Kinetic Alfv�n waves in extended radio sources

Abstract

We study a model of extended radio sources (ERS), in particular, extragalactic jets and radio lobes, which are inhomogeneous and where noncompressive Alfven and surface Alfven waves (and not shocks and magnetosonic waves) are primarily excited. We assume that a negligible thermal population exists (i.e., the ion density at the low-energy cut-off of the power law distribution is greater than the ion density of the thermal population, if present). Due to internal instabilities and/or the interaction of the ERS with the ambient medium, surface Alfven waves (SAW) are created. We show that even very small amplitude SAW are mode converted to kinetic Alfven waves (KAW) which produce large moving accelerating potentials π, parallel to the magnetic field. Neglecting nonlinear perturbations, and for typical physical parameters of ERS, we obtaineπ≳1 MeV. Wesuggest that these potentials are important in acceleration (e.g., injection energy) and reacceleration of electrons in ERS. We show that energy losses by synchrotron radiation can be compensated by reacceleration by KAW. The relation between KAW acceleration, and previously studied cyclotron-resonance acceleration by Alfven waves, is discussed.