52 research outputs found
Tables of phase functions, opacities, albedos, equilibrium temperatures, and radiative accelerations of dust grains in exoplanets
There has been growing observational evidence for the presence of condensates in the atmospheres and/or comet-like tails of extrasolar planets. As a result, systematic and homogeneous tables of dust properties are useful in order to facilitate further observational and theoretical studies. In this paper we present calculations and analysis of non-isotropic phase functions, asymmetry parameter (mean cosine of the scattering angle), absorption and scattering opacities, single scattering albedos, equilibrium temperatures, and radiative accelerations of dust grains relevant for extrasolar planets. Our assumptions include spherical grain shape, Deirmendjian particle size distribution, and Mie theory. We consider several species: corundum/alumina, perovskite, olivines with 0 and 50 per cent iron content, pyroxenes with 0, 20, and 60 per cent iron content, pure iron, carbon at two different temperatures, water ice, liquid water, and ammonia. The presented tables cover the wavelength range of 0.2–500 μm and modal particle radii from 0.01 to 100 μm. Equilibrium temperatures and radiative accelerations assume irradiation by a non-blackbody source of light with temperatures from 7000 to 700 K seen at solid angles from 2π to 10−6 sr. The tables are provided to the community together with a simple code which allows for an optional, finite, angular dimension of the source of light (star) in the phase function
Formation of recurring slope lineae on Mars by rarefied gas-triggered granular flows
Active dark flows known as recurring slope lineae have been observed on the
warmest slopes of equatorial Mars. The morphology, composition and seasonality
of the lineae suggest a role of liquid water in their formation. However,
internal and atmospheric sources of water appear to be insufficient to sustain
the observed slope activity. Experimental evidence suggests that under the low
atmospheric pressure at the surface of Mars, gas can flow upwards through
porous Martian soil due to thermal creep under surface regions heated by the
Sun, and disturb small particles. Here we present numerical simulations to
demonstrate that such a dry process involving the pumping of rarefied gas in
the Martian soil due to temperature contrasts can explain the formation of the
recurring slope lineae. In our simulations, solar irradiation followed by
shadow significantly reduces the angle of repose due to the resulting temporary
temperature gradients over shaded terrain, and leads to flow at intermediate
slope angles. The simulated flow locations are consistent with observed
recurring slope lineae that initiate in rough and bouldered terrains with local
shadows over the soil. We suggest that this dry avalanche process can explain
the formation of the recurring slope lineae on Mars without requiring liquid
water or CO2 frost activity.Comment: 15 pages, 3 figure
An all-sky radiative transfer method to predict optimal tilt and azimuth angle of a solar collector
This paper describes a radiative transfer method for calculating radiances in all-sky conditions and performing an integration over the view hemisphere of an arbitrary plane to calculate tilted irradiance. The advantage of this method is the combination of cloud parameters inside the radiative transfer model with a tilt procedure. For selected locations this method is applied with cloud, ozone, water vapour and aerosol input data to determine tilted irradiance, horizontal irradiance and optimal tilt angle. A validation is performed for horizontal and tilted irradiance against high-quality pyranometer data. For 27 sites around the world, the annual horizontal irradiation predicted by our model had a mean bias difference of +0.56% and a root-mean-squared difference of 6.69% compared to ground measurements. The difference between the annual irradiation estimates from our model and the measurements from one site that provides tilted irradiance were within ±6% for all orientations except the north-facing vertical plane. For European and African sites included in the validation, the optimal tilt from our model is typically a few degrees steeper than predictions from the popular PVGIS online tool. Our model is generally applicable to any location on the earth’s surface as the satellite cloud and atmosphere data and aerosol climatology data are available globally. Furthermore, all of the input data are standard variables in climate models and so this method can be used to predict tilted irradiance in future climate experiments
Interstellar polarization and grain alignment: the role of iron and silicon
We compiled the polarimetric data for a sample of lines of sight with known
abundances of Mg, Si, and Fe. We correlated the degree of interstellar
polarization and polarization efficiency (the ratio of to the colour
excess or extinction ) with dust phase abundances. We detect an
anticorrelation between and the dust phase abundance of iron in non
silicate - containing grains ]_\rm d, a correlation
between and the abundance of Si, and no correlation between or
and dust phase abundances. These findings can be explained if mainly
the silicate grains aligned by the radiative mechanism are responsible for the
observed interstellar linear polarization.Comment: Accepted for publication in Astronomy and Astrophysic
Cloud Coverage Acts as an Amplifier for Ecological Light Pollution in Urban Ecosystems
The diurnal cycle of light and dark is one of the strongest environmental factors for life on Earth. Many species in both terrestrial and aquatic ecosystems use the level of ambient light to regulate their metabolism, growth, and behavior. The sky glow caused by artificial lighting from urban areas disrupts this natural cycle, and has been shown to impact the behavior of organisms, even many kilometers away from the light sources. It could be hypothesized that factors that increase the luminance of the sky amplify the degree of this “ecological light pollution”. We show that cloud coverage dramatically amplifies the sky luminance, by a factor of 10.1 for one location inside of Berlin and by a factor of 2.8 at 32 km from the city center. We also show that inside of the city overcast nights are brighter than clear rural moonlit nights, by a factor of 4.1. These results have important implications for choronobiological and chronoecological studies in urban areas, where this amplification effect has previously not been considered
Motion of dust in mean-motion resonances with planets
Effect of stellar electromagnetic radiation on motion of spherical dust
particle in mean-motion orbital resonances with a planet is investigated.
Planar circular restricted three-body problem with the Poynting-Robertson (P-R)
effect yields monotonous secular evolution of eccentricity when the particle is
trapped in the resonance. Elliptically restricted three-body problem with the
P-R effect enables nonmonotonous secular evolution of eccentricity and the
evolution of eccentricity is qualitatively consistent with the published
results for the complicated case of interaction of electromagnetic radiation
with nonspherical dust grain. Thus, it is sufficient to allow either nonzero
eccentricity of the planet or nonsphericity of the grain and the orbital
evolutions in the resonances are qualitatively equal for the two cases. This
holds both for exterior and interior mean-motion orbital resonances. Evolutions
of longitude of pericenter in the planar circular and elliptical restricted
three-body problems are shown. Our numerical integrations suggest that any
analytic expression for secular time derivative of the particle's longitude of
pericenter does not exist, if a dependence on semi-major axis, eccentricity and
longitude of pericenter is considered (the P-R effect and mean-motion resonance
with the planet in circular orbit is taken into account).
Change of optical properties of the spherical grain with the heliocentric
distance is also considered. The change of the optical properties: i) does not
have any significant influence on secular evolution of eccentricity, ii) causes
that the shift of pericenter is mainly in the same direction/orientation as the
particle motion around the Sun. The statements hold both for circular and
noncircular planetary orbits.Comment: 22 pages, 12 figure
Multiple Angle Observations Would Benefit Visible Band Remote Sensing Using Night Lights
The spatial and angular emission patterns of artificial and natural light emitted, scattered, and reflected from the Earth at night are far more complex than those for scattered and reflected solar radiation during daytime. In this commentary, we use examples to show that there is additional information contained in the angular distribution of emitted light. We argue that this information could be used to improve existing remote sensing retrievals based on night lights, and in some cases could make entirely new remote sensing analyses possible. This work will be challenging, so we hope this article will encourage researchers and funding agencies to pursue further study of how multi‐angle views can be analyzed or acquired
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