Kinetic theory of small-amplitude fluctuations in astrophysical plasmas

Abstract

Electromagnetic field fluctuations are arguably among the most fundamental kinetic processes in classical, collisionless plasmas. Considering their two characteristic features, propagation and amplitude modulation, they include such eminent phenomena as waves and instabilities. This work is devoted to an extension of their theoretical understanding with particular emphasis on weakly propagating and aperiodic fluctuations and their spontaneous emission. In this regard, three main results are achieved that contribute to both generic plasma theory as well as astrophysical applications.Firstly, the fluctuation–dissipation theorem for weakly coupled thermal plasmas is generalized to arbitrary values of the complex frequency in order to rid the previous formulations of their restrictions in this respect.Secondly, the spontaneously emitted magnetic field fluctuations in the intergalactic medium are addressed. Due to a recently discovered damped and aperiodic mode, the level of magnetic noise is high enough to have the latter serving as a seed field for further amplification processes like magnetohydrodynamic dynamo action, so this spontaneous emission is of major relevance for cosmic magnetogenesis. Here, it is shown that a highly relativistic electron–positron pair beam can trigger a transition of these damped fluctuations into amplified ones in certain wavenumber ranges, thus allowing for an even higher seed level. It is found that only those fluctuations are accessible to amplification whose wavevectors are perpendicular or at least almost perpendicular to the propagation direction of the beam.Thirdly, the possibilities are investigated to obtain observational evidence for this newly discovered stable branch of the Weibel mode because an empirical confirmation of its existence is still pending today. It is shown that the mode-driven turbulence is incompressible and that, therefore, dispersion measure, rotation measure, and scintillation related techniques are not applicable. The velocity fluctuations generated by the mode, however, are shown to be large enough to qualify line broadening studies as a suitable diagnostic method to detect the mode after all.