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A moment method with three stream division of the radiation field was suggested by Wilson, Wan and Sen (1980) for solving radiative transfer problems in stationary, non-grey extended shells surrounding a central star. Use was made of the generalised Eddington relations as the closure conditions of the moment equations. In the present paper the same method has been utilised to study the radiative transfer problems in a non-grey, expanding gaseous spherical shells surrounding a central star. The transfer equation has been set in comoving frame in spherical geometry. The radiation and material quantities, angles and frequencies have been expressed in comoving frame. The mean intensity, flux and K-integrals have been calculated for extensive atmospheres in the presence of different velocity fields.

A variational technique has been developed to study the stability of the radiation field in a cylindrical homogeneous interstellar maser with steady state pumping. A dynamical potential has been constructed, the extremal of which gives the radiative transfer equations and rate equations. The dispersion equations and the stability criteria are determined from the potential for specific natures of perturbations.

Mengüç, and Viskanta (1983) examined the scope of some approximate methods for solving transfer problems in plane medium scattering anisotropically. His aim was to focus attention on methods capable of extension to multidimensional geometry. In the present paper, it is demonstrated that modified double-interval spherical harmonic method admirably suits that role. The transfer problem of Mengüç and Viskanta's model has been solved by this method. The results computed are found to be in good agreement with those obtained by other methods.

In the present paper, an inverse problem of radiative transfer in an inhomogeneous plane medium of scattering albedo ?=?0 exp(-t/s) is solved by a moment method. The results are compared with those obtained by Dunn (1983) using Monte Carlo method.

In many astrophysical problems, the study of the stability of an atmosphere in the presence of a magnetic field is of importance. In most cases the MHD instabilities of atmospheres are studied by energy principle of Bernsteinet al. (1958). In this paper, a general method for studying the stability of a system subject to MHD equations of conditions has been proposed. This is based on the local potential concept put forward by Glansdorff and Prigogine (1964). The scheme for securing stability criteria has been demonstrated in two particular cases.

The approximate method of representing the intensity by a three-stream angular division is used to compute the brightness at the zenith near the terminator of an optically thin, spherically-symmetric planetary atmosphere illuminated by parallel solar radiation. The results are compared with those of Sobolev obtained under classical Eddington approximation.

Abstract: A two-shell moment method with three stream division of the radiation field has been suggested for solving radiative transfer problems in a non-grey extended dust shell surrounding a central star. The results computed are found to be of comparable accuracy with those of Unno and Kondo, obtained by generalized Eddington approximation. A more simplified approximation has been put forward for an optically thin medium.

The radiative transfer problem in a plane medium with a highly forward-scattering phase function has been solved by Siewertet al. (1980) with reflective boundaries, and by Mengüç and Viskanta (1983) without reflective boundaries. In the present paper a moment method is devised to solve the same problem. The results computed are found to be in good agreement with the exact results.

The problem of diffuse reflection by a homogeneous, isotropically scattering, infinite cylindrical medium has been considered. The relevant auxiliary equation has been formulated, the scattering function defined and the integro-differential equation for such function deduced. For a medium having cylindrical distribution of source in addition to the incident flux at the outer surface, the integro-differential equation for the emergent intensity has been established.

In this paper, the Combined Operational Method developed by Busbridge (1961) in connection with the radiative transfer problems in plane-parallel atmospheres has been extended to similar problems in isotropic scattering, homogeneous spherical media. The relevant auxiliary equation has been formulated, the scattering function defined and the integro-differential equation for such function deduced. For a medium having radial distribution of source in addition to the incident flux at the outer surface, the integro-differential equations for source function and emergent intensity have been established.

Combined-Operations method has been utilised to solve the problem of diffuse reflection by a homogeneous, isotropic, non-coherent scattering spherical medium. The source function is considered to be frequency independent. The auxiliary equation has been formulated, the scattering function defined, and the integro-differential equation for this function deduced. A method for obtaining the emergent intensity and the internal source function for non-zero internal source distribution has been suggested for a given line profile.

The Combined Operations Method has been used to solve the diffuse radiation problems in inhomogeneous, non-coherently scattering spherical shell media involving macroscopic motion. Relevant integro-differential equations for scattering function and emergent intensity have been established.

A probabilistic model for solving transfer problems in non-homogeneous, isotropic, and non-coherent scattering cylindrical shell media has been proposed. The source function is considered to be frequency independent. The scattering and transmission functions have been defined for the case of complete redistribution in frequency. A tractable integrodifferential equation for the scattering function has been derived.