Electron-ion Coulomb scattering and the electron Landau damping of Alfvén waves in the solar wind

Abstract

[1] All Alfven waves in the solar wind have parallel electric fields, which enable Landau damping. The Alfven waves' Landau resonate with very low energy electrons; low-energy electrons are easily trapped in the Alfven waves, and at low energies electron-ion Coulomb scattering is very rapid. Analytic fluid theory and numerical solutions to the linear Vlasov equation are used to determine the properties of Alfven waves (and kinetic Alfven waves) in the solar wind. Electrostatic potentials associated with the waves' parallel electric fields are found to be relatively large. Owing to the large potentials, electrons over a broad range of velocities interact with the wave to produce Landau damping. Because of this broad range, linear Vlasov theory is invalid and the Landau-damping rates computed via linear Vlasov theory may not be accurate. Electron velocity diffusion owed to electron-ion Coulomb scattering is analyzed. Electron diffusion times in the Alfven wave Landau resonance are calculated and are found to be faster than wave periods and much faster than Landau-damping time scales. Coulomb collisions should prevent the electron distribution function from evolving away from a Maxwellian form, and thereby Coulomb collisions act to maintain Landau damping (although at a rate different than that given by linear Vlasov theory). Some complications of this Landau-damping picture arise from the interplanetary electric field competing with the Alfven wave parallel electric fields and from ion beams near the Landau resonance.