A study of solar wind acceleration based on gyrotropic transport equations

Abstract

The gyrotropic transport equations are used to describe an electron‐proton solar wind from the 500,000 K level in the upper transition region and out to 30 solar radii. These equations allow for different temperatures parallel and perpendicular to the magnetic field, as well as transport of parallel and perpendicular thermal energy along the field. We find that in models with significant coronal proton heating, the electron temperature is much lower than the proton temperature. The electron gas is collision dominated, the thermal anisotropy is small, and the heat flux is close to a “classical” heat flux. The proton gas is collision dominated in the upper transition region, but the temperature increases rapidly in the inner corona, and the protons become collisionless close to the Sun. The proton heat flux is proportional to the temperature gradient very close to the Sun, but in the extended corona it deviates substantially from a classical heat flux. In models where the proton heating is in the direction perpendicular to the magnetic field, a large perpendicular temperature is produced locally, but the perpendicular thermal motion couples into parallel thermal motion, and the parallel temperature increases outward from the Sun. We obtain a maximum parallel temperature that is comparable to the maximum perpendicular temperature. This result seems to hold for all models where the energy flux necessary to drive high‐speed wind is deposited in the corona as heat. The result is not in agreement with UVCS/SOHO observations of the 1216 Å Ly‐α line in large coronal holes. These observations are consistent with a much larger random proton motion perpendicular to the magnetic field than parallel to the field. Such anisotropies can be obtained in models of high‐speed solar wind if we allow for a significant fraction of the energy flux from the Sun to be in the form of low‐frequency, transverse waves. These waves accelerate the solar wind without heating the corona, and they contribute to the line broadening in the direction perpendicular to the magnetic field.