SPRG Seminars
October 29, 2013:
"Kinetic Instabilities Driven by Anisotropic Ion and Electron Beams in the Solar Wind"
Daniel Verscharen, University of New Hampshire
Velocity distribution functions of particles in collisionless plasmas can show significant deviations from the entropically favored Maxwellian equilibrium. In the presence of a background magnetic field, temperature anisotropies, drifts between particle species, and beam structures can all develop in certain directions with respect to the field. The solar wind typically shows these features, especially at times of fast-wind streams. If the deviations from the equilibrium become too large, the distribution functions can trigger micro-instabilities that relax the distribution functions toward a stable configuration by radiating free energy as plasma waves.
This presentation gives an overview on micro-instabilities driven by anisotropic ion beams (e.g., the drifting alpha-particle component) in the solar wind. We present an analytical framework that describes the physical mechanisms and thresholds of resonant micro-instabilities, and compare our analytical results with numerical calculations and solar-wind observations. The observations show a good agreement with our results and indicate that effects of beam structures and temperature anisotropies must be treated on equal footing. We also discuss the application of our theoretical framework to instabilities driven by the electron strahl in the solar wind. Our preliminary analysis suggests that strahl electrons are scattered into the halo by a self-induced instability of the oblique fast-magnetosonic/whistler mode. We discuss implications of this mechanism on halo formation, heat-flux regulation, and the nature of turbulent fluctuations on electron scales in the solar wind.