scholarly journals Photometric Properties of Vesta

2012 ◽  
Vol 10 (H16) ◽  
pp. 179-179 ◽  
Author(s):  
Jian-Yang Li ◽  
L. Jorda ◽  
H. U. Keller ◽  
N. Mastrodemos ◽  
S. Mottola ◽  
...  
Keyword(s):  
Multiple Scattering ◽  
Phase Function ◽  
Near Infrared ◽  
Roughness Parameter ◽  
Single Scattering ◽  
Asymmetry Factor ◽  
Linear Effect ◽  
Visible Wavelengths ◽  
Phase Angles ◽  
Geometric Albedo

AbstractThe Dawn spacecraft orbited Asteroid (4) Vesta for a year, and returned disk-resolved images and spectra covering visible and near-infrared wavelengths at scales as high as 20 m/pix. The visible geometric albedo of Vesta is ~ 0.36. The disk-integrated phase function of Vesta in the visible wavelengths derived from Dawn approach data, previous ground-based observations, and Rosetta OSIRIS observations is consistent with an IAU H-G phase law with H=3.2 mag and G=0.28. Hapke's modeling yields a disk-averaged single-scattering albedo of 0.50, an asymmetry factor of -0.25, and a roughness parameter of ~20 deg at 700 nm wavelength. Vesta's surface displays the largest albedo variations observed so far on asteroids, ranging from ~0.10 to ~0.76 in geometric albedo in the visible wavelengths. The phase function of Vesta displays obvious systematic variations with respect to wavelength, with steeper slopes within the 1- and 2-micron pyroxene bands, consistent with previous ground-based observations and laboratory measurement of HED meteorites showing deeper bands at higher phase angles. The relatively high albedo of Vesta suggests significant contribution of multiple scattering. The non-linear effect of multiple scattering and the possible systematic variations of phase function with albedo across the surface of Vesta may invalidate the traditional algorithm of applying photometric correction on airless planetary surfaces.

scholarly journals Closed-formed solutions of geometric albedos and phase curves of exoplanets for any reflection law

2021 ◽  
Author(s):  
Kevin Heng ◽  
Daniel Kitzmann
Keyword(s):  
Closed Form ◽  
Phase Function ◽  
Single Scattering ◽  
Phase Curve ◽  
Asymmetry Factor ◽  
Closed Form Solutions ◽  
James Webb Space Telescope ◽  
Phase Integral ◽  
The Relationship ◽  
Geometric Albedo

Abstract The albedo of a celestial body is the frac-tion of light reflected by it. Studying the albe-dos of the planets and moons of the Solar Sys-tem dates back at least a century [1, 2, 3, 4, 5]. Of particular interest is the relationship between the albedo measured at superior conjunction (full phase), known as the “geometric albedo”, and the albedo considered over all phase angles, known as the “spherical albedo” [2, 6, 7]. Modern astronom-ical facilities enable the measurement of geomet-ric albedos from visible/optical secondary eclipses [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20] and the inference of the Bond albedo (spherical albedo measured over all wavelengths) from in-frared phase curves [21, 22, 23, 24, 25] of transit-ing exoplanets. Determining the relationship be-tween the geometric and spherical or Bond albe-dos usually involves complex numerical calculations [26, 27, 28, 29, 30, 31, 32] and closed-form solu-tions are restricted to simple reflection laws [33, 34]. Here we report the discovery of closed-form solu-tions for the geometric albedo and integral phase function that apply to any law of reflection. The integral phase function is used to obtain the phase integral, which is the ratio of the spherical to the geometric albedos. The generality of the solu-tions stems from a judicious choice of the coor-dinate system in which to perform different parts of the derivation. The closed-formed solutions have profound implications for interpreting obser-vations. The shape of a reflected light phase curve and the secondary eclipse depth may now be self-consistently inverted to retrieve fundamental phys-ical parameters (single-scattering albedo, scatter-ing asymmetry factor, cloud cover). Fully-Bayesian phase curve inversions for reflectance maps and si-multaneous light curve detrending may now be per-formed, without the need for binning in time, due to the efficiency of computation. We demonstrate these innovations for the hot Jupiter Kepler-7b, inferring a revised geometric albedo of 0.12 ± 0.02, a Bond albedo of 0.18 ± 0.03 and a phase integral of 1.5 ± 0.1, which is consistent with isotropic scatter-ing. The scattering asymmetry factor is 0.04±0.15, implying that the aerosols are small compared to the wavelengths probed by the Kepler space tele-scope. In the near future, one may use the closed-form solutions discovered here to extract funda-mental parameters, across wavelength, from multi-wavelength phase curves of both gas-giant and ter-restrial exoplanets measured by the James Webb Space Telescope.

Parameterization for Cloud Longwave Scattering for Use in Atmospheric Models

1999 ◽  
Vol 12 (1) ◽  
pp. 159-169 ◽  
Author(s):  
Ming-Dah Chou ◽  
Kyu-Tae Lee ◽  
Si-Chee Tsay ◽  
Qiang Fu
Keyword(s):  
Multiple Scattering ◽  
Optical Thickness ◽  
Longwave Radiation ◽  
Maximum Error ◽  
Single Scattering ◽  
Asymmetry Factor ◽  
Atmospheric Conditions ◽  
Cloud Particle ◽  
Cloudy Atmosphere ◽  
Clear Atmosphere

Abstract A parameterization for the scattering of thermal infrared (longwave) radiation by clouds has been developed based on discrete-ordinate multiple-scattering calculations. The effect of backscattering is folded into the emission of an atmospheric layer and the absorption between levels by scaling the cloud optical thickness. The scaling is a function of the single-scattering albedo and asymmetry factor. For wide ranges of cloud particle size, optical thickness, height, and atmospheric conditions, flux errors induced by the parameterization are small. They are <4 W m−2 (2%) in the upward flux at the top of the atmosphere and <2 W m−2 (1%) in the downward flux at the surface. Compared to the case that scattering by clouds is neglected, the flux errors are more than a factor of 2 smaller. The maximum error in cooling rate is ≈8%, which occurs at the top of clouds, as well as at the base of high clouds where the difference between the cloud and surface temperatures is large. With the scaling approximation, radiative transfer equations for a cloudy atmosphere are identical with those for a clear atmosphere, and the difficulties in applying a multiple-scattering algorithm to a partly cloudy atmosphere (assuming homogeneous clouds) are avoided. The computational efficiency is practically the same as that for a clear atmosphere. The parameterization represents a significant reduction in one source of the errors involved in the calculation of longwave cooling in cloudy atmospheres.

scholarly journals Comparison of Aerosol Reflectance Correction Schemes Using Two Near-Infrared Wavelengths for Ocean Color Data Processing

2018 ◽  
Vol 10 (11) ◽  
pp. 1791 ◽  
Author(s):  
Jae-Hyun Ahn ◽  
Young-Je Park ◽  
Hajime Fukushima
Keyword(s):  
Radiative Transfer ◽  
Near Infrared ◽  
Ocean Color ◽  
Atmospheric Correction ◽  
Single Scattering ◽  
Weighting Factor ◽  
Simulation Code ◽  
Aerosol Model ◽  
Median Error ◽  
Visible Wavelengths

This paper reanalyzes the aerosol reflectance correction schemes employed by major ocean color missions. The utilization of two near-infrared (NIR) bands to estimate aerosol reflectance in visible wavelengths has been widely adopted, for example by SeaWiFS/MODIS/VIIRS (GW1994), OCTS/GLI/SGLI (F1998), MERIS/OLCI (AM1999), and GOCI/GOCI-II (A2016). The F1998, AM1999, and A2016 schemes were developed based on GW1994; however, they are implemented differently in terms of aerosol model selection and weighting factor computation. The F1998 scheme determines the contribution of the most appropriate aerosol models in the aerosol optical thickness domain, whereas the GW1994 scheme focuses on single-scattering reflectance. The AM1999 and A2016 schemes both directly resolve the multiple scattering domain contribution. However, A2016 also considers the spectrally dependent weighting factor, whereas AM1999 calculates the spectrally invariant weighting factor. Additionally, ocean color measurements on a geostationary platform, such as GOCI, require more accurate aerosol correction schemes because the measurements are made over a large range of solar zenith angles which causes diurnal instabilities in the atmospheric correction. Herein, the four correction schemes were tested with simulated top-of-atmosphere radiances generated by radiative transfer simulations for three aerosol models. For comparison, look-up tables and test data were generated using the same radiative transfer simulation code. All schemes showed acceptable accuracy, with less than 10% median error in water reflectance retrieval at 443 nm. Notably, the accuracy of the A2016 scheme was similar among different aerosol models, whereas the other schemes tended to provide better accuracy with coarse aerosol models than the fine aerosol models.

Aerosol Direct Radiative Effect Sensitivity Analysis

2020 ◽  
Vol 33 (14) ◽  
pp. 6119-6139 ◽  
Author(s):  
Tyler J. Thorsen ◽  
Richard A. Ferrare ◽  
Seiji Kato ◽  
David M. Winker
Keyword(s):  
Near Infrared ◽  
Single Scattering ◽  
Reanalysis Data ◽  
Asymmetry Factor ◽  
Radiative Effect ◽  
Cloud Optical Depth ◽  
Clear Sky ◽  
Crucial Component ◽  
Aerosol Scattering ◽  
Aerosol Properties

AbstractBoth to reconcile the large range in satellite-based estimates of the aerosol direct radiative effect (DRE) and to optimize the design of future observing systems, this study builds a framework for assessing aerosol DRE uncertainty. Shortwave aerosol DRE radiative kernels (Jacobians) were derived using the MERRA-2 reanalysis data. These radiative kernels give the differential response of the aerosol DRE to perturbations in the aerosol extinction coefficient, aerosol single-scattering albedo, aerosol asymmetry factor, surface albedo, cloud fraction, and cloud optical depth. This comprehensive set of kernels provides a convenient way to consistently and accurately assess the aerosol DRE uncertainties that result from observational or model-based uncertainties. The aerosol DRE kernels were used to test the effect of simplifying the full vertical profile of aerosol scattering properties into column-integrated quantities. This analysis showed that, although the clear-sky aerosol DRE can be had fairly accurately, more significant errors occur for the all-sky DRE. The sensitivity in determining the broadband spectral dependencies of the aerosol scattering properties directly from a limited set of wavelengths was quantified. These spectral dependencies can be reasonably constrained using column-integrated aerosol scattering properties in the midvisible and near-infrared wavelengths. Separating the aerosol DRE and its kernels by scene type shows that accurate aerosol properties in the clear sky are the most crucial component of the global aerosol DRE. In cloudy skies, determining aerosol properties in the presence of optically thin cloud is more radiatively important than doing so when optically thick cloud is present.

Background Fitting in the Low-Loss Region of Electron Energy Loss Spectra

1990 ◽  
Vol 48 (2) ◽  
pp. 56-57
Author(s):  
C P Scott ◽  
A J Craven ◽  
C J Gilmore ◽  
A W Bowen
Keyword(s):  
Energy Loss ◽  
Multiple Scattering ◽  
Simple Form ◽  
Single Scattering ◽  
Low Loss ◽  
Characteristic Energy ◽  
Normal Method ◽  
Fitting In ◽  
Electron Energy Loss ◽  
Electron Energy Loss Spectra

The normal method of background subtraction in quantitative EELS analysis involves fitting an expression of the form I=AE-r to an energy window preceding the edge of interest; E is energy loss, A and r are fitting parameters. The calculated fit is then extrapolated under the edge, allowing the required signal to be extracted. In the case where the characteristic energy loss is small (E < 100eV), the background does not approximate to this simple form. One cause of this is multiple scattering. Even if the effects of multiple scattering are removed by deconvolution, it is not clear that the background from the recovered single scattering distribution follows this simple form, and, in any case, deconvolution can introduce artefacts.The above difficulties are particularly severe in the case of Al-Li alloys, where the Li K edge at ~52eV overlaps the Al L2,3 edge at ~72eV, and sharp plasmon peaks occur at intervals of ~15eV in the low loss region. An alternative background fitting technique, based on the work of Zanchi et al, has been tested on spectra taken from pure Al films, with a view to extending the analysis to Al-Li alloys.

scholarly journals Retrieval of aerosol single-scattering albedo and polarized phase function from polarized sun-photometer measurements for Zanjan's atmosphere

2013 ◽  
Vol 6 (10) ◽  
pp. 2659-2669 ◽  
Author(s):  
A. Bayat ◽  
H. R. Khalesifard ◽  
A. Masoumi
Keyword(s):  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Atmospheric Aerosols ◽  
Phase Function ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Angstrom Exponent ◽  
Ångström Exponent ◽  
Sun Photometer ◽  
Ångstrom Exponent

Abstract. The polarized phase function of atmospheric aerosols has been investigated for the atmosphere of Zanjan, a city in northwest Iran. To do this, aerosol optical depth, Ångström exponent, single-scattering albedo, and polarized phase function have been retrieved from the measurements of a Cimel CE 318-2 polarized sun-photometer from February 2010 to December 2012. The results show that the maximum value of aerosol polarized phase function as well as the polarized phase function retrieved for a specific scattering angle (i.e., 60°) are strongly correlated (R = 0.95 and 0.95, respectively) with the Ångström exponent. The latter has a meaningful variation with respect to the changes in the complex refractive index of the atmospheric aerosols. Furthermore the polarized phase function shows a moderate negative correlation with respect to the atmospheric aerosol optical depth and single-scattering albedo (R = −0.76 and −0.33, respectively). Therefore the polarized phase function can be regarded as a key parameter to characterize the atmospheric particles of the region – a populated city in the semi-arid area and surrounded by some dust sources of the Earth's dust belt.

scholarly journals Lidar measurement of snow depth: a review

2013 ◽  
Vol 59 (215) ◽  
pp. 467-479 ◽  
Author(s):  
Jeffrey S. Deems ◽  
Thomas H. Painter ◽  
David C. Finnegan
Keyword(s):  
Remote Sensing ◽  
Snow Depth ◽  
Near Infrared ◽  
Ground Surface ◽  
Lidar Measurement ◽  
Snow Hydrology ◽  
Sensing Technology ◽  
Transfer Error ◽  
Scale Range ◽  
Visible Wavelengths

AbstractLaser altimetry (lidar) is a remote-sensing technology that holds tremendous promise for mapping snow depth in snow hydrology and avalanche applications. Recently lidar has seen a dramatic widening of applications in the natural sciences, resulting in technological improvements and an increase in the availability of both airborne and ground-based sensors. Modern sensors allow mapping of vegetation heights and snow or ground surface elevations below forest canopies. Typical vertical accuracies for airborne datasets are decimeter-scale with order 1 m point spacings. Ground-based systems typically provide millimeter-scale range accuracy and sub-meter point spacing over 1 m to several kilometers. Many system parameters, such as scan angle, pulse rate and shot geometry relative to terrain gradients, require specification to achieve specific point coverage densities in forested and/or complex terrain. Additionally, snow has a significant volumetric scattering component, requiring different considerations for error estimation than for other Earth surface materials. We use published estimates of light penetration depth by wavelength to estimate radiative transfer error contributions. This paper presents a review of lidar mapping procedures and error sources, potential errors unique to snow surface remote sensing in the near-infrared and visible wavelengths, and recommendations for projects using lidar for snow-depth mapping.

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