2.3 Solar wind protons and alpha particles

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Figure 3: Proton velocity distribution functions in the fast solar wind as measured by Helios at 0.5 AU (top left), 0.54 AU (top right), 0.4 AU (bottom left) and 0.3 AU (bottom right). Note in the lower VDFs a distinct temperature anisotropy in the core and the strong beam (after Marsch et al., 1982cJump To The Next Citation Point).

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 (1991aJump To The Next Citation Point,bJump To The Next Citation Point) and Feldman and Marsch (1997Jump To The Next Citation Point), 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 3View Image, after Marsch et al. (1982cJump To The Next Citation Point), 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 1.5VA. The origin of these features in the outer corona is still unclear. Two recent papers by Marsch et al. (2004Jump To The Next Citation Point) and Tu et al. (2004Jump To The Next Citation Point) 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 Tp ⊥ > Tp∥, whereas in slow wind the anisotropy is opposite, with Tp⊥ < Tp∥. Frequently, and in both types of streams, strong field-aligned proton beams occur with drift speeds larger than the local Alfvén speed.

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Figure 4: Top: The proton magnetic moment is observed to increase with heliocentric distance and indicates through its non-conservation proton heating. Bottom: Selected velocity distribution functions measured in high-speed wind. The solid isodensity contours correspond to 20% steps of the maximum, and the last broken contour is at 0.1%. Note the large temperature anisotropy in the core and the tails along the magnetic field direction (after Marsch, 1991aJump To The Next Citation Point).

The electrons are cooler than the protons in fast wind (Te = 0.1– 0.2 MK and Tp = 0.5 –0.8 MK 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 (Tp = 5 × 104 K 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., 1982bJump To The Next Citation Point).

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 4View Image, together with the radial profile of the average magnetic moment, μp = Tp⊥∕B, of the protons, which is displayed as a function of radial distance from the Sun. That μp radially increases, indicates continuous ion heating perpendicular to the magnetic field must occur. The solid line in the top frame of Figure 4View Image, 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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