Magnetohydrodynamic plasma instability driven by Alfvén waves excited by cosmic rays
Abstract
Hydrodynamical equations describing the mutual interaction of cosmic rays, thermal plasma, magnetic field and Alfvén waves scattering the cosmic rays used in cosmic ray shock acceleration theory (e.g. McKenzie & Völk 1982; Drury 1983; Webb 1983) are analysed for longwavelength linear compressive instabilities. The Alfvén wave field may contain a preexisting component as well as a component excited by the cosmic ray streaming instability. In the case of no Alfvén wave damping, adiabatic wave growth and Alfvén wave generation by the cosmic ray streaming instability, it is found that the backward propagating slow magnetoacoustic mode is driven convectively unstable by the pressure of the selfexcited Alfvén waves, provided the thermal plasma β is sufficiently large. The equations are also analysed for the case where the Alfvén wave growth is balanced by some nonlinear damping mechanisms. In the latter case both the forward and backward propagating slow magnetoacoustic modes may be driven unstable if the plasma β is sufficiently small. The conditions under which the instabilities occur are delineated, and sample calculations of growth rates given. Possible applications of the instabilities to astrophysical situations are briefly discussed.
 Publication:

Journal of Plasma Physics
 Pub Date:
 April 1984
 DOI:
 10.1017/S0022377800001628
 Bibcode:
 1984JPlPh..31..275M
 Keywords:

 Cosmic Rays;
 Interstellar Gas;
 Magnetohydrodynamic Stability;
 Particle Acceleration;
 Plasma Equilibrium;
 Astrophysics;
 Beta Factor;
 Compression Waves;
 Magnetoacoustic Waves;
 Magnetohydrodynamic Waves;
 Plasma Heating;
 PlasmaParticle Interactions;
 Self Excitation;
 Thermal Plasmas;
 Astrophysics