ON THE MAGNETIZATION OF COSMIC OUTFLOWS: PLASMA MODES AND INSTABILITIES OF UNMAGNETIZED PLASMA BEAMS

Abstract

The dissipation of the kinetic energy of cosmic outflows in interactions with ambient collision-free plasmas and the associated generation of electromagnetic plasma turbulence is a fundamental problem of modern astrophysics. Thermalization by elastic two-body Coulomb collisions is orders of magnitudes too slow as compared to interactions with electric and magnetic fields because of the generally low density of cosmic plasmas. Alternative dissipation mechanisms have to be examined such as energy diffusion by second-order Fermi interactions of charged particles with electromagnetic turbulence, which are an intrinsic property of any sufficiently agitated magnetized plasma. We consider the microphysical details of the energy conversion in relativistic and nonrelativistic outflows by investigating the solutions of the linear plasma dispersion relation in an unmagnetized anisotropic beam plasma consisting of two overall-neutral particle beams propagating with arbitrary velocities in the same direction. The general plasma dispersion relation is derived for arbitrary propagation angle θ with respect to the beam propagation direction both in the initial laboratory frame and in the counterstream frame of reference. Solutions of the linear dispersion relation are derived for parallel (θ = π/2) and perpendicular (θ = 0) propagation angle, respectively. For parallel propagation angles, the electrostatic mode is excited and its maximum growth rate depends on the relative bulk Lorentz factor of the flows and their density ratio. For perpendicular propagation angles, the aperiodic filamentation mode is excited and its maximum growth rate depends differently on the relative bulk Lorentz factor of the flow and their density ratio. The respective maximum growth rates indicate that for nonrelativistic flow velocities the electrostatic instability (EI) is excited much faster than the filamentation instability (FI), whereas for relativistic flow velocities the FI has a larger growth rate than the EI. This implies that nonrelativistic flows dissipate their kinetic energy predominantly in electric fields associated with the electrostatic mode, while relativistic flows dissipate their kinetic energy predominantly in magnetic fields. Therefore, initially unmagnetized relativistic flows are more effectively magnetized than initially unmagnetized nonrelativistic flows.