S P A C E - B A S E D M E A S U R E M E N T S OF E L E M E N T A L

A B U N D A N C E S AND THEIR R E L A T I O N TO SOLAR

A B U N D A N C E S

M. A. C O P L A N

Institute for Physical Science and Technology, University of Maryland, MD 20742-2431, U.S.A.

K. W. O G I L V I E

Laboratory for Extraterrestrial Physics, NASA/Goddard Space Flight Center, U.S.A.

P. B O C H S L E R andJ . G E I S S

Physikalisches lnstitut, University of Bern, Switzerland

(Received 14 August, 1989; in revised form 14 November, 1989)

Abstract. The solar wind provides a source of solar abundance data that only recently is being fully exploited. The Ion Composition Instrument (ICI) aboard the ISEE-3/ICE spacecraft was in the solar wind continuous- ly from August 1978 to December 1982. The results have allowed us to establish long-term average solar wind abundance values for helium, oxygen, neon, silicon, and iron. The Charge-Energy-Mass (CHEM) instrument aboard the CCE spacecraft of the AMPTE mission has measured the abundance of these elements in the magnetosheath and has also added carbon, nitrogen, magnesium, and sulfur to the list. There is strong evidence that these magnetosheath abundances are representative of the solar wind. Other sources of solar wind abundances are Solar Energetic Particle (SEP) experiments and Apollo lunar foils. When comparing the abundances from all of these sources with photospheric abundances, it is clear that helium is depleted in the solar wind while silicon and iron are enhanced. Solar wind abundances for carbon, nitrogen, oxygen, and neon correlate well with the photospheric values. The incorporation of minor ions into the solar wind appears to depend upon both the ionization times for the elements and the Coulomb drag exerted by the outflowing proton flux.

1. Discussion

Spectroscopic determinations of the abundances of the elements in the Sun require a number of assumptions about the uniformity of the material sampled and the degree to which it is representative of the Sun as a whole. Moreover, the conversion of spectro- scopic measurements to abundances requires knowledge of the oscillator strengths, and in many cases these are known only approximately, either because of the difficulty of measuring them directly in the laboratory or because the appropriate theoretical calcu- lations are too complex to perform with a high degree of accuracy. Knowledge of the physical conditions in the solar atmosphere is equally important for the accurate determination of spectroscopic abundances. Summaries of the problems associated with the derivation of solar abundances from spectroscopic observations have been made by Ross and Aller (1976) and by Grevesse (1984). The accuracy with which elemental abundances are known in the solar photosphere depends very much on the element. The noble gases are particularly difficult to determine from spectroscopic data because of the strong dependence of abundance on coronal conditions. Alternate

Solar Physics 128: 195-201, 1990. �9 1990 Kluwer Academic Publishers. Printed in Belgium.

196 M . A C O P L A N E T A L .

sources of abundance information have been considered by Anders and Grevesse (1989) in their comparison of meteoritic values with photospheric data and by Cameron (1982) who interpolated abundances from nucleosynthesis pathways.

In recent years the interplanetary medium has provided another source of informa- tion. The interplanetary medium contains at least two populations of particles whose source is the Sun, the solar wind, and solar energetic particles (SEPs). Each of these is a potential source of information about solar abundances. The derivation of solar abundances from SEP observations has recently been discussed by Stone (1988).

In this paper we present a summary of in situ measurements of the composition of the solar wind by the Ion Composition Instrument (ICI) aboard the ISEE-3/ICE spacecraft from August 1978 to December 1982 when it was in the solar wind at the forward libration point L 1 . Because these data come from sufficiently long observation periods, they can be taken as representative and compared with the results of other methods to assess their suitability for inferring solar abundances. Comparisons among different measurements draw attention to the processes by which material is transported from the photosphere to the corona and thence into the solar wind.

2 3 4 S 8

~258

_259

~g

_261

, I i , , I I i , , , I 2 3 M / O I 4 . . . . . ~

i'1,qSS-SPECTRIq [}FLY 258 TO BAY 281 1981

Fig. l. S tack plot of the mass spec t ra regis tered in the s t anda rd m o d e of the Ion Compos i t ion In s t rumen t from Sep tember 17 to Sep tember 18, 1981. Count ra tes are p lo t ted in a logar i thmic scale. 3He+ § and 4l-Ie + + are a lways clearly separa ted , and in m o s t cases 07 + (M/Q = 2.28) and 0 6 § (M/Q = 2.67) are also

resolved, I ron ions are only visible la te on day 258.

SPACE-BASED MEASUREMENTS OF ELEMENTAL ABUNDANCES 197

The Ion Composition Instrument (ICI) on the ISEE-3/ICE spacecraft described by Coplan et al. (1978) couples a high-resolution electrostatic analyzer with a stigmatic Wien velocity filter to obtain mass per charge spectra of the ions in the solar wind over a range from 1.5 (3He+ +) to 5.6 (S6Fe+ lo). An example of the quality of the data is given in Figure 1 which is a stack plot of the mass per charge spectra taken during three consecutive days. 3He+ + and 4He+ + are clearly resolved along with two oxygen charge states at mass per charge of 2.28 (O v+) and 2.67 (0 6+). When operated in a second high-resolution mode the instrument also provided abundance data for neon silicon and iron. All measurements by ICI are relative to 4He + +, the most abundant ion in the mass range covered by the instrument. To refer these measurements to H + we have used data from the plasma and electron instruments described by Bame et al.

(1978) aboard the same spacecraft. The overall measured abundance ratio is [4He+ + ]/[H + ] = 0.041 _+ 0.007. The overall uncertainty takes into account random uncertainties from background corrections and counting statistics as well as systematic errors in the normalization of the data from the two different instruments.

The ICI observations cover almost the full range of solar wind parameters rather than being based upon the analysis of a few samples taken under particular conditions. Because 3He is present only in the doubly-ionized charge state in the solar wind and is well separated in the mass per charge spectrum from the other ions, its abundance, though low, can be obtained in a rather straightforward way as described by Coplan et al. (1984). For the other ions a minimization calculation that takes into account the charge state distributions of all of the ions in the spectrum has been used by Bochsler, Geiss, and Joss (1985) and D'Annunzio, Ogilvie, and Coplan (1986). For the rarer ions, the data analysis was based on minimum variance estimation and Kalman smoothing as discussed by Schmidt, Bochsler, and Geiss (1988) and Bochsler (1989). Particular care was taken to avoid biasing the averages.

A second source of solar wind composition data is the charge-energy-mass (CHEM)

instrument that was flown on the Charge Composition Explorer (CCE) spacecraft of the AMPTE mission. The instrument developed by Gloeckler et al. (1985) can measure both the mass and charge of ions, rather than mass per charge alone, and it was possible to measure solar wind ions in the magnetosheath of the Earth with high precision. Because the apogee of the CCE orbit is 8.9 R e, this was only possible during those periods when the solar wind compressed the magnetosphere sufficiently for the space- craft to be left in the magnetosheath. The results have been summarized by Gloeckler and Geiss (1989) and are listed in Table I.

The abundances and isotopic composition of the noble gases, helium, neon, and argon, in the solar wind have been measured with high accuracy during the lunar Apollo missions with the foil collection technique of Geiss et al. (1972). The foil values are derived from several days of solar wind irradiation, and have been normalized to the ICI helium value in Table I. The agreement between the Apollo and ICI results is remarkable considering the fact that the two techniques are entirely different. The last two columns in Table I are the SEP derived coronal abundances of Breneman and Stone (1985) and the photospheric abundances of Anders and Grevesse (1989).

198 M. A COPLAN ET AL.

TABLE I

Solar wind and solar abundances

Element FIP (V) ICI Apollo AMPTE/CHEM g SEP Photosphere ~ lunar foils r derived corona h

H 13.6 1900 +_ 400 ~ - - - 1180 _+ 118 4He 24.6 75 + 20 a 75 45 _+ 5 - 115 +_ 9 3He 24.6 0.037 + 0.004 ~ 0.032 + 0.003 - - - C 11.3 - - 0.529 + 0.13 0.41 _+ 0.042 0.391 + 0.039 N 14.5 - - 0.129 + 0.008 0.123 + 0.009 0.132 + 0.013 O 13.6 1 - 1 1 1 Ne 21.6 0.17 + 0.02 c 0.139 0.106 + 0.010 0.138 + 0.014 0.145 +. 0.038 Mg 7.6 - - 0.103 + 0.011 0.192 + 0.011 0.045 + 0.005 Si 8.2 0.12 +_ 0.04 d - 0.032 + 0.009 0.176 + 0.011 0.042 + 0.005 S 10.4 - - - 0.043 +_ 0.002 0.019 + 0.003 Ar 15.8 - 0.004 + 0.001 0.124 + 0.004 0.004 + 0.001 0.004 _+ 0.001 Fe 7.9 0.19 + 0.10 e - - 0.224 _+ 0.028 0.055 + 0.004

a Ogilvie et aL (1989). b Coplan etaL (1984). ~ Bochsler, Geiss, and Kunz (1986). d Bochsler (1989). e Schmid, Bochsler, and Geiss (1988).

f Geiss etaL (1972). g Gloeekler and Geiss (1988). h Breneman and Stone (1985). i Anders and Grevesse (1989).

In compar ing the abundances from the different sources, it is important to note that

the ICI, C H E M / A M P T E , and SEP derived abundances are all normal ized in Table I

to oxygen rather than the more abundant helium. This has been done because the

correlation of the fluxes of neon and iron with helium flux is significantly weaker than

the correlation o f the same two elements with oxygen flux, despite the larger measure-

ment uncertainties. This is related to the high variability of the abundance of helium in

the solar wind.

A compar ison between the ICI and C H E M / A M P T E solar wind abundances and

SEP derived coronal abundances show general agreement; however, there are

systematic differences with respect to the photospheric abundances of Anders and

Grevesse (1989). In making this compar ison it is important to note that their quoted

abundance for helum is based on current s tandard solar models and that their neon and

argon abundances are based on local galactic values. There is an apparent correlation

between elemental abundances and first-ionization potentials (FIPs). This correlation

has been discussed by many authors including Hoves tad t (1974) and Meyer (1985).

Geiss and Bochsler (1985) have emphasized the relation among FIP, ionization times,

and ion-neutral separation in the upper chromosphere and transition region. They

calculated ionization times for nine important elements under solar surface conditions.

The times generally correlate well with F I P s ; for the case of neon the ionization time

reflects the observed relative abundance better than the FIP. More recently, yon Steiger

and Geiss (1989) have investigated models of t ransport across magnetic structures that

can be made to account for the observed relative abundances. Fract ionat ion can also

occur in the corona and Geiss, Hirt, and Leutwyler (1970) have shown that 4He + + has

SPACE-BASED MEASUREMENTS OF ELEMENTAL ABUNDANCES 199

an anomalously low Coulomb drag factor that probably contributes to the high degree of variability helium shows compared to protons and the heavier minor ions in the solar wind. The combination of long ionization times and low Coulomb drag can explain both the overall twofold underabundance of helium in the solar wind, and its abundance variability. On the other hand, elements with short ionization times such as magnesium, silicon, and iron are enriched by a factor of three to four in the solar wind. Carbon, nitrogen, oxygen, and neon with similar ionization times are not expected to be strongly fractionated with respect to each other. In this regard, neon is a particularly important element since its abundance is directly connected with opacities and the neutrino flux as discussed by Bahcall and Ulrich (1988). The neon to oxygen ratio of 0.05 of Ross and Aller (1976) was revised to 0.13 by Grevesse (1984) and Aller (1986). The latter value is consistent with the ICI value of 0.17 + 0.02 obtained by Bochsler, Geiss, and Kunz (1986) from the ratio of the fluxes of the two elements over all the spectra in their sample. The ICI neon to oxygen abundance ratio is also consistent with the neon abundances in Hn regions near the Sun as compiled by Meyer (1979) and the SEP derived photospheric value of Breneman and Stone (1985). Figure 2 is a contour plot of neon flux as a function of oxygen flux for the ICI data. The average ratio is 0.15, and the correlation coefficient between these two fluxes is 0.76, compared with 0.66 for neon and helium fluxes. It is important to note that the ICI neon to oxygen ratio is particularly well-determined because the two elements are close to each other in the mass per charge spectrum, so that there are only small differences in the instrumental transmission functions for the two elements. Furthermore, they have similar first ionization potentials and Coulomb factors, so that one expects that their ratio in the solar wind to be an accurate measure of photospheric ratios.

Fig. 2.

T I/1

E v

X

LL I

Z i i

(.9 O ._J

I I I I [ I I I I

ISEE 3 ] ICI 1980 / r - - 0 . 7 5 7 i ~ / / '

//~///#/~20 CASES/BIN

/

,,/<----(Ne) = 0 . 1 5 (O)

SIZE OF BIN ~ , ~ I I I I I I I I

8 9 10 LOG [O- Flux (m -2 s-l)]

Contour plot of the flux of neon as a function of the flux of oxygen derived from 951 individual ICI measurements.

200 M. A COPLAN ET AL.

Ramaty and Murphy (1987) have questioned the value of the solar neon abundance currently in use, and have derived a photospheric neon to oxygen ratio of 0.47 from gamma-ray observations of a selected number of solar flares. Until more data become available to establish this result, there seems to be no compelling reason to revise the solar neon abundance value at present.

2. Conclusions

Reliable, self-consistent determinations of long-term average solar wind abundances are available from ICI for several elements, with CHEM/AMPTE data for carbon, nitrogen, and sulfur adding important additional elements to the list. Foil measurements provide high precision determinations of rare gas ions for selected intervals, and SEP data give composition information from a number of solar flares over a period of several years. When the data from the different experiments are compared, the agreement is within the uncertainties of the measurements. Comparison of the solar wind abundances with photospheric abundances shows systematic differences, and theories necessary to relate abundances measured in the solar wind with solar abundances are now beginning to become available. For example, helium with the highest first ionization potential of any element and a very low Coulomb drag factor is depleted by a factor of two in the solar wind relative to the photosphere while elements such as iron, silicon, and magnesium with low first-ionization potentials are enhanced. Gravitational settling in the corona is also likely to be involved in the abundance variation observed for helium in special events. For neon, the currently accepted solar abundance agrees with solar wind and SEP measurements.

A promising new source of information on the relation between solar wind and photospheric abundances is the study of coronal mass ejection (CME) events, now being carried out by the ICI instrument, currently located off the west limb of the Sun. Because the ejecta from such events often come from deep in the corona, the com- position measurements may provide depth profiles for a limited number of important elements.

Acknowledgement

This work is supported by the National Aeronautics Space Administration and the Swiss National Science Foundation.

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