Composition of energetic particles from solar flares

Adv. Space Res. Vol. 14, No. 10, pp. (10)589--(10)598, 1994 Copyright © 1994 COSPAR Printed in Great Britain. All rights reserved. 0273-1177/94 $7.00 + 0.00

Pergamon

COMPOSITION OF ENERGETIC PARTICLES FROM SOLAR FLARES T. L. Garrard and E. C. Stone 220-47 Downs Laboratory, California Institute of Technology, Pasadena, CA 91125, U.S.A.

ABSTRACT We present a model for composition of heavy ions in the solar energetic particles (SEP). The SEP composition in a typical large solar particle event reflects the composition of the Sun, with adjustments due to fractionation effects which depend on the fLrSt ionization potential (FIP) of the ion and on the ratio of ionic charge to mass (Q/M). Flare-to-flare variations in composition are represented by parameters describing these fraetionation effects and the distributions of these parameters are presented. INTRODUCTION The purpose of this review is to describe the composition of heavy solar energetic particles (SEP) in the context of potential radiation damage to personnel and/or equipment in space. A short overview below indicates the various factors that affect the abundances of energetic particles; then additional detail is provided for those terms that are specifically related to the heavy ion composition. This review is largely based on the work of Breneman and Stone I11 and similar work with the very large flares observed by the Galileo Heavy Ion Counter 121; we relate their work to the lighter ions, hydrogen and helium, in a preliminary fashion. General Overview Solar flares in general have been recently reviewed by Kabler 131, Lee 141, and Flueckiger 151. We discuss here abundances and fluxes or fluences of SEPs from flares, with emphasis on heavy ions of sufficient energy to penetrate (at least) the minimal shielding provided on most space missions. Mason/6/reviewed SEP composition in 1987. The abundance of a particular element in the energetic particle population of a given flare is a function of many variables: time, position, energy, and atomic number (Z) being the most obvious. Considerable work has been done on proton (hydrogen) flux as a function of time, position, and energy, since the protons are, by far, the major component of the SEPs, comprising more than 90% of the flux and -80% of the energy deposited in most flares. Hydrogen fluences, spectra, temporal behavior, etc. have been extensively studied in the work of J. Feynman et al. 17, 8, 9, 10/. The contribution of heavy elements to an energy loss spectrum, even for the relatively abundant helium, is small compared to the typical variations in the contribution of the protons. Also, the hydrogen and helium have larger ranges at the same energy per nucleon (or velocity) than the heavier elements. However, because biological damage and "single event upsets" of electronic components are non-linear in energy loss or linear energy transfer (LET), there has been much interest in extending this work to heavier elements. Some preliminary, unpublished work on

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T. L Garrard and E. C. Stone

alpha particles (helium) and heavier nuclei has been done at the Jet Propulsion Laboratory (JPL)/11, 12/. I n / 1 3 / a worst case model is reported. See also/14/. It has long been accepted that these three major compositional components, hydrogen, helium, and heavier nuclei, vary widely in ratio to each other, especially hydrogen. These variations are not well understood and can only be described statistically at this time. Variations in the relative abundances among heavier nuclei are relatively small (by comparison to hydrogen) and some understanding has been achieved at least for most larger flares. Two classes of solar flares have been identified and the variations of heavy ion abundances in the class which includes essentially all large flares were characterized by Breneman and Stone IlL HEAVY ION COMPOSITION As mentioned above, two classes of flares have been identified with quite different abundances. One class, frequently labeled impulsive flares or 3He-rich flares, shows compositions which are quite unusual by comparison to common standards such as the solar photosphere or meteorites. Particle fluences tend to be relatively small in this class of flares /15/. The other class, frequently labeled gradual events, tends to be much larger and are therefore of more concern for radiation damage. The gradual events also have compositions which are more simply related to the solar composition. The flare classes differ in many characteristics, including temporal behavior (hence "impulsive" and "gradual"), associated X, ~/, and radio emissions, etc., as well as composition. The dichotomy between these two classes of flares has been extensively described by Reames and co-workers/15,16,17,18 and references therein/. In this report, we will concentrate on the larger flares (the gradual events) because they are more relevant to radiation damage and better understood. The heavy ion composition in larger flares generally reflect a fractionated solar composition. Flareto-flare variations, as illustrated in Figure 1, are interpreted as being due to variations in the fractionation processes. Breneman and Stone /1/ identified these fractionation processes as the transport processes that move and accelerate the particles from the solar photosphere to the interplanetary medium. They identified two distinct processes, one dependent upon the ratio of ionic charge to mass (Q/M) and one upon first ionization potential (FIP). They parameterized these fractionation processes, measured the parameters, and used them to correct the SEP abundances and derive SEP-based solar abundances. In the section on composition variations below, we will use that parameterization to characterize the flare-to-flare variations in terms of the fractionation parameters and the known solar composition or the Breneman and Stone average SEP composition. Fractionation due to propagation and acceleration is expected to be dependent upon particle rigidity (momentum per unit charge), hence a function of Q/M. This fractionation presumably operates between the corona, the site of the flare acceleration, and the observation point in interplanetary space. There is no reason to expect it to have anything to do with the transport from the solar photosphere to the corona. Figure 1, which shows variations in composition as a function of Q/M, clearly organizes the variations between the flares selected. One of the flares is iron rich compared to the average, the other is iron poor. The iron (with small Q/M) varies in the opposite sense from elements with large Q/M (e.g., carbon), with silicon as the normalizing element. Breneman and Stone represented this fractionation as a power-law inaQ/M with power-law slope of ~, i.e., the abundance of any particular element is proportional to (Q/M) , or, Jz/Jsi

j~/jE

r Q z / M Z llX

= p(Z,00 -- [

Qsi / Msi

J

(1)

where Jz is the abundance of element Z in a particular flare and jE is the average abundance, the SEP average for element Z, where the average is taken over many flares. Note that only abundance ratios are reported in the literature for heavy ions and that we deal basically with abundance ratios in this paper. This power-law relation is equivalent to a straight line relationship on Figure 1. The Q values

Compositionof Solar EnergeticParticles

Ca Ar

(10)591

Na _ Mg N AI \u/ /Ne

2.0 _(a) 1.0 O. W

0.5

~t-2 . 0 e- 1.0

(b)

llj

0.5

0.2

0.4

0.6

Q/M Fig. 1. Abundances of heavy SEP ions in two particular, typical flares relative to the average SEP abundances, plotted as a function of ionic charge (Q) to mass (M) ratio on a log-log scale. The data were collected during the time periods (a) 1978 April 21-29 and (b) 1977 September 24-27. It is clear that the flare-to-flare variations are organized by Q/M, with a straight line given by the power law function (Q/M) ct.

used were measured by Luhn et al. /19/ as reported in Luhn's thesis/20/. These measurements are for a limited population of flares and a limited set of elements, of which Fe is the heaviest. Interpolation is guided by the calculations of Shull and van Steenberg/21/. Having demonstrated fractionation from flare to flare correlated with Q/M, it is reasonable to expect similar fractionation between solar (coronal) abundances and the average flare abundance. In Figure 2 we show the correlation between the Breneman and Stone average SEP composition and the spectroscopically determined photospheric composition (SPC), for low FIP elements only. (Note that the accepted spectroscopic Fe abundance has changed since the publication of Ill, see /27/.) The residual fractionation is small (ix = 0.29 + 0.22), but probably statistically significant. Thus, JzE / JsE JzP / JsPi = p(Z,0.29)

(2)

for low FIP elements where JP is the SEP-based photospheric abundance, which we will, for this

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T.L. Garrardand K C. Stone

paper, treat as interchangeable with the SPC spectroscopically determined photospheric abundance. If both low and high FIP elements are corrected according to this residual Q/M effect, then the resulting corrected average SEP abundances which we identify as SEP-based solar coronal abundances (SCA or jc) correlate well with coronal composition determined with other techniques /1, 22/. They also correlate well with photospheric abundances except for the residual FIP fractionation. Then, jE/jE jzC / j c

= p(Z,0.29)

(3)

for all elements, with either high or low FIP. Fe etc

Zn Cu I

I

,

I

11I" UJ

0.3

I 0.25

I 0.30

i 0.35

J 0.40

Q/M Fig. 2. Ratio of average SEP abundances to SPC (spectroscopic photospheric) model abundances, plotted as a function of ionic charge (Q) to mass (M) ratio. Only low-FIP elements are included.

This correlation of the ratio of coronal SEP average to solar photospheric composition (SCAJSPC) with FIP is illustrated in Figure 3. Fractionation due to transport within the solar atmosphere (with its significant embedded magnetic fields) from the photosphere to the corona likely depends on charge state, which is sensitive to FIP at photospheric temperatures (i.e., at these temperatures elements with high FIP are not ionized). Again, once the correlation is identified, the SEP abundances can be corrected for FIP fractionation to yield solar photospheric abundances. This correlation has been represented as a sloping step function, with a value of 1 at low FIP, a value of S at high FIP, and a value given by interpolation for intermediate elements (P, S, and C). Thus, JzC/JsC = f(Z,S) -

JP / JP

f l if Iz < Is S if Iz > Ic

(4)

interpolated for intermediate I

where Iz is the FIP of element Z; Is is the FIP of element S (sulfur), etc.; JP are the SEP-based photospheric abundances, which are very similar to the spectroscopic photospheric abundances (SPC); and the interpolation is described in /1/. (The special case, f(He,S), will be specified below, in

Compositionof SolarEnergeticParticles

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6.) For the correlation between the corrected SEP average (SEP-based coronal or SCA or ) abundances and the SPC abundances, the parameter S = 0.26 + 0.02. Breneman and Stone's 1986 work reported solar abundances based on this technique. Their abundances are, for some elements, significantly more precise than the best spectroscopic abundances, and have had significant effect on the standard composition summaries/22/.

j e • z U a t i o n AI ~,~Fe S PCI I-''

I '1'''

I I

I''''

I''''

I

1''

I

q

10

1 ~0.3 0.1 - , , I , , l l l l l

5

Jl,~,

~ I ~,

15

20

10

?

FIP Fig. 3. Ratio of corrected average SEP abundances to SPC (spectroscopic photospheric) model abundances, where the correction removes the small residual fractionation due to effects sensitive to Q/M, plotted as a function of first ionization potential, FIP in eV.

Since then, the Galileo observations of the large October 1989 flare group have demonstrated the utility of this analysis technique for unusually large flares/2/. Also, the continuing improvement in spectroscopic measurements of solar abundances have yielded improved estimates of the SPC abundances. Since the correlation of SEP with SPC is used to derive the SEP-based coronal and photospheric abundances, the improvements in SPC (primarily a change of -10% in the Fe abundance) will lead to modified SEP-based coronal and photospheric abundances. FLARE-TO-FLARE

VARIATIONS

Variation in abundances of heavy ions from flare to flare can be represented (for large, "gradual" flares) by variations in the parameters which characterize the fractionation effects. Clearly the average flare composition can be modeled by the Breneman and Stone average, and this is the starting point for constructing a model flare composition. A variant composition would be represented by selecting a variant value of the fractionation parameters, (x and S, and operating on the average composition with the functions described above, using the change in those two parameters from those of the average SEP composition, i.e.,

Jz / Jsi f(Z,S) jE / JSEi = p(Z,a) I'(2,0.26)

(5)

where JzE /JsEi is the average SEP abundance ratio tabulated in IlL Ideally, one would know the

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T.L. Garrard and E. C. Stone

probability of any given combination of ¢t and S, but the distributions are not sufficiently well measured to assign a probability to such a combination. In order to somewhat improve the statistical significance of these distributions, we have analyzed the McGuire et aL /23/ flare data, and the Galileo observations/2/to yield data from a total of 33 different flares. The distribution of these parameters is poorly determined with only 33 flares, many of which are statistically inadequate, but the distribution of cx is clearly non-Gaussian and the distribution of S may well be non-Gaussian. The S distribution is sufficiently narrow that variations should be of little significance. The distribution of the ot parameter for all 33 flares is shown in Figure 4. deviation (rms) of the distribution are reported in Table 1, along with those of the total data set. Similarly, the distribution of the S parameter is characterized in Table 1. Additional analysis and additional measurements these distributions are well characterized.

The mean and standard of the two larger subsets shown in Figure 5 and will be necessary before

Table 1

Breneman & Stone McGuire et al. total Breneman & Stone McGuire et al. total

¢x

S

mean

rms

0.012 -0.045 -0.007 0.26 0.27 0.23

1.87 2.04 1.97 0.10 0.09 0.11

The mean and standard deviation of the distribution (rms) is given for the two fractionation parameters, for the total data set and the two subsets.

u~

10

I

I

I

I

I

I

I

I

I

1

2

i

i ['}n

3

4

(9 ~-.L~-'~

6

G)~)

4

i_¢i

JQE

-i R -4

I

-3

,

-2

-1

0

5

6

Fig. 4. A distribution histogram of cx, the number of flares observed with a particular value of o~ is shown as a function of o~ Note that this form of display does not indicate uncertainties in the value of a particular measurement of or.

Comlx~ition of Solar EnergeticParticles

I

I

I

I

I

(10)595

I

F I

I E ,-,- 2 •"'l ,,,~_

I"1

Z ~ l I

] L i

0.1

I

0.2

0.4

0.6

S, FIP step factor Fig. 5. A distribution histogram of S, the number of flares observed with a particular value of S is shown as a function of S.

NORMALIZATION TO HYDROGEN, HELIUM Of course, to completely characterize the composition of a flare, one must include the two dominant elements, hydrogen and helium. The data set of 1231 includes measurements of these two elements as well as of the heavy ions. There appears to be very little correlation between the hydrogen abundance and the abundance of helium or heavy ions. Helium is significantly better correlated with the heavy ions than hydrogen is, but the relative (to SPC) helium abundance is depleted compared to the heavier ions. If we ignore the risk of using the correlation without understanding this depletion, then we can correct the He abundance for Q/M dependent fractionation and by a value for an additional correction factor (SHe) which relates the He to the other heavies, f(Z=He,S) = S . SHe

(6)

where the value of SHe is 0.617, based on our analysis of the data in/23/. A P R O C E D U R E F O R M O D E L I N G SEP C O M P O S I T I O N To model SEP composition for a particular hypothetical flare, we first select hydrogen and helium abundances, i.e., pick JH and JHe" We assume that these two abundances are not correlated. The hydrogen abundance (fluence) is selected from the distributions of Feynman et al. /7, 8, 9/. The helium abundance is selected in a similar fashion from the Goswami et aL 1241 data. A sample of these data is shown in Figure 6. Note that the JPL group is actively working with helium fluence dislributions, and their work should be in print reasonably soon. Next, after selecting the helium abundance, we can select an a and an S to determine the relative abundances of the other heavy ions. Then we can determine any Jz in terms of J~e, normalized to the already selected He abundance (rather than Si): Jz = JHe

JzP p(Z,a+0.29) f(Z,S) Jl.lPe p(He,a+0.29) f(He,S)

(7)

Any variation in S. is subsumed into variations of S; we do not have enough data to support e . . . . determination of stilt more parameter variations. Variations m a and S can be referenced to the mean and rms reported in Table 1 or the distribution shown in Figures 4 and 5, but, as mentioned above, the

(10)596

T.L. Garrard and E. C. Stone

..,

= i,,=

I

2 5 12 21 t'-34 (D

i

=

i

i

i i i i i I

i

i

I

i

i i ii

I

Helium fluence above 10 MeV/nucleon

L

\

o 50 m66 12. 79 88 95 98

"k_

I I IIIli

i

10 5

i

i

i

itill

I

10 6

I

I

I IIII1

t

10 7

I

I

I I Illl

10 8

He Fluence (per cm 2) Fig. 6. Integral probability plot of helium fluences from Goswami et al./24/. The fluences plotted are for the energy range above 10 MeV/nucleon.

distributions are not particularly Gaussian and additional work is needed to relate a probability level to any particular parameter value. ANTICIPATED NEW RESULTS We expect to be able to improve the statistical significance of these results using additional published abundance measurements, in particular, the recent work of Mazur et al. /25, 26/. We also look forward to new measurements from the Galileo Heavy Ion Counter, the SAMPEX MAST/PET instruments, and the ACE mission. ACKNOWLEDGEMENTS This work was supported in part by NAS7-918 and NAGW-1919. The data analysis work done by Caltech student Todd Rope was invaluable. The following people were generous with their time and copy machines in assisting us to collect and evaluate the literature reviewed here, and we appreciate it: J.Feynman, S. Kahler, G. Mason, J. Mazur, R. McGuire, R. Mewaldt, E. Moebius, D. Reames, and G. Spitale. REFERENCES I. H. H. Breneman and E. C. Stone, Solar Coronal and Photospheric Abundances from Solar Energetic Particle Measurements, Astrophys. J. 299, L57 (1985). 2. T . L . Garrard and E. C. Stone, Heavy Ions in the October 1989 Solar Flares Observed on the Galileo Spacecraft, Proceedings of the International Cosmic Ray Conference, Dublin 3, 331 (1991). 3. S. W. Kahler, Solar Flares and Coronal Mass Ejections, Annu. Rev. Astron. Astrophys. 30, 113, (1992).

Composition ~ Solar Energetic Particles

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4. M. A. Lee, Solar Particle Production, Proceedings of the International Cosmic Ray Conference, Dublin 5, 293 (1991). 5. E.O. Fluecldger, Solar Cosmic Rays, Nuclear Physics (Proc. Supp.) 22B, 1 (1991). 6. G. M. Mason, The Composition of Galactic Cosmic Rays and Solar Energetic Particles, Rev. Geophysics 25, 685, (1987). 7. J. Feynman and S. Gabriel, Editors, Interplanetary Panicle Environment, Proceedings of a Conference, JPL Publication 88-28, (1988). 8. J. Feynman, T. P. Armstrong, L. Dao-Gibner, and S. Silverman, New Interplanetary Proton Fiuence Model, J. Spacecraft 27, 403 (1990). 9. J. Feynman, T. P. Armstrong, L. Dao-Gibner, and S. Silverman, Solar Proton Events During Solar Cycles 19, 20, and 21, Solar Physics 126, 385 (1990) 10. J. Feynman, G. Spitale, J. Wang, Interplanetary Proton Fluence Model: JPL 1991, submitted to J. Geophys. Res. (1992). 11. G. Spitale, J. Feynman. and J. Wang, Solar Alpha Particle Model: Progress and Problems, Jet Propulsion Laboratory, Pasadena, JPL Interoffice Memo. 5717-92-60, (1992). 12. D. R. Croley, Solar Flare Heavy Ion Model, Jet Propulsion Laboratory, Pasadena, JPL Interoffice Memo. 5215-92-042, (1992). 13. J.H. Adams, Jr., and A. Gelman, The Effects of Solar Flares on Single Event Upset Rates, IEEE Trans. Nuc. Sci. NS-31, 1212, (1984). 14. D.L. Chenette and W. F. Dietrich, The Solar Flare Heavy Ion Environment for Single Event Upsets: A Summary of Observations over the Last Solar Cycle, 1973-1983, IEEE Trans. Nuc. Sci. NS-31, 1217, (1984). 15. D.V. Reames, Energetic Particles from Impulsive Solar Flares, Astrophys. J. Supp. 73, 235 (1990). 16. D.V. Reames, Acceleration of Energetic Panicles by Shock Waves from Large Solar Flares, Astrophys. J. 358, L63 (1990). 17. D. V. Reames, I. G. Richardson and L. Barbier, On the Differences in Element Abundances of Energetic Ions from Corotating Events and from Large Solar Events, Astrophys. J. 382, L43 (1991). 18. D. V. Reames, I. G. Richardson and K. P. Wenzel, Energy Spectra of Ions from Impulsive Solar Flares, Astrophys. J. 387, 715 (1992). 19. A. Luhn. B. Klecker, D. Hovestadt, G. Gloeckler, F. M. Ipavich, M. Scholer, C. Y. Fan and L. A. Fisk, Adv. Space Research. 4, 161 (1984). 20. A. M. Luhn, Die Ladungzustaende solarer energetischer Teilchen, Max Planck Institut fuer Extraterrestrische Physik, Garching bei Muenchen, Report 195, (1986). 21. J. M. Shull and M. van Steenberg, The Ionization Equilibrium of Astrophysically Abundant Elements, Astrophys. J. Supp. 45, 95, (1982). 22. E. Anders and N. Grevesse, Abundances of the Elements: Meteoritic and Solar, Geochimica et Cosmochimica Acta 53, 197, (1989). 23. R.E. McGuire, T. T. von Rosenvinge, and F. B. McDonald, The Composition of Solar Energetic Particles, Astrophys. J. 301, 938 (1986). 24. J.N. Goswami, R. E. McGuire, R. C. Reedy, D. Lal and R. Jha, Solar Flare Protons and Alpha Particles During the Last Three Solar Cycles, J. Geophys. Res. 93, 7195, (1988). 25. J.E. Mazur, G. M. Mason. B. Klecker, and R. E. McGuire, The Energy Spectra of Solar Flare Hydrogen, Helium, Oxygen, and Iron: Evidence for Stochastic Acceleration, Astrophys. J. in press, (1992)

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26. J . E . Mazur, G. M. Mason, B. Klecker, and R. E. McGuire, The Abundances of Hydrogen, Helium, Oxygen, and Iron Accelerated in Large Solar Particle Events, submitted to Astrophys. J. (1992). 27. P. Hannaford, R. M. Lowe, N. Grevesse, and A. Noels, Lifetimes in Fe II and the solar abundance of iron, Astron. and Astrophys. 259, 301, (1992).