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

Abstract

[1] All Alfvén waves in the solar wind have parallel electric fields, which enable Landau damping. The Alfvén waves' Landau resonate with very low energy electrons; low-energy electrons are easily trapped in the Alfvén 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 Alfvén waves (and kinetic Alfvén 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 Alfvén 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 Alfvén wave parallel electric fields and from ion beams near the Landau resonance.Citation: Borovsky, J. E., and S. P. Gary (2011), Electron-ion Coulomb scattering and the electron Landau damping of Alfvén waves in the solar wind,

The Statistical Properties of Solar Wind Temperature Parameters Near 1 au

“…Thus, a sizable fraction of the time the electrons, protons, and alpha-particles are near thermal equilibrium with each other. The higher ratio for electrons and protons could result from the higher electronproton than proton-alpha and electron-alpha Coulomb collision rates in the solar wind (e.g., Spitzer & Härm 1953;Borovsky & Gary 2011; e.g., see Section 3.4 and Appendix B) or it may be a Table 2. The opposite is true for the electron-to-alpha-particle ratios.…”

The need to consider ion Bernstein waves as a dissipation channel of solar wind turbulence

“…These collisionless dissipation mechanisms are strictly valid for small amplitude waves but, nevertheless, they have been used to model turbulent dissipation in the solar wind wherein the wave amplitudes are most likely nonlinear and possibly strongly nonlinear even for wave numbers in the dissipation range [ Gary , 1999; Marsch , 1999; Leamon et al , 1999; Li et al , 2001; Stawicki et al , 2001; Cranmer and van Ballegooijen , 2003; Gary and Borovsky , 2004; Howes et al , 2008a; Sahraoui et al , 2009; Podesta et al , 2010; Sahraoui and Goldstein , 2011; Sahraoui et al , 2011]. The extent to which linear Landau damping rates remain valid for strongly nonlinear turbulent fluctuations is an important unresolved question; Borovsky and Gary [2011] have argued, for example, that Landau damping rates computed from the linearized Vlasov‐Maxwell theory are not valid at all for kinetic Alfvén waves (KAWs) in the solar wind. In addition, various nonlinear mechanisms may also play a role in the dissipation of solar wind turbulence [ Matthaeus et al , 1999b] including magnetic reconnection or nonlinear mechanisms involving wave coupling.…”

The effect of sudden wind shear on the Earth's magnetosphere: Statistics of wind shear events and CCMC simulations of magnetotail disconnections

“…It has long been recognized that the free energy of differential flow is a possible source for in situ heating of the solar wind plasma [ Coleman , 1968; Parker , 1969]. Potential mechanisms for solar wind heating associated with velocity shears are dissipation of Kelvin‐Helmholtz waves [ Korzhov et al , 1985; Neugebauer et al , 1986], the excitation and dissipation of MHD turbulence [ Roberts et al , 1992; Goldstein , 2009], the phase mixing of Alfven waves [ Ruderman et al , 1999; Kaghashvili , 1999], damping of MHD surface waves [ Hollweg et al , 1990; Yang and Hollweg , 1991], damping of shear‐driven plasma waves [ Migliuolo , 1984; Markovskii et al , 2006], and Landau‐damping of the shear structure itself [ Borovsky and Gary , 2009, 2011]. However, no heating was found in the long‐lived large‐scale shears of corotating interaction regions [cf.…”

“…Attempts to calculate an eddy‐viscosity coefficient for the solar wind from observations of the velocity fluctuations have resulted in values that differ by 8 orders of magnitude [ Korzhov et al , 1984, 1985; Verma , 1996; Borovsky , 2006]. For the collisonless solar wind, Borovsky and Gary [2009, 2011] explored methods to calculate shear viscosities from Bohm diffusion and from Landau damping, which yield values below those of eddy‐viscosity estimates. Bohm‐diffusion calculations give shear‐layer thicknesses at 1 AU of few thousand kilometers, which is in the ballpark of thicknesses observed with the high‐time‐resolution plasma measurements from the Wind spacecraft.…”