13,609 research outputs found
A stability property of a force-free surface bounding a vacuum gap
A force-free surface (FFS) is a sharp boundary separating a void
from a region occupied by a charge-separated force-free plasma. It is proven
here under very general assumptions that there is on a simple
relation between the charge density on the plasma side and the derivative
of \delta=\E\cdot\B along \B on the vacuum side (with \E denoting the
electric field and \B the magnetic field). Combined with the condition
on , this relation implies that a FFS has a general
stability property, already conjectured by Michel (1979, ApJ 227, 579): turns out to attract charges placed on the vacuum side if they are of the
same sign as . In the particular case of a FFS existing in the
axisymmetric stationary magnetosphere of a "pulsar", the relation is given a
most convenient form by using magnetic coordinates, and is shown to imply an
interesting property of a gap. Also, a simple proof is given of the
impossibility of a vacuum gap forming in a field \B which is either uniform
or radial (monopolar)
Structures observed on the spot radiance fields during the FIRE experiment
Three Spot images taken during the FIRE experiment on stratocumulus are analyzed. From this high resolution data detailed observations of the true cloud radiance field may be made. The structure and inhomogeneity of these radiance fields hold important implications for the radiation budget, while the fine scale structure in radiance field provides information on cloud dynamics. Wieliki and Welsh, and Parker et al., have quantified the inhomogeneities of the cumulus clouds through a careful examination of the distribution of cloud (and hole) size as functions of an effective cloud diameter and radiance threshold. Cahalan (1988) has compared for different cloud types of (stratocumulus, fair weather cumulus, convective clouds in the ITCZ) the distributions of clouds (and holes) sizes, the relation between the size and the perimeter of these clouds (and holes), and examining the possibility of scale invariance. These results are extended from LANDSAT resolution (57 m and 30 m) to the Spot resolution (10 m) resolution in the case of boundary layer clouds. Particular emphasis is placed on the statistics of zones of high and low reflectivity as a function of a threshold reflectivity
IT Outsourcing Risk Management at British Petroleum
This paper reports the results of a study of three successive IT outsourcing contracts at British Petroleum (BP). We offer an operational definition of IT outsourcing risk and use it to assess the risk exposure associated with each contract. We then examine how the management at BP dealt with outsourcing risk. Our results show that careful and deliberate risk management can substantially attenuate the level of risk exposure, and that IT outsourcing risks can be managed. Ce document présente les résultats d'une étude de trois contrats d'impartition successifs. L'étude fut conduite chez British Petroleum. Une définition opérationnelle du risque d'impartition est donnée. Cette définition est ensuite utilisée pour déterminer le niveau de risque associé à chacun des contrats. Les mécanismes de gestion de risque sont également identifiés. Les résultats démontrent qu'une gestion active du risque permet de réduire sensiblement les niveau d'exposition au risque, notamment dans le cas de contrats d'impartition des technologies de l'information.Outsourcing of IS, IS risk management, agency theory, transaction cost economics, case study, Impartition des systÚmes informatiques, gestion du risque d'impartition, gestion du risque, théories de l'agence et des coûts de transaction, étude de cas
Optical scatter of quantum noise filter cavity optics
Optical cavities to filter squeezed light for quantum noise reduction require
optics with very low scattering losses. We report on measured light scattering
from two super-polished fused silica optics before and after applying
highly-reflective ion-beam sputtered dielectric coatings. We used an imaging
scatterometer that illuminates the sample with a linearly polarized 1064 nm
wavelength laser at a fixed angle of incidence and records images of back
scatter for azimuthal angles in the plane of the laser beam. We extract from
these images the bidirectional reflectance distribution function (BRDF) of the
optics with and without coating and estimate their integrated scatter. We find
that application of these coatings led to a more than 50% increase of the
integrated wide-angle scatter, to 5.00+/-0.30 and 3.38+/-0.20 ppm for the two
coated samples. In addition, the BRDF function of the coated optics takes on a
pattern of maxima versus azimuthal angle. We compare with a scattering model to
show that this is qualitatively consistent with roughness scattering from the
coating layer interfaces. These results are part of a broader study to
understand and minimize optical loss in quantum noise filter cavities for
interferometric gravitational-wave detectors. The scattering measured for these
samples is acceptable for the 16 m long filter cavities envisioned for the
Laser Interferometer Gravitational-wave Observatory (LIGO), though reducing the
loss further would improve LIGO's quantum-noise limited performance.Comment: 10 pages, 3 figure
Enzyme activity below the dynamical transition at 220 K
Enzyme activity requires the activation of anharmonic motions, such as jumps between potential energy wells. However, in general, the forms and time scales of the functionally important anharmonic dynamics coupled to motion along the reaction coordinate remain to be determined. In particular, the question arises whether the temperature-dependent dynamical transition from harmonic to anharmonic motion in proteins, which has been observed experimentally and using molecular dynamics simulation, involves the activation of motions required for enzyme function. Here we present parallel measurements of the activity and dynamics of a cryosolution of glutamate dehydrogenase as a function of temperature. The dynamical atomic fluctuations faster than ~100 ps were determined using neutron scattering. The results show that the enzyme remains active below the dynamical transition observed at ~220 K, i.e., at temperatures where no anharmonic motion is detected. Furthermore, the activity shows no significant deviation from Arrhenius behavior down to 190 K. The results indicate that the observed transition in the enzyme's dynamics is decoupled from the rate-limiting step along the reaction coordinate
Resolving the terrestrial planet forming regions of HD113766 and HD172555 with MIDI
We present new MIDI interferometric and VISIR spectroscopic observations of
HD113766 and HD172555. Additionally we present VISIR 11um and 18um imaging
observations of HD113766. These sources represent the youngest (16Myr and 12Myr
old respectively) debris disc hosts with emission on <<10AU scales. We find
that the disc of HD113766 is partially resolved on baselines of 42-102m, with
variations in resolution with baseline length consistent with a Gaussian model
for the disc with FWHM of 1.2-1.6AU (9-12mas). This is consistent with the
VISIR observations which place an upper limit of 0."14 (17AU) on the emission,
with no evidence for extended emission at larger distances. For HD172555 the
MIDI observations are consistent with complete resolution of the disc emission
on all baselines of lengths 56-93m, putting the dust at a distance of >1AU
(>35mas). When combined with limits from TReCS imaging the dust at ~10um is
constrained to lie somewhere in the region 1-8AU. Observations at ~18um reveal
extended disc emission which could originate from the outer edge of a broad
disc, the inner parts of which are also detected but not resolved at 10um, or
from a spatially distinct component. These observations provide the most
accurate direct measurements of the location of dust at 1-8AU that might
originate from the collisions expected during terrestrial planet formation.
These observations provide valuable constraints for models of the composition
of discs at this epoch and provide a foundation for future studies to examine
in more detail the morphology of debris discs.Comment: 22 pages, 19 figures, accepted for publication in MNRA
The `bare' strange stars might not be bare
It is proposed that the `bare' strange matter stars might not be bare, and
radio pulsars might be in fact `bare' strange stars. As strange matter stars
being intensely magnetized rotate, the induced unipolar electric fields would
be large enough to construct magnetospheres. This situation is very similar to
that discussed by many authors for rotating neutron stars. Also, the strange
stars with accretion crusts in binaries could act as X-ray pulsars or X-ray
bursters. There are some advantages if radio pulsars are `bare' strange stars.Comment: 11 pages, 1 Postscript figures, LaTeX, Chin. Phys. Lett. 1998,
Vol.15, Nov.12, p.93
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