Heat Flux and Viscosity of Protons in the Collisionless Solar Wind

Details Heat Flux and Viscosity of Protons in the Collisionless Solar Wind Heat Flux and Viscosity of Protons in the Collisionless Solar Wind

Nov 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...453..953w&link_type=abstract

Astrophysical Journal v.453, p.953

Astronomy and Astrophysics

Astronomy

6

Hydrodynamics, Interplanetary Medium, Sun: Solar Wind

Scientific paper

Beyond 10 or 20 solar radii, the Coulomb-collision mean free path for thermal protons exceeds the scale height of the solar atmosphere. The expanding solar plasma becomes collisionless, and the kinetics of the solar wind are no longer dominated by thermalizing collisions. The usual Braginskii-type expressions for solar wind proton heat flux and viscosity are no longer valid. However, another microscale still exists in the solar wind, dictated by the gyroradius of protons in the turbulent embedded solar wind magnetic field. Wave-particle interactions will act to isotropize (but not thermalize) particle distributions, and the relevant microscale for this process is the proton gyroradius. The proton distribution can be modeled as undergoing isotropizing "collisions," with the relevant mean free path scaling with gyro radius. Here, I calculate the heat flux and viscosity expected for solar wind protons which are relaxing to isotropy on a microscale that is proportional to gyroradius. The collisionless viscosity and heat flux have a functional dependence different than their Coulombic analogues. The Coulombic expressions for proton viscosity and heat flux drastically overestimate the efficiency of diffusive energy and momentum transport actually operative in the solar wind. Because the concepts of thermal conductivity and viscosity have a validity that transcends the particular dissipation microscale, it is suggested that a transport mean free path can be identified as that microscale, whether Coulombic or not, which is governing solar wind proton dissipation.

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