Radiation, brightness temperature

Interstellar masers reveal themselves by an extremely intense emission of radiation (brightness temperatures in... 

The intensity of maser radiation the molecular aind level population analysed by the concept of radiation brightness temperature Tg and solution of the radiative transfer equation (see for general discussion e.g. Winnewisser et al., 1979 maser specific reference Elltzur, 1982). The intensity of... 

One advantage of a spectral radiation pyrometer is that the emissivity or emittance at only a specific wavelength (e.g. 0.653 pm) is of importance. A non-blackbody source will be less luminescent than a blackbody source at the same temperature. Thus, a falsely low temperature will be determined by sighting a calibrated disappearing filament pyrometer on the non-blackbody. This temperature has been referred to as the brightness temperature . 

A calorimetric method may be used where an electric heater is imbedded in the object of interest, and the power dissipated by the element is accurately calculated from voltage and current. Once steady state is established and the object is at constant temperature, the body must emit radiation at the same rate at which it is supplied. As long as conduction and convection are eliminated as mechanisms of heat transfer (e.g. vacuum conditions), the blackbody temperature is known by Rt = o"T4. The emittance can then be determined after py-rometric measurements of the brightness temperature of the object. 

If the effective temperature of our defined system is less than the universal radiation background temperature of 2.7 K, transitions between the two levels can be observed in absorption. This is the case with interstellar formaldehyde. Alternatively absorption can be observed against the continuum radiation from a nearby bright source. Spontaneous emission will always occur provided the upper of the two levels is populated, and can be observed if the populations are different. There are, in addition, examples of the exceptional situation in which N2 > N the result of this population inversion is that stimulated emission dominates, and maser emission is observed. Interstellar OH and SiO provide diatomic examples of this unusual situation, as also does interstellar H2O we shall describe the results for OH later in this chapter. Departures from local thermodynamic equilibrium are very common, and the concept of temperature in interstellar gas clouds is not simple this is a major part of astrophysics which is, however, beyond the scope of this book. 

Radiation intensities are referred to the radiation of a black body. In the radio frequency range it is convenient to express line intensities in equivalent line brightness temperatures, since the surface brightness Bv of a black body... 

In radio astronomy multichannel or autocorrelation (Fourier) spectrometers are used which simultaneously cover the whole line profile. Consider a molecular cloud observed against a source of continuum radiation of a given brightness temperature. The continuum brightness temperature is the sum of the 2.7 °K isotropic background radiation Tbh of a continuum source (such as an HII region or a supernova remnant) which may be in the line of sight and located behind the molecular cloud. A specific molecular transition with optical... 

Brightness Temperature Theoretical temperature at which a blackbody would radiate thermal energy at the rate detected by a radio telescope. 

The accuracy of the radius and temperature determination will depend on the quality of the spectral fit (and thus on the sensitivity and resolution of the spectrum), the precision of the Sun-star distance, the cloud coverage and also the distribution of brightness temperatures over the planetary surface. Assuming the effective temperature of our planet was due exclusively from radiation to the surface instead of an atmosphere layer (the average equilibrium temperature of Earth is about 265 K instead of the 288 K surface temperature), would only introduce a few percent error on the derived Earth radius. 

The planet s radius can be estimated from thermal emission of the planet, because it is a function of the planet s temperature and surface area. A low resolution spectrum in the IR can be used to derive the effective temperature of the planet and therefore one can calculate the radiating area from the IR flux. The brightness temperature provides information on the effective temperature of the atmospheric layers responsible for the emission and can provide an idea of the surface temperature if the atmosphere is see through like on Earth and Mars. 

MWR involves measuring the power in the microwave region of the natural thermal radiation from body tissues to obtain the so-called brightness temperature of the tissue under observation. Brightness temperature Tb is defined as... 

Brightness temperature The definition is not unique great care is needed to decipher the intention of a given author. The temperature at which a blackbody radiator would radiate an intensity of electromagnetic radiation identical to that of the planet for a specific frequency, frequency bandwidth, and polarization under consideration is one definition of brightness temperature. A second definition is that it is the intensity of radiation under consideration divided (normalized) by the factor jlk). The normalization factor dimensionally scales... 

Even though the planets do not radiate like a blackbody, planetary radio astronomers express the observed brightness in terms of the temperature of an equivalent blackbody that would produce the same brightness. This temperature is called the brightness temperature Tb, defined as follows [from Eq. (4)] ... 

The brightness temperature for a planet can be calculated once the flux density S and solid angle 2 are known. The brightness temperature approximates the physical temperature the more the planet behaves like a blackbody radiator. 

This equation is usually used to calculate the average disk brightness temperature for a model atmosphere for comparison with observations. Unlike the blackbody radiation model, this model predicts a wavelength dependence of... 

The effects of surface emissivity are exaggerated in high-temperature applications, and particular attention should be paid to the selection of the type of surface of the insulation system. Low-emissivity surfaces such as bright polished aluminum reduce heat loss by inhibiting the radiation of heat from the surface to the surrounding ambient... 

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