Evidence for extended solar wind heating by fast hydromagnetic waves

Abstract

In order to reduce the discrepancy between solar wind measurements at 1 AU and the corresponding predictions of the basic two-fluid model a nonthermal heating of solar wind protons in the outer corona up to distances of about 0.1 AU from the sun was postulated. In the absence of thermal energy transfer to an anisotropic plasma the quantities c∥ = T∥B²/n² and c⊥ = T⊥/B (where T∥ and T⊥ denote proton temperature components parallel and perpendicular to the magnetic field with intensity B and n denotes the proton number density) constitute adiabatic invariants. Accepting the hypothesis that damping of fast-mode hydromagnetic waves near the sun strongly dominates the heating and acceleration of the solar wind, we argue that c∥ is increased thereby. Depending on the coupling between parallel and perpendicular degrees of freedom in the turbulent solar envelope, c⊥ will be raised to some extent too. If we may neglect interplanetary irreversible processes, these values are transported to the earth and may give some direct information on the turbulent outer corona. We therefore invoke c⊥ as a new natural ordering parameter instead of the customarily employed bulk velocity in a statistical presentation of solar wind data measured on the earth-orbiting satellite Heos 2. This choice reveals completely novel features that have been missed in the conventional presentation. We interpret these features as a signature of an extended heating process which survives the transit of the solar wind plasma to 1 AU. We note that the purely wave-heated two-fluid model of Barnes and co-workers was unable to reach flow velocities exceeding 450 km/s. Our data consistently indicate that for larger flow velocities additional heating and acceleration processes (e.g., nonspherically symmetric outflow from coronal holes) come into play. We suggest that a suitable combination of a corotating model and the wave-heated two-fluid model should be able to predict both the observed macroscale relation between proton temperature and bulk velocity and the dynamic effects in the moderately active solar wind. Further, we propose that proton double streams and related deformations of the proton distribution functions might be a relic of the interaction with a fast-mode hydromagnetic wave field which propagates from the inner corona.