Modeling the Energy Budget of Solar Wind Minor Ions: Implications for Temperatures and Abundances

Abstract

The outflow of oxygen and silicon ions in the solar wind has been studied using a model that extends from the chromosphere into interplanetary space, with emphasis on understanding the energy budget of the minor ions. The model solves coupled gyrotropic transport equations, which account for temperature anisotropies and heat conduction, for all charge states, and includes ionization and recombination. The minor ions are heated with a constant heating rate per particle in the corona. In the transition region the thermal force causes minor ions to flow faster than protons, with an abundance that can be less than half of the chromospheric abundance. The ions quite suddenly decouple from the proton flow in the corona, and above this point the ion flow is independent of the proton flow. For high heating rates the coronal abundance is comparable to the chromospheric abundance, the ion terminal wind speed is high, and most of the deposited energy is lost into the solar wind. Low heating rates lead to very large coronal abundances and a low terminal flow speed, and the main energy loss is then through collisions with protons and electrons in the corona. The heavy ions become much hotter than protons in the corona, even without preferential heating of the ions. However, preferential heating is necessary to prevent a large abundance enhancement in the corona and to achieve flow speeds close to the speeds observed by the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO). The abundance enhancement implies that lowering the heating rate per particle in general leads to an increase in the total energy flux absorbed by the ions.