Radiative transfer

About: Radiative transfer is a research topic. Over the lifetime, 43287 publications have been published within this topic receiving 1176539 citations.

Transition radiation and transition scattering

Abstract: Transition radiation is a process of a rather general character. It occurs when some source, which does not have a proper frequency (for example, a charge) moves at a constant velocity in an inhomogeneous and (or) nonstationary medium or near such a medium. The simplest type of transition radiation takes place when a charge crosses a boundary between two media (the role of one of the media may be played by vacuum). In the case of periodic variation of the medium, transition radiation possesses some specific features (resonance transition radiation or transition scattering). Transition scattering occurs, in particular, when a permittivity wave falls onto an nonmoving (fixed) charge. Transition scattering is closely connected with transition bremsstrahlung radiation. All these transition processes are essential for plasma physics. Transition radiation and transition scattering have analogues outside the framework of electrodynamics (like in the case of Vavilov?Cherenkov radiation). In the present report the corresponding range of phenomena is elucidated, as far as possible, in a generally physical aspect.

Simulations of structure formation in the universe

Abstract: Cosmic structure has formed as a result of gravitational amplification of primordial density fluctuations together with the action of other physical processes (adiabatic gas dynamics, radiative cooling, photoionization and recombination, radiative transfer). These complex nonlinear processes, acting over a wide range of length scales (from kiloparsecs to tens of megaparsecs), make this a difficult problem for computation. During the last two decades, significant progress has been made in developing numerical methods and statistical tools for analyzing simulations and data. Combined with observational advances, numerical simulations have led to the demise of several formerly popular models and to an improved understanding of galaxy clusters, quasistellar object (QSO) absorption line systems, and other phenomena. This review summarizes these advances.

Assessment of a two-temperature kinetic model for dissociating and weakly ionizing nitrogen

Abstract: The validity of the author's two-temperature, chemical/kinetic model which the author has recently improved is assessed by comparing the calculated results with the existing experimental data for nitrogen in the dissociating and weakly ionizing regime produced behind a normal shock wave. The computer program Shock Tube Radiation Program (STRAP) based on the two-temperature model is used in calculating the flow properties behind the shock wave and the Nonequilibrium Air Radiation (NEQAIR) program, in determining the radiative characteristics of the flow. Both programs were developed earlier. Comparison is made between the calculated and the existing shock tube data on (1) spectra in the equilibrium region, (2) rotational temperature of the N2(+) B state, (3) vibrational temperature of the N2(+) B state, (4) electronic excitation temperature of the N2 B state, (5) the shape of time-variation of radiation intensities, (6) the times to reach the peak in radiation intensity and equilibrium, and (7) the ratio of nonequilibrium to equilibrium radiative heat fluxes. Good agreement is seen between the experimental data and the present calculation except for the vibrational temperature. A possible reason for the discrepancy is given.

The Giant Flare of 1998 August 27 from SGR 1900+14. II. Radiative Mechanism and Physical Constraints on the Source

Abstract: The extraordinary 1998 August 27 giant flare places strong constraints on the physical properties of its source, SGR 1900+14. We make detailed comparisons of the published data with the magnetar model, which identifies the soft gamma repeaters as neutron stars endowed with ~1015 G magnetic fields. The giant flare evolved through three stages, whose radiative mechanisms we address in turn. The extreme peak luminosity L > 106LEdd, hard spectrum, and rapid variability of the initial ~0.5 s spike emission all point to an expanding pair fireball with very low baryon contamination. We argue that this energy must have been deposited directly through shearing and reconnection of a magnetar-strength external magnetic field. Low-order torsional oscillations of the star fail to transmit energy rapidly enough to the exterior, if the surface field is much weaker. A triggering mechanism is proposed, whereby a helical distortion of the core magnetic field induces large-scale fracturing in the crust and a twisting deformation of the crust and exterior magnetic field. After the initial spike (whose ~0.4 s duration can be related to the Alfven crossing time of the core), very hot (T 1 MeV) plasma rich in electron-positron pairs remains confined close to the star on closed magnetic field lines. The envelope of the August 27 flare can be accurately fitted, after ~40 s, by the contracting surface of such a "trapped fireball." The form of this fit gives evidence that the temperature of the trapped pair plasma decreases outward from its center. We quantify the effects of direct neutrino pair emission on the X-ray light curve, thereby deducing a lower bound μmin ~ 1 × 1032 G cm3 to the magnetic moment of the confining field, comparable to the strongest fields measured in radio pulsars. The radiative flux during the intermediate ~40 s of the burst appears to exceed the trapped fireball fit. The lack of strong rotational modulation and intermediate hardness of this smooth tail are consistent with the emission from an extended pair corona, in which O-mode photons are heated by Compton scattering. This feature could represent residual seismic activity within the star and accounts for ~10% of the total flare fluence. We consider in detail the critical luminosity, below which a stable balance can be maintained between heating and radiative cooling in a confined, magnetized pair plasma, but above which the confined plasma runs away to a trapped fireball in LTE. The emergence of large-amplitude pulsations at ~40 s probably represents a transition to a pair-depleted photosphere whose main source of opacity is electrons (and ions) ablated from the heated neutron star surface. The best-fit temperature of the blackbody component of the spectrum equilibrates at a value that agrees well with the regulating effect of photon splitting. The remarkable four-peaked substructure within each 5.16 s pulse, as well as the corresponding collimation of the X-ray flux, has a simple explanation based on the strong inequality between the scattering cross sections of the two photon polarization modes. The width of each X-ray "jet" is directly related to the amount of matter advected outward by the high cross section ordinary mode.