Fluctuations and ion isotropy in the solar wind

The reasons the solar wind seems to behave as a fluid in spite of the near-absence of collisions are investigated. The ion distribution, which is nearly isotropic even though magnetic moment ought to be conserved, is considered quantitatively. Fluctuations and waves are often invoked to replace collisions to provide the fluid properties. Calculations based on a theory of diffusion due to electric field fluctuations are used to estimate the important ranges of fluctuations. It appears that the probable fluctuations are in the neighborhood of 1 Hz, and that such electric fields have not been sufficiently investigated. However, estimates based on observations of density and magnetic fluctuations suggest that the electric fields might be sufficient to provide the required diffusion, and are worth further study.