Solar wind ions, once being beyond the sonic critical point and detached from the Sun, behave very differently than electrons. Their VDFs are, due to weak collisionality, prone to sizable distortions in phase space, and strongly shaped in response to wave-particle interactions in the turbulent wind. For a comprehensive discussion of the phenomenology of solar wind ion VDFs we refer to the reviews by Marsch (1991a,b) and Feldman and Marsch (1997), and the many references therein.
Here we keep the discussion short and focus on the salient kinetic features. Four typical examples of proton VDFs in fast solar wind are given in Figure 3, after Marsch et al. (1982c), which shows isodensity contours in velocity space from the maximum down to the 1% level. The pertinent traits are the proton core temperature anisotropy and the proton beam travelling at about . The origin of these features in the outer corona is still unclear. Two recent papers by Marsch et al. (2004) and Tu et al. (2004) address some of the kinetic physics issues, to which we will turn in Section 6.
Obviously, the observed distributions of ions and electrons exhibit various shapes and change widely with the local in situ conditions, heliographic coordinates and the phase of the solar cycle. The proton VDFs range from Maxwellians in slow wind, embedding the heliospheric current sheet (HCS), to highly non-thermal ones in fast streams that emanate from CHs. In fast solar wind the proton temperatures are anisotropic, with , whereas in slow wind the anisotropy is opposite, with . Frequently, and in both types of streams, strong field-aligned proton beams occur with drift speeds larger than the local Alfvén speed.
The electrons are cooler than the protons in fast wind ( and at 0.3 AU), but hotter in slow wind, which is more variable in abundance, more compressive and comparatively cold, with all particle temperatures becoming minimal at the HCS ( at 1 AU). The fast wind is permeated by Alfvén waves, which are broad-band in frequency and believed to play a main role, through their dissipation, in maintaining the ion temperatures above the level expected for adiabatic cooling. Whereas high-energy extensions are a universal property of the protons, they are less frequently seen in the alpha particles (Marsch et al., 1982b).
Evidence for local perpendicular proton heating in solar wind high-speed streams was first provided by Bame et al. (1975) from observations at Earth orbit. Some typical proton distributions as measured by Helios in fast wind are presented in Figure 4, together with the radial profile of the average magnetic moment, , of the protons, which is displayed as a function of radial distance from the Sun. That radially increases, indicates continuous ion heating perpendicular to the magnetic field must occur. The solid line in the top frame of Figure 4, which is drawn through the measured points carrying standard-deviation bars, shows proton magnetic moment (temperature) resulting from a model after Tu (1988), which explains the inferred interplanetary heating by Alfvén wave damping.
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