A 16‐moment solar wind model: From the chromosphere to 1 AU

Abstract

We present a solar wind fluid model extending from the chromosphere to Earth. The model is based on the gyrotropic approximation to the 16‐moment set of transport equations, in which we solve for the density, drift speed, temperature parallel and perpendicular to the magnetic field, and transport of parallel and perpendicular thermal energy along the magnetic field (heat flux). The solar wind plasma is created dynamically through (photo) ionization in the chromosphere, and the plasma density in the transition region and corona is computed dynamically, dependent on the type of coronal heating applied, rather than being set arbitrarily. The model improves the description of proton energy transport in the transition region, where classical heat conduction is only retrieved in the collision‐dominated limit. This model can serve as a “test bed” for any coronal heating mechanism. We consider heating of protons by a turbulent cascade of Alfvén waves in rapidly expanding coronal holes. The resulting high coronal proton temperatures lead to a downward proton energy flux from the corona which is much smaller than what classical transport theory predicts, causing a very low coronal density and an extremely fast solar wind with a small mass flux. Only when some of the wave energy is forcibly deposited in the lower transition region can a realistic solar wind be obtained. Because of the poor proton heat transport, in order to produce a realistic solar wind any viable heating mechanism must deposit some energy in the transition region, either directly or via explicit heating of coronal electrons.