Seasonal buffering of atmospheric pressure on Mars

An isothermal reservoir of carbon dioxide in gaseous contact with the Martian atmosphere would reduce the amplitude and advance the phase of global atmospheric pressure fluctuations caused by seasonal growth and decline of polar CO₂ frost caps. Adsorbed carbon dioxide in the upper ～10 m of Martian regolith is sufficient to buffer the present atmosphere on a seasonal basis. Available observations and related polar cap models do not confirm or refute the operation of such a mechanism. Implications for the amplitude and phase of seasonal pressure fluctuations are subject to direct test by the upcoming Viking mission to Mars.     

Seasonal variation in the atmospheric activity of Jupiter

Periodicities of 22, 8 and 6 years have been found by a statistical analysis of the coefficient of atmospheric activity R of Jupiter for the time period 1910–1985.We have also found variations of the coefficient of asymmetry of the activity Z measured by the Observatory of Athens for the time period 1956–1985 with periodicities of 12, 8, 6 and 4 years by statistical analysis.The same periodicities have been recently found in an analysis of the total number of solar flares and in the number of high velocity solar wind streamers.     

Determination of the solar X-ray spectrum by using the atmospheric extinction

Indications about the spectral distribution of the solar radiation below 20 Å are obtained by comparing the atmospheric extinction of the solar radiation measured by three ion chambers in the satellites SOLRAD 7 and 8.The data refer to July 5, 6 and 8, 1965 and July 23, 1966 when the satellites passed from light to darkness into or out of the earth's shadow.The spectral intensity distribution is shown for the four days, and a comparison is made between active and quiet days.     

Seasonal variation of Titan's atmospheric structure simulated by a general circulation model.

The seasonal variation of Titan's atmospheric structure with emphasis on the stratosphere is simulated by a three-dimensional general circulation model. The model includes the transport of haze particles by the circulation. The likely pattern of meridional circulation is reconstructed by a comparison of simulated and observed haze and temperature distribution. The GCM produces a weak zonal circulation with a small latitudinal temperature gradient, in conflict with observation. The direct reason is found to be the excessive meridional circulation. Under uniformly distributed opacity sources, the model predicts a pair of symmetric Hadley cells near the equinox and a single global cell with the rising branch in the summer hemisphere below about z = 230 km and a thermally indirect cell above the direct cell near the solstice. The interhemispheric circulation transports haze particles from the summer to the winter hemisphere, causing a maximum haze opacity contrast near the solstice and a smaller contrast near the equinox, contrary to observation. On the other, if the GCM is run under modified cooling rate in order to account for the enhancement in nitrites and some hydrocarbons in the northern hemisphere near the vernal equinox, the meridional cell at the equinox becomes a single cell with rising motions in the autumn hemisphere. A more realistic haze opacity distribution can be reproduced at the equinox. However, a pure transport effect (without particle growth by microphysics, etc.) would not be able to cause the observed discontinuity of the global haze opacity distribution at any location. The stratospheric temperature asymmetry can be explained by a combination of asymmetric radiative heating rates and adiabatic heating due to vertical motion within the thermally indirect cell. A seasonal variation of haze particle number density is unlikely to be responsible for this asymmetry. It is likely that a thermally indirect cell covers the upper portion of the main haze layer. An artificial damping of the meridional circulation enables the formation of high-latitude jets in the upper stratosphere and weaker equatorial superrotation. The latitudinal temperature distribution in the stratosphere is better reproduced.     

The estimate of sensitivity for large infrared telescopes based on measured sky brightness and atmospheric extinction

In order to evaluate the ground-based infrared telescope sensitivity affected by the noise from the atmosphere,instruments and detectors,we construct a sensitivity model that can calculate limiting magnitudes and signal-to-noise ratio(S/N).The model is tested with tentative measurements of M'-band sky brightness and atmospheric extinction obtained at the Ali and Daocheng sites.We find that the noise caused by an excellent scientific detector and instruments at-135℃ can be ignored compared to the M'-band sky background noise.Thus,when S/N=3 and total exposure time is 1 second for 10 m telescopes,the magnitude limited by the atmosphere is 13.01~m at Ali and 12.96~m at Daocheng.Even under lessthan-ideal circumstances,i.e.,the readout noise of a deep cryogenic detector is less than 200 e~-and the instruments are cooled to below-87.2℃,the above magnitudes decrease by 0.056~m at most.Therefore,according to observational requirements with a large telescope in a given infrared band,astronomers can use this sensitivity model as a tool for guiding site surveys,detector selection and instrumental thermal-control.     

The atmospheric extinction at the complejo astronómico el leoncito and the Bosque Alegre station

The first-and second-order extinction coefficients in theUBVRI and DDO systems are derived for the first time at CASLEO. They define an extinction curve which is discussed in the framework of atmospheric absorption theory. From this curve the first-order extinction coefficients for the Washington and Strömgren systems are inferred. From a photometric point of view, CASLEO occupies a preferred place among the observatories of the world. A comparison of theUBV extinction coefficients obtained at Bosque Alegre with previous determinations supports the conclusion that the sky transparency at this station has deteriorated in the ultraviolet in the last years.Visting Astronomer, Complejo Astronómico El Leoncito operated under agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina and the National Universities of La Plata, Córdoba y San Juan, Argentina.     

On the calculation of spurious velocity signal produced by atmospheric differential extinction in whole-disk oscillation measurements

The systematic diurnal signal drift in full-disk solar oscillation measurements has been calculated taking into account differential rotation, the inclination of the rotational axis of the Sun with respect to the picture plane, the limb-darkening function and a realistic estimation of the sky transparency, slightly variable during the day. An illustration of this method on the Kumbel (U.S.S.R.) IRIS data is presented and discussed.     

Evolving interstellar extinction

High brightness temperatures ( T _b) implied by quasar intraday variability may be explained by coherent emission, or else by physically implausible bulk relativistic Lorentz factors Γ ≥ 100. Previous theory asserts that various absorption mechanisms will block escape of such coherent, high-brightness sources. Yet this same theory fails to account for laboratory experiments detecting collective emission. Probably this is because present theory is inadequate, and should not be used to rule out collective radiation processes.     

The anomalous extinction law

In this paper we present further results of the calculation of anomalous extinction laws following Steenman and Thé (1989). The extinction laws in this article are calculated by enlarging the lower size cutoff of the particle size distribution, as opposed to the laws in the previous paper, which were obtained by making the upper size cutoff larger. The extinction laws we have derived have three important properties: (1) the change in the infrared is similar as in the case of changing the upper size cutoff; the difference increases as the value ofR becomes larger, (2) in the ultraviolet the 2200 Å bump disappears gradually as the lower size cutoff is made larger, (3) in the far-UV the extinction curves become more and more flat. The ultraviolet extinction laws in the direction of the stars HD 200775 and HD 259431 are lower than the Savage and Mathis (1979) galactic interstellar extinction law.     

The anomalous extinction law

In this paper we present the results of the calculations of anomalous extinction laws characterized by different values of the ratio of the total to selective extinctionR=A _v/E(B-V). As far as we are aware, no such theoretical extinction laws have been published before. The calculation is based on the Mie theory of light scattering by small particles assuming a two-grain dust model of Mathiset al. (1977) and by adopting theri particle size (a) distribution, given byn(a)=a ^–3.5, in which the minimum and maximum size is taken to be 0.005 and 0.22 m, respectively. Furthermore, we use the dielectric functions for the two types of grains, graphite and astronomical silicate, as derived by Draine and Lee (1984), published in tabular form by Draine (1985). Following Mathis and Wallenhorst (1981), we then obtained the anomalous extinction laws by changing the upper size cutoff of the particles.A comparison shows that the calculated anomalous extinction laws agree quite well with the corresponding laws derived observationally using the colour-difference method. Finally, a trial and error method for the determination of the anomalous extinction law based on a comparison of the observed extinction-free spectral energy distribution with the corresponding theoretical Kurucz (1979) model, is explained.     

Far-ultraviolet extinction in σ Scorpii

It was found earlier from OAO-2 data (Bless and Savage, 1972) that considerable variability with direction in space is present in both the shape and level (relative toB-V color excess) of the interstellar extinction curve in the far ultraviolet. The star σ Sco was shown to be a case of extremely low UV extinction, but there was some question of whether this could be due to scattered nebular light entering the large entrance slit of the Wisconsin spectrometer aboard OAO-2. We have obtained UV data on σ Sco usingCopernicus (OAO-3), which has an entrance slit on the order of 10³ times smaller in projected area than that of OAO-2, so that the contribution to the signal from scattered nebular light would be correspondingly smaller. We find very good agreement with the extinction curve of Bless and Savage, confirming the low UV extinction in the line of sight to σ Sco. The curve is extended down to 100 Å, showing a continued rise towards short wavelengths.     

A model for interstellar extinction

From an analysis of the interstellar extinction we conclude that interstellar grains are of three main kinds: graphite spheres of radii 0.02 m making up 10% of the total grain mass, small dielectric spheres of radius about 0.04 m making up 25% of the mass, and hollow dielectric cylinders containing metallic iron with diameters of 2/3 m making up 45% of the mass. The remaining 20% consists of other metals and metal oxides. The main dielectric component of the grains appears to be comprised of organic material.     

The extinction of the mammoths

Two kinds of blanket of fine grains in the Earth's upper atmosphere are possible: a blanket of reflective dielectric particles and one of absorbing metallic particles. The first is a trigger for the onset of an ice-age, and the second a trigger for the disappearance of an ice-age. The facts support the view that it was a blanket of absorptive particles which caused the extinction of the mammoths.     

Analysis of dust through extinction

Extinction is treated through the optical thickness . Some important characters of dust, such as the composition and the size of grains, are reviewed in the light of observational evidence. Extinction is investigated through three kind of parameters: (1) those affecting it directly such as the geometrical cross sections of the grains (through the radius) and the wavelength of radiation interacting with them; (2) those connected witht the solid state constitution of grains, such as the dielectric constant and the conductivity; and (3) those connected with the distribution of dust in the intergalactic medium (IGM), such as the dust density and the geometrical thickness of dust slabs. The results are discussed and compared with observations.     

The extinction of starlight revisited

The rigorous Kerker-Matijevi formulae for light scattering by co-axial double cylinders are used to calculate the extinction properties of hollow organic grains. A size distribution of such particles together with iron whiskers of radii 0.01 m, silica spheres of radius 0.03 m and free aromatic molecular clusters comprised of 50–100 atoms yield excellent agreement with data on the extinction of starlight. The mass ratios of silica to organics and of iron to organics are in good accord with cosmic abundance constraints.     

Patchy structure and selective extinction

Based on diurnal values of the total radio-flux density at 10.7 cm as well as on corresponding Zürich numbers the relation giving the radio-flux as function ofR is established. The behaviour of low and high values ofR andF ₂₈₀₀ during the time interval 1957–1976 is studied. Preliminary conclusions drawn by other investigators are confirmed. A prediction of the total radio-flux for the 22nd solar cycle is given.     

Internal extinction of disc galaxies – I. High-resolution extinction map of NGC 6946

Traditionally, it has been believed that extinction effects due to dust within the interstellar medium of external galaxies are rather small and can largely be ignored. Over the last 10 years, however, considerable doubt has been cast over the evidence to support this comfortable idea, and it has become clear that a more detailed analysis is required. Here, a new technique for mapping the extinction in disc galaxies with high resolution is presented.
This technique has been applied to the Sc galaxy NGC 6946. The results show that dust extinction significantly affects both the overall brightness and appearance of the galaxy. The total extinction is found to be A_B =0.45 – somewhat larger than the value of A_B =0.2 usually quoted for an Sc galaxy. When corrected for dust the morphology more closely resembles that of an Sb galaxy rather than an Sc galaxy.
The most surprising result of this work is finding interarm regions that suffer high extinction. It appears that these regions appear faint because of the high extinction and not as a result of low stellar density. There are also interarm regions that suffer little extinction; these are therefore truly regions of low stellar density.     

Features of the interstellar extinction curve

The extinction curves for spherical particles are subject to the errors of the particle material's refractive index. Their sensitivity to these errors has been investigated and is found to be dependent upon wavelength. For graphite, significant errors are produced in the far ultraviolet part of the extinction curve; for silicates, in the near ultraviolet; while for iron the error is relatively small. The wavelength dependence of the 10 μm and 20 μm absorption bands of small silicate spheroids upon their shape and alignment has been studied. It is found that the bands can be displaced by ～1 μm towards longer wavelengths from their positions for corresponding spheres: and that a further, though small, displacement can be superimposed upon this by their subsequent alignment.     

Unusual extinction profiles by RAYLEIGH scattering

The extinction efficiencies for small smooth metallic particles (granules) have been calculated using rigorous MIE theory of scattering. Strong size dependent resonances for spheres composed of sodium, potassium, and calcium have been found. The results seem to have relevance to the problems of interstellar grains especially with reference to structures in the observed extinction curve, diffuse bands and anomaly of cosmic abundances of elements.     

Intrinsic variations of interstellar extinction laws

The intrinsic ultraviolet colours of early type stars have been derived from the rocket data of J. W. Campbell and A. M. Smith. There exists a considerable spread in these colours. One possible source is the predicted rms variation due to the effect of rotation. It is necessary to recognise that rotation is an important source of uncertainty of the interstellar extinction curves derived from the comparison of pairs of stars.Predicted rms errors due to different sources (rotation, uncertainty of MK spectral type, photometric errors) have been compared with the dispersion of the observed extinction data and it is found that part of the large scatter could be due to intrinsic differences of the properties of interstellar grains.