SOLAR W I N D O B S E R V A T I O N S WITH THE ION C O M P O S I T I O N

I N S T R U M E N T ABOARD THE I S E E - 3 / I C E S P A C E C R A F T

K. W. O G I L V I E

NASA/Goddard Space Flight Center, Laboratory for Extraterrestrial Physics, Code 692, Greenbelt, MD 20771, U.S.A.

M. A. C O P L A N

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

and

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

Physikalishes Institut, University of Bern, Bern, Switzerland

(Received 20 February, 1989)

Abstract. In this paper we use the observations of solar wind helium ions made by the Ion Composition Instrument (ICI) on the ISEE-3/ICE spacecraft to study the variation of helium abundance in the solar wind and to arrive at an average value of that quantity for the period August 1978 to December 1982. The abundance varies in a similar way to that observed in the previous solar cycle, but more detailed dependence on velocity and solar cycle epoch is observed. The long-term average helium abundance is used in conjunction with long term abundances of 3He, O, Ne, Si, and Fe, measured with respect to helium using the same instrument, to compile abundances with respect to hydrogen which can be reliably compared with solar system abundances. With the extended data set we are able to show Si and Fe to be overabundant by a factor of three with respect to solar system abundances and He underabundant by a factor of two.

1. Introduction

Long-term variations of solar wind properties have been studied and interpreted, for example, by Feldman et al. (1978). Earlier work used observations made during the 11 year solar activity cycle 20 (1965-1976). Activity was comparatively low during those years, which only constitute half of a full 22-year magnetic activity cycle. The observations were made by a number of instruments of various designs, for shorter periods, many without overlap in time. More suitable instrumentation for a study of the (smaller than expected) effects on solar wind properties of the solar activity cycle was carried by the long-lived ISEE-3 spacecraft. In this paper we discuss observations of variations of helium abundance made with the Ion Composition Instrument (ICI) on the spacecraft ISEE-3 during the period September 1978 to the end of 1982. This period covers the rise, maximum, and part of the decline of solar cycle 21, as measured in the conventional way by the sunspot number.

In the first part of this paper we discuss our observations of 4He + § both to study and interpret the variations of helium abundance, and to arrive at a value of the long-term solar wind helium abundance to use in the second part. Then we discuss our recent determinations of abundances of other species (3He, O, Ne, Si, and Fe) and

Solar Physics 124: 167-183, 1989. �9 1989 Kluwer Academic Publishers. Printed in Belgium.

168 K . W . OGILVIE ET AL.

compare these with the abundances of Solar Energetic Particles (SEP). For these species we have obtained from the ISEE-3 observations representative long-term average abundances which are typical of the solar wind for the 4�89 year period. We find good agreement between the solar wind and the SEP populations, which both show overabundance of silicon and iron with respect to solar system abundances. By the same criterion helium is markedly underabundant in the solar wind and, as is well known, Neugebauer (1981), more variable in flux than other ions. As previous observers have, we found a number of examples of transient helium abundance increases, some of which are associated with material in unusually high charge states. These will be discussed in a subsequent paper.

2. Instrumentation

The Ion Composition Instrument (ICI) on the ISEE-3/ICE spacecraft couples a high- resolution electrostatic analyzer with a stigmatic Wien velocity filter (Coplan et aL, 1978) to obtain M/Q spectra of the ions in the solar wind. Ions entering the instrument pass through the filter before entering the analyzer. The narrow (2.5 7o) passband of the filter can be centered at velocities within the range 300 to 620 km s- 1. By combining the knowledge of ion velocity and energy per charge, the mass/charge of the ions whose flux is measured can be determined, within the resolution of the instrument, under most solar wind conditions. The ICI has operated successfully from launch in August 1978 to the present time. In addition to solar wind measurements, it has made the first observations of the composition of the distant magnetotail of the Earth (Ogilvie and Coplan, 1984), and of the composition of the coma of comet Giacobini-Zinner (Ogilvie et al., 1986; Coplan et al., 1987; Geiss et al., 1986). The solar wind data set discussed here covers the period from August 1978 to December 1982, when the spacecraft was in transit to, or at, the foreward libration point, L1, 1.5 x 10 6 km upstream from the Earth.

The high-velocity limit of the ICI is 620 km s - 1. Though higher velocities occurred during the period of interest, this limit allowed observations to be made for over 90~o of the time, including many examples of high speed streams and shocks. At the highest velocities, the energies of heavy ions such as Fe are sufficient to be detected by solid state detectors, such as those of Mitchell et aL (1981) and Ipavich et al. (1986), providing an independent check on the results of plasma instruments by an entirely different technique. Another more extensive check can be carried out by comparing the 3He, 4He, and Ne results with observations of these ions carried out on the Apollo flights by the foil method of Geiss et al. (1972). The good agreement among the results of these three different techniques gives us confidence in the accuracy of our knowledge of these abundances and their variability. In order to reduce dynamic range requirements in the detector, the ICI does not measure protons, roughly 20 times as dense as 4He + +. The abundances of heavier ions have been referred to helium, so that to convert to abun- dances with respect to hydrogen, we require a hydrogen density as a function of time. There is a plasma instrument on the ISEE-3 spacecraft (Bame et al., 1978), and for the earlier part of the measurement period proton densities Np were available. Due to an

SOLAR WIND OBSERVATIONS 169

instrument problem, these were not available after February 1980. However, solar wind densities derived from the electron observations have been available continuously, and agree on average within ten percent with the proton densities. To produce a consistent data set, the electron derived densities have been used for the present work.

The ICI instrument, because of the characteristics of the stigmatic Wien filter, has an acceptance angle parallel to the spin axis of the spacecraft (maintained perpendicular to the ecliptic plane to + �88 deg) of + 8 deg, (above and below the ecliptic). The solar wind plasma instrument, Bame et al. (1978), has a larger acceptance angle (+ 20 ~ in the same direction. As a result, the flux of helium ions can occasionally be cut off from the ICI by deflection of the solar wind in the N - S direction, whereas such a cut off of proton or electron flux from the solar wind plasma instrument almost never occurs. Thus, determinations of the helium abundance at times of large N - S deflections are lower limits, and this could introduce a bias in average values, which we have investi- gated. Individual values and short-term averages of helium density in interaction regions may be considerably affected by deflection of the solar wind in the N - S direction, and each case must be treated individually. Of course, the north-south deflections affect the fluxes of other minor ions measured by ICI in the same way as helium, so that abundances determined with respect to helium are not affected. North-south deflec- tions predominantly occur in the interaction regions of high-speed streams, and exceed eight degrees for less than 2% of the total observing time (Roberts, 1988), see Figure 1. Though the helium ion velocity direction does not coincide exactly with the proton velocity direction due to magnetic field components out the ecliptic, this is a _+ 2 ~ effect. Because this difference in direction is just as likely to increase the flux of helium ions into the ICI as decrease the flux, the overall effect can be neglected in the long-term averages. Overall, any effect of N-S deflections on the helium abundance is within the uncertainties of the measurement. Effects of deflection in the E - W direction are irrelevant because of the spacecraft spin. The long-term average densities we quote are

40

2o

0 0 5

100

80

60

I . . . . I . . . . I . . . .

10 15 20

ANCLS (nSC) 25

Fig. 1. The latitude angle of the solar wind direction using ISEE-3 data obtained during 1978. The open circles represent the percentages of observations with deflections in the two degree angular range centered at the point, the solid circles are the percentages with deflection greater than the corresponding angle.

170 K. W. OGILV1E ET AL.

affected much less by N-S deflections of the solar wind than, for example, by determi- nations of the instrument functions for both instruments.

We emphasize that relative abundance determinations using observations by two different instruments introduce uncertainties that do not apply to determinations made with a single instrument. For the abundance measurements made by ICI alone ([3He]/[4He], [O]/[He], [Ne]/[He], [Si]/[He], and [Fe]/[He]) there is no such problem. This is underlined by the excellent agreement between the ICI and the foil-determined abundances demonstrated in Figure 2. When different instruments are used, uncertainties in the knowledge of the instrument functions become important. For the ICI, the instrument function was carefully measured before launch, including determinations of the transmission, the detection efficiency and the effective solid angle subtended by the limiting stop at the input collimator, using pencil beams of several ionic species (Coplan et al., 1978). Nevertheless, differences between this quantity and the effective instrument function in the solar wind could amount to + 20 70, and there may be some similar uncertainties in the calibration of the solar wind instrument.

9

14,000 OBSERVATIONS AUG 78-JUL 82

4He/3He = 2000 /

D 7 _1 8 - u_

§ I

(.9 o . J

7 1 10 11 12

LOG (*He++FLUX)

Fig. 2. Contour plot of tog (Flux 3He + + ) vs log (Flux 4He + + ) in the solar wind. The contour levels are marked on the figure, and the results of five Apollo foil experiments are shown as dots labeled 11 through

16. The line corresponds to 4He/3He = 2000. (From Coplan et al., 1984).

S O L A R W I N D O B S E R V A T I O N S 171

The channeltron detectors in the ICI, which have an average counting rate of ~ 100/s, have remained saturated up to the present time and there is no evidence of changes in instrument properties with time. The abundances of helium, N J N p , where N~ is the ICI

derived density and Np the proton density provided by the solar wind instrument, have

been normalized by the constant factor 1.3 to match up smoothly with the observations

made during solar cycle 20 and with (six-month average) observations made by

Voyager 2 when that spacecraft was not too far from the Earth. All the subsequent

discussion applies to the normalized data set.

The data have been compiled into six-month collections of hourly averages.

Six months is larger than a solar rotation period, but small enough to display the

expected solar cycle variation. Note that the six-month period also approximately

divides the data by heliographic latitude; the first six months of the year are predomi-

nately at negative, and the second six months at positive, heliographic latitudes. No very

evident changes in the abundances or other properties appear to be associated with

heliographic latitude. Figure 3 shows the hourly average 4He abundance data for the

3

72 0 ~s

1

4 . . . . I ' ' ' ' 1 ' ' ' ' 1

0 i i • i I i i i i I i i i i I

2.5 3 3 .5 4 LOG (.,V)

1.5 . l l l l [ ' l ' " ' l l l i ' ' ' l l l l l l

.5

I I [ .

i i I -

4.5

I I I

M E A N - 0 . 0 ~

I - lO?O

V < 4 5 0

.05 .1 .15 .2 N,,/Np

I I I I i I I I

. 06

.02

o , , , , I , , , ~ l ~ j ~ 3 0 0 4 0 0 5 0 0 6 0 0

V 1.5 , 0 0 , 1 , , , , l , , , , i , , , , i , , , i

- 0.045 jl . 386

l

1 ~ g > 4 5 0

.5

.,•04 Z

0 ) 0 J i i , , ,-J~ n ) 0 .25 0 .05 .1 .15 .E .25

ISEE-3 1978.75 N,,/Np

Fig. 3. Helium abundance data for the period August 1978 to December, 1978. Top left: flux contour plot. Top right: apparent dependence of helium abundance on solar wind speed in km s - i. The solid line is from Hirshberg et al. (1972, Equation (2)). Bottom left: normalized histogram of relative helium abundance for speeds below 450 km s - i with mean value and number of hourly average observations. Bottom right: normalized histogram of relative helium abundance for speeds above 450 km s - 1 with mean value and

number of hourly average observations.

172 K. W. OG1LVIE ET AL.

period August to December 1978. At the top left-hand corner of the figure is a contour plot, on a logarithmic scale, of the flux of helium versus that of hydrogen. Below are two normalized histograms of the distribution of helium abundance for the velocity ranges 300 < U_< 450 km s - 1 and 451 < U < 620 km s- 1, respectively. At the top right-hand corner is a plot of the apparent variation of helium abundance with bulk speed. The error bars are the probable errors of the means of the observations for the

six months. Figures 4, 5, 6, and 7, in the same format, cover the rest of the observing period in six-month intervals. Figure 8 shows the mean value of helium abundance for each six-month collection of hourly averages, plotted together with previous determi-

4

3

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'''i''''l .... i'

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.05 ,1 .15 .2 N./N,

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' 0 ,25 .05 .1 .15 .2 .25

~ - 3 1 ~ 9 , 2 5 N~Np

3

>.

~2 o o

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4 . , , , , 1 , , , , I , , , , I , , , ,

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U~ I,,V) 1.5 '''l''''l''''l''''i'*'

I " 11k~

I

1 ~ Y < 450

0 .05 .I .15 .2 .25

06~'|' ' ' I ' ' ' ' j ' ' ' '-

.02 ~

O , , , , , I , , , , I , , , , 300 400 500 600

Y 1.5

i "" ....

V > 460 "1

1

o P ' ' ' ' I J . , q ' f ' , ~ - ' . J , , . . - , I . , , , t 0 .05 .1 .15 .2 .25

N"/Ne ISEE-3 1979.75 NJN~

Fig. 4. Helium abundance data for the year 1979. Same arrangement as in Figure 3.

SOLAR WIND OBSERVATIONS 173

3

~2

8

1

4 . , , , , [ r , , , | , , , , I , , ,

2.5 3 3.5 LOG (",~0

1.S ' ' ' 1 ' ' " 1 ' ' " 1 " ' ' 1 " '

1

1 g V< 450

0 ,,,I,,,~I,,,,I,,, 0 .05 .1 .15 .2

NJN,

Fig. 5.

'! a!

~2

1

Iii i i 4 4.6

' ' ' 1 . . . . t . . . . -4

.02

0 t ~ , I i I t , I , , t , 300 400 500 600

V 1.5

"I'' .... ,"L",' !

V > 450

.5

o , ~ - I , - , . , I ~ ~ ~

.25 0 .05 .1 .15 .2 .25

ISEE-3 1980.25 N./Np

0 I I I l [ l l l l [ l l l [ [ l l l �9 5 3 ~G 4

LOG ( N , ~

1.5 ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' lawi - o.~i

1 L V < 450

.5

0 .05 .1 .15 .2

4.5

.25

L ' ' ' ' I ' ' ' ' I ' ' ' '~

300 400 500 600 V

1

1 ~ V > 450

.5

0 ~l,,,~I~,, .05 A .15 .2 .25

N./Np ISEE-3 1980.75

Helium abundance data for the year 1980. Same arrangement as in Figure 3.

nations from solar cycle 20, obtained from Neugebauer (1981), versus time for the period 1962 to 1984. The two points derived from Voyager observations are indicated

(Lazarus, 1985). Sunspot number is also plotted as a measure of solar activitY. We note that, although the variations of helium abundance and of sunspot number are both manifestations of solar activity, they are not necessarily directly related by a physical mechanism�9

174 K . W . OGILVIE ET AL.

F ig . 6.

3

~ 2

1

1.5

o 2.5

3

2

1

0 0

1.5

0 2.5

''I''''I'''' '''

, , I , , , , l ~ t t , l , , , , - 3 3.5 4 4.5

LOG (.,V)

141~ . OJ~,O ! - I ' ~ 1

t

Y < 450

, , I , , , I , , ,

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300 4 ~ 500 600 V

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# - ~G |

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.5

I l l l l , �9 25 0 . ~ .1 .15 .2 .25

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,,,,I1,,rl,,,+

,,,I,,,,I,,,, 3 3.G

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4,5

, , , i , , , , i , , , , i , , , , i , , , ~ - o.o41)

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' ' ' 1 . . . . I ' ' ' '

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.02 -

0 I I t I * I I I I I I I 1

300 4.00 500 600 V

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dl - 7,1h~

I ~ u ~ V > 450

\ ,5

0 , , I , , , , u ~ . . n ~ l ~ . ~ _ , I , , ,

.25 .05 .1 .15 .2 2 5

ISEE-3 1981,7t NJNp

H e l i u m a b u n d a n c e d a t a for t h e y e a r 1981. S a m e a r r a n g e m e n t as in F i g u r e 3.

3. Discussion

These figures clearly show an apparent dependence of the relative abundance of helium on solar activity, sustained over a 22-year period, which is most pronounced, as noted by Neugebauer (1981), during the years 1968 to 1972 and 1978 to 1981, close to the solar maxima. In solar cycle 20, Feldman et al. (1978) found that the maximum helium abundance occurred when the sunspot number had decreased by a factor of 2-3 from its maximum value, creating a time lag between the two maxima. Figure 8 indicates a much smaller lag for solar cycle 21. In the years 1978 to 1981, the pattern of inter-

S O L A R W I N D O B S E R V A T I O N S 1 7 5

3

2 2

.a

1 -

2.5 3 3.5 4 4.5 LOC (N,V)

1.5 '"I .... i .... i .... J'"

E - t;~

.5

0 1 , ~ , 0 .05 .!

V<450

,i,,,,l,,, .15 .;~ .25

.O6

.02

' '''I''''I' ' ''

0 i i ,4 I , ,,~ I , , , ~ 3O0 4OO 5OO 6OO

V 1.5 "'l''''i'"'i'"'l'''

MEA~ ~ O.04t /I . itB4

#

1 ' A V > 450

.5

0 I , , , , I , , .05 .I .15 .2 .25

NJN, ISEE-31~.~ N ~

4

3

2 2

1

0 2.5

1.5

- ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' '

9 .06

m.

Z

.02

0 3 3 . 5 4 4.5

LOC (HpV)

' ' ' 1 . . . . i . . . . I . . . . I . . . . -[

.05 .1 .15 .2 .25 0 0

0

_ ' ' ' ' 1 ' ' ' ' 1 ' ' ' -

I I I I l l l i i l l i l i 300

1.5

400 500 600 V

,.. . . . . [ . . . . [ ' ' ' ' I i ~ . 'm I ~ . ~ ' J'"~ I # - 674

t V > 450 1

,, I ~-,..oA~ ,J .... I ,, ,,'~ .05 .1 .15 .2 .25

NJN~ ISE E -3 1982.75 NJN~

Fig. 7. Helium abundance data for the year 1982. Same arrangement as in Figure 3.

planetary magnetic sector structure was quite unstable (Couzens and King, 1986), but it had started to be more regular by 1982. Thus most of our observations refer to the

most active part of the solar cycle, and examination of them as a time series shows that much of the abundance variation was associated with interplanetary disturbances such as shocks, coronal mass ejections and noncompressive density enhancements. Feldman et al. (1978) state that helium abundance variation is most pronounced at low solar wind speeds; the histograms in Figures 4 to 7 do not support this contention for the first half of solar cycle 21, at least in the velocity range covered by this instrument.

The line in Figure 8, drawn by eye, between the points representing ICI observations

P~ z

.04 z < 2;

.03 I

.02 -

~120 I

~ 80

z 40 co

0162 64

I I I I I o'5 t i L t I ~ t i t i t / t t t E~4 1 �9 ~ ~X~

�9 I \ / f 0~ 0~ E43 i~ I ~

~ x tct (x / I / T\ I I / , j vs/ ~, ~

~ _ v3/ / \ ~ I

FROM NEUGEBAUER, 1981 g~/~- VOYAGER 2

176 K. W. OGILVIE ET AL.

I - -

1.3)

66 68 70 72 74 76 78 80 82 84 YEAR

Fig. 8. Above: Variation of helium abundance during solar cycles 20 and 21. Early observations taken from Neugebauer (1981). M2 - Mariner 2, V3 - Vela 3, E34 - Explorer 34, 05 - OGO 5, H1 - Heos 1, E43 - Explorer 43, I - Imps 6 and 8. Later observations are from Voyager 2 and the present work, which are average values of six-monthly periods. Below: Sunspot number for the same period as an indicator of solar

activity.

shows a rapid rise in the early part of 1979, followed by a statistically significant dip and secondary maximum. Such a rapid rise, dip, and secondary maximum are also suggested in the results for solar cycle 20, if these are taken at their face value. However, to do so is probably not warranted by the accuracy and precision of the data.

Hirshberg, Asbridge, and Robbins (1972), using data from the Vela spacecraft for the

period 1967 to 1969, obtained a relation

(N~/Up) • 102 = 1.02 x 10-2v - 0.39 (2)

between solar wind helium abundance and speed, v. This relation is shown in each of Figures 4 to 7 and agrees remarkably well with our data in the analogous period, 1978 and the first part of 1979, when activity was rising at the start of the solar cycle, but is a poor fit to the rest of our data. This indicates the increases in helium abundance around solar maximum were often produced by the transients associated with high speeds (flares, CMEs, etc.) followed by a longer period when abundance variations were also associated with disturbances at low speeds - possibly quasi-static structures, Borrini et al. (1981, 1982a, b). This scenario helps to reconcile conflicting claims in the literature, based upon data from different phases of the solar cycle.

S O L A R W I N D O B S E R V A T I O N S 177

From observations made during the period 1971 to 1974, when solar activity was

dropping to a minimum, Bame, Asbridge, and Gosling (1977) deduced the existence of

a structure-free state in the solar wind at high speeds, associated with the contemporary

long lived coronal hole structures, and characterized by a relatively constant value of

helium abundance. Bame's 'high speed' data set consisted of time intervals when the

solar wind speed exceeded 650 km s - 1, with a mean value of 702 km s - 1, above the

range of the ICI. During the years 1978 through 1982, the solar wind speed exceeded

the upper limit of our inst rument only 4 ~o of the time. We have divided our observations,

shown in Figures 4 through 7, at 450 km s - 1. The lower speed range contains solar wind

not associated with coronal holes, while in the upper range there are many examples

of shock and coronal hole associated flows. Thus we might expect to see a trend towards

constancy in abundance in the higher speed range during the last two years of our

period.

The abundance observed in high speed streams by Bame, Asbridge, and Gosling

(1977) was 0.048 + 0.005. Our normalized mean value for speeds between 450 and

600 km s 1 of 0.045 + 0.010, where the uncertainty is a measure of variability which

in turn depends upon the averaging period employed. The histograms in the lower

right-hand corner of Figures 4 to 7 show a definite trend with time to a narrower

distribution about the mean value.

Table I is a summary of the observations. The quantity cr is the full width at half

maximum of the abundance distribution for the corresponding time period; and A(v) is

a measure of the velocity dependence of the abundance during that period. This quantity

seems to be small after the second part of 1979, and is perhaps increasing towards the

TABLE I

A summary of the observations

Helium/hydrogen Hel ium/hydrogen Helium/hydrogen Helium/hydrogen ratio ratio ratio ratio a

<450 km s 1 a <450km s i a All A(v)

1978.75 0.032 0.039 0.045 0.060 0.035 0.0313 0.029 1979.25 0.044 0.054 0.056 0.054 0.050 0.020 0.049 1979.75 0.053 0.050 0.055 0.045 0.053 - 0.004 0.054 1980.25 0.037 0.050 0.039 0.040 0.038 - 0.003 - 1980.75 0.035 0.045 0.040 0.053 0.037 0.010 - 1981.25 0.040 0.050 0.049 0.060 0.042 0.017 - 1981.75 0.040 0.040 0.045 0.035 0.041 0.012 - 1982.25 0.031 0.020 0.041 0.020 0.036 0.021 - 1982.75 0.030 0.018 0.038 0.023 0.035 0.011 -

Mean 0.038 0.04 0.045 0.043 0.041

a = FWHM of distribution, mostly a measure of variability. A(v) = (abundance at 600 km s-~) - (abundance at 300 km s 1) from the data. Long period average = 0.041 + 0.007, error estimate includes an estimate of the error in the normalization. a Could only be calculated for the first three intervals, using measured proton densities.

178 K. W. OGILVIE ET AL.

end of our observing interval. The eighth column gives abundances determined using the proton densities when they were available and is to be compared to the first three entries in column six; the agreement is good.

Both Ogilvie and Hirshberg (1974) and Feldman et al. (1978) discussed the depend- ence of helium abundance on solar wind speed. Ogilvie and Hirshberg, using measure- ments acquired near solar maximum, found the maximum dependence, expressed as the slope of a regression line, to occur at the time of solar maximum. Feldman et al. (1978), using measurements made during the decreasing part of the solar cycle between 1971 and 1976, found the maximum difference between the helium abundances in high and low speed wind to occur near solar minimum. As shown in Table I, the differences, A(v),

between the helium abundances, measured by the present instrument at the extremes of its velocity range, are smallest during the period late-1979 to the beginning of 1982, suggesting that the least dependence on velocity occurs during the years of decreasing solar wind activity when Feldman's measurements commenced, but before solar minimum. This is perhaps due to the decreasing rate of flare associated transients during that period. In the rest of this paper we will use for the long-term average helium abundance in the solar wind in this epoch

[He]/[H] = 0.041 + 0.007.

We suggest that this value is consistent with other observations, and is determined over a sufficiently long period of time to be representative of all states of the solar wind. When dealing with the high speed wind from coronal holes we consider Bame, Asbridge, and Gosling's (1977) result

[He]/[H] = 0.048 + 0.005

to be the best value.

4. Abundances of Ions Other Than Helium

With an average helium abundance for the solar wind covering the period from August 1978 to December 1982, we can further discuss observations which were made with the ICI of the species 3He, O, Ne, Si, and Fe during the same period. For each of these species the 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, and can, therefore, be compared both to the abundances of solar energetic particles and to solar system abundances.

In the individual publications, referred to in Table II, the results were exhibited as contour plots showing the log of the flux of the minor ion versus the log of the 4He flux, with contours connecting regions of constant number of observations. As an example we show the plot for 3He, taken from Coplan et al. (1984), in Figure 2. This figure was constructed from 14,000 hourly average observations. The shapes of the contours reflect the convolution of measurement uncertainties and natural variability.

The dots indicate the results of individual 3He and 4He determinations by the foil

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180 K . w . OGILVIE ET AL.

method during the Apollo flights, identified with the corresponding Apollo flight number (Geiss et al., 1972). The agreement between these two determinations of [3He]/[4He] by entirely different techniques is remarkable, and similar agreement was also obtained between the observations ofNe (Bochsler, Geiss, and Kunz, 1986). Coplan et al. (1984) concluded that the best value of the average [4He+ § ]/[3He+ +] was 2050 _+ 200. A small variation with solar activity was observed, and the ratio is more nearly constant at high than at low solar wind speeds. The pear shaped contours probably reflect the fact that the uncertainties in the measurements are greatest at low fluxes. Examination of the measurements as a time series indicates that there are periods of substantial intrinsic variability. To obtain an abundance of 3He referred t o l l , we require a value of [4He + + ]/[H +]. Normally this ratio is highly variable, but B ame, Asbridge, and Gosling (1977) have shown that the helium abundance in high speed solar wind flows at 1 AU has a relatively constant value of 0.048 + 0.005. Such flows, originating predominately from coronal holes from where the magnetic field lines are open, may provide the best conditions for solar helium abundance measurement. Taking our average value for the ratio [4He]/[3He] in high speed streams (1900 + 200) and the Bame, Asbridge, and Gosling (1977) value for [4He+ § ]/[H + ] in high speed streams, 0.048, we obtain a relative abundance of 3He § § of (2.6 _+ 0.2) x 10- 5 Taking the average from Coplan etal. for [4He]/[3He] to be (2050 + 200) and our long-term average helium to hydrogen ratio (0.041 + 0.007) we obtain [3He]/[H] = (1.95 _+ 0.5) x 10 -5.

The abundances of the minor ions other than helium are obtained by a minimization calculation. The charge state distribution of an element depends nonlinearly on the 'freezing-in' temperature (Hundhausen, 1972). By determining the best fit (in the least- squares sense) between predicted and observed counts at given M/Q settings, both abundances and an estimate of 'freezing-in' temperature, assumed the same for all ions, was obtained. This procedure takes into account the variation of instrument parameters with ionic mass, charge, and velocity (Bochsler, Geiss, and Kunz, 1986). In doing this for the rarer ions, the averaging period was adjusted to prevent statistical uncertainties of individual measurements becoming dominant. Table II is a compilation of the results, showing the averaging period and the estimated uncertainties for the species measured by the ICI.

Table II also compares these results with two recent compilations of abundances derived from observations of SEPs by Cook, Stone, and Vogt (1984) and McGuire, von Rosenvinge, and McDonald (1986). Agreement among the measurements, made under a wide variety of solar wind conditions, is well within the experimental uncertainties, except for oxygen where the agreement is at the limit of the uncertainties; we conclude that both SEPs and the solar wind seem to be drawn from the same population, which we know for the solar wind is the corona.

Now, using our long-term average value for [He]/[H], 0.041 + 0.007, we can convert to abundances with respect to hydrogen and compare these with solar system abundances. The result is shown in Table III.

Comparing the solar wind and solar system abundances, we see that there are two

SOLAR WIND OBSERVATIONS

T A B L E I I I

Abundances wi th respect to hydrogen

Element Solar wind ( ICI) Solar sys tem

H 1 1 3He (1.9 + 0.5) x 10 . 5 (2.3 + 1.2) x 10 - s a

4He (4.0 + 0.7) x 10 . 2 c (8 __ I) x 10 - 2 b

O (5 + 1) x 10 4 (7.4 + 1.5) • 10 . 4 b

Ne (9 + 3) x 10 . 5 (1.4 ~ 0.5) • 10 - 4 b

Si (1.3 -~ 0.7) X 10 4 d (3.6 + 0.2) X 10 S b

Fe (1.1 Jr 0.5) • 10 4d (3.3 _+ 0.1) X 10 - s b

a Hua and Lingenfel ter (1987); indi rec t pho tospher ic measurement .

b F r o m Ander s and Eb iha ra (1982).

~ U n d e r a b u n d a n t in solar wind.

a Ove rabundan t in solar wind.

i81

major discrepancies. Helium is definitely underabundant in the solar wind as concluded by many authors (Neugebaner, 1981). It has been proposed that this is due to the reduced drag force preferentially exerted by the protons on the helium ions (Geiss, Hirt, and Leutwyler, 1970) to extract them from the gravitational potential well in the lower corona. Although we find oxygen and neon solar wind abundances in good agreement with those quoted for the solar system, both silicon and iron are overabundant in the solar wind by a factor of about three. It has been suggested that the SEP abundance data can be organized according to first ionization potentials (Hovestadt, 1974), ions having low first ionization potentials being overabundant (Si, 8.15 eV; Fe, 7.87 eV). A mechanism in which ionization early in the process of solar wind acceleration would favor incorporation of a species into the flow has been suggested to explain the observations (Meyer, 1985; Geiss and Bochsler, 1986).

5. Conclusions

During solar cycle 21 we observed the expected variation of helium abundance with solar activity. We also observed that near the maximum of activity the variation at high speeds associated with transients predominated, whereas away from the maximum the helium variations at low speeds became more important, in agreement with the con- clusions of Feldman et al. (1978). The existence of a 'structure-free' solar wind asso- ciated with coronal hole flows and a relative abundance narrowly distributed about a value near 0.045 is consistent with this data set. A number of periods of high helium abundance were observed, and these will be analyzed in a subsequent paper.

Abundances of 3He, O, Ne, Si, and Fe have been derived from ICI measurements, and these are based upon a sufficiently long observing period for them to be charac- teristic of the solar wind and to be compared to other abundance compilations. There is some evidence that it is helium alone (presumably because of its unusual mass/charge

182 Ir W. O G I L V I E ET AL.

combination) whose abundance varies appreciably with solar activity. For example, Bochsler (1989) found that the He/O ratio is related to solar activity,

log([He]/[O]) = 1.6 x 10-3Rz + 1.692,

where R~ is the sunspot number. This relation predicts a variation of the [He]/[O] ratio from about 50, at solar

minimum, to about 90, at solar maximum, in a typical solar activity cycle. The [He]/[H] abundance also increases with solar activity by about a factor of two, so that the oxygen abundance (with respect to hydrogen) remains nearly constant, independent of activity, as one might expect for a value characteristic of the solar corona.

The abundances measured by ICI agree very well with those derived for the same species in SEP events. Comparison with solar system abundances, however, shows helium to be underabundant, and Si and Fe to be overabundant in the solar wind. The former discrepancy is thought to be understood as principally resulting of gravitational fractionation, although the details are not known precisely. The latter discrepancy seems to result from a mechanism that links a low first ionization of a species to incorporation into the solar wind. Advances in the theory are now required with predictions amenable to quantitative verification.

The ICI represents an important advance over earlier instruments, and has made possible the determination of truly representative solar wind abundances for a small but important sample of ions. In the near future, these results will be supplemented by measurements of charge and mass at higher resolution, to be carried out by the next generation of instruments.

Acknowledgements

The authors thank L. Klein for the construction of Figures 3 through 7, and D. Roberts for the data incorporated in Figure 1. They acknowledge helpful discussions with L. Burlaga. P. Bochsler and J. Geiss acknowledge financial support from the Swiss National Science Foundation.

References

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