eaa.iop.org
DOI: 10.1888/0333750888/2303
Solar Wind Composition
Peter Bochsler
From
Encyclopedia of Astronomy & Astrophysics
P. Murdin
© IOP Publishing Ltd 2005
ISBN: 0333750888
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Institute of Physics Publishing
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Solar Wind Composition
To first order the solar wind composition reflects the composition of the source material, which is photospheric (SOLAR ABUNDANCES). However, there are some important distinctions between the solar wind and solar abundances which will be discussed in the following paragraphs. Since solar wind particles feed theCORONA, and solar energetic particles (SEPs; seeSOLAR WIND: ENERGETIC PARTICLES) largely originate in the corona, elemental and isotopic abundances of matter in the corona, in the solar wind and in solar energetic particles have much in common.
The first solar wind composition measurements were carried out soon after the experimental discovery of the solar wind in the early 1960s. Gradually improved techniques have revealed many details about the acceleration and heating processes operating in the solar atmosphere and corona. With recent improvements of resolution in mass spectrometers carried on theWIND,
SOHO and ACE missions, it has also become possible to study abundances of the major isotopic species and hence to gain direct information about the isotopic composition of solar matter. This information is not accessible with optical methods because the natural line widths are much wider than isotopic shifts in spectra. Thus the interest in solar wind composition is twofold and the topics of interest are interrelated. Depending on the viewpoint and the application, studying the solar wind composition and its variability provides on the one hand knowledge about the ways solar matter propagates into the interplanetary space. On the other hand, it provides access to the nuclidic composition of solar matter, information which is relevant for a multitude of astrophysical applications since solar matter is the most important reference point for the isotopic evolution of the solar system. An example of a high-resolution solar wind mass spectrum is shown in figure 1.
An interesting minority within the solar wind population are the so-called ‘pick-up ions’ which mainly originate from interstellar gas flowing through the inner heliosphere. The elements helium and neon, having the highest first ionization potentials (FIPs), penetrate as neutrals to radial distances of less than 1 AU before they become ionized and can be picked up by the magnetic field which is traveling with the solar wind. Depending on their location of ionization, they relax more or less efficiently to local conditions in the plasma. However, they generally remain in low ionization states which leaves them distinguishable from the rest of the solar wind population. Freshly picked-up particles perform cycloidal motions about the outwards propagating magnetic field;
hence their energy distributions reach beyond typical solar wind energies. They are thus particularly susceptible to acceleration in the fields associated with interplanetary shock fronts which are formed e.g. from coronal mass ejections (CMEs) or co-rotating interaction regions (CIRs).
In addition to interstellar pick-up ions, other species with low FIPs have been found in the pick-up ion population.
These particles originate from evaporating dust grains released from cometary debris and from interstellar dust particles which also can penetrate deep into the inner solar system.
Feeding of the corona with solar matter Coronal structure and solar wind composition
The corona exhibits a complex spatial and temporally variable structure which becomes visible to the naked eye during solar eclipses. The coronal structure with many details and its dynamic evolution are essentially determined by the configuration of the surface
SOLAR MAGNETIC FIELD. Coronal structure and solar wind composition are interrelated in many ways. For a rough characterization two terms are frequently used:
‘interstream solar wind’ and ‘coronal-hole-associated streams’.
Interstream solar wind is believed to be supplied from the fringes of the equatorial closed loop system. Solar wind particles travelling along open structures on the borderline of magnetic loops exhibit strong variations in composition. This observation has been taken as evidence for multiple sources rapidly connecting and disconnecting thereby producing expanding plasma blobs with complex spatial and magnetic topology which move outwards through the corona. On the other hand, coronal-hole- associated fast streams are generally much more stable in their composition.
The balance between the gravitational attraction of the Sun and the Coulomb friction force exerted from outwards travelling protons on minor species seems to play a crucial role in establishing solar wind abundances.
In a corona at thermal equilibrium with a temperature of typically 106 K, only protons (together with electrons to maintain charge neutrality) reach the escape velocity and move into the interplanetary space. One possibility to visualize the interaction between coronal structures and solar wind composition, which has been extensively studied, uses coronal magnetic flux tubes as elements that guide the solar wind flow. Depending on the flux density or—more precisely—on the phase space density (m−6 s−3) in a given flux tube, and depending on the divergence of the flux tube in the solar wind acceleration region, protons couple to minor ions via Coulomb collisions and tend to drag heavy species more or less efficiently according to the individual Coulomb drag factors of each species. An alternative to interplay between coronal structures and the solar wind plasma to populate the solar wind with minor ions involves magnetic fields as carriers of magnetohydrodynamic waves which interact with charged particles. This seems particularly important for coronal-hole-associated solar wind where Coulomb friction seems insufficient to drag minor species.
Nevertheless, in this type of solar wind, minor ions are at least as abundant as in the interstream wind, and even helium—although still depleted relative to its solar surface abundance—is better represented in coronal-hole- associated wind than in the typical interstream wind.
Hence, momentum added through waves to protons, to
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Figure 1.Solar wind elements and isotopes as observed with the MTOF sensor of the CELIAS experiment on SOHO. Elements and isotopes which have been identified for the first time in the solar wind thanks to isochronous mass spectrometry are shown in grey.
helium and to minor species seems crucial to obtain the typical properties of coronal-hole-associated solar wind.
This has recently been confirmed with optical observations from the SOHO spacecraft, indicating that ion-cyclotron resonance wave–particle interaction brings minor ions to temperatures as much as a hundred million kelvins, thus exceeding the main gas temperature by one or more orders of magnitude.
Magnetic reconnection is another process to link the magnetic field structures to solar wind abundances.
It occurs in the innermost corona and this link to abundances has not yet been extensively explored.
Rapidly changing magnetic topologies in reconnection regions around the equatorial streamer belt, which supplies matter to the interstream solar wind, lead to dissipative heating, generation of EUV and soft x-ray radiation, and electric fields which could selectively accelerate particles according to their charge per mass ratio. The EUV radiation originating in the dissipation region and in shock fronts controls the ionization of matter in the underlying chromosphere and thus selectively regulates the replenishment of fresh ions in the corona.
Fractionation effects Elements
As discussed in more detail elsewhere in this encyclopedia (seeTRANSITION REGION: FIRST IONIZATION POTENTIAL EFFECT), the
most important elemental fractionation process occurs in the ionization layer situated in the upper chromosphere where species with short ionization times (or low FIPs) are enriched over species with long ionization times typically by factors of 3–5 over photospheric abundances in the interstream solar wind. This effect seems much weaker in coronal-hole-associated wind.
Further up in the atmosphere, it appears that wave–
particle interaction, which plays a dominant role in the acceleration of coronal-hole-type solar wind, acts in a much more democratic manner than Coulomb drag—
including also weakly ionized species—which would otherwise hopelessly remain behind. A summary of solar wind elemental abundances is given in table 1. One conspicuous feature of abundances, in both types of solar wind, is the depletion of helium by typically a factor of 2 relative to hydrogen. Since this feature of helium depletion is also found in the otherwise ‘democratically elected’ solar abundance representation in coronal holes, it is generally ascribed to the ion–neutral separation process which leaves its imprint on all types of solar wind.
Isotopes
Table 2 is a compilation of recent data on the isotopic composition of the solar wind. The refractory elements which have uniform isotopic composition throughout the
Table 1.Solar wind elemental abundances.
Abundance (logarithmic scale), Abundance (linear scale),
[O]≡ 8.78 [O]≡ 1000
Element Z FIP (eV) FIT (s) Interstream Coronal hole Interstream Coronal hole H 1 13.6 70 12.21 ± 0.13 11.89 ± 0.12 2 700 000± 600 000 1 280 000± 320 000 He 4 24.5 260 10.68 ± 0.10 10.52 ± 0.10 80 000± 20 000 55 000± 20 000
C 6 11.2 20 8.60 ± 0.07 660± 100
O 8 13.6 74 ≡8.78 ≡1000
Ne 10 21.5 81 8.01 ± 0.06 170± 20
Mg 12 7.6 0.3 8.01 ± 0.10 7.69 ± 0.10 171± 35 81± 18
Si 14 8.1 0.6 8.08 ± 0.10 7.49 ± 0.08 200± 30 52± 7
S 16 10.3 11.6 7.55 ± 0.05 7.50 ± 0.06 59± 7 53± 6
Ar 18 15.7 50 6.35 ± 0.13 3.7 ± 1.0
Fe 26 7.8 2 7.84 ± 0.10 7.58 ± 0.10 115± 15 63± 8
Kr 36 14.1 20.3 3.25 3.0 × 10−3
Xe 54 12.1 10.1 2.63 7.1 × 10−4
This is a compilation of results obtained with the Apollo/SWC experiment, from lunar soil investigations, from ISEE3/ICI, the SWICS Instrument and the Plasma Ion Spectrometer on Ulysses, and from CELIAS on SOHO (see Bochsler 2000 for more details and a complete list of references). [O]≡ 8.78 is used as a normalized value for both types of solar wind. It reflects the photospheric abundance of oxygen in dex units as recommended by Holweger (private communication). The standard first ionization times (FITs) are taken from Geiss (1998) and references therein.
solar system and whose solar isotopic composition is presumably well known (e.g. Mg and Si) show no evidence of fractionation between the solar surface and the solar wind to the level of precision which has been achieved to date. In contrast, the volatile elements, i.e. helium, neon and nitrogen, show rather strong variations in the isotopic composition in different solar system samples, such as terrestrial and solar wind. In view of the absence of fractionation effects in the case of the refractories, it can be concluded that the solar wind is generally a faithful representation of the isotopic composition of the solar surface. For the case of the very light element helium with a large relative mass difference between the two stable isotopes, variations of the 4He/3He ratio of the order of 10% have to be taken into account under normal conditions. Hence, the observed differences between isotopic abundances of solar wind volatiles from planetary volatiles are differences which originate at the source; they reflect different histories of solar volatiles and volatiles in the rest of the solar system. Isotopic solar wind data represent important benchmarks for models of the early geochemical evolution of the solar system and also put rigid constraints for models describing the acceleration of heavy species in the inner corona.
Since all quantitative models on the FIP effect published heretofore ascribe the depletion of high-FIP elements in the corona essentially to atomic properties, it is no surprise that they predict virtually no isotopic fractionation concomitant with the ion–neutral separation.
In view, however, of the sometimes strong variability of the He/H ratio it is possible than an overall isotopic fractionation of the order of 1% per mass unit results for medium-mass elements between the solar surface and the solar wind due to selective acceleration in the corona. This effect is at present, however, still below the detection limit.
Table 2.Isotopic abundances.
Solar wind Terrestrial
4He/3He 2350± 70 740 000
14N/15N 200± 55 272
16O/18O 450 498.8
20Ne/22Ne 13.7 ± 0.3 9.80
22Ne/21Ne 31± 4 34.5
24Mg/Mg 0.7857 ± 0.0076 0.7899
25Mg/Mg 0.1020 ± 0.0048 0.1000
26Mg/Mg 0.1123 ± 0.0071 0.1101
28Si/Si 0.9220 ± 0.0026 0.9223
29Si/Si 0.0454 ± 0.0020 0.0467
30Si/Si 0.0326 ± 0.0021 0.0310
36Ar/38Ar 5.58 ± 0.03 5.32
40Ca/42Ca 128± 47 149.8
40Ca/44Ca 50± 8 46.47
54Fe/Fe 0.0604 ± 0.0053 0.0580
56Fe/Fe 0.9148 ± 0.0053 0.9172
78Kr/86Kr 0.019 45 ± 0.000 15 0.019 95 ± 0.000 08
80Kr/86Kr 0.1274 ± 0.0006 0.1296 ± 0.0004
82Kr/86Kr 0.6573 ± 0.0020 0.6617 ± 0.0016
83Kr/86Kr 0.6586 ± 0.0017 0.6600 ± 0.0014
84Kr/86Kr 3.279 ± 0.007 3.273 ± 0.007
124Xe/130Xe 0.0290 ± 0.0007 0.023 35 ± 0.0014
126Xe/130Xe 0.0259 ± 0.0009 0.021 76 ± 0.0014
128Xe/130Xe 0.5038 ± 0.0028 0.4708 ± 0.0017
129Xe/130Xe 6.354 ± 0.0017 6.505 ± 0.0015
131Xe/130Xe 4.988 ± 0.0011 5.224 ± 0.0012
132Xe/130Xe 6.062 ± 0.0016 6.614 ± 0.0012
134Xe/130Xe 2.239 ± 0.0008 2.567 ± 0.0007
136Xe/130Xe 1.818 ± 0.0006 2.182 ± 0.0006 For details and a complete list of references see Bochsler (2000).
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Ionic composition
Solar wind particles (as well as SEPs), after passage through the coronal temperature maximum, are usually strongly ionized. Figure 2 is an illustration showing a mass versus mass/charge distribution as obtained with the SWICS instrument of ACE. Protons are suppressed in this instrument to avoid counting overflows; helium appears exclusively as4He2+together with some traces of3He2+. Carbon and oxygen are almost fully ionized whereas the heavier species usually conserve their innermost electron shells, e.g. Fe is most conspicuous in the mass per charge range from 4.67 to 7.0 (Fe12+through Fe8+). The ionization balance of minor species is predominantly established via electron collisions in the corona at about a distance of 1R from the solar surface. The ionization rates, and to some degree also the recombination rates, depend on the local electron temperatures. In the dense, inner parts of the corona, collisions are sufficiently frequent to establish a quasi-equilibrium distribution of charge states. As the ions move further out, the ambient electron density decreases, collisional reactions are gradually supressed and the result is a relatively wide distribution of charge states which ‘freezes’ and remains unaltered throughout the heliosphere. Charge states of minor species measured far from the Sun provide a useful means to deduce the electron temperatures in the corona. Another application is reminiscent of hydrological methods: because of their conservative properties, charge states of minor ions are now frequently used as reliable tracers to map distorted solar wind flux tubes back to their source in the solar corona.
Coronal mass ejection related solar wind composition
Only recently has it become possible to reliably determine the rapidly varying composition of minor ions in CME- related solar wind. A long-known feature is the sometimes enhanced He/H ratio which has been taken as evidence for gravitational stratification in the undisturbed corona. Similar effects might be responsible for the sometimes observed mass-dependent fractionation. The most surprising recent discovery, however, is a temporary strong enrichment of 3He, probably due to the same mechanism which produces the even more dramatic enrichments of 3He in impulsive flare particles. With the improved time resolution of modern instrumentation, less surprisingly, extremely unequilibrated charge state distributions, e.g. large amounts of O2+,3+,4+together with otherwise common solar wind species O6+and O7+, have been observed with ACE and WIND.
Historical solar wind
Lunar soil and also the soil in the surface layers of asteroids are constantly exposed to the solar corpuscular irradiation.
Particles at solar wind energies penetrate typically to depths of 0.1 µm into dust grains and are stored within these grains for extended periods depending on their diffusive behavior within the mineral and depending on
the further history of the grains. Lunar soil undergoes continuous sputtering from solar and cosmic ray particles, combined with erosion from micrometeorite impacts.
Because of the occasional covering of soil layers with freshly excavated material from larger impacts, it is possible that some solar particle records are buried within the asteroidal and lunar regolith for billions of years, thus storing an archive of most of the solar wind history in the near-Earth environment and the inner solar system.
Some tracers such as 40Ar, which is a decay product from40K, and volatile fission products from decaying235U emanating from the lunar interior and re-implanted with the solar wind into surface material make it possible to reliably date the irradiation epochs. The lunar record has been intensively investigated. The results are somewhat controversial. One major problem is the allocation of suprathermal particles and SEPs which exhibit somewhat different isotopic abundances and seem overabundant with respect to the solar wind population. There is general agreement that the isotopic composition of the solar wind has not changed significantly during the last few billion years, except for the case of the 15N/14N abundance ratio which, however, must be attributed to extrasolar contributions. The apparent invariance of the3He/4He abundance ratio in the solar wind over this extended period bears some significance for the history of the Sun, since it indicates that the outer part of the Sun (from Mr ≥ 0.5Mto the base of the outer convective zone) has never undergone mixing since the ignition of the nuclear fuel.
The elemental abundances of the heavy noble gases indicate, however, that there has been a secular modification in solar wind composition over the past few billion years which is possibly related to a change in solar activity (i.e. that the ancient, in-ecliptic solar particle fluence contained significantly more CME-related, transient solar wind than the more recent record). This provides an additional boost to the study of CME-related composition which has only recently become possible owing to the considerably improved time resolution of modern particle instrumentation.
Bibliography
Bochsler P 2000 Charge states and abundances of particles in the solar wind Space Sci. Rev. (in preparation) Geiss J 1998 Constraints on the FIP mechanisms from
solar wind abundance data Solar Composition and its Evolution—from Core to Corona ed C Fr ¨ohlich, M C E Huber, S K Solanki and R von Steiger (Dordrecht: Kluwer) pp 241–52
Von Steiger R, Geiss and Gloeckler G 1997 Composition of the solar wind Cosmic Winds and the Heliosphere ed J R Jokipii, C P Sonett and M S Giampapa (Tucson, AZ: University of Arizona Press) pp 581–616 Wieler R, Kehm K, Meshik A and Hohenberg Ch
1996 Secular changes in the xenon and krypton abundances in the solar wind recorded in single lunar grains Nature 384 46–9
Figure 2.Mass versus mass per charge distribution in the solar wind as observed on day 29, 1998, with ACE/SWICS. This figure is reproduced as Color Plate 56.
Peter Bochsler
