Collisionless solar wind, 2, Variable electron temperature

Abstract

We consider a two-component ‘model’ for the solar wind, in which the protons become collisionless beyond r0≥10 RS, where they are already highly supersonic. The proton temperatures are found from the double adiabatic equation of state. The electrons are highly subsonic, and their temperature profile is prescribed ad hoc. Solar rotation is considered in a semi-self-consistent fashion. The momentum equations for the electrons and protons are solved subject to the conditions of quasi neutrality and zero charge efflux from the sun. The principal results are the following. (1) The proton thermal anisotropy is substantially reduced when solar rotation is considered. We find Tp∥/Tp⊥ < 3 if r0 = 40 RS, while Tp∥/Tp⊥ < 2 if r0 = 55 RS. Wave-particle interactions may therefore play a less significant role in destroying proton anisotropy than has been heretofore thought. (2) The observed dependence of Tp∥/Tp⊥ on solar-wind speed is consistent with collisionless flow, and the double adiabatic equation of state, beyond r0=55 RS, if r0 and υ0 = υ(r0) do not change. (3) Solar rotation leads to significantly lower mean proton temperatures, Tp = ( Tp∥ + 2Tp⊥)/3, than are obtained when the magnetic field is radial. Thus the apparent advantage of collisionless models in reproducing the observed solar-wind conditions does not persist when solar rotation is included. (4) Even slight electron anisotropy, Te∥/Te⊥ ≌ 1.2, in the region r>10 RS reduces the solar-wind speed at the earth by 10–15%, thus worsening the disagreement between observations and the two-fluid model. (5) The electron temperature profile in the supersonic region is the primary parameter determining flow acceleration there; we urge that the details of the electron energy balance, and the possibility of electron heating, be carefully examined in future work.