SOLAR WIND COMPOSITION AT SOLAR MAXIMUM

PETER BOCHSLERPhysikalisches Institut, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland

Abstract. Although coronal mass ejections have traditionally been thought to contribute only aminor fraction to the total solar particle flux, and although such events mainly occur in lower helio-graphic latitudes, the impressive spectacle of eruptions – observed with SOHO/LASCO even at timesof solar minimum – indicates that an important part of the low-latitude solar corona is fed with matterand magnetic fields in a highly transient manner. Elemental and isotopic abundances determined withthe new generation of particle instruments with high sensitivity and strongly enhanced time resolutionindicate that, apart from FIP/FIT-fractionation, mass-dependent fractionation can also influence thereplenishment of the thermal ion population of the corona. Furthermore, selective enrichment of thethermal coronal plasma with rare species such as 3He can occur. Such compositional features haveuntil recently only been found in energetic particles from impulsive flare events. This review willconcentrate on this and other aspects of the present solar maximum and conclude with some out-look on future investigations of near-terrestrial space climate (the generalized counterpart of ‘spaceweather’).

1. Introduction

Ulysses with the SWICS instrument of Gloeckler et al. (1983) has opened a new di-mension in solar wind composition analysis. Whereas the use of energy per chargeanalyzers, combined with Wien filters, allowed a one-dimensional description ofthe solar wind in the M/Q-space, the application of time of flight spectrometrynow routinely makes it possible to analyze simultaneously M and M/Q with a verylow intrinsic background. With this improvement in instrumentation the study ofcharge state distributions and compositional analysis is now generally accepted asan important diagnostic tool in heliospheric research. It is worthwhile rememberingat this point that the sources and many of the features of heliospheric particle pop-ulations would have gone unnoticed without investigation of their compositionalsignatures.

Minor species have been used as tracers of dynamic processes occurring in theinner corona (wave/particle interaction, Coulomb collisions, shock acceleration).Their charge state, which remains unchanged throughout the heliosphere, is indica-tive for electron temperatures and, more generally, for electron energy distributionsin the inner corona. Since dynamic properties of major and minor species arestrongly altered during their transfer into the interplanetary plasma and continueto be modified also beyond the Earth’s orbit, backmapping of solar wind streamsto the source regions on the basis of purely dynamic properties, such as wind

Space Science Reviews 97: 113–121, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

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velocities, becomes increasingly problematic with large solar distances. For thisand for other reasons the identification of stream sources and stream boundariesusing composition information has become particularly popular with the Ulyssesmission, because Ulysses spends most of its time at solar distances beyond 3 AU.

The fundamental astrophysical and cosmochemical aspect of solar wind compo-sition analysis has been emphasized on other occasions, however, it is less relevantfor this review: the solar wind carries information on the solar composition, es-pecially on the solar isotopic composition, which cannot be obtained with othermeans. Understanding the fractionating processes, which occur during injection ofsolar matter into the corona, is essential and complementary to the fundamentalendeavor of investigating the isotopic composition of the solar atmosphere. Thisremains a primary objective of solar research, which is pursued with the CELIASexperiment on SOHO, and with the MASS experiment on ACE. It has originallybeen introduced with the foil collection technique during the Apollo missions, andthis technique will also be applied for the GENESIS mission of NASA. On theother hand, the secondary goal, which is to use the wind composition as tracer anddiagnostics for coronal processes and for the identification of solar wind sourceswill be relevant for future solar missions of this decade. The payload of STEREO,the Solar Probe, and of the Solar Orbiter include solar wind composition packageswith various degrees of sophistication. All solar wind composition experimentshave essentially been included to serve as diagnostic tools.

This volume and the present review focus on the solar maximum. What con-cerns the solar wind, solar activity triggers coronal mass ejections, and even at highheliographic latitudes, slow solar wind dominates the scene during solar maximum.Ulysses has impressively shown that the coronal-hole-associated high-speed windis rather uniform, not only with respect to its dynamic properties, but also in itscomposition. In contrast, low-speed solar wind shows comparatively high variabil-ity in all aspects. This variability, especially the variability of dynamic propertiesrenders the analysis of composition data with in-situ mass spectrometers difficult,and furthermore, instruments on spinning spacecraft with low duty cycles havelimited time resolving capacity, aggravating the problem of compositional analy-sis. Thus, it is often quite difficult to distinguish variability, which is caused frominstrumental properties under different solar wind conditions (flow angles, solarwind velocity, anisotropic velocity distributions with varying principal directions)from real variations in composition. On the other hand, although the analysis ofCME-related solar wind data suffers from similar difficulties, the variability inthese events is apparently so large that its determination is relatively little affectedby instrumental effects. Whereas the source of the fast solar wind is unequivocallyattributed to coronal holes, the identification of the sources of the slow wind is stillsomewhat controversial. Traditionally, the boundaries between closed loops andcoronal holes have been considered to be the source of the low-speed solar wind.More recently, it has become evident that continuous reconnection and openingof large-scale structures contributes significantly to this type of solar wind (e.g.,

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Figure 1. Abundance ratios and freeze-in temperatures of 40 CMEs observed between 1990 and1996 with Ulysses/SWICS. From compositional features, charge states of C and O, and from thevelocity of the ambient plasma, three groups of CMEs can be distinguished: One group (indicatedby triangles) is related to high-latitude CMEs. The remaining two groups are related to low-latitudeCMEs, ejected from a heliomagnetic sector boundary or its vicinity. Group 2 (open diamonds) haveapparently undergone strong mass fractionation accompanied by an enrichment of He and a depletionof H. The last group (crosses) exhibits a composition and charge state distribution closely resemblinglow-speed solar wind (adapted from Neukomm, 1998).

Axford et al., 1999). In this picture, the variability of the composition of the low-speed solar wind is not primarily due to the variation of some dynamical processesoccurring in the acceleration region such as Coulomb friction or wave-particleinteraction, but rather to the variability of the composition in its building blocks;i.e., the loops which are intermittently tapped and emptied into the corona, fromwhere ions emanate along open magnetic field lines into the interplanetary plasma.

2. Building Blocks of the Solar Wind at Solar Maximum

Since large-scale loops are considered to be the principal agents feeding the tran-sient, CME-related solar wind, it seems natural to assume a relationship betweenthe CME-associated wind and the slow solar wind, at least with respect to itscomposition. Correspondingly, this review focuses on the composition of coro-nal mass ejections as observed during their transit across spacecraft. The varioustypes of composition observed in CMEs are considered to be extreme cases and torepresent the underlying texture for variability of the low-speed solar wind. From

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this simplifying picture we will make an attempt to compare the variability of thetwo types, CME-related wind and low-speed wind, using a scheme of decayingturbulent structures in the slow solar wind.

2.1. COMPOSITION OF SOLAR WIND ASSOCIATED WITH CORONAL MASS

EJECTIONS

Neukomm (1998) has derived the composition of forty CME-related solar windflows from Ulysses/SWICS recorded from 1990 through 1996 and compared theabundance pattern with those of slow and high-speed solar wind. An impressionof the variability of CME abundance is given in Figure 1. Neukomm distinguishesthree groups, which are indicated by different symbols. From the analysis it emergesthat the CME-related composition of the solar wind can not simply be representedby the conventional simultaneous relative variation of the two groups of low- andhigh-FIP elements; rather, it is the result of the superposition of FIP-fractionationand mass fractionation. Where do these processes occur? The clear connection oflow freeze-in temperatures of elements contained in the magnetic clouds of CMEswhich originated from high latitudes suggests that these clouds carry material rep-resentative of the corona at the location of freezing of charge states, i.e., at about1 R�. This does not necessarily imply that high-latitude CMEs are replenished withcoronal material at altitude of 1 R� above the solar surface. However, it shows thatthere must be some communication of conditions in the ambient corona, namely theelectron temperatures or electron energy distributions, into the interior of magneticclouds.

2.2. MASS FRACTIONATION IN CORONAL LOOPS

Wurz et al. (1997) reported mass-dependent variations of CME-related solar windabundances, which they ascribed to mass-dependent fractionation, superposed onthe normal FIP-related fractionation. The process is supposed to operate in theloop from which the CME-bubble finally forms. More recently, Wurz et al. (2000)reproduced the observed abundance pattern of one CME-event in a physical model.The abundance pattern can be interpreted as the result of a process, whereby highlycharged particles diffuse across a concentration gradient, which is perpendicularto the magnetic field in loop-like structures. Figure 2 illustrates the abundancesfound in the CME (normalized to typical interstream solar wind abundances) incomparison with the calculated model values (full dots) as a function of the atomicmasses of the relevant elements. In order to reach sufficiently strong depletion oflight particles at the prevailing densities, it is important to keep the ion temperaturerelatively low (typically 105 K). The bulk of the electrons remains at a similar tem-perature whereas a small fraction of electrons must reach temperatures as high as2.1×106 K in order to produce the observed ionization states of minor species. Theauthors reconcile the apparent discrepancy of two different electron temperatures inan otherwise collision-dominated regime by arguing that charged particles in loops

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Figure 2. Comparison of the observed elemental pattern of the January 6, 1997, CME (normal-ized to interstream abundances) with the result of a diffusion model. Diffusion occurred during27.5 hours across a magnetic structure of 10 km scale. The proton density within the loop wasassumed to be 4.6 × 1017 m−3, the proton-, electron- and ion-temperatures were assumed asTp = Te,core = Tion = 105 K, the magnetic induction amounted to B = 10−3T , the halodistribution of electrons had a temperature of Te,halo = 2.1 × 106 K (adapted from Wurz et al.,2000).

are continuously heated with MHD waves, thus maintaining a regime far fromthermodynamic equilibrium. We note in passing that such a scenario seems not un-realistic in view of the low phase space densities of suprathermal electrons and thesmall overlap between the two electron populations. Having a fraction of electronsat a temperature, which differs from the temperature of the bulk of the electronsand ions might be relevant and applicable for deriving ionization temperatures andelemental abundances from optical observations of coronal loops. However, to ourknowledge, a self-consistent physical model of a wave-heated stationary loop witha core/halo energy distribution of electrons still awaits development.

2.3. ISOTOPE FRACTIONATION IN CORONAL LOOPS

In the previous sections it has been shown that the CME-related, transient solarwind is not simply the result of a FIP-effect with alternating strength but thatmass fractionation, possibly occurring in coronal loops, can affect the solar windcomposition as well. Considering such loops as the structural elements feedingthe solar wind at solar maximum activity, another issue merits attention. With theanalysis of CME-related wind with enhanced time resolution it has recently beenfound that strong isotopic enrichments occur not only in the relatively minor pop-ulation of solar energetic particles heated and accelerated in impulsive flares, butalso in the bulk of particles flowing at solar wind energies. Ho et al. (2000) foundsix periods of enhanced 3He2+ associated with the passage of CME-ejecta. The3He-enrichment relative to normal solar wind composition ranged from a factor offour to a factor of ten. All periods of enhanced 3He were associated with unusualcharge state distributions of helium, carbon, oxygen, and iron. The fact that weakly

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Figure 3. Correlation of blob size and energy content as result of a simulation experiment. Thecascade begins with blobs of 1 R�, which decay after some time and generate a pair of new blobswith different size but conserving the total energy which is distributed according to a Kolmogorovlaw. The distributions surviving out to 1 AU and 45 R�, respectively, is strongly skewed towards bigstructures with the longest lifetimes.

charged ions such as C2+, He+, and Fe6+ would appear together with typical solarwind ions such as O6+ can be explained in terms of rapidly expanding plasmoidsin the solar corona. Such a process has, in fact, been postulated by Neukomm andBochsler (1996). Mechanisms enriching 3He together with heavy ions in impulsiveflares have been designed and discussed by Riyopoulos (1991), Temerin and Roth(1992), and Miller and Vinas (1993). The presence of isotopically enriched 3He inthe CME-related wind suggests that the impulsive flare mechanism may operate –although to a minor degree – in any building block contributing to the active-regionsolar wind, including the regular low-speed solar wind. The occasionally strongenrichment of 3He in the thermal solar wind population is another indication forthe expected complexity of the solar wind feeding process. This complex picture,if correct, will emerge more clearly, once the necessary instrumental sensitivityand time resolution is achieved. The above observations should also be taken asa warning sign against the prejudice of homogeneous isotopic composition of thesolar wind at large (e.g., Bochsler and Kallenbach, 1994).

3. Synthesis of Solar Wind Composition at Solar Maximum

In an attempt to investigate the natural variability of the low-speed solar wind,building blocks as defined in the previous section have been used to simulatecompositional variations in a toy model. Consider a simple situation with loopsproducing blobs of different compositions, which propagate into the interplanetaryspace. Will these blobs retain their compositional identity as they move throughthe inner heliosphere, or will they decay and form new structures according to the

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concepts of MHD-turbulence? Assuming the latter case, the model simulates struc-tures erupt from the solar surface and propagate with typical solar wind speeds.The structures expand with increasing solar distance and split into randomly sizedfragments. The energy contained in the original blob is split according to the sizeof the fragments following a Kolmogorov law, i.e., according to E ∝ l5/3. Thecharacteristic decay time is determined from the size l of the blob and the turnovervelocity v, which in turn is directly related to the energy E. The expectation valueof the decay time τ scales with the turnover time, i.e., τ ∝ l/v. With this schemea hierarchy of blobs is generated. The result of a first test is illustrated in Figure 3.The biggest blobs have the longest life expectancy, whereas small blobs tend todecay rapidly into smaller blobs and their compositional identity is soon lost ina smear of background of average slow wind composition. Simulation of an arti-ficial time series with variable composition using the compositional variations asobserved in CMEs confirm this expectation. Thus far the toy model reproducessome dynamic features of MHD turbulence in the solar wind, but with the givenset of parameters it does not reproduce the compositional variations quantitativelyas typically observed in the slow solar wind. Possible improvements are underinvestigation.

4. Space Climate in the Past

Mechanisms which shape the solar cycle are far from being understood. Predic-tions of solar activity are based entirely on phenomenology. The fact that onlya few centuries ago, during several decades, no sunspots were observed, whilethe galactic cosmic-ray intensity still underwent some modulation by the varyinginterplanetary magnetic field or by coronal mass ejections, justifies the questionhow the amplitude of the solar activity will vary in the future. The past and thepresent solar cycle have yielded a far better picture of heliospheric energetic parti-cle generation and acceleration than ever existed before, and compositional studieshave been particularly useful. Unfortunately, the lunar record of past heliosphericparticle flows is difficult to interpret. Understanding the energetic particle recordis particularly difficult as there is an unexplained discrepancy of more than anorder of magnitude between the record attributed to the long-range fluence andthe present-day fluxes.

Wimmer-Schweingruber and Bochsler (2000) have recently discussed the pos-sibility of some non-negligible fraction of the implanted particle record in lunarsoils originating from pick-up ions. Among others Talbot and Newman (1977) havestudied the likelihood of passages of the solar system through dense molecularclouds in the past and concluded that such events occurred quite often during thesolar lifetime. The mechanism of Wimmer-Schweingruber and Bochsler (2000)makes use of the fact that an increased inflow of interstellar neutrals and dust grainswill have two effects that reinforce each other: An enhanced production of pick-up

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ions and the corresponding increased mass loading of the interplanetary magneticfield will lead to enhanced turbulence (e.g., Chalov and Fahr, 2000). The two effectscombined then lead to an enhanced pick-up ion flux and to an even more enhancedACR-flux at 1 AU. Whereas encounters of the solar system with dense molecularclouds very likely produce different compositional signatures in suprathermal andenergetic heliospheric particles, it seems less likely that enhanced solar activity inthe past will have produced noticeable variations in the energetic heliospheric par-ticle composition. From the considerations in the previous sections, we concludethat although some differences between the CME-related, transient solar wind andthe normal low-speed solar wind exist on short time scales, fundamental changesof compositional pattern in long-time averages are less likely to occur. The veryactive Sun continues to eject solar particles with essentially solar abundances. Todetect the subtle, solar-cycle-related variations in the long-time records in lunar orasteroidal soils is a formidable task, which probably will not be achieved in thenear future.

Acknowledgements

The author gratefully acknowledges helpful discussions with Johannes Geiss, EckartMarsch, Rosmarie Neukomm, Ruedi von Steiger, Robert Wimmer-Schweingruberand with Peter Wurz. This work was supported by the Swiss National ScienceFoundation.

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