In briefly summarizing our present understanding of the role of plasma waves and microinstabilities in the solar wind, we can generally state that most VDFs are found to be stable or marginally stable. However, comparatively many proton VDFs are prone to the core temperature anisotropy instability. The four salient wave modes (and free energy sources) are: (1) ion acoustic wave (ion beam, electron heat flux); (2) electromagnetic ion Alfvén-cyclotron wave (proton beam and core temperature anisotropy); (3) magnetosonic wave (proton beam, ion differential streaming); (4) whistler-mode and lower-hybrid wave (core-halo drift, electron heat flux). The quasilinear evolution of these waves and instabilities, let alone their non-linear evolution or possible saturation, and the associated spatial evolution of the VDFs in the non-uniform corona and interplanetary medium have not yet been explored.
High-frequency plasma waves (most likely in the Alfvén/ion-cyclotron mode) have been suggested to heat the corona through rapid dissipation within a fraction of a solar radius. This idea of Axford and McKenzie (1992) was corroborated in a two-fluid turbulence model (Tu and Marsch, 1997), including parametric studies of the wind properties (Marsch and Tu, 1997) in dependence on the average wave amplitude at the coronal base. Resonant cyclotron-wave absorption was already shown by Marsch et al. (1982a) to heat the interplanetary solar wind. It is now generally believed, that high-frequency Alfvén waves may through reconnection (Axford and McKenzie, 1997) originate from the flaring magnetic network in the lower solar transition region. A key feature of this scenario is that the damping at the cyclotron frequency of Alfvén waves propagating in a rapidly declining magnetic field will through the frequency sweeping mechanism provide strong heating close to the Sun.
In an empirical model (Cranmer et al., 1999) of a polar coronal hole, spectroscopic constraints were placed on the cyclotron-resonance heating. Cranmer (2000) further investigated the ion-cyclotron wave dissipation in the solar corona by a consideration of the summed effect of more than 2000 ion species. There is an increasing awareness that cyclotron resonance may play an important role in heating all ions in coronal holes and the fast solar wind. For a recent review see Hollweg and Isenberg (2002). Most work on cyclotron-resonant interactions published so far has concentrated on the perhaps unrealistic case of wave propagation along the ambient magnetic field. However, a paper by Hollweg and Markovskii (2002) offers a comprehensive discussion of how an ion in cyclotron resonance will behave for oblique wave propagation. In particular it is shown how the resonances at harmonics of the cyclotron frequency come about. The linear-theory result of Gary and Borovsky (2004), who showed that proton cyclotron damping is essentially independent of , implies that the consequences of cyclotron damping should be similar for both parallel and obliquely propagating fluctuations. Because of the great analytical simplification that parallel wave propagation gives us, we subsequently concentrate on this transparent situation.
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