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Solar Energetic Particles

Solar Energetic Particles. and Shocks. What are Solar Energetic Particles?. Electrons, protons, and heavier ions Energies Generally KeV – MeV Much less energetic than GCRs GLE protons from .3 – 1.8 GeV. What are Solar Energetic Particles?. Important Ions 3 He/ 4 He

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Solar Energetic Particles

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  1. Solar Energetic Particles and Shocks

  2. What are Solar Energetic Particles? • Electrons, protons,and heavier ions • Energies • Generally KeV – MeV • Much less energetic than GCRs • GLE protons from .3 – 1.8 GeV

  3. What are Solar Energetic Particles? • Important Ions • 3He/4He • Charge state of atoms • Fe/O

  4. Two Types of SEP Events • Gradual • Generated by CME-driven shocks • Associated with Type II & IV bursts • Created at heights of 2 – 5 solar radii • Solar wind composition • Duration - several days at MeVenergies • Impulsive • Generated in magnetic reconnection regions in flares • Associated with Type III radio bursts • Created at heights of 1 solar radii or lower • 3He/4He 1000 times background, Fe/O 10 times

  5. Types of SEPs

  6. Revised SEP Event Classification

  7. SEP Escape Conditions • SEPs require open magnetic field lines to escape into interplanetary space • Spacecraft at L1 are well connected to 60° W

  8. Acceleration Mechanisms • Constant Electric Field • Sub-Dreicer Field • Super-Dreicer Field • Magnetic X and O points • Time Varying Electric Fields • Betatron acceleration • Increasing B(x) or B(t) • Coalescence and X-point collapses • Non-linear oscillations; double peaks • Match timescales of Radio and X-ray observations

  9. Accelerating Mechanisms • Alternating Currents • Electromagnetic waves transfer energy to particles • Gyroresonant wave-particle interaction • EM wave frequencies match particle gyrofrequencies • Interacting waves can cause both constructive and destructive interference  stochastic

  10. Stochastic Acceleration • Ions & electrons acceleratedby different mechanisms • Alfven waves can resonate at gyrofrequencies of ions • Electromagnetic waves can resonate at gyrofrequenciesof electrons • Can explain enhanced ion abundances seen in SEPs • Have produced observed behavior • Problem: Need another mechanism to pre-accelerate ions and electrons

  11. Acceleration at Shocks • Shocks are waves with nonlinear amplitudes that propagate faster than the sound speed of the medium • Described by: • Particle velocity distribution: collisionless or collisional • Ion acceleration:subcritical or supercritical • Driving agent: blast or piston-driven wave

  12. Acceleration at Shocks • Fermi acceleration • Magnetic cloud with regions of high B-field • Moving magnetic “mirror” imparts momentum to particle • Single Interaction • Diffusive Shock Acceleration • Particles scatter many times • Cumulative energy gain • Turbulence in magnetic reconnection events or shock fronts

  13. Acceleration Mechanisms • Diffusive shock acceleration  Strong turbulence regime • Gyroresonant wave particle interaction  Weak turbulence regime

  14. The Gradual Event Question • Gradual Events not so predictable • Observe high charge states of many ions like in impulsive events • Enhanced 3He/4He • High Fe/O • Events consistent with multiple temperaturestates

  15. Gradual Event Solution? • Suprathermal particles populate the solar wind • Accelerated by CME during gradual event • A flare occurs as the CME propagates creating a superposition? • Small flares populate solar wind withsuprathermals?

  16. Are SEPs Generated in Flares or CMEs? • Study shows CME shock alone not enough to generate SEP event • Study of CME-less shocks • 15 events 2000 – 2005 • None produced CMEs • CME-less flares were effective accelerators of electrons, as evidenced by microwave bursts • Electrons remained confined in corona • Too deep to access open field lines and escape

  17. Are SEPs Generated in Flares or CMEs? • Tempting to think that CMEs must be present to open field lines • Another possibility: CMEs are generally generated near the periphery of an active region where open field lines are easier to access

  18. The Most Intense SEPs - GLEs • Ground Level Events (GLE) • Prompt component – Stochastic acceleration • Delayed component – Trapping • Only a dozen GLE events per year • ~1 GeV – largest energies produced in solar system

  19. GLE Source Region? • Half of the GLEs studied appeared to come from shocks generated at 2 – 5 solar radii • Half appeared to come from the flare at .05 solar radii • Consistent with flare accelerating and trapping • Cannot rule out secondary CME acceleration

  20. Other Questions • Type II bursts due to shocks generated by: • Blast wave from flare region? • CME piston shock? • Small scale ejecta drive piston shocks? • What happens to SEPs during transport? • Conflicting studies about the ability of ions and electrons to diffuse across magnetic field lines

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