Insights on Shock Acceleration from Solar Energetic Particles

Transcription

1 Insights on Shock Acceleration from Solar Energetic Particles aka Reading Shakespeare letter-by-letter (J. Vink, ) Allan J. Tylka Code 672, NASA Goddard Space Flight Center, Greenbelt, MD USA Two points: (1) The role of suprathermal ions as seed particles for shock acceleration. (2) The role of shock geometry (Ɵ Bn ) in spectral and compositional variability. Shock Acceleration: from the Solar System to Cosmology The Lorentz Institute, Leiden, The Netherlands, 4-10 January 2015

2 Important Background Information There are at least two distinct acceleration mechanisms for SEPs: (1) at flares, in association with reconnection, probably through resonant wave-particle interactions ( impulsive events): ~1000s events per year, small intensities, duration~hours, ion energies rarely reach ~10 MeV, longitudinally limited (2) at CME-driven shocks, through first-order Fermi and shock-drift acceleration ( gradual events): ~10s events per year, large intensities, duration~days, proton energies ~ 10 GeV, broad range of longitudes 2

3 Flares, Coronal Mass Ejections (CMEs), and Solar Energetic Particles (SEPs): The Big Picture Shocks driven by fast CMEs produce the large intensities of high-energy solar energetic protons, ions, and electrons. CMEs are the energy source for SEPs. In the largest SEP events, ~5-10% of the CME kinetic energy goes into energetic particles (Mewaldt et al. 2005; Emslie et al. 2008) Particles from flares are important contributors to the seed population upon which the CME-driven shocks act. Flares are a matter source for SEPs. 3

4 Outline 1. Brief look at relevant characteristics of flare-accelerated ions 2. Observational Evidence for Suprathermal Seed Particles (including those from flares) in SEPs 3. Does shock geometry matter? Results from Earth s bow shock Results from ESP events 4. The Challenge of Event-to-Event Variability in high-energy SEPs (>30 MeV/nucleon) 5. A Model for SEP Spectral and Compositional Variability at High Energies 6. Conclusions & Caveats Tylka & Lee (2006) Sandroos & Vainio (2007, 2009)

5 Relevant Characteristics of Flare-Accelerated (aka Impulsive ) SEP Events Impulsive SEP Event Spectra Average X/O, relative to the corona Heavy-Ion Enhancements High Ionization States Mason et al., ApJ, 574, 1039 (2002) In the solar wind: Fe/O ~ 0.1 3He/4He ~ Reames et al., Sol Phys, 289, 4675 (2014) Average over 111 events, , ~2-10 MeV/nuc

6 Suprathermal Seed Population for Large SEP Events There s lots of circumstantial evidence that suprathermals rather than the thermal solar-wind -- are the primary seed population for CME-driven shocks: Other than CME speed, the intensity of remnant energetic particles form previous events is the only factor that helps to organize event-to-event variablity in peak SEP intensityes (Kahler et al. 1999; Kahler 2001 Large SEP events are generally preceded by another CME erupting from the same active region within the previous 24 hours.(gopalswamy et al. 2004; Kahler & Vourlidas 2005) The average SEP elemental composition average solar-wind elemental composition (numerous papers) Here are two more arguments:

7 Observational Evidence for Suprathermal Seed Population Mewaldt et al Mason et al. 2005

8 Observational Evidence for Suprathermal Seed Population: This time from flare suprathermals: 3 He in shock-accelerated SEP Events Timelines for 4He and 3He Intensities 3He/4He Ratio in a GLE ~ 1 MeV/nuc Mason et al Tylka & Lee Note: Curve is from a model discussed later. Remember: in the thermal solar-wind, average 3He/4He ~ 0.04% (Gloeckler & Geiss 1998)

9 Theoretical Evidence for the Potential Importance of Suprathermal Seed Particles Laming et al., ApJ 770:73 (2013); Ng & Reames, ApJ 686:123 (2008)

10 Does shock geometry matter? Meziane, Hull, Hamza, & Lin, JGR 107, 1243 (2002) 211 Wind Crossings of Earth s Bow Shock Protons (#/cm2-sr-s-kev) 10 * Energy (kev) Energy (kev) = quiet solar wind; O = in the presence of ambient energetic particles

11 Shock-Geometry at Traveling Interplanetary Shocks Reames., ApJ 757, 93 (2012) 258 IP shocks at 1 AU measured by the Wind spacecraft, , with a complete plasma analysis (Kaspar 2005) 39 shocks found to be associated with an increase in He ions at ~2-10 MeV/nucleon Fraction of Shocks with >2 MeV/nuc He: Ɵ Bn = 0-30 deg: % Ɵ Bn = deg: % Ɵ Bn = deg: % 11

12 SEP Variability at High Energies (above a few tens of MeV/nuc) Enhanced Fe/O at lowenergies is probably a transport effect. (only GLE of 2002) (biggest proton-fluence event of 2002) CMEs, flares, and SEPs below ~10 MeV/nuc are very similar in these two events. But Fe/O diverges at high energies: -- GLE approaches impulsive values at high energies. -- Big fluence (>30 MeV protons) event is Fe poor at high energies High-energy particles were produced mostly when the CME was at <15 Rs 12

13 The Link Between SEP Composition & Spectral Shape Tylka et al., Astrophysical Journal Suppl., 164, , (2006) 13 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.

14 The Link Between SEP Composition and Spectral Shape: Results from a Survey of the Largest SEP Events of Tylka et al., Astrophysical Journal 625, (2005) Fe/O at MeV/nucleon v. γ 2 - γ 1 Event Selection: >2 x 10 5 protons/cm 2 -sr above 30 MeV High-energy Fe/O ratio varies by nearly 3 orders of magnitude. 14

15 The Link Between SEP Composition and Spectral Shape: Results from a Survey of the Largest SEP Events of Tylka et al., Astrophysical Journal 625, (2005) Fe/O at MeV/nucleon v. γ 2 - γ 1 Event Selection: >2 x 10 5 protons/cm 2 -sr above 30 MeV High-energy Fe/O ratio varies by nearly 3 orders of magnitude. Spectral character varies from power laws to nearly exponential rollovers. 15

16 The Link Between SEP Composition and Spectral Shape: Results from a Survey of the Largest SEP Events of Tylka et al., Astrophysical Journal 625, (2005) Fe/O at MeV/nucleon v. γ 2 - γ 1 Event Selection: >2 x 10 5 protons/cm 2 -sr above 30 MeV High-energy Fe/O ratio varies by nearly 3 orders of magnitude. Spectral character varies from power laws to nearly exponential rollovers. Event size spans more than 3 orders of magnitude. Note the correlation coefficient. There is a coupling that links composition, spectral shape, and event size. To understand Fe/O variability, we must also understand spectral variability. 16

17 SEP Variability at High Energies (above a few tens of MeV/nuc) (only GLE of 2002) Let s begin with this one. (biggest proton-fluence event of 2002) 17

18 Spectral Origin of Highly Suppressed Fe/O at High Energies F x (E) ~ E -γ exp(-e/e 0x ) with E 0x Q x /A x (Ellison & Ramaty 1985) Power-law indices nearly the same for all species. But the e-folding energies differ trans- Fe Data from 8 instruments, 5 satellites Oxygen charge state measured by SAMPEX 18 Others inferred by scaling according to fitted E 0

19 Solar-Wind-Like Seed Population in the 2002 April 21 Event Use theoretical calculations to determine source plasma temperatures from the mean ionic charge states Arnaud & Rothenflug 1985 Arnaud & Raymond 1992 All species consistent with 1.4 MK, typical of the solar wind. These charge states are our justification for identifying solar wind (suprathermals) as the dominant seed population in this event. 19

20 Solar-Wind-Like Seed Population in the 2002 April 21 Event Caveat: Although the charge states are those of the bulk SW, there are still puzzles here. Most vexing is C/O: SEPs = SW = Use theoretical calculations to determine source plasma temperatures from the mean ionic charge states Arnaud & Rothenflug 1985 Arnaud & Raymond 1992 All species consistent with 1.4 MK, typical of the solar wind. These charge states are our justification for identifying solar wind (suprathermals) as the dominant seed population in this event. 20

21 SEP Variability at High Energies (above a few tens of MeV/nuc) (only GLE of 2002) Understanding falling Fe/O seems relatively easy. (biggest proton-fluence event of 2002) But how to get Fe/O increasing with energy is a harder puzzle! 21

22 Pre-Event Particle Background Blue: Oxygen Red: Iron 2002 April August Nominal Fe/O ~ 0.1 in the 4 days preceding the event Enhanced Fe/O ~ 1 in the 4 days preceding the event These two CME-driven shocks likely launched into very different seed populations. 22

23 A Model for Spectral and Compositional Variability at High Energies in Large Gradual SEP Events Tylka & Lee, ApJ, 646, (2006) Two variable factors in the model: Zank et al Range of θ Bn variation (i.e., initial perpendicular phase or not) Relative sizes of flare and coronal seed components 23

24 A quasi-perp shock preferentially reveals the characteristics of the higher-speed seed particles. A quasi-parallel shock has access to a larger seed population and is likely to give larger fluences.

25 A Model for SEP Variability at High Energies Tylka & Lee, Astrophysical Journal, 646, (2006) Power-law index the same for all species. Species X and θ Bn dependence reside in rollover, E 0X A heuristic implementation of suppressed injection 25

26 A Model for Spectral and Compositional Variability at High Energies in Large Gradual SEP Events Two large SEP events that illustrate the extremes of high-energy variability in elemental composition and spectral shape. Tylka et al., Astrophysical Journal Suppl., 164, , (2006) The model, by exploiting parameter variation that is inherent in shock acceleration, naturally accounts for both event morphologies. Model Data (from ACE & Wind) 26 Tylka & Lee, Astrophysical Journal, 646, , (2006)

27 The Zoo of Fe/O vs. Energy Modeling Tylka & Lee, Astrophysical Journal, 646, (2006) Data (ACE and Wind) Vary shock geometry and size of flare component in seed population: 27 R = (Seed Pop Flare O) / (Seed Pop Coronal O)

28 Energy-Dependent SEP Charge States 2001 April 15: another GLE, like 2002 August 24, but bigger 2001 September 24: another suppressed Fe/O event, like 2002 April 21, but bigger >30 MeV protonsfluence Model Curves from Tylka & Lee (2006) 28 SAMPEX: Labrador et al Mazur, private communication.

29 How does the SEP radiation hazard evolve in time? Proton Intensity vs. Time quasi-parallel near the Sun quasi-perp near the Sun 29 Tylka et al., Astrophysical Journal Suppl., 164, , (2006)

30 Enhanced High-Energy Fe/O in Ground-Level Events We have Fe/O measurements above 30 MeV/nucleon for 42 GLEs in /42 (~80%) of the GLEs have Fe/O > 3x corona at >30 MeV/nucleon. compared to 15/61 (~25%) for all large >30 MeV proton-fluence events, No GLE has been observed with suppressed Fe/O.

31 Enhanced Fe/O in Shocks at 1 AU ( Energetic Storm Particle (ESP) Events ) 31

32 Further Development of the Shock-Geometry Model: Tylka & Lee (2006) presented a simplified analytic formulation for high-energy SEP variability due to evolving shock geometry and a compound suprathermal seed population. Sandroos & Vainio (2007) did a more sophisticated implementation: Test-particle simulation of DSA Semi-realistic solar-coronal model: radial field Semi-realistic evolving coronal shock: spherical shock Compound suprathermal seed population, like Tylka & Lee (2006) Spectra of oxygen from coronal (left) and flare (right) suprathermals Fe/O vs. Energy (left) & <QFe> vs. energy (right) 32

33 Further Development of the Shock-Geometry Model: Sandroos & Vainio (2007) Tylka & Lee (2006) Sandroos & Vainio (2007) We find that the critical assumptions of the [Tylka & Lee (2006)] model are correct and that, despite it simplicity, it may provide a realistic description of the compositional and spectral variability of heavy ions in SEP events. BUT 33

34 BUT Sandroos & Vainio (2007) did not include a self-consistent treatment of wave-growth at the shock. Instead, they assumed a scattering law that was equivalent to a turbulence spectrum I(k) ~1/k Lee (2005) showed that this wave spectrum would give an e-folding energy that scaled as (Q/A)(secƟ Bn ) λ, where 0 < λ < 2. (Tylka & Lee (2006) used λ =1.) But when Sandroos & Vainio (2009) assumed other slopes for the turbulence spectrum, the Q/A - scaling of the e-folding energy changed, causing in some cases the model to no longer produce energydependent Fe/O similar to the data. -- The model results are sensitive to details of the near-shock scattering. Is this a bad thing? Or are the SEP data telling us something about wave-growth in the corona? Perhaps Solar Probe and Solar Orbiter will clarify. 34

35 Conclusions & Caveats 1. At high energies, large SEP events reveal a complicated phenomenology of event-to-event variability in spectral shape, energy-dependent composition, and event size. 2. It has been shown that a simple shock model (Tylka & Lee 2006) can semi-quantitatively explain this phenomenology, by invoking the interplay of two two factors: A compound seed population, including both the suprathermal tail of the solar-wind and suprathermals from flares, with the latter dominating at higher energies. An evolving shock-normal angle as the shock moves outward from the Sun. 3. There are good reasons, both observational and theoretical, for believing that both of these factors exist. The question is the interplay: in particular, the injection threshold vs. shock-normal angle. 4. The model needs further development, with more sophisticated numerical implementation. Attempts to-date to do this (Sandroos & Vainio, 2007, 2009) are encouraging. See also Yang, Lembege, & Lu, JGR 116, A10202, CAVEAT: Direct validation of the necessary conditions (i.e., near-sun suprathermal seed population & shock-normal evolution) are difficult from 1 AU and at least at present rely on modeling of the coronal magnetic field. (See C. Cohen s talk tomorrow.) Solar Orbiter & Solar Probe will help. 6. CAVEAT: In the SEP community, there is a competing hypothesis a direct component of high-energy particles from the flare, mixed with energetic particles form the CME-driven shock. See backup slides for one of the problems with that hypothesis.

36 Backups

37 A Direct Flare Component at High Energies? γ 2 - γ 1 < 0 would indicate hardening 37

38 A Direct Flare Component at High Energies? Search for Hardening in the Fe Spectrum 3-10 MeV/nuc MeV/n γ 2 - γ 1 < 0 would indicate hardening Only one event (at E09 o, with a large ESP component) shows this. 38