3,289 research outputs found
Glory revealed in disk-integrated photometry of Venus
Context. Reflected light from a spatially unresolved planet yields unique
insight into the overall optical properties of the planet cover. Glories are
optical phenomena caused by light that is backscattered within spherical
droplets following a narrow distribution of sizes; they are well known on Earth
as localised features above liquid clouds. Aims. Here we report the first
evidence for a glory in the disk-integrated photometry of Venus and, in turn,
of any planet. Methods. We used previously published phase curves of the planet
that were reproduced over the full range of phase angles with model predictions
based on a realistic description of the Venus atmosphere. We assumed that the
optical properties of the planet as a whole can be described by a uniform and
stable cloud cover, an assumption that agrees well with observational evidence.
Results. We specifically show that the measured phase curves mimic the
scattering properties of the Venus upper-cloud micron-sized aerosols, also at
the small phase angles at which the glory occurs, and that the glory contrast
is consistent with what is expected after multiple scattering of photons. In
the optical, the planet appears to be brighter at phase angles of 11-13 deg
than at full illumination; it undergoes a maximum dimming of up to 10 percent
at phases in between. Conclusions. Glories might potentially indicate spherical
droplets and, thus, extant liquid clouds in the atmospheres of exoplanets. A
prospective detection will require exquisite photometry at the small
planet-star separations of the glory phase angles.Comment: In press. Astronomy & Astrophysics. Letter to the Editor; 201
Coupling of morphology to surface transport in ion-beam irradiated surfaces. I. Oblique incidence
We propose and study a continuum model for the dynamics of amorphizable
surfaces undergoing ion-beam sputtering (IBS) at intermediate energies and
oblique incidence. After considering the current limitations of more standard
descriptions in which a single evolution equation is posed for the surface
height, we overcome (some of) them by explicitly formulating the dynamics of
the species that transport along the surface, and by coupling it to that of the
surface height proper. In this we follow recent proposals inspired by
``hydrodynamic'' descriptions of pattern formation in aeolian sand dunes and
ion-sputtered systems. From this enlarged model, and by exploiting the
time-scale separation among various dynamical processes in the system, we
derive a single height equation in which coefficients can be related to
experimental parameters. This equation generalizes those obtained by previous
continuum models and is able to account for many experimental features of
pattern formation by IBS at oblique incidence, such as the evolution of the
irradiation-induced amorphous layer, transverse ripple motion with non-uniform
velocity, ripple coarsening, onset of kinetic roughening and other.
Additionally, the dynamics of the full two-field model is compared with that of
the effective interface equation.Comment: 23 pages, 14 figures. Movies of figures 6, 7, and 8 available at
http://gisc.uc3m.es/~javier/Movies
Multilayer hazes over Saturnâs hexagon from Cassini ISS limb images
ProducciĂłn CientĂficaVer abstract
The Simple Non-degenerate Relativistic Gas: Statistical Properties and Brownian Motion
This paper shows a novel calculation of the mean square displacement of a
classical Brownian particle in a relativistic thermal bath. The result is
compared with the expressions obtained by other authors. Also, the
thermodynamic properties of a non-degenerate simple relativistic gas are
reviewed in terms of a treatment performed in velocity space.Comment: 6 pages, 2 figure
High-Order SUSY-QM, the Quantum XP Model and zeroes of the Riemann Zeta function
Making use of the first- and second-order algorithms of supersymmetric
quantum mechanics (SUSY-QM), we construct quantum mechanical Hamiltonians whose
spectra are related to the zeroes of the Riemann Zeta function .
Inspired by the model of Das and Kalauni (DK), which corresponds to this
function in the strip , and taking the factorization energy equal to
zero, we use the wave function , , as a seed solution
for our algorithms, obtaining XP-like operators. Thus, we construct SUSY-QM
partner Hamiltonians whose zero energy mode locates exactly the nontrivial
zeroes of along the critical line in the complex plane.
We further find that unlike the DK case, where the SUSY-QM partner potentials
correspond to free particles, our partner potentials belong to the family of
inverse squared distance potentials with complex couplings
Coulomb explosion sputtering of selectively oxidized Si
We have studied multiply charged Arq+ ion induced potential sputtering of a
unique system comprising of coexisting Silicon and Silicon oxide surfaces. Such
surfaces are produced by oblique angle oxygen ion bombardment on Si(100), where
ripple structures are formed and one side of each ripple gets more oxidized. It
is observed that higher the potential energy of Arq+ ion, higher the sputtering
yield of the non conducting (oxide) side of the ripple as compared to the
semiconducting side. The results are explained in terms of Coulomb explosion
model where potential sputtering depends on the conductivity of the ion impact
sites.Comment: 9 pages and 3 figure
Beyond genomic selection: the animal model strikes back (one generation)!
Genome inheritance is by segments of DNA rather than by independent loci. We introduce the ancestral regression (AR) as a recursive system of simultaneous equations, with grandparental path coefficients as novel parameters. The information given by the pedigree in the AR is complementary with that provided by a dense set of genomic markers, such that the resulting linear function of grandparental BV is uncorrelated to the average of parental BV in the absence of inbreeding. AR is then connected to segmental inheritance by a causal multivariate Gaussian density for BV. The resulting covariance structure (Σ) is Markovian, meaning that conditional on the BV of parents and grandparents, the BV of the animal is independent of everything else. Thus, an algorithm is presented to invert the resulting covariance structure, with a computing effort that is linear in the number of animals as in the case of the inverse additive relationship matrix.Instituto de Genética Veterinari
Modelling of tungsten erosion and deposition in the divertor of JET-ILW in comparison to experimental findings
The erosion, transport and deposition of tungsten in the outer divertor of JET-ILW has been studied for an H-Mode discharge with low frequency ELMs. For this specific case with an inter-ELM electron temperature at the strike point of about 20 eV, tungsten sputtering between ELMs is almost exclusively due to beryllium impurity and self-sputtering. However, during ELMs tungsten sputtering due to deuterium becomes important and even dominates. The amount of simulated local deposition of tungsten relative to the amount of sputtered tungsten in between ELMs is very high and reaches values of 99% for an electron density of 5E13 cm(-3) at the strike point and electron temperatures between 10 and 30 eV. Smaller deposition values are simulated with reduced electron density. The direction of the B-field significantly influences the local deposition and leads to a reduction if the E x B drift directs towards the scrape-off-layer. Also, the thermal force can reduce the tungsten deposition, however, an ion temperature gradient of about 0.1 eV/mm or larger is needed for a significant effect. The tungsten deposition simulated during ELMs reaches values of about 98% assuming ELM parameters according to free-streaming model. The measured WI emission profiles in between and within ELMs have been reproduced by the simulation. The contribution to the overall net tungsten erosion during ELMs is about 5 times larger than the one in between ELMs for the studied case. However, this is due to the rather low electron temperature in between ELMs, which leads to deuterium impact energies below the sputtering threshold for tungsten.EURATOM 63305
Overview of progress in European medium sized tokamaks towards an integrated plasma-edge/wall solution
Integrating the plasma core performance with an edge and scrape-off layer (SOL) that leads
to tolerable heat and particle loads on the wall is a major challenge. The new European
medium size tokamak task force (EU-MST) coordinates research on ASDEX Upgrade
(AUG), MAST and TCV. This multi-machine approach within EU-MST, covering a wide
parameter range, is instrumental to progress in the field, as ITER and DEMO core/pedestal
and SOL parameters are not achievable simultaneously in present day devices. A two prong
approach is adopted. On the one hand, scenarios with tolerable transient heat and particle
loads, including active edge localised mode (ELM) control are developed. On the other hand,
divertor solutions including advanced magnetic configurations are studied. Considerable
progress has been made on both approaches, in particular in the fields of: ELM control with
resonant magnetic perturbations (RMP), small ELM regimes, detachment onset and control,
as well as filamentary scrape-off-layer transport. For example full ELM suppression has now
been achieved on AUG at low collisionality with n = 2 RMP maintaining good confinement
HH(98,y2) 0.95. Advances have been made with respect to detachment onset and control.
Studies in advanced divertor configurations (Snowflake, Super-X and X-point target divertor)
shed new light on SOL physics. Cross field filamentary transport has been characterised in a
wide parameter regime on AUG, MAST and TCV progressing the theoretical and experimental
understanding crucial for predicting first wall loads in ITER and DEMO. Conditions in the
SOL also play a crucial role for ELM stability and access to small ELM regimes.European Commission (EUROfusion 633053
- âŠ