Radiation Transfer and the Possibility of Negative Absorption in Radio Astronomy

Abstract

Stimulated transitions are relatively enormously more probable at radio than at optical frequencies and it is this which makes it possible for negative absorption to arise at radio wavelengths when the medium will behave like an amplifier to the incident radiation. A necessary condition for the existence of this phenomenon is that the kinetic energy distribution F(?) of the radiating electrons be markedly non-thermal with an appreciable excess of high energy electrons such that ¶F/¶? is positive over a finite range of the kinetic energy ?. However, this condition is not sufficient, since it is shown that an electron gas in which free·free transitions provide the dominant radiation process can never exhibit negative absorption whatever the form of F(?), and it is further necessary that the stimulated transition probability should have a maximum at some finite value of the kinetic energy, the most favourable case occurring when this maximum is a sharp one at the value of ? at which ¶F/¶? has a positive maximum. These conditions can both be met in principle for the cases in which the dominant radiation process is due (a) to Cerenkov effect, (b) to gyro radiation by non-relativistic electrons, (c) to synchrotron-type radiation by highly relativistic electrons, and it is shown that negative absorption can arise in all these cases; the relevance of these results to radio astronomy is discussed briefly.