Multi-Species Instability Thresholds in the Solar Wind

Abstract:

There has been much recent investigation into how various pressure anisotropy and drift instabilities constrain the distribution of solar wind parameters such as temperature anisotropy, beta, and differential flow between species. These investigations have tended to look at the major solar wind species (protons, alphas, electrons) separately, and how the distribution of the parameters of each species is separately constrained by the kinetic instability arising from free energy in its own distribution. However, the stability of the plasma depends on all species together, and here we present a first investigation of this using a combined proton/alpha/electron data set from the 3DP and SWE instruments on the Wind spacecraft. When all species are combined, the distributions appear to be remarkably well constrained by the fluid firehose and mirror thresholds. Non-proton species and drifts make significant contributions to the plasma being marginally firehose unstable. Theoretical implications of these results will be discussed.