Measurements of Heavy Elements and Isotopes in Small Solar Energetic Particle …authors.library.caltech.edu/27836/1/SLOaipcp01.pdf · 2012. 12. 26. · small solar energetic particle

Measurements of Heavy Elements and Isotopes in SmallSolar Energetic Particle Events

RL. Slocum*, E.R. Christian^ C.M.S. Cohen**, A.C. Cummings**, R.A. Leske**,R.A. Mewaldt**, E.G. Stone**, T.T. von Rosenvinget and M.E. Wiedenbeck*

*Jet Propulsion Laboratory, Pasadena, CA USA^NASA/Goddard Space Flight Center, Greenbelt, MD USA

** California Institute of Technology, Pasadena, CA USA

Abstract. Using the Solar Isotope Spectrometer on the Advanced Composition Explorer, we have examined the~ 10-20 MeV/nucleon elemental and isotopic composition of heavy (Z>6) energetic nuclei accelerated in 30small solar energetic particle (SEP) events which occurred between 31 March 1998 and 2 January 2001. We havemeasured the average heavy element content, the 22Ne/20Ne ratio, and the 26Mg/24Mg ratio in these events, andfind good agreement with past studies. We have categorized the events according to their 3He/4He ratios, and findsignificant enhancements in the neutron-rich heavy isotopes of Ne and Mg in the combined 3 He-rich data set:22Ne/20Ne=0.17±0.05 and 26Mg/24Mg=0.25±0.05. We discuss the implications of these measurements for theacceleration of energetic nuclei in SEP events.

INTRODUCTION

Measurements of the elemental and isotopic compo-sition of energetic nuclei from solar energetic particle(SEP) events can provide information about the accel-eration mechanisms in these kinds of occurrences. SEPevents have been classified into two main types: impul-sive and gradual [1]. During gradual events, which havea duration on the order of days, the abundances of heavySEPs have been shown to vary according to their charge-to-mass ratio [2]. The acceleration of the nuclei duringthese events is understood to occur at shock waves whichare driven by coronal mass ejections (CMEs) [3,4, 5, 6].On average, the heavy element composition of SEP ma-terial from gradual events is expected to reflect thatof the corona. Impulsive SEP events, however, com-monly contain enhancements in ~MeV Fe (^10x) andin r^MeV Ne, Mg, and Si (~3 x) relative to coronal abun-dances [7, 8]. Additionally, impulsive events frequentlyhave 3He/4He ratios which are up to 3-^ orders of mag-nitude larger than the solar wind value of 0.0004 [9]. Im-pulsive SEP events typically last on the order of hours.

The observed differences between impulsive and grad-ual SEP events suggest that each has a distinct ac-celeration mechanism. As stated above, gradual eventsare known to be associated with CME-driven shocks.However, while the 3He found in impulsive events maybe selectively enhanced by ion cyclotron wave reso-

nances [10, 11] or by cascading Alfven waves [12], theexact acceleration mechanism in impulsive events is notpresently well understood. In this paper we have exam-ined the heavy elemental and isotopic content of 30 smallSEP events. We have classified the small events into twosub-groups according to their 3He content, and have dis-cussed the implications of their elemental and isotopiccomposition for possible acceleration models.

EVENT SELECTION

The Solar Isotope Spectrometer (SIS) consists of a parrof silicon solid state detector telescopes which measurethe energy loss (E) and the rate of energy loss (dE/dx) ofincident energetic nuclei with 10<E< 100 MeV/nucleon.Then, using the measured values of dE/dx and E, thecharge and mass of the incident nuclei are derivedthrough an iterative mathematical algorithm [13].

The 30 SEP events selected for this study were cho-sen according to three criteria. First, a set of 98 daysbetween 31 March 1998 and 31 December 2000 wereidentified by requiring that their daily averaged SIS 11.0-26.5 MeV/nucleon Fe or 8.6-19.3 MeV/nucleon Mgfluxes be greater than a threshold of 5 x 10~7 particles/(scm2sr MeV/nuc), and that their daily averaged SIS 11.0-26.5 MeV/nucleon Fe fluxes be less than 2.5 x 10~6 par-ticles/(s cm2sr MeV/nuc). Second, the days which over-

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lapped with any of the known large, gradual SEP eventswere excluded from the study. Finally, the SIS ~3-5 MeV/nucleon He fluxes on the selected days were ex-amined subjectively for well-defined SEP event onsettimes and terminations. The 30 events with well-definedonset times and terminations became the final data set ex-amined in this study, while the days which did not showwell-defined time profiles in the ̂ 3-5 MeV/nucleon Heflux were discarded. Even so, due to the frequent occur-rence of this type of small SEP event, it is still probablethat some of the selected "good" events contain multipleinjections from the same region on the Sun. This problemis not expected to generate any significant uncertainty inthe results of this study, however, because we are only re-porting abundances which have been averaged over manySEP events.

TABLE 1. Event times, ^4.5-5.5 MeV/nucleon3He/4He ratios, and 11-22 MeV/nucleon Mg fluxes in(crn2sr s MeV/nuc)"1 for the 30 SEP events included inthis study.

Event # I Time (YeanDay) I ^e/^He I MgFlux01234567891011121314151617181921222324252627282930

1998: 93.1-104.81998:119.7-121.31998:121.3-122.21998:147.4-148.61998:167.7-170.61998:249.1-251.21998:251.6-254.21998:270.2-272.11998:294.1-297.01998:309.7-311.71998:311.7-316.41998:327.5-334.91999:147.3-148.41999:172.7-176.01999:320.7-321.81999:321.7-327.21999:362.0-364.22000: 9.2- 13.52000: 67.2- 68.82000: 82.4- 87.22000:125.3-126.72000:136.7-138.82000: 138.8-140.72000:144.7-147.92000:169.2-172.92000:175.3-177.22000:177.2-180.62000:224.0-225.42000:225.4-226.62000:226.6-228.6

0.0350.0450.0410.0660.0420.1000.1770.1100.0370.0450.0620.0410.0390.0640.0620.0430.0530.0370.0900.0410.0710.0380.0390.1960.0540.0510.0320.0380.0450.044

1.7e-071.9e-075.7e-077.6e-077.3e-072.5e-074.3e-07l.Oe-061.2e-071.6e-061.5e-062.3e-073.9e-07l.le-073.5e-073.4e-071.2e-063.9e-074.2e-07l.le-074.0e-066.4e-077.7e-071.9e-071.6e-071.4e-063.9e-07l.Oe-062.2e-077.1e-07

The final set of 30 events, listed in Table 1, were sub-divided into two groups according to their total ~4.5-5.5 MeV/nucleon ^e/^He ratios. Of the 30 events, 13had 3He/4He ratios greater than or equal to 0.065, while17 had 3He/4He ratios which were less than 0.065. In

general, the latter 17 events had 3He/4He ratios whichwere measured between 0.04 and 0.065. This apparentlower limit of 4% is probably due to spillover contam-ination of the 3He peak by the 4He peak. Although thenomenclature "3 He-rich" has generally been reserved forSEP events with 3He/4He>0.1, we have designated thefirst sub-group with 3He/4He>0.065 as "3He-rich" andthe second as "3He-poor" in order to preserve adequatemeasurement statistics in the 3 He-rich sub-group.

While the unweighted average ^4.5-5.5 MeV/nucleon 3He/4He ratio in the 3He-rich and3He-poor groups is ^12% and ^5% respectively, andimpulsive events are expected to have higher 3He/4Heratios than gradual events, it is not clear that our twoevent sub-groups are cleanly divided as such. For exam-ple, the unweighted average duration of the events in the3He-rich (3He-poor) subset is 2.4 (3.4) days, which islonger than expected for impulsive SEP events. It couldbe that while the 3He-rich events contain a relativelylarge fraction of material which was originally acceler-ated in impulsive SEP events, they are not necessarilyimpulsive events. This idea stems from the theory byMason et al. [15], which says that residual materialfrom past impulsive SEP events may provide a sourcepopulation for further acceleration by CME-drivenshocks associated with gradual events. Experimentalobservations of SEP composition during large gradualevents [14, 15, 16, 17] and during times of low solaractivity [18,19] have supported this theory.

LOW SOLAR ACTIVITY SPECTRA

The contributions to the small SEP event spectra fromgalactic cosmic rays (GCR) and anomalous cosmic rays(ACR) [20] have been estimated from SIS measure-ments during 241 days of low solar activity between9 April, 1998 and 25 December, 2000. The 241 dayswere identified by examining the proton fluxes from theElectron, Proton, and Alpha Monitor (EPAM) instru-ment on board ACE, and requiring that the daily aver-aged 1.06-4.75 MeV proton flux be less than 0.16 (cm2

sr s MeV/nuc)"1. Next, the 241 days were groupedchronologically into three time periods of relatively con-stant solar modulation, based on inspection of SIS ~7-10 MeV/nucleon O fluxes. These time periods were9 April, 1998-25 November, 1998,26 November, 1998-14 January, 2000, and 14 January, 2000-25 Decem-ber, 2000. For each of the three time periods, the av-erage heavy element spectra were extracted from thedaily measurements. These average heavy element spec-tra were weighted with the temporal fraction of the 30small SEP events which occurred during each time pe-riod, and were subsequently summed for subtraction

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from the small SEP event spectra. NEON AND MAGNESIUM ISOTOPERATIOS

HEAVY ELEMENT MEASUREMENTS

The average energy spectra for the elements C, O, Ne,Na, Mg, Al, Si, and Fe were measured for the 30 smallSEP events, as well as for the "3 He-rich" and "3 He-poor"subsets. These spectra were derived by taking the rawnumber of nuclei for each species detected during all ofthe SEP events combined, and dividing that number bythe total amount of instrument livetime. Finally, the OCRand ACR components of the spectra were subtracted asdescribed above to yield the SEP fluxes for each element.Figure 1 depicts the relative abundances of nine heavy el-ement ratios in the energy range ^11-22 MeV/nucleon,normalized to coronal abundances [21], for both the 3 He-rich and 3 He-poor SEP event data sets.

The isotope ratios of ~ 15-24 MeV/nucleon Ne and^16-26 MeV/nucleon Mg have been extracted from theSIS data using a superposition of Gaussian fits to the de-rived charge histograms. Figure 2 shows the two chargehistograms of Ne and Mg in these energy ranges. Withthe iterative charge calculation used, different isotopesof an element are assigned charges differing by M).lcharge units for each 1 amu mass difference. Thus thefive peaks representing 20Ne, 22Ne, 24Mg, 25Mg, and26Mg are shown in the figure, and have been fit witha function composed of five Gaussian curves of iden-tical width and uniformly spaced isotopes. Because the25 Mg peak was not well-constrained by the data in theenergy range ^16-26 MeV/nucleon, its height was fixedat 12.7% of the 24Mg peak height in accordance with

O<S7S: 3He/4He 2 0.065

S: 3He/4He < 0.065

NeC

MgC

FeC

Na Na A! FeMg Ne Mg Mg

FIGURE 1. 11.0-21.8 MeV/nucleon abundances of O, Ne,Mg, Si, and Fe with respect to C, as well as the Na/Mg,Na/Ne, Al/Mg, and Fe/Mg ratios, normalized to coronal abun-dances [21], for the 3 He—rich (open diamonds) and 3 He—poor(filled circles) data sets.

From the figure, it is apparent that while the compo-sition of the 3 He-poor data set generally reflects that ofthe corona, the 3 He-rich data set contains an average en-hancement in Fe with respect to C of ^4x, as well asmore modest enhancements in Ne, Mg, and Si of ^2x.The uncertainties shown in the plot are statistical in na-ture, and do not reflect any contribution from a possiblenon-statistical population spread in the events. This trendin the heavy element abundances is in agreement withpast studies, which associate large 3He/4He ratios in SEPevents with Fe, Ne, Mg, and Si enhancements at ^MeVenergies [7, 8], although the magnitude of the enhance-ments are smaller in the above SIS data. Also of note isthat since the coronal values chosen for normalization inthis figure were derived from large gradual SEP events,it is likely that the 3 He-poor events consist primarily ofsmall "gradual" events.

SIS Range 2

1 r

10 11 12 13Estimated Nuclear Charge

SIS Range 3

1 r

9 10 11 12 13Estimated Nuclear Charge

FIGURE 2. Estimated nuclear charge in the energyrange 14.7—19.3 MeV/nucleon (top panel) and 17.6-24.1 MeV/nucleon (bottom panel) measured using SIS duringthe 30 small SEP events. The 20Ne, 22Ne, 24Mg, 25Mg, and26Mg peaks have been fit with a superposition of five Gaussiancurves of identical width and uniformly spaced isotopes. The25Mg/24Mg ratio of peak heights has been fixed at the averagesolar system value of 0.127 [22]

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°Ne Spectra10 u

io-7

m-9

'• A SIS Flux During -Small SEP Events"

• OSISLow; * ' ; »

10Energy (MeV/nuc)

22Ne Spectra10 6

10-7

-8

Flux DuringSolar Activity"

. •:

1C

: A SIS Flux During -Small SEP Events'

OSISLow

-

r ^ 6 $ $

Flux DuringSolar Activity"

-

. ^« «1

10Energy (MeV/nuc)

100

FIGURE 3. Average 20Ne and 22Ne energy spectra for the 30small SEP events contained in this study (solid circles), and forthe days of low solar activity over the same time period (opencircles).

the average solar system value reported by Anders andGrevesse [22]. It is important to note that a factor of twovariation about the assumed abundance of 25Mg changesthe 26Mg/24Mg result less than the statistical uncertaintyin that ratio.

The measurements of the Ne and Mg isotope spectraare shown in Figures 3 and 4. The filled circles repre-sent SIS data during the 30 small SEP events, while theopen circles show the SIS measurements during periodsof low solar activity between the small SEP events (seeabove). Because the 26Mg quiet-time spectrum was diffi-cult to extract below ^40 MeV/nucleon due to statisticallimitations, the higher energy data points were fit witha power-law function which was extrapolated down tors.20 MeV/nucleon. Conversely, the Ne quiet-time spec-trum contains a contribution from the ACRs which re-sults in the beginning of a turn-up in the quiet-time spec-trum at about 20-30 MeV/nucleon. All of the isotopespectra show clear low-energy (<30 MeV/nucleon) turn-ups in the SEP event spectra, which is evidence for thepresence of solar particles in numbers well above the lowsolar activity background.

With the event spectra and low solar activity spectrashown in Figures 3 ,̂ one can derive the small SEP event(with OCR and ACR components subtracted) ̂ Ne/^Neand 26Mg/24Mg isotope ratios. This has been done forboth the 3 He-rich and 3 He-poor event data sets. Figure 5shows the results of the isotope ratio measurements be-low ~30 MeV/nucleon for each data set. On each plot,the average of the two data points, with one standarddeviation of statistical uncertainty, is represented by thegrey shaded area.

From the two top panels in Figure 5, it is apparentthat the average SEP 22Ne/20Ne ratio is at least slightlyenhanced over the solar wind value of 0.073 [23] for boththe 3 He-rich and 3 He-poor event data sets. In the data setwith 3He/4He>0.065, the 22Ne/20Ne ratio is found to beenhanced over the solar wind value by a factor of ̂ 2.4 x,with significance at the 20 level, while in the 3He-poordata set the ratio is only enhanced by la. Similarly, inthe bottom panels the 26Mg^4Mg ratio in the 3 He-rich

10

10

-64Mg Spectra

10-8

10_g

• SIS Flux During :Small SEP Events

OSIS Flux DuringLow Solar Activity

10 100

10

> 10

-6

Energy (MeV/nuc)

26Mg Spectra

10-8

10_g

• SIS Flux DuringSmall SEP Events'

OSIS Flux DuringLow Solar Activity

10Energy (MeV/nuc)

100

FIGURE 4. Average 24Mg and 26Mg energy spectra for the30 small SEP events contained in this study (solid circles),and for the days of low solar activity over the same timeperiod (open circles). The solid line represents the extrapolatedgalactic cosmic ray (GCR) spectrum from the fit with a power-law function to the three highest energy low solar activity datapoints on the plot.

180


0.40

15 20Energy (MeV/nuc)

15 20Energy (MeV/nuc)

0.40

o.oo E10 15 20

Energy (MeV/nuc)15 20Energy (MeV/nuc)

FIGURE 5. Top panels: 22Ne/20Ne ratios for the 3He-rich and 3He-poor data sets. The horizontal dotted line respresents thesolar wind value of 0.073 [23], Bottom panels: 26Mg/24Mg ratios for the 3He-rich and 3He-poor data sets. The horizontal dottedline respresents the average solar system value of 0.127 [22]. The shaded regions in each plot denote the average value of the twodata points, with an uncertainty of one standard deviation.

SEP events shows an enhancement over the solar systemvalue of 0.127 [22] by a factor of ^2 x, at a significancelevel of 2,50. The data set with 3He/4He<0.065 yields ameasurement of 26Mg/24Mg which is consistent with thesolar system value.

While the 3 He-rich events have significant enhance-ments of neutron-rich heavy isotopes of Ne and Mg, it isimportant to note again that our definition of "3 He-rich"is less restrictive than the widely-accepted requirementthat 3He-rich SEP events have 3He/4He>0.1. For thisreason, our "3 He-rich" subset of events may contain asmaller fraction of impulsive SEP material than it wouldhave had with a more stringent cut on the 3He content. Itis possible that the heavy isotopic enhancements wouldhave been even larger if statistics had allowed for such astudy.

CONCLUSIONS

The average heavy element content of these 30 smallSEP events has been shown to change significantly with

the event 3He/4He ratio. Of the 30 events selected forthe study, the 17 with 3He/4He<0.065 have an average~ 11 -22 MeV/nucleon heavy element composition whichis very similar to the solar corona. The other 13 3He-richevents contain average enhancements in Fe/C of ̂ 4, andenhancements in Ne/C, Mg/C, and Si/C of ~2. This trendis in qualitative agreement with past studies of 3He-richevents at lower energies [7, 8].

The average ^15-25 Me V/nucleon Ne and Mg iso-topic content of these SEP events also changes withthe 3He/4He ratio. The 22Ne/20Ne ratio increases from0.13±0.06 in the 3He-poor events to 0.17±0.05 inthe 3 He-rich data set, which is consistent with pastmeasurements of "3 He-rich" events at ^MeV ener-gies (22Ne^°Ne=0.29±0.10) [24]. Similarly, we findthat the ^Mg/^Mg ratio increases to 0.25±0.05 inthe 3 He-rich data set, consistent with previous studies(26Mg/24Mg=0.36±0.21) [24].

The observations of 3 He-rich SEP events with en-hancements in neutron-rich heavy isotopes is consistentwith the model of Miller [12]. This model predicts thatin 3 He-rich events, isotopes with a lower charge-to-mass

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ratio should be enhanced relative to those with a higherratio due to interactions with cascading Alfven wave tur-bulence. While the exact magnitudes of the expected en-hancements have not been determined, we find that theisotopic enhancements reported using SIS data are inqualitative agreement with this model and are quantita-tively consistent with those of past measurements.

ACKNOWLEDGMENTS

This research was supported by NASA at Caltech(under grant NAG5-6912), JPL, and GSFC. We thankthe EPAM science team for providing the proton fluxesused to define the time periods of low solar activity inthis paper.

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10.

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12.

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Measurements of Heavy Elements and Isotopes in Small Solar Energetic Particle …authors.library.caltech.edu/27836/1/SLOaipcp01.pdf · 2012. 12. 26. · small solar energetic particle