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

SH14A-07. Measurements of Ultra-Heavy Elements in Solar Energetic Particle Events Above 10 MeV/nucleon. R.A. Leske , C.M.S. Cohen, A.C. Cummings, R.A. Mewaldt, and E.C. Stone, California Institute of Technology, Pasadena, CA USA M.E. Wiedenbeck

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

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  1. SH14A-07 Measurements of Ultra-Heavy Elements in Solar Energetic Particle Events Above 10 MeV/nucleon R.A. Leske,C.M.S. Cohen, A.C. Cummings, R.A. Mewaldt, and E.C. Stone, California Institute of Technology, Pasadena, CA USA M.E. Wiedenbeck Jet Propulsion Laboratory, Pasadena, CA USA T.T. von Rosenvinge NASA/Goddard Space Flight Center, Greenbelt, MD USA

  2. Introduction/Motivation Observations from Wind/LEMT (Reames, ApJL 540, L111 [2000]) at 3.3-10 MeV/nucleon and ACE/ULEIS (Mason et al. ApJ 606, 555 [2004]) at ~400 keV/nucleon have found surprisingly large enhancements (factors of ~100-1000) in the abundances of ultra-heavy (UH) elements (with Z>30) relative to O in some impulsive SEP events. Flare material may be present in large gradual events, and may dominate the composition above 10 MeV/nucleon, either via direct access from the flare site (Cane et al., GRL 30 [2003]) or by way of acceleration of flare suprathermals at quasi-perpendicular shocks (Tylka et al., ApJ 625, 474 [2005]). Using ACE/SIS we cannot study the full range of UH nuclei since the PHAs saturate for stopping particles with Z~40 or so (with greatly reduced efficiency beyond, which has not yet been properly modeled in detail). However, we can probe the low end of the UH regime at energies inaccessible to the previous studies (>10 MeV/nucleon) with significantly better resolution.

  3. ACE/SIS can observe at least the first UH element group bump reported at lower energies by Wind/LEMT and ACE/ULEIS Wind/LEMT 3.3-10 MeV/nucleon Reames, ApJ 540, L111 (2000) Mason et al., ApJ 606, 555 (2004)

  4. On average (i.e., summing over all detected particles from large and small SEP events as well as quiet time, mission-to-date) the ~10-50 MeV/nuc UH composition out to at least Z=40 appears similar to that of standard solar system abundances…

  5. …but there is significant variability in UH abundances from one SEP event to another.

  6. Most >10 MeV/nuc UH nuclei seem to arrive on days with high count rates (i.e., during large SEP events) and have abundances that cluster near the standard solar values.

  7. Browsing through Z vs time plots gives a sense of the time distribution of UH particles. We see a few UH nuclei in the large 15 April 2001 gradual event, and about as many in the much smaller impulsive event of 14 April.

  8. The 20 August 2002 event was the largest impulsive event observed above 10 MeV/nucleon this solar cycle. Along with the other unusual impulsive events of August 2002, it included many UH nuclei.

  9. The most extreme UH-rich event we’ve found so far at >10 MeV/nuc occurred on 23 July 2004. The number of UH particles is similar to that in the 20 August 2002 event, but the Fe intensity is ~50 times lower.

  10. Particle time profiles for the 23 July 2004 event are all “gradual”. The 3He/4He ratio at several MeV/nuc is a few % at most. The 4 impulsive M-class flares during this period originate near central meridian (W04). Wind/WAVES report a poorly-defined type II and clear type IV burst.SOHO finds a halo CME and partial halo within 2 hours of each other, brightest towards the WSW.

  11. Reames & Ng have made a detailed survey of UH composition in gradual events (through mid 2003) and impulsive events (through 2002) below 10 MeV/nucleon.

  12. During the time periods of the events in the Reames & Ng study, we find Zn above 10 MeV/nuc in many of the gradual events and several of the impulsive events, with Zn/O correlated with Fe/O. The average enhancement of Zn is similar to that of Fe in both gradual and impulsive events.

  13. Z=34-40 enhancements above 10 MeV/nuc seem similar to those observed at lower energies, for both gradual and impulsive events. On average, this enhancement is ~100 times greater in impulsive events than in gradual events.

  14. Summary/Conclusions • Long-term average composition of ~10-50 MeV/nucleon particles from the Fe group up through Z~40, summing over numerous large and small SEP events, appears to agree reasonably well with standard solar system values. • Most large gradual SEP events (25/27 with finite detections) and many large impulsive events (10/17) with UH composition studied by Reames & Ng at 3.3-10 MeV/nucleon also have detectable UH particles (at least Zn) at 10-50 MeV/nucleon. • Preliminary results indicate that impulsive events above 10 MeV/nucleon are more enriched in UH abundances than are gradual events, although in an absolute sense most UH nuclei arrive during the largest (gradual) events. • Although many of its characteristics appear to make it a gradual event, the 23 July 2004 event has a Z=34-40 enhancement (relative to O) of ~350 at >10 MeV/nucleon and seems even more enriched for Z>40. • Much more work remains to be done to characterize UH efficiencies in SIS and obtain quantitative UH abundance enhancements.

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