Solar wind heavy ions

2.4 Solar wind heavy ions

Other than for protons and alpha particles, for solar wind heavy ions the three-dimensional VDF have not been measured. There are many results from in situ measurements of modern ion spectrometers flown on various spacecraft. However, the main objectives of those measurements usually were to analyse the chemical composition and ionization state (von Steiger et al., 1997

) of the solar wind, and not the kinetic properties (other than simple energy spectra) of the heavy ions. They usually come in various ionization stages and have about coronal abundances in fast wind, but show distinct element fractionation in slow wind (for a review see, e.g., the paper of Peter, 1998 ). Helium is always an exception Neugebauer (1981 ), in that its sizable relative abundance of about 3 – 5% on average differentiates it from a minor constituent. In slow wind and the current sheet, the higher density and lower temperature there may enforce a state in which all ions are near collisional equilibrium. The composition of the solar wind and the abundances of heavy ions, and their variation with the wind stream structure, was extensively discussed by von Steiger et al. (1997 ). Many important solar and heliospheric processes can be inferred from solar wind composition measurements, as was demonstrated by Geiss et al. (1994 ).

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

where $Z$ is the charge and $Ai$ the atomic mass number of ion species $i$ . This relation implies that $Ti > AiTp$ . Since Coulomb collisions would make $Ti → Tp$ and $Vi → Vp$ , wave heating must play a crucial role in lifting the heavy species out of the corona, with low heat transfer occurring in the wave dissipation region. The corresponding unknown minor ion heating rate, $Qi$ , should reflect this requirement, i.e., $Qi > AiQp$ . Concerning the overall energy budget of solar wind minor ions and their temperatures and abundances in the corona, Lie-Svendsen and Esser (2005

) have recently modelled these features by treating the heavy ions as test particles in a prescribed collisional proton-electron solar wind. They found that minor ions are always hotter than protons, even with lower heating rates per ion than proton. However, to avoid too large abundances and obtain faster flows of the heavy ions, preferential heating is necessary.

Figure 5:

Velocity distribution functions of helium (top), oxygen (intermediate) and neon (bottom curves) ions as measured by the ion mass spectrometer on the WIND spacecraft for various solar wind speeds. Note the extended power-law tails in the VDFs which are fitted well by kappa functions, in particular for helium (after Gloeckler et al., 2001

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, $Vi ≤ Vp + VA$ (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 $Ti ≥ AiTp$ (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 $κ$ 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.