1 SEP Spectral and Compositional Variability at High Energies Tylka et al., ApJ 625, (2005) Tylka et al, ApJS 164, (2006) Tylka and Lee, ApJ 646, (August 1, 2006) Tylka et al., poster at this conference Allan J. Tylka US Naval Research Laboratory, Washington DC

2 A Model for Spectral and Compositional Variability at High Energies in Large Gradual SEP Events A. J. Tylka & M. A. Lee, ApJ, August 2006 Tylka et al Event-to-event variability in high-energy SEP spectral & compositional characteristics due to the interplay of two factors:  Evolution in the geometry of the CME-driven shock as it moves outward from the Sun, (quasi-perpendicular v. quasi-parallel)  A compound seed population, typically comprising at least suprathermals from flares and suprathermals from the corona and/or solar-wind.  Bn vs. R S

3 Fe/O vs. Energy Spectral Variability 3 He/ 4 He vs. Energy Average Fe/O Enhancement vs. Energy vs. Energy A Model for SEP Variability at High Energies, A. J. Tylka & M. A. Lee (ApJ, Aug. 2006) The model addresses a number of issues in high-energy SEP phenomenology, with both qualitative and quantitative successes.

4 Model parameters chosen to match spectral shape of O and Fe (next slide). Using nominal composition and charge states, the model reproduces the SEP heavy-ion fractionation effects discovered by Breneman & Stone. C, N, O, Ne, Mg, Si, S, Ca, Fe SIS data provided by C.M.S. Cohen A Model for SEP Variability at High Energies, A. J. Tylka & M. A. Lee (ApJ, Aug. 2006) The Origin of Breneman & Stone (1985) Fractionation

5 Model parameters were selected to roughly match spectral shapes of Fe and O above a few MeV/nucleon. Model normalized to oxygen data at ~8 MeV/nucleon Normalizations of all other species fixed automatically by the model. A Model for SEP Variability at High Energies, A. J. Tylka & M. A. Lee (ApJ, Aug. 2006) The Origin of Breneman & Stone (1985) Fractionation

6 These calculations are the first quantitative explanation of the Breneman & Stone effect. A Model for SEP Variability at High Energies, A. J. Tylka & M. A. Lee (ApJ, Aug. 2006)

7 Enhanced Fe/O Asymptote at High Energies Regardless of the size of the flare- component in the seed population, re- acceleration by a quasi-perp shock give an asymptotic Fe/O value at high-energies: (Fe/O) shock = (Fe/O) flare [Q/A] Fe, Flare / [Q/A] O, Flare = 8 (20/56) / (8/16) = 8 (20/56) / (8/16) = 5.9 = 5.9 using average values for impulsive events: (Fe/O) flare = 8x =20 =8 (Fe/O) flare = 8x =20 =8 Do we see this in the data? Modeling (Tylka & Lee 2006)

8 Average Fe/O Enhancement at High Energies Based on characteristics of flare ions, we’ve made a prediction for the value of enhanced Fe/O at high- energies in gradual events. Based on average characteristics of flare ions, we’ve made a prediction for the average value of enhanced Fe/O at high- energies in gradual events. Thus, we can understand this average behavior as the consequence of flare seed particles accelerated to high energies by shocks. It is a challenge for other models. Numbers on the datapoints tell how many events were used in the average MeV/nuc MeV/nuc MeV/nuc

9 Very Large SEP Events of Cycle 23 ACE Fe/O (normalized to corona) at MeV/nuc In : 13 out of 38 events with Fe/O > 4x corona Sole Event Selection Criterion: >30 MeV GOES proton fluence > 2 x 10 5 /cm 2 -sr An unbiased survey of high-energy heavy-ions in the Cycle’s largest events

10 Very Large SEP Events of Cycle 23 ACE Fe/O (normalized to corona) at MeV/nuc In : 13 out of 38 events with Fe/O > 4x coronal In : 0 out of 20 events with Fe/O > 4x coronal Where has all the Fe gone? ? Sole Event Selection Criterion: >30 MeV GOES proton fluence > 2 x 10 5 /cm 2 -sr An unbiased survey of high-energy heavy-ions in the Cycle’s largest events

11 Flare Sizes and Longitudes in the Very Large SEP Events of Cycle 23 In both time periods, most of the events are associated with large flares at western longitudes. It appears that differences in characteristics of the associated flare cannot explain the disappearance of events with enhanced Fe/O.

12 In 1997 – 2002, 17 out of 35 events had CME Speed < 1500 km/s In 2003 – 2005, 2 out of 17 events had CME Speed < 1500 km/s Why have the ‘slow’ CMEs almost disappeared from our event sample?* * had 5 more very fast (>2500 km/s) CMEs than we saw in But this fact has no bearing on this question. CME Speeds in the Very Large SEP Events of Cycle 23 ?

13 CME Speeds in the Very Large SEP Events of Cycle 23 Although SEP event statistics are limited in , it appears that the probability of getting a very large SEP event with a CME at ~ km/s dropped by nearly an order of magnitude late in the Cycle.

14 Hypotheses on the Origin of Enhanced Fe/O at High Energies in Very Large SEP Events 1. A direct flare component that dominates at high energies: Either from the “impulsive phase” of the flare Or from reconnection beneath the CME after launch (Cane et al. 2003; 2006) According to Cane et al., this direct flare component is most likely to be observed when the associated CME is comparatively slow. 2. CME-driven shocks accelerate “fresh” suprathermal seed ions that have escaped from the flare that accompanied the CME launch. Li & Zank 2005 Requires the existence of open field lines connecting the flare site to the region upstream of the shock 3.CME-driven shocks accelerate “remnant” suprathermal flare ions, left over in the corona and interplanetary medium from earlier impulsive SEP events. Mason et al. 1999; Tylka et al The flare that accompanied the CME-launch does not contribute to the shock’s seed population because it energized particles mostly on closed loops (Reames 2002).

15 Unless the magnetic topology of CME-source regions or some relevant flare characteristic is fundamentally different late in the Cycle, hypotheses 1 & 2 cannot explain the disappearance of Fe-rich events in But hypothesis 3 may be consistent with the disappearance: The overall level of flare activity started to decline in 2003; The remnant flare suprathermal population (at least near Earth) also declined. Monthly Flare Count (SGD Online) Hypotheses on the Origin of Enhanced Fe/O at High Energies in Very Large SEP Events Wind Suprathermal Fe (Desai et al. 2006)

16 Why are slower (< 1500 km/s) CMEs less effective at producing very large SEP events late in the Cycle? Our event selection requires that the fluence of >30 MeV protons exceed 2 x 10 5 /cm 2 -sr Tylka & Lee (2006) suggested that slower CMEs can produce large numbers of high-energy SEPs only if the shock is quasi-perpendicular near the Sun. Otherwise, the SEP spectra are too steep at high energies. Late in the Cycle, the suprathermal seeds needed by these quasi-perp shocks have diminished. These CMEs therefore become less effective in producing large fluences of high energy SEPs. (In some events – such as 2005 January 20, which may have been quasi-perp near the Sun – the suprathermal seeds probably came from previous gradual SEP events.)

17 Some Caveats … An implication of this proposed scenario is that the population of suprathermal protons has also decreased in But this has not yet been proven: Desai et al. [2006] showed only that the population of remnant suprathermal flare Fe has apparently diminished. It is hoped that Wind/3DP and ACE/EPAM can address the Solar Cycle dependence of ~100 keV protons during solar-quiet times. But this will depend upon the long-term stability and background levels in these instruments. It will also be important to see how the absolute intensity and spectral shape of the suprathermal populations have evolved. Thus, although this idea appears compelling, there are still open issues requiring further investigation.

18 CME Speeds in the Very Large SEP Events of Cycle 23 Note the data points at km/s: : 5 SEP events in 5 years : 1 SEP event in 3 years Only one “very slow” CME in (925 km/s, on 2004 Nov 01) produced a SEP event big enough to meet our criterion. This particular event was preceded by three days of high flare activity and large impulsive events. This event has the second largest Fe/O at 30 MeV/nuc in the event sample, with even larger Fe/O at ~3 MeV/nuc (see table below). This event also has the highest 3 He/ 4 He (= %) among the 64 large SEP events surveyed by Desai et al. 2006b. This exceptional event requires further study.

19 Cane et al. (GRL 30, 8017, 2003) wrote: “It has also been suggested that enrichments of 3 He (and Fe) in large gradual events could result from shock acceleration of remnant flare suprathermal particles in the interplanetary medium from previous small flares (Mason et al. 1999). However, since it has now been shown that flare particles escape directly in all SEP events (Cane et al. 2002), we see no need to invoke a population from earlier, smaller flares.”

20 Cane et al. (GRL 30, 8017, 2003) wrote: “It has also been suggested that enrichments of 3 He (and Fe) in large gradual events could result from shock acceleration of remnant flare suprathermal particles in the interplanetary medium from previous small flares (Mason et al. 1999). However, since it has now been shown that flare particles escape directly in all SEP events (Cane et al. 2002), we see no need to invoke a population from earlier, smaller flares.” Cane et al. 2002: type IIIL radio emissions: Implicit assumption here: the acceleration process that makes ~keV electron beams also makes >30 MeV/nuc ions (esp. Fe). All the events of have type IIILs; but there are no Fe enhancements. This assumption requires reconsideration.

21Summary A simple analytical model based on two factors: Evolution in the geometry of the CME-driven shock as it moves outward from the Sun, (quasi-perpendicular v. quasi-parallel) A compound seed population, typically comprising at least suprathermals from flares and suprathermals from the corona and/or solar-wind. accounts qualitatively and quantitatively for many aspects of high-energy SEP variability. This suggests that these ideas should be pursued more rigorously. The disappearance of Fe-rich events late in Cycle 23 may serve to clarify various hypotheses in the origins of these events. In particular, it favors shock acceleration of remnant flare suprathermals

22 Backups

23

24 Effects of Averaging over Shock-Normal Angle Tylka & Lee, submitted to ApJ Power-law index the same for all species. Species “X” and  Bn dependence reside in rollover, E 0X SW component has steeper spectrum by E -1 Flare & SW components have different Q/A dependence

25 How these calculations work… When we average over  =cos  Bn, we pull a term ~(Q/A)  out of the integral. Neglecting non-leading terms, For our example,  = 1.5 This is close to what we observe for the pure coronal case. But we still have a positive slope for the flare component!

26 How these calculations work… But in the flare component of the seed particles, the composition already has an inherent Q/A dependence: (Glenn will explain this!) Substituting this in, for flare particles processed through a shock, we get: By parameter variation, we can get various slopes on the Breneman & Stone plot.

27 Energy Dependence of 3 He/ 4 He and Energy Dependence of 3 He/ 4 He and Model calculations roughly reproduce the observed values and energy dependence, except below ~1 MeV/nucleon. Our calculations overestimate the flare component of the seed population at low energies. This problem can probably be solved with a more careful treatment of the shock’s actual  Bn evolution. Data from G. Mason, M. Wiedenbeck, and W.F. Dietrich Data from J. Mazur and A. Labrador. Tylka & Lee 2006

28 Spectral and Compositional Variability at High Energies  The event with suppressed high-energy Fe/O :  Exponential rollovers at high energies  Fe softer than O.  The event with enhanced high-energy Fe/O :  Power-law spectra at high energies  Fe harder than O.  This correlation is a general characteristic of the SEP data.

29 Correlations in SEP Characteristics at High Energies vs. Fe/O vs. Fe/O We thus have correlations among measures of composition (Fe/O), charge states, spectral shape, and event size. These correlations are powerful constraints for models. Fe/Ovs. Spectral Steepening Fe/O vs. Spectral Steepening

30 The “Zoo” of Fe/O vs. Energy Data (ACE and Wind) Modeling (Tylka & Lee 2006) Vary shock geometry and size of flare component in seed population: Vary shock geometry and size of flare component in seed population: R = (Seed Pop Flare O) / (Seed Pop Coronal O)

31 Spectral Characteristics Tylka et al. [2005] characterized spectra by quantifying the steepening of the oxygen spectrum between Wind/LEMT (at ~3-10 MeV/nuc) and ACE/SIS (at >30 MeV/nuc). Tylka et al. [2005] found an anti-correlation between high-energy Fe/O and the steepening of the oxygen spectrum. -- events with strong Fe/O enhancements tended to be power-laws In , only one event (2005 January 20) showed a power-law spectrum over the combined Wind-ACE energy range of ~3-100 MeV/nuc.

32 Very Large SEP Events of Event list for published in Tylka et al., ApJ 625, (2005).