COUPLING OF THE CORONAL HELIUM ABUNDANCE TO THE SOLAR-WIND

Models of the transition region-corona-solar wind system are investiga ted in order to find the coronal helium abundance and to study the rol e played by coronal helium in controlling the solar wind proton flux. The thermal force on alpha-particles in the transition region sets the flow of helium into the corona. The frictional coupling between alpha -particles and protons and/or the electric polarization field determin es the proton flux in the solar wind as well as the fate of the corona l helium content. The models are constructed by solving the time-depen dent population and momentum equations for all species of hydrogen and helium in an atmosphere with a given temperature profile. Several tem perature profiles are considered in order to vary the roles of frictio nal coupling and electric polarization field in the solar wind, and th e thermal force in the transition region. Steady-state solutions are f ound for coronae with a hydrogen flux at 1 AU of 1.0 x 10(9) cm-2 s-1 or larger. For coronae with lower hydrogen fluxes, the helium flux int o the corona is larger than the flux ''pulled out'' by the solar wind protons, and solutions with increasing coronal helium content are foun d. The timescale for forming a helium-filled corona, that may allow fo r a steady outflow, is long compared to the mixing time for the corona .