Frank Varosi

Thermospheric gravity waves: observations and interpretation using the transfer function model (TFM)

Gravity waves are prominent in the polar region of the terrestiral thermosphere, and can be excited by perturbations in Joule heating and Lorents force due to magnetospheric processes. We show observations from the Dynamics Explorer-2 satellite to illustrate the complexity of the phenomenon and review the transfer function model (TFM) which has guided our interpretation. On a statistical basis, the observed atmospheric perturbations decrease from the poles toward the equator and tend to correlate with the magnetic activity index, Ap, although individual measurements indicate that the magnetic index is often a poor measure of gravity wave excitation. The theoretical models devised to describe gravity waves are multifaceted. On one end are fully analytical, linear models which are based on the work of Hines. On the other end are fully numerical, thermospheric general circulation models (TGCMs) which incorporate non-linear processes and wave mean flow interactions. The transfer function model (TFM) discussed in this paper is between these two approaches. It is less restrictive than the analytical approach and relates the global propagation of gravity waves to their excitation. Compared with TGCMs, the TFM is simplified by its linear approximation; but it is not limited in spatial and temporal resolution, and the TFM describes the wave propagation through the lower atmosphere. Moreover, the TFM is semianalytical which helps in delineating the wave components. Using expansions in terms of spherical harmonics and Fourier components, the transfer function is obtained from numerical height integration. This is time consuming computationally but needs to be done only once. Once such a transfer function is computed, the wave response to arbitrary source distributions on the globe can then be constructed in very short order. In this review, we discuss some numerical experiments performed with the TFM, to study the various wave components excited in the auroral regions which propagate through the thermosphere and lower atmosphere, and to elucidate the properties of realistic source geometries. The model is applied to the interpretation of satellite measurements. Gravity waves observed in the thermosphere of Venus are also discussed.

ANALYTICAL APPROXIMATIONS FOR CALCULATING THE ESCAPE AND ABSORPTION OF RADIATION IN CLUMPY DUSTY ENVIRONMENTS

We present analytical approximations for calculating the scattering, absorption, and escape of nonionizing photons from a spherically symmetric two-phase clumpy medium, with either a central point source of isotropic radiation, a uniform distribution of isotropic emitters, or uniformly illuminated by external sources. The analytical approximations are based on the mega-grains model of two-phase clumpy media, as proposed by Hobson & Padman, combined with escape and absorption probability formulae for homogeneous media. The accuracy of the approximations is examined by comparison with three-dimensional Monte Carlo simulations of radiative transfer, including multiple scattering. Our studies show that the combined mega-grains and escape/absorption probability formulae provide a good approximation of the escaping and absorbed radiation fractions for a wide range of parameters characterizing the clumpiness and optical properties of the medium. A realistic test of the analytic approximations is performed by modeling the absorption of a starlike source of radiation by interstellar dust in a clumpy medium and by calculating the resulting equilibrium dust temperatures and infrared emission spectrum of both the clumps and the interclump medium. In particular, we find that the temperature of dust in clumps is lower than in the interclump medium if the clumps are optically thick at wavelengths at which most of the absorption occurs. Comparison with Monte Carlo simulations of radiative transfer in the same environment shows that the analytic model yields a good approximation of dust temperatures and the emerging UV-FIR spectrum of radiation for all three types of source distributions mentioned above. Our analytical model provides a numerically expedient way to estimate radiative transfer in a variety of interstellar conditions and can be applied to a wide range of astrophysical environments, from clumpy star-forming regions to starburst galaxies.

Mid-Infrared (4.8 – 12.4 and 20.0 µm) Images of the Galactic Center: Modeling the Energetics and Morphology of the Central Parsec

We have modeled the mid-infrared emission from the Galactic Center using our array camera images at eight wavelengths. The results suggest that the high infrared luminosity of the region is provided by a cluster of luminous stars. There is no direct indication in the new model results of a very luminous object or “central engine” near Sgr A*.

The Effect of Dust Evolution on the Spectral Energy Distribution of Galaxies

The spectral energy distribution of galaxies is strongly affected by the abundance, composition, and size distribution of their dust content. In addition, the morphology of a galaxy and the clumpiness of its interstellar medium play a crucial role in the attenuation of starlight and in its conversion to infrared emission. In this contribution we present simple models for the evolution of dust in normal spiral galaxies and pristine starburst regions, and examine how the dust affects the attenuation and spectral energy distribution of these systems.

Radiative transfer in clumpy and fractal media

A Monte Carlo model of radiative transfer in multi-phase dusty media is applied to the situation of stars and clumpy dust in a sphere or a disk. The distribution of escaping and absorbed photons are shown for a range of clump filling factors and densities. Analytical methods of approximating the escaping fraction of radiation, based on the Mega-Grains approach [4], are discussed. Comparison with the Monte Carlo results shows that the escape probability formulate provide a reasonable approximation of the escaping/absorbed fractions, for a wide range of parameters characterizing the clumpy dusty medium. A possibly more realistic model of the interstellar medium is one in which clouds have a hierarchical structure of denser and denser clumps within clumps [2], resulting in a fractal distribution of gas and dust. Monte Carlo simulations of radiative transfer in such multi-phase fractal media are compared with the two-phase clumpy case.

Mid-Infrared (4.8 – 12.4 and 20.0 μm) Imaging of the Galactic Center: Modeling the Dust Emission and Energetics of the Central Parsec

We have modeled the mid-infrared emission from the Galactic Center using our array camera images at eight wavelengths. The results suggest that the high infrared luminosity of the region is provided by a cluster of luminous stars. There is no direct indication in the new model results of a very luminous object or “central engine” near Sgr A*.

Survey of Fine-Scale Structure in the Far-Infrared Milky Way

What is the general morphology of the diffuse interstellar medium? Is it mostly uniform or clumpy? Are the clumps mostly in the form of spheroidal clouds, sinuous filaments, extended sheets, or discrete shells? And do the clumps or the voids better define the overall structure? By addressing these morphological questions, one can better constrain the dynamical processes that are most responsible for shaping; and energizing the ISM.