solar wind composition from the moon

Pergamon Adv. SpaceRes. Vol. 14, No. 6, pp. (6)161--(6)173, 1994

Copyright © 1994 COSPAR Printed in Great Britain. All rights reserved.

0273-1177/94 $6.00 + 0.00

SOLAR WIND COMPOSITION FROM THE MOON

P. B o c h s l e r

Physikalisches Institut, Universitat Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland

ABSTRACT

The lunar regolith contains the best accessible record of the solar wind composition of the past few billion years. Interpreting this record crucially depends on our understanding of the implantation mechanisms, potential alternative sources other than the solar wind, storage and degradation processes, and transport- and loss-mechanisms of trapped particles in the regolith. We therefore suggest that a future mission to the Moon should contain the following objectives: I) A thorough in-situ investigation of the contemporary solar wind composition by means of long-duration exposure experiments with various techniques as baseline for investigation of the historic and ancient solar wind. 2) A multidisciplinary program, involving an experimental investigation of implantation-, storage- and loss-processes of solar particles at the conditions of the lunar environment. This program is complementary to an elaborated sys- tematic sampling of all layers of the lunar regolith, based on the experience from the Apollo- and the Luna-missions. Difficulties 'with the interpretation of the lunar record are illuminated in the case of surface correlated nitrogen. 3) A complementary goal for the extensive sampling of the lunar surface is the documentation of the lunar regolith for future generations, prior to extended human activities which could have detrimental effects to the lunar environment.

INTRODUCTION

The p u r p o s e o f t h i s p a p e r is to p r o m o t e t he ca se f o r t h e i m p o r t a n c e o f t h e Moon as s i t e f o r t h e i n v e s t i g a t i o n o f h i s t o r i c s o l a r wind. The l u n a r r e g o l i t h has been e x - posed to t h e f l o w o f s o l a r p a r t i c l e s f o r t he l a s t f e w b i l l i on y e a r s , and i t ha s been d e m o n s t r a t e d t h a t t he s o l a r r e c o r d w i th in t he l u n a r so i l can be d e c i p h e r e d by a n a - l y s i s o f e l e m e n t s i m p l a n t e d and t r a p p e d a t t he s u r f a c e o f l u n a r g r a i n s . I t is f a r beyond t h e s cope o f t h i s p a p e r to s u m m a r i z e t he r e l e v a n t w o r k which h a s been ach i eved in t h i s v e r y a c t i v e f i e l d d u r i n g the l a s t two decades s ince t h e A p o l l o - and t h e L u n a - mi s s ions . We r e f e r to two r e c e n t r e v i e w s in t he book 'The Sun in T ime ' / 1 , 2 / which a l so a d d r e s s some c o n t r o v e r s i a l a s p e c t s . In t h i s p a p e r , we make a f e w r e m a r k s con - c e r n i n g o b s e r v a t i o n s o f t he c o n t e m p o r a r y s o l a r wind f r o m the Moon and we g ive some t h o u g h t s to p o s s i b l e f u t u r e d i r e c t i o n s o f r e s e a r c h on h i s t o r i c s o l a r wind.

At t h i s t i m e , t h e l u n a r r e g o l i t h is c o n s i d e r e d to be t he b e s t a r c h i v e f o r s a m p l e s o f a n c i e n t s o l a r wind. Dec iphe r ing the l una r r e c o r d is, however , no t s imple . The usua l a p p r o a c h is t o s e a r c h f o r s u r f a c e c o r r e l a t e d g a s e s on l u n a r g r a i n s , e i t h e r by g r a i n - s i ze s e p a r a t i o n , o r by e t ch ing t echn iques o r by s t e p w i s e h e a t i n g o r p y r o l y s i s . These m e t h o d s have been s u c c e s s f u l w i th v o l a t i l e e l e m e n t s such a s t h e nob le g a s e s , and to some e x t e n t w i t h n i t r o g e n and ca rbon . However , i t is qu i t e c l e a r t h a t f o r c o n d e n s a b l e e l e m e n t s , o t h e r s o u r c e s t h a n the s o l a r wind become d o m i n a n t a s c o n t r i b u t o r s o f s u r - f a c e c o r r e l a t e d componen t s . Thus, in each ca se a c a r e f u l a s s e s s m e n t o f p o s s i b l e a l - t e r n a t i v e s o u r c e s is n e c e s s a r y b e f o r e s u r f a c e c o r r e l a t e d o r ' t r a p p e d ' c o m p o n e n t s can be a t t r i b u t e d to t h e s o l a r wind.

(6)161

(6)162 P. Bochsl~

One has to keep in mind that despite the fac t that the Moon is a relat ively inactive p lanetary body, outgassing of various species f rom the lunar inter ior can contr ibute to the existence of a tenuous lunar atmosphere. From there, implan ta t ion into the lunar regoli th is possible via ionization and acceleration of charged par t ic les with the in terplanetary magnetic field, embedded within the moving solar wind plasma /3 ,4 / . Condensable elements can also be redistr ibuted as surface corre la ted components by evaporat ion f rom impacts of meteori tes and micrometeori tes or by direct out - gassing f rom the lunar crust. Furthermore, as will be discussed below, it is well possible tha t a non-vanishing f rac t ion of certain surface corre la ted elements in the lunar regoli th must be a t t r ibuted to the te r res t r ia l magnetotail and not to the solar wind. Until now, documented sampling of the lunar regoli th is limited to a relatively na r row belt around the lunar equator and sites near subter res t r ia l longitudes. Samp- ling on a large scale, with a wide selenographic distribution, would cer ta inly help to identify potential competing surface correlated components other than the solar wind.

In the second par t of this paper, we i l lustrate some of the diff icul t ies related to the deciphering of the lunar record. We discuss the case of ni t rogen in lunar soils and we give some additional information on a topic which we had raised in an earl ier paper with J. Geiss / 2 / . We give a quantitative assessment of the likelihood of a t r a n s f e r of t e r r e s t r i a l ni trogen to the lunar surface.

From the discussion of these topics it will be evident tha t the lunar regoli th is a sensitive monitor of all possible entries, including those generated f rom human ac t i - vities. We the re fo re advocate an extremely restr ic t ive use of the Moon fo r scientif ic purposes in order not to endanger research on the lunar h is tory and on the his tory of the solar system fo r fu ture generations.

In conclusion, we condense the outcome of the discussion into a list of objectives fo r Moon-based solar wind research.

INVESTIGATIONS OF THE HISTORIC SOLAR WIND

The Moon is the only place to find documented samples of ancient solar wind. The oldest exposures might date back as f a r as 4 Gy. It is probably premature to develop an elaborated sampling s t ra tegy at this time, instead, we concentra te on a few guidelines. The best approach to disentangle the sources contr ibuting to sur face co r re l a t - ed concentra t ions of cer tain elements in lunar soils is to obtain a wide selenographic dis tr ibut ion with respect to longitude, latitude and depth. The potential influence of the or ientat ion of a sample site with respect to the position of the Earth and the t e r res t r i a l magnetotail require samples f rom the f a r side of the Moon, or at least a wide longitudinal distribution. With respect to sampling of the la t i tu- dinal distribution, it seems unlikely that the lunar poles have been i r radia ted in the recent past by solar particles. On the other hand, it is natural to expect the largest amounts of indigenous lunar condensable gases, including heavier noble gases, at the lunar poles. Furthermore, it cannot be excluded that the lunar poles have been i r radia ted with solar part icles in the distant past. Taking this into account and considering the fac t tha t large-scale lateral t ranspor t of c ra te r e jec ta seems to be of minor importance fo r the format ion of the lunar regoli th (cf . /S / ) , it cer tainly seems worthwhile to thoroughly investigate samples and drill cores f rom both lunar poles. The spatial distr ibution of investigations must, of course, also include the third dimension, i.e. depth. The s t ra t igraphy of the lunar regoli th should be sampled at various places down to the bedrock. This seems the best approach to obtain the widest possible distribution, also with respect to exposure ages. It is self-evident tha t in order to follow the lunar and solar his tory in detail, reliable determinations of exposure ages are required. Different techniques for dating exposure periods exist as outlined in Table I. However, existing age determinations of regoli th samples are of ten ra the r ambiguous. This is not surprising if one keeps in mind that each sample of regoli th consists of a multitude of grains and t iny rock fragments , each of them with of ten ra ther complicated histories, sometimes involving multiple exposures. Hopefully, it will be possible to fur ther improve dating methods to the

Solar Wind Composition (6)163

stage where reliable exposure ages can be obtained from individual grains.

Table I: Dating methods for surface exposure age of lunar soils

• Maturity index I,/FeO: Measures integrated duration of residence in the

topmost layers of regolith.

• Exposure to galactic cosmic rays yields integrated 'surface' exposure time via production of rare nuclides in the lunar soil. 'Surface' means:

Sample was at a depth ~ 150 g/cm z. In principle, it is not known when sam-

ple was exposed to solar wind.

• 4°Ar/36Ar exposure age: Samples exposed in the early h is tory of the lunar

regol i th contain surface correla ted radiogenic 4°Ar f rom the 4°K-decay of long-lived natural K and subsequent outgassing f rom the lunar interior . Applicability of this method depends on various assumptions, cf. / 7 / .

• Fission xenon dating: Due to the decay of mother isotopes with different yields and different decay rates into different xenon isotopes, absolute dating of surface exposures is possible. This method is only applicable to samples with ages > 2.5 Gy /8/.

The investigation of samples with exposure to extremely ancient solar wind might pose additional problems of interpretation, since such samples could have experienced their irradiation under quite different conditions than recently exposed samples. First, it cannot be excluded a priori that, earlier, a more intense intrinsic magnetic field prevailed at the lunar surface. This would, among others, have the effect that such samples were shielded from the less energetic solar wind particles, whereas solar particles at higher energies could still penetrate, thus yielding a higher fraction of energetic particles relative to the total dose in the distant past. The information available from noble gas analysis of lunar samples exposed earlier in the lunar history certainly does not exclude such a scenario. Second, as discussed by Geiss and Bochsler /2/ and as will be elaborated in more detail in the following, it is well possible that the influence of the terrestrial magnetotail was more important - if not dominant - in earlier irradiation scenarios of the lunar regolith. Third, it seems plausible that the regolith contains traces from passages through dense, interstellar clouds. It has been suggested that the solar system, during its entire history, has traversed approximately ten such dense molecular clouds with the effect that the radius of the heliospheric bubble shrunk to less than one astronomical unit, thereby allowing the 'contamination' of the lunar surface among others with various interstellar molecular species /cf. 9/. Fourth, it is generally accepted that the early bombardment of the lunar surface with cometary debris, asteroids and meteorites was much more intense than what is observed today. This would again lead to complete- ly different irradiation scenarios for early regolith samples as compared to those which were exposed to contemporary solar wind.

THE MOON AS SITE FOR OBSERVATIONS OF THE CONTEMPORARY SOLAR WIND

During the Apollo-missions, the Moon has repeatedly been used as si te f o r observa- t ions of the contemporary solar wind with the foil collection technique / 6 / . In fact , the best and to -da t e most reliable isotopic measurements have been obtained with this technique. However, despite the great success of this experiment, and in view of possible in ter ferences on such measurements, it is evident tha t the Moon is not the f i r s t choice fo r observations of the contemporary solar wind. A recoverable, axis- stabil ized spacecraf t , f a r f rom the influence of a planetary body, would be be t te r suited f o r such observations. Although one would possibly like to take advantage of

(6)164 P. Bochsler

available t r anspor t capacity to and f rom a future lunar station, one encounters ser i - ous disadvantages fo r pure solar wind investigations f rom a lunar base: The monthly revolution of the Moon around the Earth and its monthly spin-period would render a continuous integrat ion of the solar wind flux f rom one single location impossible. More seriously, the existence of a tenuous lunar atmosphere could hamper investiga- t ions of the contemporary solar wind. Furthermore, the Moon crosses once per revolu- t ion the t e r r e s t r i a l magnetotail and, with some non-vanishing probability, the plasmasheet. It is conceivable, and it will be discussed in a following section that t e r - res t r ia l ions f rom the plasmasheet could contribute to the part icle f low to the lunar surface in the I to I0 keY/u-range. On the other hand, the impact of micrometeori tes re leas ing small plasma clouds would probably not impose serious problems fo r a Moon- based solar wind composition experiment. The above mentioned adverse e f fec t s were extensively studied during the design of the Apollo foil experiment. T h e y did not have a negative influence due to the fac t that the Moon never approached or touched the t e r r e s t r i a l magnetotai l during the exposure periods and that the t rapping e f f i - ciency of the Al-foils ideally matched the energy range of solar wind ions. For a t - mospheric low energy ions trapping was practically negligible. Of course, it is possible to design also fu ture experiments in such a way that in terferences can be mini- mized. However, the principal conclusion remains valid: There i s no n e e d to go to the Moon t o i n v e s t i g a t e t h e c o n t e m p o r a r y so lar w ind c o m p o s i t i o n p e r se .

Figure l: E.E. Aldrin implanting the Solar Wind Composition Ex- periment of J. Geiss and co-workers at Tranquilli ty Base during the Apollo 11 mission. (Photography by N.A. Armstrong)

The above argumentat ion is, however, not complete. Since a concentra ted scientif ic in teres t fo r samples of ancient solar wind in lunar regoli th exists, it is essential to deepen our understanding of the conditions under which sampling of solar wind in lunar grains occurs. We suggest the following four cornerstones as basis fo r investi- gat ions of the contemporary solar wind f rom the Moon:

• Monitoring of the solar wind flow of major elements by in-si tu mass-spec t romet ry , if possible, with fine time resolution and f rom dif ferent sites, evenly distr ibuted on the lunar surface,

• investigation of r a r e elements and isotopes in long-durat ion experiments applying the foil collection technique,

Sol& Wind Compc~ition (6)165

• inclusion of samples wi th well known t r app ing and s to rage p r o p e r t i e s f o r long- du ra t i on expe r imen t s on (unmanned} p recu r so r missions,

• thorough inves t iga t ion of t r app ing p rope r t i e s of a wide va r i e ty of minera l g ra ins under lunar condit ions.

With the f i r s t ob jec t ive we intend to obtain a continuous t ime se r i e s of the l a r g e - sca le ' he l iospher i c c l imate ' as r e fe rence f o r al l complementary s tudies . The second and the t h i r d t a sk encompass s imi la r object ives . Long-dura t ion expe r imen t s - a l though i t might be p r e f e r a b l e to rea l i ze them on a f r e e - f l y e r - wil l be of high sc i en t i f i c i n t e r e s t - independent of the r ea l i za t ion of fu tu re lunar missions. Since a sub- s t a n t i a l i n t e r e s t ex i s t s f o r the implanta t ion mechanisms also of minor species (e.g. Kr and Xe), i t is abso lu te ly necessary to have such exper iments as complement to the inves t iga t ion of h i s to r i c so la r wind in lunar regol i th samples. I t must be emphasized t h a t such exper imen t s also have the aim to es tab l i sh a da ta base f o r s o l a r p a r t i c l e s in the s u p r a t h e r m a l energy range. Although it is expected t ha t knowledge about the s u p r a t h e r m a l p a r t i c l e composi t ion will be g r e a t l y improved in the near f u t u r e by the WIND- and the SOHO-missions, there will s t i l l be la rge white a r e a s on the map to e x - p lore the ea r ly h i s to ry of the so la r system. With the fou r th objec t ive , we envisage a mul t i tude of con t ro l l ed exper iments to inves t iga te t rapping , s to r ing , and loss p r o - cesses of so l a r p a r t i c l e s in the lunar regol i th .

TRANSFER OF NITROGEN FROM THE TERRESTRIAL ATMOSPHERE TO THE MOON: A CASE STUDY

Kerr idge / I 0 / was the f i r s t to point out t ha t the i sotopic composi t ion of n i t rogen t r a p p e d in lunar soi ls exhib i t s a secular t rend. He showed t h a t a l inear c o r r e l a t i o n

between exposure age and the ISN/14N isotopic r a t i o exis ts . Recently, a more de ta i l ed t r end as shown in Figure 2 was presented by the same au thor /11/. Ni t rogen f rom cu r -

r en t l y exposed samples is enriched in ISN over 14N re la t ive to the t e r r e s t r i a l s t a n d -

a rd by 127., whereas the l ightes t samples show a deplet ion in ISN by a pp r ox i m a t e l y 2.57.. The or ig ina l i n t e r p r e t a t i o n / 1 0 / invoked a secular change of the i so top ic com- pos i t ion of the source ma te r i a l , i.e. the photosphere, or of the f r a c t i o n a t i o n mechanism, ope ra t i ng a t the so la r sur face . One d i f f i cu l ty wi th the i n t e r p r e t a t i o n of lunar n i t rogen l ies in the f ac t t ha t n i t rogen in lunar soi ls is overabundant wi th r e spec t to the noble gases which a re c lear ly of so la r wind origin. An i l l u s t r a t i o n to th i s obse rva t ion is given in Figure 3 (adapted f rom Kerr idge /12/) . Apollo 16 soi ls show a

c l ea r c o r r e l a t i o n of the so la r wind implanted 36Ar with n i t rogen, so one is t empted to ass ign the n i t rogen content of these samples to the so la r wind as well. The p r o b - lem, however, is r e l a t ed to the quant i ta t ive aspects : If one der ives a corona l abun-

dance r a t i o of 36Ar/N f rom the work of Breneman and Stone /13/ , one ob ta ins a c o r r e - l a t ion line wi th a slope which is one order of magnitude s t eepe r than the i n f e r r e d r a t i o f rom the Apollo 16 soils. This d iscrepancy has been a t t r i b u t e d to the d i f f e r - ences in r e t e n t i v i t y between the noble gas Ar and the chemical ly ac t ive n i t rogen (see / 1 / f o r a more extensive discussion). On the o ther hand, r ecen t expe r imen t s / 1 4 / in- d ica te t h a t a r t i f i c i a l l y implanted Ar and N in olivine and i lmeni te minera l s exhib i t s imi l a r r e l ea se pa t t e rn , and there is no indicat ion of d i f f e r ences in r e t en t iv i ty . However, the r e su l t is of p re l iminary nature; the exper iment should a t l e a s t be r e - pea ted wi th minera l samples, a r t i f i c i a l l y ac t iva ted with an equivalent so l a r dose of hydrogen, be fo re a val id comparison of r e t en t iv i t i e s f o r d i f f e r en t gases can be made.

Geiss and Bochsler / 15 / challenged the hypothesis of Kerr idge about a secu la r v a r i a - t ion of the s o l a r wind source and promoted an explana t ion invoking two components:

So la r wind enr iched by approx imate ly 127. in ISN, and a second, ' p l a n e t a r y ' component,

deple ted by a t l eas t 307. in ISN re la t ive to air . More recent ly , Geiss and Bochsler / 2 / a lso d iscussed the poss ib i l i ty t ha t th is p l ane t a ry component might in f a c t be the t e r r e s t r i a l a tmosphere . In the fol lowing, we p resen t an o r d e r - o f - m a g n i t u d e e s t ima t ion of the p r e s e n t - d a y n i t rogen cont r ibut ion of the t e r r e s t r i a l a tmosphere to the lunar

( 6 ) 1 6 6 P . B o c h s l e r

L.

i sO

100

-I00

-200

-300

1.0 I I I

!

Antiquity, Gyr 2.0 3.0

I I I

Apollo 16 bulk soils

, .s

o ' ' ;

I

Apollo regolith samples

' 6 ' 8' ' 10 Trapped 4OAr/36Ar

i

12

Figure 2: (from Kerridge /II/). Isotopic ratios of trapped nitrogen in lunar regolith samples versus antiquity (or trapped

4°Ar/S6Ar). Open symbols denote the lightest component in the sample, released at moderate temperatures, filled symbols denote the heaviest nitrogen component, released at low extraction temperatures.

Adapted from Kerridge (1980)

SeAr 11o'~mal,

0

0 50 100 150 N [ ppm I

F i g u r e 3: Abundance of 36Ar versus N in Apollo 16 soils and the coronal cor re la t ion line derived f rom /13/ . From the diFFerences in slopes, it seems that N is overabundant by at least a factor of 14. (Adapted from Kerridge /12/).

surface, and we speculate about the importance of this component in the past. A gen- eral outline of the scenario is illustrated in Figure 4 (from Geiss and Bochsler /2/). Finally, we delineate some observational consequences following from this hypothesis.


UV

TRANSFER OF14N FROM EARTH TO MOON?

Figure 4: (from Geiss and Bochsler /2 / ) . It is assumed tha t the Moon frequent ly crosses the te r res t r ia l magnetotail and the plasmasheet, whereby nitrogen ions originating in the t e r r e s t r i a l atmosphere will be t r ans fe r red to the lunar surface.

Est imation of the fluxes of atmospheric nitrogen f rom the existing obse rva t iona l data in the t e r r e s t r i a l magnetotail and in the plasmasheet is not simple. First , ion-f lows in the plasmasheet are not a s teady-s ta te phenomenon but s tochast ic with enormous f luctuat ions. The plasmasheet, to be visualized as layer of typically a few R E

(=Earth-Radii) thickness, frequently changes direction. Hence, the residence of the Moon within the plasmasheet is diff icult to be predicted f rom case to case. However, since we are interested in long- term effects , the s p a t i a l motion of the sheet and the momentary f lux distr ibution within the sheet is not important fo r our purpose. We

only need good temporal averages. The approach is the following: We use O+-ions which have repeatedly been measured in the plasmasheet as t racers . These ions can unambig- uously be a t t r ibu ted to the te r res t r ia l atmosphere /16/. Since we know of no in-si tu determinat ions of n i t r o g e n in the magnetotail , we use a measurement of the magne- tospheric N/O abundance ra t io to derive the nitrogen density. We also have to make some assumptions about the average spatial distribution of p lasmasheet - f lows within the t e r r e s t r i a l magnetotail . We then compare the average t e r r e s t r i a l ni t rogen f lux (computed f rom average oxygen density × N/O-rat io × average f low speed) with the average solar wind ni trogen flux. We apply the following relat ion fo r the mean ra t io of the t e r r e s t r i a l to the solar wind nitrogen flux r :

~ t a l l R t a i l r = , ( 1 )

~solar wind " ( R o r b l t - R t a l l )

~tall is the average nitrogen flux in the plasmasheet of the magnetotail , ~solar wind

is the average solar wind nitrogen flux (we assume a value of 10 s m-Zs-1), Rtall is

the (average) radius of the plasmasheet at the lunar orbi t which we assume to be 5 RE, Rorbl t is the radius of the lunar orbit (at present 60 RE). The above expression

can be derived f rom simple geometrical considerations, assuming tha t the plasmatail has a cylindrical shape and taking into account tha t the solar wind f low is blocked while the Moon crosses this cylinder. Naturally, due to its orbi tal inclination re la - t i re to the ecliptic plane and due to the i rregular motion of the plasmasheet, such crossings might not occur during every lunar revolution about the Earth. On the other hand, as long as the cross-sec t ion of the plasmasheet is not s t rongly enlarged re la - t i re to the place where the average fluxes are taken, and as long as the average la t - eral motion of the plasmasheet is comparable to the excursions of the Moon due to its orbi tal motion, the est imate of the long-time average will not be s t rongly influenced

by these uncertainties. O+-densities within the plasmasheet have been published in d i f fe ren t publications. The problem, here, is to find a reliable average. We find

tha t the safes t approach is to adopt the average H+-density f rom Baumjohann /17 / who

gives a value of typically 0.35 cm -3 in the plasmasheet, and to take a conservative

(6)168 P. Bochsler

e s t i m a t e w i t h an a v e r a g e O + / H + - r a t i o o f 1/50 given by S h a r p e t a l . / 1 8 / f o r low

m a g n e t i c a c t i v i t y . The e s t i m a t e d O+-dens i ty is t hen 0 .007 cm -3. This e s t i m a t i o n is c o n s i s t e n t w i t h n u m b e r s de r i ved f r o m p u b l i c a t i o n s by M6bius e t al . / 1 9 / o r by L e n n a r t s s o n / 2 0 / (his F i g u r e 3). We a d o p t a va lue o f 0 .05 f o r t he a v e r a g e N / O - r a t i o f r o m / 2 1 / , t he on ly i n f o r m a t i o n on m a g n e t o s p h e r i c N - c o n t e n t we could f i n d in t he l i t - e r a t u r e . F o l l o w i n g t h e concep t o f t he ' c l e f t ion f o u n t a i n ' ( L o c k w o o d e t al . / 2 2 / ) , we have to a s s u m e t h a t t h i s r a t i o cou ld be s u b j e c t to r a t h e r l a r g e v a r i a t i o n s , t y p i c a l l y by a f a c t o r 3, depend ing on the a l t i t u d e o f t he s o u r c e w i t h i n t he t e r r e s t r i a l i o n o - s phe re . The l a s t n e c e s s a r y i ng red i en t , t he a v e r a g e t a i l w a r d f l o w speed w i t h i n the p l a s m a s h e e t , is d i f f i c u l t to e s t i m a t e due to the s t o c h a s t i c n a t u r e o f t h e f l o w r e l a - t e d to t h e o c c u r r e n c e o f m a g n e t i c r econnec t i on , and to t he f o r m a t i o n o f p l a smoids . M e a s u r e m e n t s show a wide s p r e a d o f va lues , and keeping in mind t h a t f o r m i n g a v e r a g e s in such a s i t u a t i o n is p r o b l e m a t i c , we a s sume a speed o f 100 k m / s which we be l i eve is no t o v e r e s t i m a t e d : I t is a f r a c t i o n o f a t y p i c a l A l f v e n - s p e e d w i t h i n t h e p l a s m a s h e e t and a s m a l l f r a c t i o n o f a t y p i c a l s o l a r wind speed. Note, t h a t t h i s a s s u m p t i o n does no t imply m a g n e t o s p h e r i c n i t r o g e n to be l ess e n e r g e t i c t han s o l a r w ind ions. On the c o n t r a r y , t he a c c e l e r a t i o n mechan i sms in the m a g n e t o t a i l can a t t i m e s p r o d u c e p a r t i - c l e s f a r above t h e I keV/u r ange . In f a c t , the l igh t componen t in F i g u r e 2 s eems m o r e deep ly i m p l a n t e d in t he l u n a r r e g o l i t h g r a i n s /11/ . However , i t is ou r a im to make a c o n s e r v a t i v e e s t i m a t e a b o u t t he f l u x . I n s e r t i n g the numbe r s g iven above in to e x p r e s - s ion (I), r e s u l t s in a b e s t e s t i m a t e of r = 0 .04. The u n d e r l y i n g a s s u m p t i o n s to t h i s e s t i m a t e a r e s u m m a r i z e d in Tab le 2.

T a b l e 2: I n g r e d i e n t s f o r t h e e s t i m a t i o n o f t h e p r e s e n t - d a y

n i t r o g e n f l u x in t h e t e r r e s t r i a l m a g n e t o t a i l

a v e r a g e H + - d e n s i t y : a v e r a g e H + / O + - d e n s i t y r a t i o : a v e r a g e O + / N + - d e n s i t y r a t io : a v e r a g e t a i l w a r d f l o w s p e e d : r a d i u s o f p l a s m a s h e e t a t 60 RE: a v e r a g e s o l a r w i n d n i t r o g e n f l u x :

0 . 035 c m - 3 50 20 I 0 0 k m / s 5 R E I . 108 m - Z s -1

R e s u l t : The a v e r a g e c o n t r i b u t i o n o f t e r r e s t r i a l n i t r o g e n t o t h e l u n a r r e g o l i t h i s a p p r o x i m a t e l y 4g o f t h e t o t a l n i t r o g e n i n p u t ( w i t h i n l a r g e u n c e r t a i n t y l i m i t s )

Clea r ly , a t e r r e s t r i a l c o n t r i b u t i o n of a f ew p e r c e n t to n i t r o g e n in t h e l u n a r r e g o - l i t h wou ld n o t e x p l a i n t he n i t r o g e n ove rabundance , even in r e c e n t l y i r r a d i a t e d s a m - p ies , a s d e s c r i b e d above. This is no t t he i m p o r t a n t po in t . We r e p e a t t h a t t h e f l u x o f heavy ions w i t h i n t h e t e r r e s t r i a l m a g n e t o t a i l is by no means a s t e a d y - s t a t e p r o c e s s , and one shou ld no t be s u r p r i s e d i f , even in t he r e c e n t p a s t , t h i s f l u x d i f f e r e d t e m - p o r a r i l y by o r d e r s o f m a g n i t u d e f r o m th i s a v e r a g e e s t i m a t e . I t is we l l e s t a b l i s h e d

t h a t t h e a b u n d a n c e o f heavy a t m o s p h e r i c ions such as 0 + a t g e o s t a t i o n a r y o r b i t s is s t r o n g l y c o r r e l a t e d to t he s o l a r EUV-f lux / 2 3 , 2 4 / , hence, a l so to t he s o l a r cyc le and, to a l e s s e r e x t e n t , t o t he s o l a r wind p r o p e r t i e s .

Now we a r e in a s i t u a t i o n to make an a t t e m p t to e s t i m a t e t he i s o t o p i c c o m p o s i t i o n o f t he c o n t e m p o r a r y t e r r e s t r i a l n i t r o g e n componen t in t he l u n a r r e g o l i t h . The i s o t o p i c c o m p o s i t i o n o f m a g n e t o s p h e r i c n i t r o g e n is p r o b a b l y e s t a b l i s h e d in a s i m i l a r w a y as

t h e N + / O + - r a t i o in t h e t e r r e s t r i a l e x o s p h e r e v ia g r a v i t a t i o n a l l a y e r i n g . I t wou ld t hen be c o n s i s t e n t to a s s u m e a s m a l l e r i so top i c f r a c t i o n a t i o n f a c t o r when l a r g e n i - t r o g e n f l u x e s ( f r o m d e e p e r ' a t m o s p h e r i c w e l l s ' ) occur , and we would e x p e c t n i t r o g e n

f l o w s , s t r o n g l y d e p l e t e d in ISN, in t i m e s o f low a t m o s p h e r i c in f lux . To r e m a i n c o n -

s i s t e n t w i t h t h e a s s u m e d N + / O + - r a t i o in t he m a g n e t o s p h e r e o f 0 .05 , we e s t i m a t e a

6 N m - v a l u e e s t a b l i s h e d in t he i o n o s p h e r e a t a p p r o x i m a t e l y 1500 km a l t i t u d e , which is t y p i c a l l y -SO0 to - 8 0 0 p e r mil. F o r p e r i o d s o f high m a g n e t o s p h e r i c a c t i v i t y t h i s v a l - ue cou ld a p p r o a c h 0 p e r mil . F o r the c o n t e m p o r a r y t e r r e s t r i a l n i t r o g e n inpu t which we e s t i m a t e d to be only a f e w p e r c e n t s o f t he t o t a l input in to t he l u n a r r e g o l i t h , we


obtain only a small modification of the overall lunar isotopic composition - despite the large fractionation factors in the terrestrial component - of -20 to -30 per mil. Nevertheless, such an effect could be detected in measurements in lunar fines. We must, however, emphasize at this occasion that we still have no direct information about the isotopic composition of nitrogen in the solar wind, nor do we know the photospheric isotopic composition of nitrogen; and this situation might last to the end of this decade.

It is important tha t the te r res t r ia l input - although f rom a d i f fe ren t reservoi r than solar ni t rogen - could well be correla ted with the solar activity. One could speculate tha t the Sun 'biows te r res t r i a l nitrogen to the near-s ide of the Moon, thus mi- micking solar wind' with a non-solar isotopic composition. The other impor tant point is tha t in the dis tant past, the influx of t e r res t r ia l ni trogen to the lunar surface might have been much more important than it is today. This argument has already been presented by Geiss and Bochsler / 2 / . Here, we give a somewhat more detailed outline. Figure 5 i l lus t ra tes the network of possible interdependencies.

I Transfer of Nitrogen from Earth to Moon ( a case study )

Decrease of I Deceleration of terrestrial 1 Solar UV-F ux t rotation duo to tidal actionJ

I t I [Decrease of ter-~strial] [ SeparatTon of 1 I I dynamo action and of ~ IMoon from Earth / I I terrestrial B-Field ]

LIl°wer '°n flux I ~ I ~l in magnot°ta'll I ~ I

Deoreuo of so ar I occultation from I I wind f,ux I I I solar wind I ' |

J~ J I diminishes I

F i w e 5: Transfer of ter rest r ia l nitrogen to the Moon. Possible interrelat ions between t idal damping of the te r res t r ia l rotat ional energy and the secular decrease of the solar act ivi ty. See tex t f o r explanations.

We consider two fundamental processes as basis fo r the development of the t ransfer mechanism of te r res t r ia l nitrogen to the Moon in the past. Both processes are well established and generally accepted, the interaction of them remains to be studied in detail. First , f rom young solar type stars i t is known that the act iv i ty and the EUV- output is strongly enhanced, typical ly by two orders of magnitude, i f compared to the present-day Sun /25-28/ . At the time of the f i r s t format ion of the lunar regol i th, the EUV-output of the Sun was st i l l typical ly a_n order of magnitude above i ts present level /25 / . As we have discussed in the previous paragraph, this has consequences fo r

the content of O + (and N +) in the ter rest r ia l magnetosphere and magnetotail. I t is safe to assume that wi th the aging of the Sun the t ransfer of ni trogen to the Moon has decreased. Second, ever since the separation of the Earth-Moon system has the Moon exerted t idal action on the Earth. The result of this interact ion was a decel- erat ion of the ter res t r ia l rotat ion and, because of the conservation of angular momentum wi th in the Earth-Moon system, this has lead to an increasing distance between Earth and Moon. Here, the time scale of the process was probably shorter than in the case of the decrease of the solar EUV-output. Adopting the value of 2.4 ms/century fo r the lengthening of the day at present /29 / , we derive a present-day ret reat ing rate of the Moon of approximately 4 m per century. A constant deceleration of te r res- t r ia l angular velocity leads to an accelerated separation, however a backward extra- polation is problematic. In any case, the increasing separation of the Moon-Earth system has led to a decrease of the occultation of the solar wind by the terrestrial magnetotail. A second consequence is more speculative but seems sound and is probably more important: Due to the deceleration of the terrestrial rotation, we have to expect a secular decrease in the dynamo action on the terrestrial magnetic field, lead- ing ultimately to a weakening of the terrestrial magnetotail. This would again have

JASR 14:6-L

(6)170 P. Bochsler

an i m p a c t on the f r a c t i o n r , i .e . t he r a t i o o f t e r r e s t r i a l n i t r o g e n inpu t to s o l a r input . We m u s t a l so t a k e in to accoun t t h a t mos t l ike ly t he s o l a r w ind f l u x o f n i t r o - gen has a l so d e c r e a s e d du r ing the s o l a r m a i n - s e q u e n c e evolu t ion . However , i f we c o m - p a r e t h e l a r g e f l u c t u a t i o n in t he s o l a r EUV-outpu t to t he r a t h e r s m a l l f l u c t u a t i o n s in t h e s o l a r w ind m a s s f l u x du r ing a s o l a r cyc le a t p r e s e n t , we i n f e r t h a t du r ing l a r g e f r a c t i o n s o f t h e s o l a r evo lu t ion the m o d i f i c a t i o n in s o l a r w ind m a s s f l u x was r a t h e r m o d e s t , p o s s i b l y l ess t h a n a f a c t o r o f two. At t h i s t ime , we c a n n o t g ive a q u a n t i t a t i v e a s s e s s m e n t o f the m o d i f i c a t i o n o f r du r ing the l a s t f e w Gy, bu t i t is p r o b a b l y s a f e to a s s u m e t h a t t he t r a n s f e r mechan i sm of n i t r o g e n f r o m t h e E a r t h to the Moon w a s much m o r e e f f i c i e n t in t he pas t .

OBSERVATIONAL CONSEQUENCES

The n e x t q u e s t i o n is: Are t h e r e o t h e r i n d i c a t i o n s f o r such a m e c h a n i s m ? The a n s w e r

is: P r o b a b l y no. P r i m e c a n d i d a t e s f o r such o b s e r v a t i o n s would be o x y g e n and 4°Ar. However , s i nce t h e l u n a r r e g o l i t h c o n s i s t s to a p p r o x i m a t e l y 407, o f oxygen , i t wi l l p r o b a b l y neve r be p o s s i b l e to d i s t i n g u i s h s o l a r o r t e r r e s t r i a l oxygen f r o m the i n d i g -

enous oxygen in t he l u n a r r ego l i t h . Exces s -4°Ar has been found , but , a s d i s c u s s e d above , t h i s e x c e s s is g e n e r a l l y a t t r i b u t e d to t he o u t g a s s i n g f r o m the i n t e r i o r o f t he Moon.

A s u m m a r y o f t h e o b s e r v a t i o n a l consequences is given in t he fo l l ow ing :

• Overabundance o f n i t rogen compared to predic t ions f r o m solar wind f l u x e s . The s c e - n a r i o d e s i g n e d above should consequen t l y l ead to an o v e r a b u n d a n c e o f n i t r o g e n c o m p a - r e d to s p e c i e s o f pu re s o l a r wind or ig in . I t r e m a i n s to be shown w h e t h e r t he e f f e c t is s u f f i c i e n t to e x p l a i n the d i s c r e p a n c y no ted by K e r r i d g e e t al . / i / and K e r r i d g e / 1 2 / .

• S trong v a r i a b i l i t y o f the p r e s e n t - d a y and o f the h i s tor ic t e r r e s t r i a l n i t r o g e n in- f l u x to the lunar surface . As ou t l i ned above, the mechan i sm o f n i t r o g e n t r a n s f e r f r o m E a r t h to Moon is no t smooth . On the c o n t r a r y , due to t he dynamic n a t u r e o f t h e p r o - cess , we e x p e c t r a t h e r l a r g e f l u c t u a t i o n s on t ime s c a l e s o f h o u r s and days . We a l so e x p e c t a c l o s e c o r r e l a t i o n o f t h i s f l u x to the s o l a r EUV-ou tpu t and, to a l e s s e r e x - t e n t , t o t h e s o l a r w ind m a s s f lux . Genera l ly , we e x p e c t a s e c u l a r d e c r e a s e o f t he t e r r e s t r i a l n i t r o g e n in f lux .

• S trong v a r i a b i l i t y o f the i sotopic composi t ion of the t e r r e s t r i a l n i t r o g e n component . I t is no t j u s t by m e r e i g n o r a n c e t h a t we have d i f f i c u l t i e s to p r e d i c t t he i s o -

t o p i c c o m p o s i t i o n . Judg ing f r o m the s t r o n g v a r i a t i o n s o f t he H + / O + - r a t i o in t he p l a s - m a s h e e t and j u d g i n g f r o m the mechan i sms which f e e d the m a g n e t o s p h e r e w i t h a t m o s p h e r i c

ions , we e x p e c t r a t h e r s t r o n g v a r i a t i o n s f r o m ~N Is ~ 0 to va lues a s low as ~N Is = - 8 0 0 p e r mil . We e x p e c t some c o r r e l a t i o n o f the i s o t o p i c c o m p o s i t i o n w i t h t h e f l u x and w i t h t h e s o l a r a c t i v i t y .

• Longi tudina l var ia t ion o f the t e r r e s t r i a l N component in the lunar rego l i th . Since t h e f l u x o f ions in t h e p l a s m a s h e e t a t 60 R E is g e n e r a l l y t a i l w a r d , we e x p e c t con -

s i d e r a b l y l e s s t e r r e s t r i a l n i t r o g e n in s amp le s f r o m t h e l u n a r f a r s ide . Th is p r o b a b l y a l so ho lds f o r s a m p l e s i r r a d i a t e d in t he d i s t a n t p a s t , s ince we do no t e x p e c t t h a t t h e Moon changed i t s o r i e n t a t i o n w i th r e s p e c t to t he E a r t h in t he l a s t f e w Gy. This p r e d i c t i o n is p r o b a b l y the only one which could give some conc lus ive ev idence on the v a l i d i t y o f t h e s c e n a r i o d e s c r i b e d above.

• Weak v a r i a b i l i t y o f the i sotopic composi t ion of p r e s e n t - d a y and h i s t o r i c so lar wind. This p r e d i c t i o n is no t d i r e c t l y r e l a t e d to t he t e r r e s t r i a l t r a n s f e r mechan i sm. I t j u s t r e p r e s e n t s t h e a u t h o r ' s p e r s o n a l j u d g m e n t on the l i ke l ihood o f s t r o n g f r a c - t i o n a t i n g m e c h a n i s m s o p e r a t i n g on co rona l i s o t o p i c abundances . In p r i n c i p l e , i t is

n o t ev iden t why t h e s o l a r wind ISN/14N-rat io (or the r e c o r d in t he l u n a r r e g o l i t h o f

i t ) shou ld behave c o m p l e t e l y d i f f e r e n t f r o m the Z°Ne/ZZNe- o r t h e m C / I Z C - r a t i o . (An


exception to this rule might hold fo r suprathermal and solar energetic par t ic les as br ief ly outlined below). Reliable measurements of these ra t ios in the contemporary solar wind, or, a t least, in solar energetic part icles f rom shock accelera t ion in co- ro ta t ing interact ion events are badly needed.

Concluding this section, we emphasize that it is certainly premature to say tha t the scenario described above is the solution to the lunar nitrogen problem. We should f i r s t t ry be t te r and quanti tat ive assessments of the processes involved and one still needs to look fo r other solutions, one possible al ternative will be mentioned in the next paragraph. The important point here is tha t we believe tha t we have a process in which the ni trogen t r ans f e r f rom Earth to Moon, amplified and t r iggered by the solar activity, could produce the enigmatic e f fec ts observed in the ni t rogen content and the isotopic composition of the lunar regolith.

ISOTOPIC FRACTIONATION OF NITROGEN IN SOLAR ENERGETIC PARTICLES7

Fisk / 3 0 / has proposed an explanation for the observed enrichment of 3He-ions in

f la re associated part icle events. In this scheme, 3He is selectively heated by its

ion-cyclo t ron resonance generated in a plasma mixture of protons and 4He-ions. The same mechanism might be responsible for the observed enrichment of heavy species in

3He-rich events / c f . 31/. An abundant charge s ta te of ni trogen in solar f la res is

probably N s+ because of its He-like electron shell. Now, lSNS+ has an M/Q-rat io of 3,

whereas 3He2+ has a ra t io of 1.5, hence the second harmonic of the cyclotron f r e -

quency of ISNS+ is exactly at the f i r s t harmonic of the cyclotron f requency of 3HeZ+.

ISNS+ might thus interactively resonate with the waves responsible f o r the p r e f e r -

ential heating and pre-accelera t ion of 3He2+. One would thus not be surprised to find

a sys temat ic overabundance of lSN over 14N in solar energetic part icles, especially

in 3He-rich events. Available information f rom spacecraf t measurements is cer ta inly not contradic t ing this idea: Mewaldt and Stone / 3 2 / repor t an isotopic ra t io of ni-

t rogen of ISN/14N = 0.008 with large experimental uncertainties. This value, taken at

face value, would represent an enrichment of ISN by a f ac to r of 2. The conclusion fo r ni t rogen in the lunar regoli th is clearly that the resonant wave-heat ing process should be considered in more details. Possibly, this process provides a viable a l t e r - native explanation to the nitrogen problem.

PROTECTION OF THE LUNAR ENVIRONMENT

In this paper, we have made an at tempt to establish some guidelines f o r the study of his tor ic solar part icles which are recorded in the lunar regolith. At the case of surface cor re la ted nitrogen, we have shown how important the potential of informat ion about the h is tory of the inner solar system could be. In concluding this paper, we must emphasize again how sensitive the lunar record to human inferences might be. The solar ni t rogen input to the entire lunar surface amounts to less than 2 kg per day. It is not diff icul t to visualize that any human activity on the Moon might rapidly overwhelm the natural nitrogen entry into the lunar regolith, thereby destroying the possibili ty fo r fu ture generat ions to decipher this important parameter fo r the his- to ry of the solar system. Nitrogen, as a volatile element, has a high likelihood fo r migrat ing into the tenuous lunar atmosphere f rom where it is ul t imately lost by ioni- zat ion and incorporat ion into the solar wind. The problem here is tha t a large f r a c - t ion of the 'human' ni trogen will be reimplanted ra ther eff ic ient ly into the r e - golith, and indistinguishable f rom all natural entries. It is not too early to begin the discussion whether mankind should really take the ' l as t giant leap' and colonize the lunar surface.

ACKNOWLEDGEMENTS

I am grea t ly indebted to Hans Balsiger, Fr i tz Biihler, Otto Eugster, Johannes Geiss,

(6)172 P. Bochsler

Ernest Kopp, Andreas Weigel, and to Rainer Wieler fo r helpful discussions and sug- gestions. John F. Kerridge has reviewed this paper and contr ibuted with cr i t ical r e - marks to a more equilibrated view in several aspects of the nitrogen problem. Grace Troxler checked the final versions of the manuscript. This work is supported by the Swiss National Science Foundation.

REFERENCES

I. J.F. Kerridge, P. Signer, R. Wieler, R.H. Becket, and R.O. Pepin, Long- term changes in composition of solar part icles implanted in ex t r a t e r r e s t r i a l materials , in: The Sun in Time, eds. C.P. Sonett, M.S. Giampapa, and M.S. Matthews, The Univer- si ty of Arizona Press, Tucson, pp. 389-412 (1991).

2. J. Geiss and P. Bochsler, Long time variations in solar wind propert ies: Possible causes versus observations, in: The Sun in Time, eds. C.P. Sonett, M.S. Giampapa, and M.S. Matthews, The University of Arizona Press, Tucson, pp. 98-117 (1991).

3. R.H. Manka and F.C. Michel, Lunar ion energy spectra and surface potential , Geo- chim. Cosmochim. Acta 3, 2897-2908 (1973).

4. R.H. Manka, J.W. Freeman, F.C. Michel, R.C. Elphic, D.J. McComas, R.R. Hodges, J.L. Burch, and R.E. Johnson, Lunar atmosphere, plasma and fields, this issue.

S. Y. Langevin, Remote sensing of the lunar surface f rom a lunar polar orbi ter , this issue.

6. J. Geiss, F. Btihler, H. Cerutti, P. Eberhardt, and Ch. Filleux, Solar wind composition experiment, Section 14 of Apollo Preliminary Science Report, NASA SP-31S (1972).

7. O. Eugster, J. Geiss, and N. GrSgler, Dating of early exposure and the evolution

of t rapped 4°Ar/36Ar with time, Proc. Lunar and P lanetary Sc i ence Conf. XIV, Lun. Planet . Inst . , Houston, pp. 177-178 (1983).

8. O. Eugster, P. Eberhardt, J. Geiss, and N. GrSgler, Neutron-induced fission of uranium: A dating method fo r lunar surface material, Science 219, 170-172 (1983).

9. D.M. Butler, M.J. Newman, R.J. Talbot, Inters tel lar cloud material : Contribution to planetary atmospheres, Science 201, 522-525 (1978).

I0. J.F. Kerridge, Solar nitrogen: Evidence for a secular increase in the ra t io of ni t rogen-IS to nitrogen-14, Science 188, 162-164 (19"/5).

II. J.F. Kerridge, Isotopic systematics of lunar surface nitrogen: A reappraisal , T w e n t y - t h i r d lunar and p lane tary sc ience c o n f e r e n c e , Lun. Planet. Inst., Houston, pp. 683-684 (1992).

12. J.F. Kerridge, Secular variat ions in composition of the solar wind: Evidence and causes, in: 'The Ancient Sun ' , eds. R.O. Pepin, J.A. Eddy, and R.B. Merrill, 475-489 (1980),

13. H.H. Breneman and E.C. Stone, Solar coronal and photospheric abundances f rom solar energet ic part icle measurements, Astrophys. J. 299, LST-L61 (1985).

14. N. Sugiura, T. Futagami, and S. Zashu, Nitrogen implantation: Experimental simulation of the solar wind, Meteor i t ics 27, 293 (1992).

IS. J. Geiss and P. Bochsler, Nitrogen isotopes in the solar system, Geochim. Cosmo- chim. Acta 46, 529-548 (1982).


16. S. Orsini, K. Altwegg, and H. Balsiger, Composition and p lasma p rope r t i e s of the p lasma sheet in the E a r t h ' s magnetota i l , Annales Geophys icae 4, 391-398 (1986).

17. W. Baumjohann, Plasmamessungen im Magnetosph"Irenschweif (Habi l i ta t ionsschr i f t ) , MPE-Report 233, Max-Planck- lns t i tu t fu r E x t r a t e r r e s t r i s c h e Physik, 1992.

18. R.D. Sharp, W. Lennartsson, W.K. Peterson, and E.G. Shelley, The or igins of the p lasma in the d i s tan t p lasma sheet, J . Geophys. Res. 87, 10420-10424 (1982).

19. E. MSbius, M. Scholer, B. Klecker, D. Hovestadt, G. Gloeckler, and F.M. Ipavich, Accelerat ion of ions of ionospheric origin in the plasma sheet during subs to rm ac t i - vity, in: M a g n e t o t a i l Physics , ed. A.T.Y. Lui, The Johns Hopkins Univers i ty Press , pp. 231-234 (1987).

20. W. Lennar tsson, Dynamical f e a t u r e s of the p lasma-shee t ion composit ion, density, and energy, in: Magne to t a i l Physics , A.T.Y. Lui ed., The Johns Hopkins Univers i ty Press , pp. 35-40 (1987).

21. C.R. Chappell, R.C. Olsen, J.L. Green, J.F.E. Johnson, and J.H. Waite Jr . , The discovery of n i t rogen ions in the E a r t h ' s magnetosphere, Geophys . Res. L e t t e r s 9, 937-940 (1982).

22. M. Lockwood, M.O. Chandler, J.L. Horwitz, J.H. Waite, T.E. Moore, and C.R. Chappell, The c le f t ion fountain, J. Geophys. Res. 90, 9736-9748 (1985}.

23. M. Stokholm, H. Balsiger, J. Geiss, H. Rosenbauer, and D.T. Young, Var ia t ions of the magnetospher ic ion number densit ies near geos ta t ionary orb i t with so la r act iv i ty , Annales G e o p h y s i c a e 7, 69-76 (1989}.

24. D.T. Young, H. Balsiger, and J. Geiss, Correla t ions of magnetospher ic ion composi t ion with geomagnet ic and solar act ivi ty, J. Geophys. Res. 87, 9077-9096 (1982).

25. F.M. Walter and D.C. Barry, P re - and main-sequence evolution of so la r act iv i ty , in: The Sun in Time, eds. C.P. Sonett , M.S. Giampapa, and M.S. Matthews, The Univer- s i ty of Arizona Press , Tucson, pp. 633-657 {1991).

26. S. Baliunas, The past , present and fu tu re of so lar magnetism: S te l l a r magnet ic act ivi ty , in." The Sun in Time, eds. C.P. Sonett , M.S. Giampapa, and M.S. Matthews, The Univers i ty of Arizona Press, Tucson, pp. 809-831 (1991).

27. J.R. S t a u f f e r and D.R. Soderblom, The evolution of angular momentum in so la r mass s t a r s , in: The Sun in Time, ed. C.P. Sonett , M.S. Giampapa and M.S. Matthews, The Univers i ty of Arizona Press , Tucson, pp. 832-847 (1991}.

28. S.H. Saar , The t ime evolution of magnetic f ields on so la r - l ike s t a r s , in: The Sun in Time, ed. C.P. Sonett , M.S. Giampapa and M.S. Matthews, The Univers i ty of Arizona Press , Tucson, pp. 848-858 (1991).

29. F.R. Stephenson and L.V. Morrison, Long- te rm changes in the ro t a t i on of the Earth: 700 B.C. to A.D. 1980, Phil . T rans . R. Soc. Lond. A 313, 47-70 (1984).

30. L.A. Fisk, 3He-rich f lares : A possible explanation, Astrophys. J. 224, 1048-1055 (1978).

31. D.V. Reames, R. Ramaty, and T.T. yon Rosenvinge, Solar neon abundances f r o m

g a m m a - r a y spec t roscopy and 3He-rich par t ic le events, As t rophys . J . 332, L87-L91 (1988).

32. R.A. Mewaldt and E.C. Stone, Isotope abundances of so la r coronal ma te r i a l derived f r o m so la r energet ic pa r t i c le measurements , As t rophys . J. 337, 959-963 (1989).

solar wind composition from the moon

Documents