Solar Energetic Particles
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Transcript of Solar Energetic Particles
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Solar Energetic Particles
and Shocks
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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
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What are Solar Energetic Particles?
• Important Ions– 3He/4He– Charge state
of atoms– Fe/O
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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– 3He/4He 1000 times background, Fe/O 10 times
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Types of SEPs
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Revised SEP Event Classification
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SEP Escape Conditions
• SEPs require open magnetic field lines to escape into interplanetary space
• Spacecraft at L1 are well connected to 60° W
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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
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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
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Stochastic Acceleration
• Ions & electrons acceleratedby 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
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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
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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
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Acceleration Mechanisms
• Diffusive shock acceleration Strong turbulence regime
• Gyroresonant wave particle interaction Weak turbulence regime
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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
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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?
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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
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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
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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
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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
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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