Adv. Space Res. Vol.1, pp.65—68. 0273—1177/8l/0201—0065$05.O0/O

© COSPAR, 1981. Printed in Great Britain.

ON COMPOSITIONAL VARIATIONSOF HEAVY IONS DURING SOLARPARTICLE EVENTS

B. Klecker,1M. Scholer,’D. Hovestadt,1C. Y. Fan,2L. A. Fisk,3G. Gloeckler,4F. M. Ipavich4andJ. J. O’Ga11agher~

1 Max-Planck-Institutfür ExtraterrestrischePhysik,

8046 Garching, FRG2 University of Arizona Tucson AZ 85721 USA

~Universityof NewHampshire, Durham, NH 03824, USAUniversityof Maryland, CollegePark, MD 20742, USAUniversityof Chicago, IL 60637, USA

ABSTRACT

Intensity-time profiles of protons, alpha particles, and heavy ions (C, 0, Fe) inthe MeV/nucleon energy range have been analyzed for one solar particle event fol-lowing the solar flare on September 23, 1978. The data have been obtained with thewide angle double dE/dx-E sensor of the Max—Planck-Institut/tJniversity of Marylandexperiment onboard ISEE-3. We found time variations in the iron to helium ratio ofup to 2 orders of magnitude and a significant variation of the 0/He ratio duringthis event, whereas the C/0—ratio at the same energy/nucleon appears to be tine in-dependent. We investigated the influence of a rigidity dependent mean free path ininterplanetary space and of rigidity dependent coronal propagation on heavy ionratios during solar particle events. We found that both the magnitude and timescale of the ratio changes during the September 23 event cannot be explained by ri-gidity dependent interplanetary or coronal propagation alone. These ratio changesare probably caused by multiple injection at the sun.

INTRODUCTION

In recent years tine variations of the composition of low energy solarcosmic raysduring solar particle events have been studied by several authors (e.g. [i], [2],[3]). These ratio changes are generated during the propagation of the particlesfrom the flare site to the observer. Therefore, compositional changes of particlesof different rigidity, e.g. of protons, alpha particles, or partially ionized heavyions at the same energy/nucleon, are generally interpreted as being due to rigiditydependent processes in the solar corona and/or in interplanetary space.

In this paper we will present the relative abundances of heavy ions during one (outof several) solar particle event in 1978. The September 23 event was selected be-cause it shows the most dramatic ratio changes of heavy ions observed so far. Wewill compare the data with model calculations, extending previous work by includingthe effect of rigidity dependent release times at the sun. We will show that rigi-dity dependent propagation in interplanetary space and/or in the corona alone can-not explain the large ratio changes as observed during the September 23-27 solarparticle event The data have been obtained with the wide angle double dE/dx-Esensor of the Max-Planck—Institut/University of Maryland experiment onboard ISEE—3For a detailed description of the experiment see Hovestadt et al. [4],.

65

66 B. Klecker et al.

OBSERVATIONS

Figure 1 shows in the lower panel the intensity—time profile of the counting rateof heavy ions (Z > 2) with energies greater than 0.3 MeV/nucleon. The upper panelspresent the relative abundances of heavy ions (C, 0, Fe) and of protons in theenergy range 0.6—1.0 MeV/nucleon. All ratios have been normalized to helium (andnot to oxygen) because helium in this energy range is most probably fully ionized.The position of ISEE—3 during the September 23—27, 1978 time period was approxima-tely 1 5 x 106 km from the earth about 200 from the earth—sun line

The September 23 event was most probably

MPE/UoMd EXPERIMENT ISEE-3 produced by a 2B flare at heliographic

SEPT23 - SEPT 27 1978 position N35, W50 with the maximum in ~lot at 1000 UT. However, there had been two

w~~_--—~ ~ ~ more flares during the time period Sep—— + tember 23—26, labeled (2) and (4) in Fl—

101 — C/U gure 1. Flare (2) was of importance lB— —~ .J,,_. and occurred on September 24, 210 UT at

- the same MacMath Plage region as the~ i~’ T ~. ~fl-~ first flare. Flare (4) on September 24,

—± 0/He 1920 UT was located at N25W07. Label (3)— indicates the passage of an interplane—

~ io~• - — — tary magnetic field sector boundary and+ - Fe/He label (5) the transition of an interpla—

— - netary shock wave at the spacecraft,

102 p/He ± respectively.

.4 -f-_. .—‘ — The experimental results presented in Fi—

— —.I ______ — ______ gure 1 can be summarized as follows:~—--- —~-----.-. (1) During the September 23—27 event a

— I/ ‘—.- dramatic decrease of the Fe/He ratio

~ 10~ by almost two orders of magnitude isI observed.

~ia~ ~-~iJ~ ~ ~ _____ (2) The 0/He ratio varies by a factor of266 267 268 269 270 10 and is also decreasing.

DAY (3) The ratio changesoccur on the verylong time scale of 2—3 days.

(4) The decrease in the Fe/He ratio isFig. 1 Heavy ion ratios for the accompanied by a.decrease in the He/September 23—27 flare event, p-ratio. However, the variation of

the He/p—ratio is much smaller.(5) There is no systematic variation in

the dO-ratio.

DISCUSSION

In the following we will discuss several processeswhich could have produced theobserved variation of the heavy ion ratios.

The effect of rigidity dependent interplanetary propagation on the time variationof heavy ion relative abundances has beem discussed in great detail by Scholer atal. [2]. They showed that a rigidity dependent mean free path A (e.g. A R°’

5,R = particle rigidity) could produce ratio changesOf — 1 order of magnitude duringthe first 12—24 hours after flare onset. Thus the variation of the Fe/O—ratio ontime scales of 12—24 hours as observed for several solar particle events [2]could easily be explained by rigidity dependent interplanetary propagation, sub-ject to the requirement that Fe is not fully ionized, which is in agreement withdirect measuremamts of charge states [5], [9]. However, rigidity dependent

Compositional Variations of Heavy Ions 67

propagation in interplanetary space does not provide an explanation for the large(factor — 100) ratio changes on time scales of — 2—3 days as observed during theSeptember 23-27, 1978 event investigated here.

Ratio changes caused by adiabatic deceleration in interplanetary space due to dif-ferent injection spectra at the sun can also be excluded, because the spectralslopes for the different species turn out to be very similar (e.g. y = 1.5 for Feand 0 in the September 23—27 event).

Having dismissed interplanetary propagation as a possible source for the ratiochanges during the September 23—27 event we will now discuss the effect of rigidi-ty dependent coronal propagation. In a simple one dimensional model including dif-fusive transport and loss in the solar corona, the particle density 1(1, t) at atime t and distance 1 from the flare position is given by

1(1, t) = /(Ks t) exp C— ~ — (1)

K5 is the diffusion coefficient at the sun and 1/i- the loss rate [6]. The location

of the flare on September 23, 1978 lies well within the so—called preferred con—nection region [7], so that the effect of the diffusive propagation will be small.However, even with 1 = 0 in equation (1), (thus assuming direct connection for alltimes), there still remains the influence of a possibly rigidity dependent releasetime r. With equation (1), and 1 = Owe calculated the expected compositional va-riations, for different values of T. We used for the interplanetary propagationthe approximate analytical solution of Owens [8] which includes diffusion, con-vection, and adiabatic deceleration. We present in Figure 2 relative abundances,

10-1 -

io2~~2)

TIME [DAYS]

Fig. 2 Variation of heavy ion ratios, calculated for different release times atthe sun and assuming a constant mean free path in interplanetary space.

calculated with values of 2 and 8 hours (1), 2 and 16 hours (2), and 2 and 24hours (3) for the release time t. For the solar wind velocity and the scatteringmean free path in interplanetary space 350 km/sec and 0.05 AU, respectively, havebeen used. We see that for ratio changes of almost 2 orders of magnitude verylarge differences in the release time constants are necessary, implying a strongrigidity dependenceof r. Therefore, the charge state of oxygenmustbe considerablylower than 8+ to get any variation of the 0/He—ratio. However, a direct determina-tion of the charge state in the 1.1eV/nucleon—energyrange for the same flare, whichis now for the first time available [9], revealed a mean charge state of 7.3+0.1for oxygen. Therefore, it is unlikely that the observed ratio changesare due tocoronal propagation.

68 B. Klecker at al.

SUMMARY

We have shown that the large time variations of the Fe/He and 0/He-ratios in theSeptember23—27 solar particle event cannot be explained by rigidity dependentpro-pagation in interplanetary space or in the solar corona alone. Other possibilities,as e.g. multiple injection at the sun, are presently under investigation. A preli-minary analysis shows that injection of low energy particles during the flares onSeptember24 (labeled (2) and (4) in Figure 1) could account for the observed ratiochangesand intensity—time profiles, subject to the requirement that the relativeabundancesin these flares are different from the flare on September 23. This isnot unlikely, because flare (4) originated in a different region of the sun. How-ever, both flares on September 24 are only observed in H

0 and are not correlatedwith emission of large intensities of X—rays or high energy particles as e.g. theSeptember23 flare. Therefore, the question whether multiple injection at the suncan account for the observed ratio changesdeserves further investigation.

Acknowledgements

The authors are grateful to the many individuals at the Max-Planck—Institut, theUniversity of Maryland and other institutions who contributed to the successofthe ISEE-l/-3 experiments. This work has been supported by the Bundesministeriumfür Forschung und Technologie, Germany (contract RV 14-B8/74) and by NASA (con-tract NAS 5-20062).

References

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