Leske, R A

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29th International Cosmic Ray Conference Pune (2005) 1, 107110Ultra-Heavy Elements Above 10 MeV/nucleon in Solar EnergeticParticle EventsR.A. Leske , C.M.S. Cohen , A.C. Cummings , R.A. Mewaldt , E.C. Stone ,M.E. Wiedenbeckand T.T. von Rosenvinge(a) California Institute of Technology, Mail Code 220-47, Pasadena, CA 91125 USA(b) Jet Propulsion Laboratory, MS 169-327, Pasadena, CA 91109 USA(c) NASA/Goddard Space Flight Center, Code 661, Greenbelt, MD 20771 USAPresenter: R.A. Mewaldt (ral@srl.caltech.edu), usa-leske-R-abs2-sh12-posterRecent measurements from Wind/LEMT and ACE/ULEIS show that elements heavier than Zn (Z=30) can beenhanced by factors of  100 to 10,000 in impulsive solar energetic particle (SEP) events at energies belowseveral MeV/nuc. Using data from the Solar Isotope Spectrometer (SIS) on ACE at energies from  10 to 100 MeV/nuc, we find that even large gradual events at these energies often are very iron rich and mayappear similar in composition to impulsive events. Since August 1997, SIS has recorded  1000 nuclei with Zof 29 or greater, including measurable quantities of Zn, Ge and Se (Z=30, 32, and 34). Although quantitativeanalysis to obtain ultra-heavy abundances remains to be done, we present sample observations that establishthe potential of extending ultra-heavy SEP measurements up to higher energies in order to test models ofacceleration and abundance enhancements in both gradual and impulsive events.1. Introduction and Data AnalysisThe abundances of ultra-heavy (UH) elements (with Z  30) relative to oxygen have been found to be enhancedby surprisingly large factors (  100 to 10,000) in some impulsive solar energetic particle (SEP) events at  400keV/nuc by ACE/ULEIS [1] and at 3.3-10 MeV/nuc by Wind/LEMT [2], with much smaller (factors of a few)enhancements observed in some large gradual events below 10 MeV/nuc [2]. If the observed increase in Fe/Oratios with increasing energy above 10 MeV/nuc in some large gradual events [3] is due to the presence of flarematerial, either via direct access from the flare site [4] or by way of reacceleration of flare suprathermals atquasi-perpendicular shocks [5], one might expect to find very large UH enhancements in some gradual eventsat higher energies.The Solar Isotope Spectrometer (SIS) instrument on ACE consists of two stacks of silicon solid state detectorsand is primarily designed to measure the isotopic composition of species from He (Z=2) through Ni (Z=28)at energies of  10 to several hundred MeV/nuc using the dE/dx vs. residual energy technique [6]. To achievethe required resolution over this dynamic range, the pulse height analyzers were designed specifically for thislimited interval in Z and saturate above certain energy deposits in the detectors, typically for particles heavierthan Z  40 (Zr) at large incident angles penetrating deep into a detector segment. The resulting loss of detectionefficiency gradually increases for Z  40 and has not yet been fully modeled in detail.Although limited in UH sensitivity, SIS attains excellent elemental resolution in this regime compared withprevious studies at lower energies [1, 2]. Figure 1 shows histograms of nuclear charge in the UH range forall particles recorded by SIS during the ACE mission to date. Standard data cuts were applied requiring thatparticles stop in the instrument and that multiple calculations of nuclear charge using different combinationsof energy-loss measurements be consistent in order to reject interacting particles and/or chance coincidencesof multiple particles. For the lowest energy (“range 0”) particles, only two detectors are triggered, resultingin only one charge determination with no consistency check possible. Under low rate conditions, this is not aproblem; however at high rates (such as during very large SEP events) a large-angle, out-of-geometry nucleus108 R.A. Leske et al.Figure 1. Nuclear charge histograms of particles recorded by SIS from 27 August 1997 through 21 June 2005. Left: Datafrom ranges 0-7 (  10-168 MeV/nuc for Fe), center: data with cut on range 0 (see text), right: data from ranges 1-7 (16-168MeV/nuc for Fe).such as Fe stopping in the first detector in coincidence with a H or He nucleus stopping in the second detectorcan mimic the signal pattern expected for range 0 ultraheavies. This background can be greatly reduced byrequiring that the energy deposit in the second detector for any range 0 particle exceeds the maximum expectedfor a He nucleus in that detector. (This drastic cut automatically rejects all range 0 He as well, but this isnot a concern for this study.) Applying this cut results in the center-panel histogram in Figure 1, which hasa noticeably smaller background in the Z=30-35 region. Rejecting all range 0 particles (right panel) furtherimproves the resolution but with a significant loss of statistical accuracy and energy coverage. For this studywe use the cuts illustrated in the center panel.2. DiscussionTaking the detected events from the center panel of Figure 1 and omitting the highest energy ranges to rejectthe bulk of the galactic cosmic rays results in the histogram shown in Figure 2. This represents a summationover many SEP events, both very large and small, without any accounting for variations in instrument livetime,sampling and telemetry limitations, UH detection efficiencies, or differences in energy intervals and/or spectralshapes for the different elements. Nevertheless, it is instructive to compare the relative peak heights withthose expected from standard solar system abundances [7]. Since SEP material is relatively underabundantin elements with a high first ionization potential (FIP), we have adjusted the expected abundances downwardby a factor of 4 for all elements with a FIP greater than 10 eV. Note that because SEP intensity spectra fallsteeply with energy and the measurements were made at a constant range for all elements, those elementslighter (heavier) than Fe will have lower (higher) energies and thus higher (lower) abundances than expectedusing a normalization at Fe, qualitatively as observed. From this figure, we conclude that the long-term, overallaverage composition of  10-50 MeV/nuc particles out to at least Z  40 agrees reasonably well (within a factorof  2) with standard solar system values.In the near future we will survey the entire data set in detail to study the UH composition of individual SEPevents as well as the average UH composition of various types of events (e.g., impulsive, Fe-poor gradual, andUltra-Heavy SEPs Above 10 MeV/nucleon ... 109Figure 2. Mission-to-date nuclear charge histogram from ACE/SIS for ranges 0-4 (corresponding to  10-50 MeV/nucat Fe), compared with standard solar system (SS) abundances [7] normalized to Fe (diamonds) and SS abundances afteraccounting for FIP fractionation (connected squares).Fe-rich gradual). This study is not yet complete, but the data at hand indicate that such a project is feasible andthat the results should prove interesting. This is illustrated by several examples in Figure 3. The 6 November1997 event (top left) shows clear peaks for Zn and Ge (Z=30 and 32); UH abundances from this event havebeen previously reported [8]. The large 21 April 2002 event (top right) shows no particles heavier than Z=34,indicating that even in an event this large, background contamination in the UH region is negligible. Note thata measurement of the Zn abundance will be possible in this event. By some standards, the 20 August 2002event (lower left) is the largest impulsive event of this past solar cycle [9]; in addition to Zn and Ge we find 15particles between Z=33 and 47 (As and Ag).The most remarkable UH event we have discovered so far in this study is that of 23 July 2004 (lower right). Thisis a very small event, with a GOES  10 MeV proton intensity which peaked at only  4 pfu; the 11 particlesbetween Z=33 and 50 (As and Sn) therefore cannot possibly be due to chance coincidence background. Thetime profile is very rounded and extends for 1.5 days, looking nothing like a classic “impulsive” event. Aseries of 4 very short duration M-class x-ray flares occurred during this time from an active region near centralmeridian (W04 to W20), but with no obvious particle velocity dispersion and UH particle arrival times spreadover nearly a day it is unclear which one or ones are associated with the SEP event. The 12-60 MeV/nucFe/O ratio is only about half that of the 20 August 2002 event and the Fe intensity about 50 times less, but theUH intensity is comparable, indicating that the UH enhancement does not correlate with the Fe/O ratio; this isconsistent with a study of other events in Wind/LEMT [2]. The Fe spectrum is very soft, with a power-law indexof  4.7, which is also consistent with the report that the largest UH enhancements are associated with eventswith the softest Fe spectra [2]. Simply taking the ratio of particles between Z=33 and 50 to Fe and normalizingto solar, without correcting for different energy intervals, reduced UH efficiency, or FIP fractionation (all ofwhich would serve to increase the enhancement) yields a (33  Z  50)/Fe enhancement of  80. Since the Fe/O110 R.A. Leske et al.Figure 3. Nuclear charge histograms over ranges 0-4 for 4 selected time intervals, each 1.5 days long. The 23 July 2004event (lower right) shows the largest UH enhancement uncovered in this study so far.ratio in this event is  0.76, or 5.7 times the average SEP value, this means the (33  Z  50)/O enhancement is 400-500! Clearly this event merits further study, but already it demonstrates that large UH enhancements canexist in SEP events at energies  10 MeV/nuc.3. AcknowledgementsThis work was supported by NASA at Caltech (under grant NAG5-6912), JPL, and GSFC.References[1] G.M. Mason et al., Ap. J. 606, 555 (2004).[2] D.V. Reames, Ap. J. Lett. 540, L111 (2000).D.V. Reames and C.K. Ng, Ap. J. 610, 510 (2004).[3] C.M.S. Cohen et al., Adv. Space Res. 32, 2649 (2003).C.M.S. Cohen et al., J. Geophys. Res., accepted (2005).A.J. Tylka et al., Ap. J. Lett. 558, L59 (2001).[4] H.V. Cane et al., Geophys. Res. Lett. 30, SEP 5-1, doi:10.1029/2002GL016580 (2003).[5] A.J. Tylka et al., Ap. J. 625, 474 (2005).[6] E.C. Stone et al., Space Sci. Rev. 86, 357 (1998).[7] K. Lodders, Ap. J. 591, 1220 (2003).[8] L.S. Sollitt et al., 26th ICRC, Salt Lake City (1999) 6, 143.[9] R.A. Leske et al., 28th ICRC, Tsukuba (2003) 6, 3253.M.E. Wiedenbeck et al., 28th ICRC, Tsukuba (2003) 6, 3245.

Ultra-Heavy Elements Above 10 MeV/nucleon in Solar Energetic Particle Events

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