9,128 research outputs found
Spatial sampling of the thermospheric vertical wind field at auroral latitudes
Results are presented from two nights of bistatic Doppler measurements of neutral thermospheric winds using Fabry!Perot spectrometers at Mawson and Davis stations in Antarctica. A scanning Doppler imager (SDI) at Mawson and a narrow-field Fabry-Perot spectrometer (FPS) at Davis have been used to estimate the vertical wind at three locations along the great circle joining the two stations, in addition to the vertical wind routinely observed above each station. These data were obtained from observations of the 630.0 nm airglow line of atomic oxygen, at a nominal altitude of 240 km. Low!resolution all-sky images produced by the Mawson SDI have been used to relate disturbances in the measured vertical wind field to auroral activity and divergence in the horizontal wind field. Correlated vertical wind responses were observed on a range of horizontal scales from ~150 to 480 km. In general, the behavior of the vertical wind was in agreement with earlier studies, with strong upward winds observed poleward of the optical aurora and sustained, though weak, downward winds observed early in the night. The relation between vertical wind and horizontal divergence was seen to follow the general trend predicted by Burnside et al. (1981), whereby upward vertical winds were associated with positive divergence and vice versa; however, a scale height approximately 3â4 times greater than that modeled by NRLMSISE-00 was required to best fit the data using this relation
Dipoleâdipole interaction in superparamagnetic nanocrystalline Fe63.5Cr10Si13.5B9Cu1Nb3
Cr-substituted Finemet-type nanocrystalline alloy (Fe63.5Cr10Si13.5B9Cu1Nb3) has been studied by
differential scanning calorimetry, x-ray diffraction, Mošssbauer spectroscopy, and magnetic
measurements. The Curie temperature of the remaining amorphous phase decreases as the
crystalline volume fraction increases, reaching values below room temperature. This feature makes
the alloy adequate for studying the magnetic decoupling of the ~Fe,Si! nanocrystals at moderated
temperatures and, in particular, the superparamagnetic relaxation in broad temperature and
crystalline fraction ranges. It was shown that the anomalous dependence of the coercive field on the
annealing temperature can be satisfactorily explained assuming a dipolar-type interaction between
the crystallites.DGES del Gobierno español-PB97-1119-CO2-01Fondo de investigaciĂłn del gobierno hĂșngaro-OTKA T-030753Academia de Ciencias de HungrĂa-AKP 98-25 2,
Thermospheric winds and temperatures above Mawson, Antarctica, observed with an all-sky imaging, Fabry-Perot spectrometer
A new all-sky imaging Fabry-Perot spectrometer has been installed at Mawson station (67&deg;36' S, 62&deg;52' E), Antarctica. This instrument is capable of recording independent spectra from many tens of locations across the sky simultaneously. Useful operation began in March 2007, with spectra recorded on a total of 186 nights. Initial analysis has focused on the large-scale daily and average behavior of winds and temperatures derived from observations of the 630.0 nm airglow line of atomic oxygen, originating from a broad layer centered around 240 km altitude, in the ionospheric F-region. <br><br> The 1993 Horizontal Wind Model (HWM93), NRLMSISE-00 atmospheric model, and the Coupled Thermosphere/Ionosphere Plasmasphere (CTIP) model were used for comparison. During the geomagnetically quiet period studied, observed winds and temperatures were generally well modelled, although temperatures were consistently higher than NRLMSISE-00 predicted, by up to 100 K. CTIP temperatures better matched our data, particularly later in the night, but predicted zonal winds which were offset from those observed by 70â180 ms<sup>&minus;1</sup> westward. During periods of increased activity both winds and temperatures showed much greater variability over time-scales of less than an hour. For the active night presented here, a period of 45 min saw wind speeds decrease by around 180 ms<sup>&minus;1</sup>, and temperatures increase by approximately 100 K. Active-period winds were poorly modelled by HWM93 and CTIP, although observed median temperatures were in better agreement with NRLMSISE-00 during such periods. <br><br> Average behavior was found to be generally consistent with previous studies of thermospheric winds above Mawson. The collected data set was representative of quiet geomagnetic and solar conditions. Geographic eastward winds in the afternoon/evening generally continued until around local midnight, when winds turned equatorward. Geographic meridional and zonal winds in the afternoon were approximately 50 ms<sup>&minus;1</sup> weaker than expected from HWM93, as was the transition to equatorward flow around midnight. There was also a negligible geographic zonal component to the post-midnight wind where HWM93 predicted strong westward flow. Average temperatures between 19:00 and 04:00 local solar time were around 60 K higher than predicted by NRLMSISE-00
Kicking Off in Brazil: Manifesting Democracy
On 6 June, nine days before the Confederations Cup was due to kick off in BrasĂlia, the first of a series of public protests â âmanifestaçÔesâ â began in SĂŁo Paulo. They quickly swept across Brazil, rippling through at least 70 cities, including Rio de Janeiro, Belem, Belo Horizonte, Salvador and BrasĂlia. The largest protests that the country, and indeed Latin America, has witnessed in more than twenty years, the manifestaçÔes have received widespread media attention and critical commentary both within Brazil and overseas. This dispatch reflects on these recent protests, signposting and discussing a number of issues that they raise in Brazil's current conjuncture and more broadly, such as urban and class politics, state violence, political mediation and representation, and the uneven developmental politics of Brazil's so-called âemerging economyâ identity. It suggests that the manifestaçÔes pose challenging questions for all of these issues
Caso ClĂnico ARP nÂș 21: Qual o seu DiagnĂłstico?
info:eu-repo/semantics/publishedVersio
Eclipsing Binaries Showing Light Time Effect
Four eclipsing binaries, which show apparent changes of period, have been
studied with respect to a possible presence of the light time effect. With a
least squares method we calculated new light elements of these systems, the
mass function of the predicted third body, and its minimum mass. We discuss the
probability of the presence of such bodies in terms of mass function, changes
in radial velocity and third light in solution of light curves.Comment: 4 pages, 4 figures, 1 table, conference proceeding
Detailed theoretical predictions of the outskirts of dark matter halos
In the present work we describe the formalism necessary to derive the
properties of dark matter halos beyond two virial radius using the spherical
collapse model (without shell crossing), and provide the framework for the
theoretical prediction presented in Prada et al. (2005). We show in detail how
to obtain within this model the probability distribution for the
spherically-averaged enclosed density at any radii P(delta,r). Using this
probability distribution, we compute the most probable and mean density
profiles, which turns out to differ considerably from each other. We also show
how to obtain the typical profile, as well as the probability distribution and
mean profile for the spherically averaged radial velocity. Two probability
distributions are obtained: a first one is derived using a simple assumption,
that is, if Q is the virial radius in Lagrangian coordinates, then the enclosed
linear contrast delta_l(q,Q) must satisfy the condition that delta_l(q=Q) =
delta_vir, where delta_vir is the linear density contrast within the virial
radius Rvir at the moment of virialization. Then we introduce an additional
constraint to obtain a more accurate P(delta,r) which reproduces to a higher
degree of precision the distribution of the spherically averaged enclosed
density found in the simulations. This new constraint is delta_l(q,Q) <
delta_vir for all q > Q, which means that there are no radii larger than Rvir
where the density contrast is larger than that used to define the virial
radius. Finally, we compare in detail our theoretical predictions for the
probability distributions with the results found in the simulations.Comment: 12 pages, 8 figures, 1 table, replaced to match the published versio
Rudiments of Holography
An elementary introduction to Maldacena's AdS/CFT correspondence is given,
with some emphasis in the Fefferman-Graham construction. This is based on
lectures given by one of us (E.A.) at the Universidad Autonoma de Madrid.Comment: 60 pages, additional misprints corrected, references adde
Photoluminescence Stokes shift and exciton fine structure in CdTe nanocrystals
The photoluminescence spectra of spherical CdTe nanocrystals with zincblende
structure are studied by size-selective spectroscopic techniques. We observe a
resonant Stokes shift of 15 meV when the excitation laser energy is tuned to
the red side of the absorption band at 2.236 eV. The experimental data are
analyzed within a symmetry-based tight-binding theory of the exciton spectrum,
which is first shown to account for the size dependence of the fundamental gap
reported previously in the literature. The theoretical Stokes shift presented
as a function of the gap shows a good agreement with the experimental data,
indicating that the measured Stokes shift indeed arises from the electron-hole
exchange interaction.Comment: 8 pages, 4 figures, LaTe
- âŠ