2,462 research outputs found
Spectropolarimetric signatures of Earth-like extrasolar planets
We present results of numerical simulations of the flux (irradiance), F, and
the degree of polarization (i.e. the ratio of polarized to total flux), P, of
light that is reflected by Earth-like extrasolar planets orbiting solar-type
stars, as functions of the wavelength (from 0.3 to 1.0 micron, with 0.001
micron spectral resolution) and as functions of the planetary phase angle. We
use different surface coverages for our model planets, including vegetation and
a Fresnel reflecting ocean, and clear and cloudy atmospheres. Our
adding-doubling radiative transfer algorithm, which fully includes multiple
scattering and polarization, handles horizontally homogeneous planets only; we
simulate fluxes and polarization of horizontally inhomogeneous planets by
weighting results for homogeneous planets. Like the flux, F, the degree of
polarization, P, of the reflected starlight is shown to depend strongly on the
phase angle, on the composition and structure of the planetary atmosphere, on
the reflective properties of the underlying surface, and on the wavelength, in
particular in wavelength regions with gaseous absorption bands. The sensitivity
of P to a planet's physical properties appears to be different than that of F.
Combining flux with polarization observations thus makes for a strong tool for
characterizing extrasolar planets. The calculated total and polarized fluxes
will be made available through the CDS.Comment: 31 pages text, 17 figures, 1 table Submitted to A&
Modeled flux and polarisation signals of horizontally inhomogeneous exoplanets, applied to Earth--like planets
We present modeled flux and linear polarisation signals of starlight that is
reflected by spatially unresolved, horizontally inhomogeneous planets and
discuss the effects of including horizontal inhomogeneities on the flux and
polarisation signals of Earth-like exoplanets. Our code is based on an
efficient adding--doubling algorithm, which fully includes multiple scattering
by gases and aerosol/cloud particles. We divide a model planet into pixels that
are small enough for the local properties of the atmosphere and surface (if
present) to be horizontally homogeneous. Given a planetary phase angle, we sum
up the reflected total and linearly polarised fluxes across the illuminated and
visible part of the planetary disk, taking care to properly rotate the
polarized flux vectors towards the same reference plane. We compared flux and
polarisation signals of simple horizontally inhomogeneous model planets against
results of the weighted sum approximation, in which signals of horizontally
homogeneous planets are combined. Apart from cases in which the planet has only
a minor inhomogeneity, the signals differ significantly. In particular, the
shape of the polarisation phase function appears to be sensitive to the
horizontal inhomogeneities. The same holds true for Earth-like model planets
with patchy clouds above an ocean and a sandy continent. Our simulations
clearly show that horizontal inhomogeneities leave different traces in flux and
polarisation signals. Combining flux with polarisation measurements would help
retrieving the atmospheric and surface patterns on a planet
The influence of forward-scattered light in transmission measurements of (exo)planetary atmospheres
[Abridged] The transmission of light through a planetary atmosphere can be
studied as a function of altitude and wavelength using stellar or solar
occultations, giving often unique constraints on the atmospheric composition.
For exoplanets, a transit yields a limb-integrated, wavelength-dependent
transmission spectrum of an atmosphere. When scattering haze and/or cloud
particles are present in the planetary atmosphere, the amount of transmitted
flux not only depends on the total optical thickness of the slant light path
that is probed, but also on the amount of forward-scattering by the scattering
particles. Here, we present results of calculations with a three-dimensional
Monte Carlo code that simulates the transmitted flux during occultations or
transits. For isotropically scattering particles, like gas molecules, the
transmitted flux appears to be well-described by the total atmospheric optical
thickness. Strongly forward-scattering particles, however, such as commonly
found in atmospheres of Solar System planets, can increase the transmitted flux
significantly. For exoplanets, such added flux can decrease the apparent radius
of the planet by several scale heights, which is comparable to predicted and
measured features in exoplanet transit spectra. We performed detailed
calculations for Titan's atmosphere between 2.0 and 2.8 micron and show that
haze and gas abundances will be underestimated by about 8% if
forward-scattering is ignored in the retrievals. At shorter wavelengths, errors
in the gas and haze abundances and in the spectral slope of the haze particles
can be several tens of percent, also for other Solar System planetary
atmospheres. We also find that the contribution of forward-scattering can be
fairly well described by modelling the atmosphere as a plane-parallel slab.Comment: Icarus, accepted for publicatio
Looking for the rainbow on exoplanets covered by liquid and icy water clouds
Looking for the primary rainbow in starlight that is reflected by exoplanets
appears to be a promising method to search for liquid water clouds in
exoplanetary atmospheres. Ice water clouds, that consist of water crystals
instead of water droplets, could potentially mask the rainbow feature in the
planetary signal by covering liquid water clouds. Here, we investigate the
strength of the rainbow feature for exoplanets that have liquid and icy water
clouds in their atmosphere, and calculate the rainbow feature for a realistic
cloud coverage of Earth. We calculate flux and polarization signals of
starlight that is reflected by horizontally and vertically inhomogeneous
Earth--like exoplanets, covered by patchy clouds consisting of liquid water
droplets or water ice crystals. The planetary surfaces are black. On a planet
with a significant coverage of liquid water clouds only, the total flux signal
shows a weak rainbow feature. Any coverage of the liquid water clouds by ice
clouds, however, dampens the rainbow feature in the total flux, and thus the
discovery of liquid water in the atmosphere. On the other hand, detecting the
primary rainbow in the polarization signal of exoplanets appears to be a
powerful tool for detecting liquid water in exoplanetary atmospheres, even when
these clouds are partially covered by ice clouds. In particular, liquid water
clouds covering as little as 10%-20% of the planetary surface, with more than
half of these covered by ice clouds, still create a polarized rainbow feature
in the planetary signal. Indeed, calculations of flux and polarization signals
of an exoplanet with a realistic Earth--like cloud coverage, show a strong
polarized rainbow feature.Comment: accepted for publication in Astronomy & Astrophysic
Scattering matrices and expansion coefficients of Martian analogue palagonite particles
We present measurements of ratios of elements of the scattering matrix of
Martian analogue palagonite particles for scattering angles ranging from 3 to
174 degrees and a wavelength of 632.8 nm. To facilitate the use of these
measurements in radiative transfer calculations we have devised a method that
enables us to obtain, from these measurements, a normalized synthetic
scattering matrix covering the complete scattering angle range from 0 to 180
degrees. Our method is based on employing the coefficients of the expansions of
scattering matrix elements into generalized spherical functions. The synthetic
scattering matrix elements and/or the expansion coefficients obtained in this
way, can be used to include multiple scattering by these irregularly shaped
particles in (polarized) radiative transfer calculations, such as calculations
of sunlight that is scattered in the dusty Martian atmosphere.Comment: 34 pages 7 figures 1 tabl
Obfuscation for Cryptographic Purposes
An obfuscation of a function F should satisfy two requirements: firstly, using it should be possible to evaluate F; secondly, should not reveal anything about F that cannot be learnt from oracle access to F. Several definitions for obfuscation exist. However, most of them are either too weak for or incompatible with cryptographic applications, or have been shown impossible to achieve, or both.
We give a new definition of obfuscation and argue for its reasonability and usefulness. In particular, we show that it is strong enough for cryptographic applications, yet we show that it has the potential for interesting positive results. We illustrat
Large atom number Bose-Einstein condensate of sodium
We describe the setup to create a large Bose-Einstein condensate containing
more than 120x10^6 atoms. In the experiment a thermal beam is slowed by a
Zeeman slower and captured in a dark-spot magneto-optical trap (MOT). A typical
dark-spot MOT in our experiments contains 2.0x10^10 atoms with a temperature of
320 microK and a density of about 1.0x10^11 atoms/cm^3. The sample is spin
polarized in a high magnetic field, before the atoms are loaded in the magnetic
trap. Spin polarizing in a high magnetic field results in an increase in the
transfer efficiency by a factor of 2 compared to experiments without spin
polarizing. In the magnetic trap the cloud is cooled to degeneracy in 50 s by
evaporative cooling. To suppress the 3-body losses at the end of the
evaporation the magnetic trap is decompressed in the axial direction.Comment: 11 pages, 12 figures, submitted to Review Of Scientific Instrument
The dark days are overcast : iron-bearing clouds on HD 209458 b and WASP-43 b can explain low-dayside albedos
Funding; This project has received funding from STFC, under project number ST/V000861/1. Ch.H. and L.C. acknowledges funding from the European Union H2020-MSCA-ITN-2019 under grant agreement no. 860470 (CHAMELEON). K.L.C further acknowledges funding by UK Research and Innovation (UKRI) under the UK governmentâs Horizon Europe funding guarantee as part of an ERC Starter Grant [grant number EP/Y006313/1].We simulate the geometric albedo spectra of hot Jupiter exoplanets HD 209458 b and WASP-43 b, based on global climate model (GCMs) post-processed with kinetic cloud models. We predict WASP-43 b to be cloudy throughout its dayside, while HD 209458 b has a clear upper atmosphere around the hot sub-solar point, largely due to the inclusion of strong optical absorbers TiO and VO in the GCM for the latter causes a temperature inversion. In both cases our models find low geometric albedos - 0.026 for WASP-43b and 0.028 for HD 209458 b when averaged over the CHaracterizing ExOPlanet Satellite bandpass of âŒ0.35 - 1.1 ÎŒm - indicating dark daysides, similar to the low albedos measured by observations. We demonstrate the strong impact of clouds that contain Fe-bearing species on the modelled geometric albedos; without Fe-bearing species forming in the clouds, the albedos of both planets would be much higher (0.518 for WASP-43 b, 1.37 for HD 209458 b). We conclude that a cloudy upper or mid-to-lower atmosphere that contains strongly absorbing Fe-bearing aerosol species, is an alternative to a cloud-free atmosphere in explaining the low dayside albedos of hot Jupiter atmospheres such as HD 209458 b and WASP-43 b.Peer reviewe
Effect of solder volume on joint shape with variable chip-to-board contact pad ratio
The objective of this paper is to investigate the effect of the pad size ratio between the chip and
board end of a solder joint on the shape of that solder joint in combination with the solder volume available.
The shape of the solder joint is correlated to its reliability and thus of importance. For low density chip bond
pad applications Flip Chip (FC) manufacturing costs can be kept down by using larger size board
pads suitable for solder application. By using âSurface Evolverâ software package the solder joint
shapes associated with different size/shape solder preforms and chip/board pad ratios are predicted.
In this case a so called Flip-Chip Over Hole (FCOH) assembly format has been used. Assembly trials
involved the deposition of lead-free 99.3Sn0.7Cu solder on the board side, followed by reflow, an
underfill process and back die encapsulation. During the assembly work pad off-sets occurred that
have been taken into account for the Surface Evolver solder joint shape prediction and accurately
matched the real assembly. Overall, good correlation was found between the simulated solder joint
shape and the actual fabricated solder joint shapes. Solder preforms were found to exhibit better
control over the solder volume. Reflow simulation of commercially available solder preform volumes
suggests that for a fixed stand-off height and chip-board pad ratio, the solder volume value and the
surface tension determines the shape of the joint
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