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 – Charge state of atoms – Fe/O

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 MeV energies Impulsive – Generated in magnetic reconnection regions in flares – Associated with Type III radio bursts – Created at heights of 1 solar radii or lower – 3 He/ 4 He 1000 times background, Fe/O 10 times

Types of SEPs

Revised SEP Event Classification

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

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

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

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

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

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

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

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

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 with suprathermals?

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

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

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

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

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