The spiral field inhibition of thermal conduction in two‐fluid solar wind models

Abstract

The two‐fluid solar wind equations, including inhibition of heat conduction by the spiral magnetic field, have been solved for steady radial flow, and the results are compared with those of our previous study of two‐fluid models with straight interplanetary field lines. The main effects of the spiral field conduction cutoff are to bottle up electron heat inside 1 AU and to produce adiabatic electron (and proton) temperature profiles at large heliocentric distances. Otherwise, the spiral field models are nearly identical with straight field models with the same temperatures and velocity at 1 AU, except for models associated with very low coronal base densities (n0 ∼ 106 cm−3 at 1 Rs). Low base density spiral models give a nearly isothermal electron temperature profile over 50–100 AU together with high velocities and temperatures at 1 AU. In general, high‐velocity models do not agree well with observed high‐velocity streams; lower‐velocity states can be represented reasonably well at 1 AU, but only for very high proton temperatures (Tp ∼ 2Te) at the coronal base. For spherically symmetric base conditions the straight field and spiral field models can be regarded, in lowest order, as approximations to the polar and equatorial three‐dimensional flows, respectively. This viewpoint suggests a pole to equator electron temperature gradient in the region 1‐10 AU, which would be associated with a meridional velocity of ∼0.5–1.0 km/s, diverging away from the equatorial plane. The formalism developed in this paper shows rather stringent limits to the mass loss rate for conductively driven winds and, in particular, illustrates that putative T Tauri outflows could not be conductively driven.