Helium abundance in the corona and solar wind: Gyrotropic modeling from the chromosphere to 1 AU

We have developed a solar wind model including helium that extends from the chromosphere to 1 AU. The model is based on the gyrotropic approximation to the 16-moment set of fluid transport equations, which allows it to accommodate temperature anisotropies, as well as nonclassical heat transport. We find that, as in a pure electron-proton solar wind, the flow geometry close to the Sun also has a large impact on helium. In a radially expanding flow, downward proton heat conduction from the corona leads to a high transition region pressure and a large thermal force that pulls helium ions into the corona. In this case alpha-particles may easily become the dominant species in the corona, resulting in a polar wind type of solar wind in which the light protons are accelerated outward in the electric field set up by the alpha-particles and electrons. By contrast, applying the same form for the coronal heating in a rapidly expanding geometry intended to simulate a coronal hole, protons become collisionless closer to the Sun, and therefore the downward proton heat flux is smaller, resulting in a lower transition region pressure and a lower thermal force on helium. In this case the helium abundance is low everywhere and helium is unimportant for the acceleration of the solar wind. For the low coronal proton and alpha-particle densities found in the rapidly expanding flow, where asymptotic flow speeds are typically significantly higher than the gravitational escape speed at the solar surface, the solar wind helium mass flux is determined by the amount of helium available at the top of the chromosphere. In the radially expanding flow, with asymptotic flow speeds lower than the escape speed, the helium mass flux depends on the amount of energy available in the corona to lift helium out of the gravitational potential. In both cases the frictional coupling between helium and hydrogen in the chromosphere, using currently accepted elastic cross sections, is too weak to pull a sufficient number of helium atoms up to the top of the chromosphere and thus obtain a mass flux in agreement with observations. Abetter understanding of the chromosphere is therefore called for.