Proton temperature anisotropy effect on the heat flux instability in the fast solar wind

A possible source of the ion cyclotron waves that energize the protons in the fast solar wind is the heat flux launched by reconnection events at the base of coronal holes. A strong enough heat flux becomes unstable and generates the cyclotron waves interacting with the protons. When the protons are energized via the cyclotron resonance, they can develop considerable temperature anisotropy. At the very low plasma beta close to the Sun, the electrostatic heat-flux instability dominates other modes. This instability is known to be stabilized by the proton anisotropy, which limits the effect of the proton energization. However, further out in the corona, the electromagnetic ion cyclotron instability becomes the dominant mode. We will discuss how the electromagnetic mode is affected by the anisotropy and how it interacts with anisotropy-driven instabilities. The latter instabilities will also constrain the anisotropy produced by the cyclotron energization of the protons. We will show that, with the temperature anisotropy resulting from the interplay of these processes, the heat- flux instability is strong enough to convert the entire heat flux launched at the coronal base to the kinetic energy flux of the protons within the acceleration region of the solar wind. This confirms that the heat-flux instability is a viable means of generation of the fast solar wind.