for the accurate prediction of strain-charge mobility relations and should provide a useful tool for the emerging field of molecular strain engineering
Measurements with the Chandra X-Ray Observatory's flight contamination monitor
NASA's Chandra X-ray Observatory includes a Flight Contamination Monitor
(FCM), a system of 16 radioactive calibration sources mounted to the inside of
the Observatory's forward contamination cover. The purpose of the FCM is to
verify the ground-to-orbit transfer of the Chandra flux scale, through
comparison of data acquired during the ground calibration with those obtained
in orbit, immediately prior to opening the Observatory's sun-shade door. Here
we report results of these measurements, which place limits on the change in
mirror--detector system response and, hence, on any accumulation of molecular
contamination on the mirrors' iridium-coated surfaces.Comment: 7pages,8figures,for SPIE 4012, paper 7
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