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1.
Stephen J. Keihm 《Icarus》1982,52(3):570-584
The effects of volume scattering on the lunar microwave brightness temperature spectrum are evaluated for a broad range of plausible scattering fragment populations. Mie-scattering phase functions and the radiative transfer method are utilized. Results indicate that emission darkening of ～1–7°K is to be expected over the wavelength range 3–30 cm, dependent on the total volume fraction of centimeter-sized and larger fragments. Spectral variations can occur if the size distribution of scatterers is nonuniform in a power law sense. For mare regions representative of the Surveyor III, V, and VI sites, an increase in brightness temperature with wavelength is predicted which is smaller than the predicted spectral variation due to planetary heat flow. The amplitude of lunation variation in brightness temperature is particularly sensitive to the fraction of fragments in the upper 10-cm diurnal layer. Deductions of electrical loss based on purely absorptive models can overestimate loss tangent values by a factor of 1.5 or more if scattering effects are not accounted for. The absence of anomalies exceeding ～2°K in lunar night-time γ3.55-cm brightness temperature maps requires a remarkable uniformity of the surface layer (upper 10 cm) scattering properties on a 250-km scale.  相似文献   

2.
The evaluation of the planetary Fourier spectrometer performance at Mars is presented by comparing an average spectrum with the ISO spectrum published by Lellouch et al. [2000. Planet. Space Sci. 48, 1393.]. First, the average conditions of Mars atmosphere are compared, then the mixing ratios of the major gases are evaluated. Major and minor bands of CO2 are compared, from the point of view of features characteristics and bands depth. The spectral resolution is also compared using several solar lines. The result indicates that PFS radiance is valid to better than 1% in the wavenumber range 1800-4200 cm−1 for the average spectrum considered (1680 measurements). The PFS monochromatic transfer function generates an overshooting on the left-hand side of strong narrow lines (solar or atmospheric). The spectral resolution of PFS is of the order of 1.3 cm−1 or better. A large number of narrow features to be identified are discovered.  相似文献   

3.
We present a first detailed spectrum synthesis calculation of a supernova-fallback disk composed of iron. We assume a geometrically thin disk with a radial structure described by the classical α-disk model. The disk is represented by concentric rings radiating as plane-parallel slabs. The vertical structure and emission spectrum of each ring is computed in a fully self-consistent manner by solving the structure equations simultaneously with the radiation transfer equations under non-LTE conditions. We describe the properties of a specific disk model and discuss various effects on the emergent UV/optical spectrum. We find that strong iron-line blanketing causes broad absorption features over the whole spectral range. Limb darkening changes the spectral distribution up to a factor of four depending on the inclination angle. Consequently, such differences also occur between a blackbody spectrum and our model. The overall spectral shape is independent of the exact chemical composition as long as iron is the dominant species. A pure iron composition cannot be distinguished from silicon-burning ash. Non-LTE effects are small and restricted to few spectral features.  相似文献   

4.
During the descent of the Huygens probe through Titan's atmosphere in January 2005, the Descent Imager/Spectral Radiometer (DISR) will perform upward and downward looking measurements at various spectral ranges and spatial resolutions. This internal radiation density could be estimated by radiative transfer calculations for Titan's atmosphere. However, to do this, the optical properties—i.e. volume extinction coefficient, single scattering albedo and scattering phase function—have to be prescribed at every altitude, and these are apriori not known. Herein, an inverse approach is investigated, which retrieves the single scattering albedo and the phase function of the aerosols from DISR observations. The method uses data from a DISR subinstrument, the Solar Aureole imager (SA), to estimate the optical properties of the atmospheric layer between two successive observation altitudes. A unique solution for one layer can in principle be calculated directly from a linear system of equations, but due to the sparseness of the data and the unavoidable noise in the measurements, the inverse problem is ill-posed. The problem is stabilized by the regularization method requiring smoothness of the resultant solution. A consistent set of solutions for all layers is obtained by iterating several times downward and upward through the layers. The method is tested in a simulated radiation density scenario for Titan, which is based on a microphysical aerosol model for the haze layer. Within this scenario, the expected coverage of SA data allows a reconstruction of the angular dependence of the scattering phase function with an explained variance better than 90%.  相似文献   

5.
The possibility of using broad absorption lines (on the order of 100 km/sec or more) of the Mg II 2800 doublet, formed in regions of planetary nebulae with fast collimated flows and, probably, manifested against the background of the short-wavelength wing of the Balmer continuum of the spectrum as indicators of such flows, is discussed. For this purpose, using existing analytical (dynamical) models of mass-loaded isothermal flows and numerical (photoionization) models of planetary nebulae, theoretical line profiles are calculated in the Sobolev approximation and their dependence on certain parameters is shown. An advantage of the proposed method consists in the possibility of revealing fast flows in faint, star-like planetary nebulae. Translated from Astrofizika, Vol. 42, No. 4, pp. 519–530, October–December, 1999.  相似文献   

6.
We investigate the use of diffraction theory as an alternative to solving the full electromagnetic problem of near-forward scattering by a collection of dielectric spheres of arbitrary spacing and orientation. We define an amplitude screen, which is constructed by projecting the shadows of a cluster of spheres onto a plane perpendicular to the wave vector of the incident radio wave. The far-field (Fraunhofer) diffraction pattern of the amplitude screen is then computed and compared with the Mie-theoretic result. We show that for EM scattering from a cluster of electrically large spheres—both singly-sized and belonging to a size distribution—there is excellent agreement between the exact Mie solution and its diffraction theory approximation when near-forward scattering is the angular range of interest. This excellent agreement holds over a broad range of particle separation and orientation configurations relative to the incidence direction. It is also achieved at a much reduced computational cost compared with an exact solution of the electromagnetic interaction problem. Fortified by these results, the authors have applied diffraction theory to the analysis of Cassini radio occultation data, thereby detecting fine-scale structure in Saturn’s rings [Thomson, F.S., Marouf, E.A., Tyler, G.L., French, R.G., Rappoport, N.J., 2007. Periodic microstructure in Saturn’s rings A and B. Geophys. Res. Lett. 34, L24203].  相似文献   

7.
The sub-millimetre/microwave range of the spectrum has been exploited in the field of Earth observation by many instruments over the years and has provided a plethora of information on atmospheric chemistry and dynamics—however, this spectral range has not been fully explored in planetary science, having been exclusively employed to carry out ground-based measurements. To this end, a sub-millimetre instrument, the Orbiter Terahertz Infrared Spectrometer (ORTIS), is studied by the University of Oxford and the Rutherford Appleton Laboratory, to meet the requirements of the European Space Agency's Cosmic Visions 2015-2025 programme—in particular, the Europa Jupiter System Mission (EJSM), which has the European Space Agency and the National Aeronautics and Space Administration as partners. ORTIS is designed to measure atmospheric temperature, the abundance of stratospheric water vapour and other jovian gases, and is intended to be capable of retrieving vertical profiles of horizontal windspeed in the stratosphere for the first time, from Doppler-shifted emission lines measured at high spectral resolution. In this work, a preliminary study and implementation of the estimation of windspeed profiles on simulated spectra representative of Jupiter is presented, detailing the development of the retrieval algorithm, showing that a sub-millimetre instrument such as ORTIS should be able to retrieve windspeed profiles to an accuracy of about 15 m/s between 70 and 200 km/0.1-10 mb using a single near-limb measurement, for expected noise amplitudes.  相似文献   

8.
The large stellar/planetary flux ratio (>106) and small angular separation (0.1 arcsec when observed from 10 parsecs) make it difficult to study Earthlike extrasolar planets. Hybrid coronographs with apodized masks and nulling by Earth based interferometric techniques could reduce the flux ratio by 3 orders of magnitude. Further reduction of starlight is possible with frequency filters. Due to large (upto 30 km/s) differences in radial velocities the specific spectral line for a particular molecule will be Doppler shifted by different amounts depending on from where, the star or the planet, the emission originates. The stellar spectrum itself could be used as a dynamic reference to determine the differential Doppler shift and define the frequency search space for the sought after planetary spectral line. The Differential Radial Velocity Spectrometer (DRVS) could use a heterodyne receiver with steep skirted filters and a laser local oscillator tracking the stellar spectrum. Several planetary spectral line windows should be searched and correlation/code gain techniques used to enhance detection capabilities.  相似文献   

9.
The closing years of the 20th century have allowed us, for the first time, to seriously discuss interferometric instruments deployed in space. With the express purpose of achieving unprecedented spatial resolution, these missions will lead to new astrophysics. Especially—and most challenging—we expect to be able to carry out the first studies of terrestrial exoplanets. The detection and study of the latter promises to usher in a new era in science and will affect a broad spectrum of science and technology. For the first set of interferometric missions—the precursor missions such as SMART and ST-3 and the astrometric SIM, the time line for implementation is such that it could be about 5-10 years until we receive the first results from them. In this review, we describe the impact of interferometry from space on the topic of terrestrial exoplanets. We also briefly review the state of the art of the study of exoplanets as well as discuss the potential impact of several different techniques for their study.  相似文献   

10.
R.J. de Kok  D.M. Stam 《Icarus》2012,221(2):517-524
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 μm 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. This potentially reduces the need for a full three-dimensional Monte Carlo code for calculating transmission spectra of atmospheres that contain forward-scattering particles.  相似文献   

11.
The study of the minor constituents of the planetary atmospheres from the analysis of the scattered light properties requires the knowledge of the absolute incident solar irradiance at high resolution. The data were obtained from the UVSP experiment on board the Solar Maximum Mission satellite in the 184.5–232.5 nm spectral range. We have reconstituted the solar spectrum measured in three different regions of the solar disk with a spectral resolution of 0.01 nm and a spatial resolution of 3 arc sec. The wavelength scale was determined with a standard deviation of 0.0025 nm. The comparison of the relative intensities in three locations of the solar disk with those obtained by other authors allowed us to determine these positions accurately and to derive the integrated spectrum of the whole disk. Finally, the resulting spectrum has been expressed in absolute units using the spectral irradiance by the SOLSPEC and SUSIM spectrometers, respectively operated with the ATLAS 1 mission and from the Upper Atmosphere Research Satellite. We obtained the absolute solar irradiance with an accuracy of 10% in the 184.5–232.5 nm spectral range with a spectral resolution of 0.01 nm for the first time using data from space observations. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1017976515168  相似文献   

12.
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13.
I present a model for the ionization cones of NGC1068, and, by extension, for Seyfert Galaxies in general. In this, the cones represent the hypersonic interaction of a sheet of material expelled from close to the galactic nucleus with the interstellar medium of the galaxy. Such a model produces a turbulent magnetically-supported shearing mixing layer with a Kelvin-Helmholtz unstable outer boundary. This drives strong shocks into the dense wall material, and generates fast stand-off shocks in the entrainment flow. The EUV photons produced in the fast shock ionize both the wall material and the entrainment layer to generate the broad highly excited lines that are such a characteristic of the emission line spectrum. This model is consistent with the energy budget, the observed spectrum, the emission line widths, and provides sufficient column density to produce an electron scattering "mirror".  相似文献   

14.
Ben Zellner 《Icarus》1973,18(4):661-664
New polarization observations of Titan in three spectral regions are presented. The results are not consistent with scattering from either an ordinary planetary surface or a pure molecular atmosphere. Apparently an opaque cloud layer with a strongly uv-absorbing constituent is needed.  相似文献   

15.
Computations of the equivalent widths of absorption lines as a function of planetary phase angle are made for a homogeneous cloud with particles having the properties (shape, refractive index, and size distribution) deduced from polarimetry of Venus. The computed equivalent widths show an “inverse phase effect” comparable to that which is observed for CO2 lines on Venus. This result verifies a recent suggestion of Regas et al. that the existence of an inverse phase effect does not by itself imply the presence of multiple layers of scattering particles in the atmosphere of Venus.  相似文献   

16.
We present a Chandra image of the quasar, jet, and lobes of PKS 1354+195 (=4C 19.44). The radio jet is 18 arcsec long, and appears to be very straight. The length gives many independent spatial resolution elements in the Chandra image while the straightness implies that the geometrical factors are constant along the jet although their values are uncertain. We also have 4 frequency radio images with half to one arcsecond angular resolution, and use HST and Spitzer data to study the broad band spectral energy distributions. The X-ray and radio spectra are both consistent with a spectrum f ν ∝ ν −0.7 for the integrated jet. Using that spectral index, the model of inverse Compton scattering of electrons on the cosmic microwave background (IC/CMB) gives magnetic field strengths and Doppler factors that are relatively constant along the jet. Extended X-ray emission is evident in the direction of the otherwise unseen counter-jet. X-ray emission continues past the radio jet to the South, and is detected within both the southern and northern radio lobes.  相似文献   

17.
Multiple resonance scattering of spectral line radiation is examined in atmospheres with uniformly distributed sources of unpolarized radiation. It is assumed that the profile of the absorption coefficient is lorentzian and that scattering involves complete frequency redistribution. The polarization characteristics of the emerging radiation are determined by iterative solution of a nonlinear Ambartsumyan-Chandrasekhar matrix integral equation. In particular, it is found that for pure scattering the maximum polarization at the limb of the disk is 5.421%. The polarization characteristics of the emerging radiation are compared for three different absorption profiles: Lorentz, Doppler, and rectangular (monochromatic radiation). __________ Translated from Astrofizika, Vol. 50, No. 2, pp. 199–217 (May 2007).  相似文献   

18.
Thomson scattering is often invoked to explain broad wing features that are seen in various objects including active galactic nuclei and symbiotic stars. Despite the wavelength-independent scattering cross-section of Thomson scattering, the line flux may exhibit wavelength-dependent linear degree of polarization, because various parts of emission wings are contributed by photons with different scattering numbers. Specifically, more scattered and hence more weakly polarized photons tend to fill the farther wing parts from the line centre, while the neighbourhood of the line centre is dominated by less-scattered photons with higher degree of polarization. Using a Monte Carlo technique, we investigate the polarization structure of Thomson-scattered line radiation. A detailed analysis of polarization structure formation is conducted by investigating the dependence of the polarization and profile width on the scattering number for various finite electron scattering slabs. Significantly varying degree of polarization is obtained when the scattering medium has Thomson optical depth  τTh≥ 1  . We present our high-resolution spectrum of the symbiotic star V1016 Cyg obtained with the Bohyunsan Optical Echelle Spectrograph (BOES) in order to fit the broad profile around Hα by electron scattering wings adopting an oblate spheroidal geometry with Thomson optical depth  τTh= 0.5  and electron temperature   T e= 6.2 × 104  K  . Local maxima in the linear degree of polarization of Thomson-scattered line radiation are expected to appear in the spectral regions characterized by the average scattering number ≃1.  相似文献   

19.
Stellar activity is the major astrophysical limiting factor for the study of planetary atmospheres. Its variability and spectral characteristics may affect the extraction of the planetary signal even for moderately active stars. A technique based on spectral change in the visible band was developed to estimate the effects in the infrared due to star activity. This method has been purposely developed for the EChO mission which had the crucial characteristics of monitoring simultaneously a broadband from visible to infrared. Thanks to this capability the optical spectrum, whose variations are mainly due to stellar activity, has been used as in an instantaneous calibrator to correct the infrared spectrum. The technique is based on principal component analysis which significantly reduces the dimensionality of the spectra. The method was tested on a set of simulations with realistic photon noise. It can be generalized to any chromatic variability effects provided that optical and infrared variations are correlated.  相似文献   

20.
We present an XMM–Newton observation of the bright, narrow-line, ultrasoft type 1 Seyfert galaxy Ton S180. The  0.3–10 keV  X-ray spectrum is steep and curved, showing a steep slope above 2.5 keV  (Γ∼ 2.3)  and a smooth, featureless excess of emission at lower energies. The spectrum can be adequately parametrized using a simple double power-law model. The source is strongly variable over the course of the observation but shows only weak spectral variability, with the fractional variability amplitude remaining approximately constant over more than a decade in energy. The curved continuum shape and weak spectral variability are discussed in terms of various physical models for the soft X-ray excess emission, including reflection off the surface of an ionized accretion disc, inverse Compton scattering of soft disc photons by thermal electrons, and Comptonization by electrons with a hybrid thermal/non-thermal distribution. We emphasize the possibility that the strong soft excess may be produced by dissipation of accretion energy in the hot, upper atmosphere of the putative accretion disc.  相似文献   

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