We discuss the physics of the hot-spots of extended extragalactic radio sources. Although the canonical model assumes that high-energy electrons are injected at the shock, consideration of high Mach number hydromagnetic shocks suggests that this is an over-simplification. Observational and experimental evidence indicates that such shocks break into macroscopic turbulence and that particle processes are inadequate to thermalize the flow energy.We propose a model in which both particle and magnetic field energy are derived from these macroscopic motions. Waves, generated when the shock breaks, give rise to the large-scale magnetic field by dynamo action and steepen as they propagate, producing a distribution of secondary shocks within which electrons are accelerated to the threshold for Fermi acceleration. It is shown that, as a consequence of this, there will be approximate energy equipartition between waves, particles and large-scale magnetic field. An illustrative model of the head is constructed that allows for spatial variation of the electron spectrum whilst producing the observed radiation spectrum. © 1981 Royal Astronomical Society. Provided by the NASA Astrophysics Data System.
