The energy requirements on heavy ions are tough, given the notion that Coulomb friction (Geiss, 1982) in the dilute, hot corona is usually too weak to couple the ions together tightly. It appears rather difficult to drag out such heavy ions as He+, or He2+, or multiply-charged ions of any heavier element, against the Sun’s gravitational attraction. To achieve equal proton and heavy-ion bulk speeds in the distant wind (Ryan and Axford, 1975), their coronal velocity distributions should overlap sufficiently, which roughly requires about equal effective thermal speeds of a proton-electron pair and a heavy ion dressed by its electron cloud, and which means we must require that
The heavy minor ions seem to act as ideal tracers of wave effects in the wind. There is ample evidence that waves do preferentially heat heavy ions in interplanetary fast streams as observed by Helios (Marsch, 1991a) and Ulysses (von Steiger et al., 1995). They also stream faster than protons (Asbridge et al., 1976) by a fraction of the local Alfvén velocity, (Marsch et al., 1981, 1982b; Neugebauer et al., 1994), and are sometimes found to surf on the ubiquitous Alfvén waves without participating in the wave motion (Marsch et al., 1981). This differential streaming presumably originates in the outer corona and is observed by Helios to fade away with increasing heliocentric distance. Smaller speed differences between protons and alpha particles are seen also by Ulysses beyond 1 AU (Neugebauer et al., 1996; Reisenfeld et al., 2001), and sometimes appear to be generated locally at shocks and stream interaction regions. In fast solar wind streams, heavy ions have a high kinetic temperature, with (von Steiger et al., 1995). In the past years, new and more detailed observations became available (von Steiger et al., 1995; Steinberg et al., 1996; Hefti et al., 1998), and thus new theoretical work on an old subject was stimulated.
One of the interesting salient features, detected by modern ion spectrometers in the suprathermal domain of the heavy-ion energy spectra in the solar wind, are the extended tails which link the thermal keV-energy range of the solar wind with the energetic particle range with MeV energies and beyond. Figure 5 shows the speed (energy) distribution functions of helium, oxygen and neon in the solar wind, after measurements from the WIND spacecraft at 1 AU according to a paper by Collier et al. (1996). This paper also gives the relative abundances and the temperature ratios of these three species. Note the pronounced suprathermal tails appearing in the energy (speed) distributions, which are well fitted by a convected kappa function after Equation (6), with ranging here between 2.5 and 4. Remember in this context that in their exospheric model Pierrard et al. (2004) assumed that such non-thermal VDS of heavy ions already prevailed in the solar corona, in which case, as they showed, the heavy ions were driven even faster than the protons out of CHs.
The kinetic features and speed distributions of heavy interplanetary ions are not the subject of this article. But for the interested reader we refer to the review of Gloeckler et al. (2001), who discuss at length the heliospheric and interstellar phenomena revealed from observations of pick-up ions. Heavy ions in the solar wind may not only originate from the corona but for example as pick-up ions from cometary dust and various other sources in the inner heliosphere.
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