Radial evolution of solar wind thermal electron distributions due to expansion and collisions

Abstract

ISEE 3 electron observations near 1 AU show that the solar wind thermal electron temperature anisotropy, T∥/T⊥ is typically 1.0 to 1.5, with densest distributions most nearly isotropic, but is sometimes much higher when density is low. For a small observational subset characterized by high density and low bulk speed, T⊥ can exceed T∥. Based on these and other observations, we propose a simple model for radial evolution of thermal electrons in a structureless solar wind under the influence of Coulomb collisions and geometric expansion in a spiral interplanetary magnetic field (IMF). We use the Chew‐Goldberger‐Low double adiabatic invariants, modified by a collisional relaxation term, to develop coupled differential equations for dT∥/dr and dT⊥/dr which can be numerically integrated from an assumed isotropic starting point near the Sun. The model, which satisfactorily explains the 1 AU observations, shows that the evolution of electron temperature and T∥/T⊥ is controlled by plasma density, flow speed, and initial temperature, with density the most important factor. Highest density plasmas cool most rapidly and remain nearly isotropic, while lower density plasmas can develop large thermal anisotropies due to their low collision frequencies and thus can maintain a higher total temperature. Convection speed influences the radial evolution by (1) dictating the IMF spiral, and (2) controlling the time available for collisional relaxation. The model makes predictions for radial distances other than 1 AU and for high solar latitudes. In the inner solar system, where the IMF is nearly radial, the prevailing T∥/T⊥ should be greater than at 1 AU. At greater radial distances in the solar equatorial plane, the increasing inclination of the IMF to the radial direction plus the cumulative effects of collisional relaxation reduce the anisotropy; eventually T⊥ consistently exceeds T∥. At higher solar latitudes the more nearly radial IMF produces a higher anisotropy at all radial distances than at the equator, and the total temperature decreases more slowly with increasing heliocentric distance. We expect that the Ulysses mission, which will return the first full electron distributions from beyond 1 AU and at high heliographic latitudes, will allow tests of these predictions.