Alfv�n instabilities in streaming plasmas with anisotropic pressures and their relevance for the solar wind

Abstract

Low frequency or Alfven waves in streaming plasmas can become unstable when the square of the Alfven velocity is smaller than the mean square of the bulk motion in a co-moving reference frame, 〈(u ‖−〈u ‖〉)2〉, whereu ‖ stands for the bulk velocity of each species and 〈u ‖〉 is the average bulk velocity of the plasma as a whole. For these new Alfven instabilities the streaming effects can be enhanced by a suitable pressure anisotropy. Perpendicular pressure effects are stabilizing, parallel pressure effects are destabilizing, as in the usual firehose instability. The observed velocity differences between helium and the main (hydrogen) flow in the solar wind plasma are such that the Alfven waves are getting close to marginal instability. These new Alfven instabilities limit the velocity differences between helium and hydrogen and thus provide a possible mechanism for accelerating the helium particles up to the order of the main flow velocity.