On the excitation of lower levels of singlet helium in quiescent prominences

The paper deals with the excitation of the helium singlet level 2¹ P in the homogeneous and filamentary models of quiescent prominences with following parameters: the optical thickness at the limit of helium Lyman continuum _1c ^M = 0.1–100, T _e = 7000 K, n _e = 5 × 10¹⁰ cm^–3. Assuming a model He atom with seven discrete levels (1¹ S, 2³ S, 2¹ S, 2³ P, 2¹ P, 3³ D, 3¹ D) and the continuum the steady state equations for the levels 2³ S, 2¹ P and the continuum have been solved together with the radiative transfer equations for the line 584 Å and the continuum 504 Å. The variations with depth of the functions n ₂ ³ _S /n ₁ ¹ _S (₁ c), n ₂ ¹ _P /n ₁ ¹ _S (_1c ), and n ⁺ _He n _e /n ₁ ¹ S(_1c ) as well as the intensities of the triplet (D₃, 10830 Å) and singlet (16678, 20581 Å) lines have been calculated. Comparison with observations leads to the following conclusions: (1) The line intensities calculated for filamentary models of prominences agree better with observations than those for homogeneous ones. (2) The helium level 2¹ P is excited by diffuse field 584 Å being formed by recombinations and spontaneous transitions 2¹ P – 1¹ S and escaping from the prominence into the space between the filaments and to the surface. (3) Underpopulation of the singlet level 2¹ P may be explained by combination of weak excitation mechanism (recombinations and formation of the diffuse field 584 Å) and strong deexcitation mechanism (spontaneous transitions into the level 1¹ S).     

On the probable change of the radius and nature of aerosol particles in the deep layers of Jupiter’s atmosphere

When analyzing the pressure dependences of the aerosol volume scattering coefficient calculated from the measurement data on the geometric albedo of Jupiter obtained in 1993 in the methane absorption bands at 619, 727, and 842 nm, the signs of probable changes in the parameters of aerosol particles in the deep atmospheric layers were detected and the first estimates of the magnitude of these changes were obtained. It has been found that, in the pressure interval from 4 to 14 bar, the effective radius of particles may increase twofold and more (larger than 0.73 μm) and the real part of the refractive index may grow by 10% (from 1.44 and higher) relative to the values of these parameters in the upper atmosphere. If we take into account these changes, we find no signs of aerosol deep in the atmosphere of Jupiter.     

Parameters of Dust Particles in the Martian Atmosphere

A critical analysis of the methods and results of estimating the optical thickness of the dust component in the Martian atmosphere ₀, the particle size r ₀, and the imaginary part of the refractive index n _ihas shown the following. (1) Observational data on the brightness distribution over the Martian disk as well as the phase dependences of diffusely reflected light and the azimuthal dependences of diffusely transmitted light are most appropriate to use only for verifying the reliability of the aerosol parameters determined by other methods. (2) If the morning and evening fogs in the atmosphere are disregarded, the Bouguer–Lambert–Beer method used to analyze the solar-brightness attenuation measured on the planetary surface yields overestimated extraatmospheric solar intensity I ₀and atmospheric optical depth ₀. At the Viking 1landing site, I ₀and ₀could be overestimated by a factor of 1.7 and by 0.35, respectively. (3) The aerosol size determined by analyzing measurements of the azimuthal dependences for the Martian sky brightness at low elevations of the Sun most likely corresponds to the fog particles. (4) If overestimated values of I ₀were used to standardize the observations of the solar radiation transmitted by the Martian atmosphere, then n _iwere also overestimated; using overestimated ₀also affected the reliability of the latter. (5) The problem of reliability of the available ₀and r ₀estimates for periods of high atmospheric transparency is yet to be solved. For the highest activity of the dust storm in 1971, it was found that 4.5 r ₀ 7.5 m for the lognormal particle size distribution with ²= 0.2 and the optical thickness of a dust cloud ₀ 15. (6) The spectral values of the apparent albedo of Mars measured in October 1971 at a phase angle of 42° in the spectral range 0.250 0.717 allowed the imaginary part of the refractive index to be estimated in terms of a model of a dust cloud composed of spherical particles with the lognormal size distribution with r ₀= 4.5 m and ²= 0.2.     

Physical Properties of Dust in the Martian Atmosphere: Analysis of Contradictions and Possible Ways of Their Resolution

Atmospheric aerosols play an important role in forming the Martian climate. However, the basic physical properties of the Martian aerosols are still poorly known; there are many contradictions in their estimates. We present an analytical overview of the published results and potentialities of various methods. We consider mineral dust. Zonally averaged data obtained from mapping IR instruments (TES and IRTM) give the optical thickness of mineral aerosols ₉ = 0.05–0.1 in the 9-m band for quite atmospheric conditions. There is a problem of comparing these estimates with those obtained in the visible spectral range. We suggest that the commonly used ratio _vis/₉ >2 depends on the interpretation and it may actually be smaller. The ratio _vis/₉ 1 is in better agreement with the IRIS data (materials like montmorillonite). If we assume that _vis/₉ = 1 and take into account the nonspherical particle shape, then the interpretation of ground-based integrated polarimetric observations ( < 0.04) can be reconciled with IR measurements from the orbit. However, for thin layers, the sensitivity of both methods to the optical thickness is poorly understood: on the one hand, polarimetry depends on the cloud cover and, on the other hand, the interpretation of IR measurements requires that the atmospheric temperature profile and the surface temperature and emissivity be precisely known. For quite atmospheric conditions, the local optical-thickness estimates obtained by the Bouguer–Lambert–Beer method and from the sky brightness measured from Viking 1 and 2 and Mars Pathfinder landers are much larger: = 0.3–0.6. Estimates of the contrasts in images from theVikingorbiters yield the same values. Thus, there is still a factor of 3 to 10 difference between different groups of optical-thickness estimates for the quiet atmosphere. This difference is probably explained by the contribution of condensation clouds and/or by local/time variations.     

The Effect of the Shape of Dust Aerosol Particles in the Martian Atmosphere on the Particle Parameters

The influence of the shape of dust aerosol particles in the Martian atmosphere on the imaginary part of the refractive index n _i as derived from photometric observations during the period of the highest activity of the dust storm in 1971 was studied and exemplified for particles of spherical and oblate spheroidal shape. A similar analysis was performed for mean particle radii r ₀ and optical thicknesses ₀ of the dust layer estimated from polarization observations for periods of high atmospheric transparency. It was demonstrated that the values obtained for these optical parameters are affected by the adopted aerosol shape. Namely, the values of n _i, r ₀, and ₀ found for spheroidal particles proved to be nearly twice as large as those for spheres. However, they are still much less than the available estimates of these parameters inferred, in particular, from interpreting space experiments. The reason for this difference needs further investigation.     

Phase effect for the brightness coefficient of the central disk of Saturn and features of Jupiter's disk

Spectral reflectivities ?(λ) of the features on the disk of Jupiter (NTrZ, NEB, EZ, SEB, STrZ) and the center of the disk of Saturn were measured for different phase angles. The phase dependence of ?(λ) is shown to correspond to the shape of the phase function.     

New determination of monochromatic methane absorption coefficients with regard to the thermal conditions in the atmospheres of giant planets. IV. Jupiter and Saturn

Based on the data on the wavelength dependence of geometrical albedo for the disks of Jupiter and Saturn, we determined the trends in the height variation of the aerosol optical depth in the upper atmospheric layers of these planets, the fractional methane concentration in the Jovian atmosphere (0.00125), and the monochromatic methane absorption coefficients (or the superposition of these coefficients for methane and ammonia) typical of the thermal conditions in the atmospheres of Jupiter and Saturn in the wavelength range from 527 to 956 nm.     

On the efficiency of polarization measurements while studying aerosols in the terrestrial atmosphere

It has been shown that the orbital polarization measurements of the Earth in the spectral range λ > 300 nm do not allow the sets of the Stokes parameters satisfying the homogeneity requirement for the optical properties of the “atmosphere + surface” system to be retrieved. Due to this, the atmospheric and surface contributions cannot be correctly separated and the physical properties of the atmospheric aerosol cannot be determined. This is caused by the optical heterogeneity of the system, the different nature of aerosol above different relief features, and the poorly predictable temporal changes of the optical properties of the “atmosphere + surface” system. Observations at λ < 300 nm are more acceptable, since not only the surface but also the tropospheric layer of the atmosphere, which are both mostly subjected to the effects of horizontal inhomogeneity and temporal variations, become practically invisible due to a high absorption by the ozone layer. Because of this, from the scans along specified latitude zones, one may obtain the quasi-homogeneous dependences of the second Stokes parameter Q(α) (U(α) = 0) suitable for estimating the physical characteristics of the stratospheric aerosol and revealing their horizontal and temporal variations.     

On intensity ratios of helium and hydrogen lines in quiescent prominences

Observations of the lines He i 3888 and H8 in 80 quiescent prominences by the author, and in other prominences by Kubota et al. (1972) and Morozhenko (1971), have been used to derive the dependences of I(3888)/I(H8) on I(H8), N ₂ ³ _s on ₀ (H), and N ⁺ n _e on ₀(H) (Figures 1, 2, 3 and 4). The equations of ionization equilibrium and triplet system steady state for a helium atom (27 levels and continuum were considered) were solved together with the radiation transfer equation in the helium Lyman continuum. As given n _e () distribution with depth and T _e =7500 K were assumed. The 2³ S level population N₂ ³ s, helium emission measure N ⁺ n _e and the intensity ratios of the He i 3888 and H8 lines were calculated and compared with observation (Figures 2, 3 and 4, solid lines). The figures show that in bright prominences the observed values of N ₂ ³ s and N ⁺ n _e are systematically higher than the calculated ones. These deviations cannot be eliminated by decreasing n _e . One can make the calculations and observations agree for bright prominences by increasing the UV radiation which penetrates into the prominence.     

Emission of helium in prominences and the chromosphere

The excitation of the levels (2¹ P, 3¹ D, 5¹ D, 6¹ D and 7¹ D) of helium in quiescent prominences and the chromosphere is considered using the observations made during the 1952, 1961 and 1970 solar eclipses (Thomas and Athay, 1961; Athay and Orrall, 1957; Rigutti and Russo, 1964; Kubota et al., 1972) as well as by means of high altitude coronographs (Jefferies and Orrall, 1962). It is shown that the lower levels of the singlet series of helium, like the lower levels of hydrogen, are excited by resonance (λ = 584 Å) and photospheric radiation. The higher levels are populated by transitions from the triplet system, so that a relative state LTE is established between the systems for n ? 6.