Solar wind plasma : kinetic properties and micro-instabilities

Abstract

The kinetic properties of ions in the solar wind plasma are studied. Observations of solar wind +H and +2He by the Faraday Cup instrument component of the Solar Wind Experiment on the Wind spacecraft show that these ions have magnetic field-aligned, convected, bi-Maxwellian velocity distribution functions. The analysis yields the best-fit values of the bulk velocity, U, number density n, and parallel T and perpendicular T temperatures of each of the ion species. The accuracy of each of these measurements is studied and an absolute calibration of the Faraday Cup is performed, demonstrating the accuracy of the densities to =/< 2%. The range of the proton temperature anisotropy Rp= - Tp/Tp is explored, and it is demonstrated that thermodynamic concepts such as the double adiabatic equations of state are insufficient approximations for a kinetic description of the solar wind plasma. It is shown that Rp is constrained on macroscopic timescales by Coulomb relaxation and the expansion of the solar wind, and on kinetic timescales by the mirror, cyclotron, and firehose plasma micro-instabilities. Electromagnetic fluctuations generated by growing mirror and cyclotron modes are detected in the solar wind. The first detailed observations of the firehose instability are presented. The limiting bounds to Rp imposed by each of these instabilities are measured and compared with the theoretical predictions of fluid magnetohydrodynamics, linear kinetic Vlasov theory, and numerical simulations. It is shown that the predictions of linear theory and the simulations are in agreement with the observations.

(cont.) A new proton temperature anisotropy driven instability in the regieme Rp < 1, βP < 1 is discovered. The kinetic properties of +H and +2He are compared. For the first time a cyclotron resonant instability driven by the proton temperature anisotropy is demonstrated to limit the differential flow U=Uα-Up attainable in the solar wind, in confirmation of recent theoretical predictions. It is shown that the +2He temperature anisotropy Rα=Tα/Tα is also constrained by micro-instabilities, and the first observations of the+2He cyclotron and firehose instabilities are presented. The parallel and perpendicular temperatures of +H and +2He are compared, and evidence of cyclotron-resonant heating of +2He preferrentially to +H in the interplanetary medium is presented.