Origin of the fast solar wind: From an electron - Driven wind to cyclotron resonances

Even before the discovery of the fast solar wind in the mid - 1970s, it was known that even the average solar wind could not be well explained by models in which electron heat conduction was the energy source and the electron pressure gradient was the principal accelerating force. The outward - propagating Alfven waves discovered around 1970 were thought for a while to provide the sought - after additional energy and momentum, but their wave pressure ultimately failed to explain the rapid acceleration of the fast wind close to the Sun in coronal holes. By the late 1970s, various in situ data were suggesting that protons and heavy ions were being heated and accelerated by the ion - cyclotron resonance far from the Sun. This notion was soon applied to the acceleration region in coronal holes close to the Sun. The models which resulted suggested that the fast wind could be driven mainly by the proton pressure gradient (which is mainly the mirror force if the anisotropy is large), and that the high temperatures and flow speeds of heavy ions could originate within a few solar radii of the coronal base; these models also emphasized the importance of treating the extended coronal heating and solar wind acceleration on an equal footing. By the mid 1990s, SOHO, especially the UVCS (Ultraviolet Coronagraph Spectrometer), provided remarkable data which have given great impetus to studies of the ion cyclotron resonance as the principal mechanism for heating the plasma in coronal holes, and ultimately driving the fast wind. We will discuss the basic ideas behind current research, emphasizing the particle kinetics. We will discuss remaining problems such as the source of the ion - cyclotron resonant waves (direct launching, turbulence, microinstabilities), problems concerning OVI and MgX, the roles of inward - propagating waves and instabilities, the importance of oblique propagation, and the electron heating. Some alternatives, such as shock heating and turbulence - driven magnetic reconnection, will also be reviewed.