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Magnetic Reconnection in Solar Flares

Magnetic Reconnection in Solar Flares. Hao-Yi Wu Department of Physics. Physics 312: Basic Plasma Physics March 1, 2007. Outline. Introduction Basics of solar physics Phenomenon of solar flares Magnetic reconnection Basic ideas, Sweet-Parker model The Petschek model

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Magnetic Reconnection in Solar Flares

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  1. Magnetic Reconnection in Solar Flares Hao-Yi Wu Department of Physics Physics 312: Basic Plasma Physics March 1, 2007

  2. Outline • Introduction • Basics of solar physics • Phenomenon of solar flares • Magnetic reconnection • Basic ideas, Sweet-Parker model • The Petschek model • Models for solar flares • Eruptive flares • Compact flares • Summary

  3. Introduction I: Basics of Solar Physics • Solar Structure • Solar interior • Solar atmosphere • Chromosphere • Corona • Solar activity • Sunspots, spicules, prominences, flares, etc. • 11 years of sunspot cycle, 22 years of solar magnetic field cycle • Entanglement of magnetic field lines due to differential rotation Credit: NASA

  4. Introduction II: Solar Flares • Violent energy release in solar atmosphere • Energy: a billion megatons (TNT) or 1031 ergs • Time: 103 to 104 sec • Size: 104 km • Particle speed: 0.1%c • Emission in H-, hard X-rays and soft X-rays • Often accompanied by coronal mass ejection (CME) or prominence eruption • Affecting the Earth’s magnetosphere, harming communication devices and astronauts Photo Credit: NASA & TRACE http://www.nasa.gov/vision/universe/solarsystem/flare_sept7.html

  5. Magnetic Reconnection: Basic Ideas (Sweet-Parker Model) • Flux freezing approximation: • For , flux freezing no longer holds and a pair of lines can break and reattach with each other • In reconnection region, plasma is heated to balance the magnetic energy outside • Reconnection rate: (η:magnetic viscosity, S: Lindquist number)

  6. Magnetic Reconnection: Petschek Model • Sweet-Parker model predicts too long time scale for solar flares • Shock fronts are required for fast reconnection • Reconnection of two field lines occurs in a shorter diffusion region • Reconnected field lines provide magnetic tension to drive plasma out of diffusion region • Reconnection rate: Credit: Priest & Frobes

  7. Case I: Eruptive Solar Flares • Storage model: • Continual emergence of new flux from convection zone and movement of footprints increase magnetic stress • When the stress exceed the equilibrium threshold, the field erupts • Two scenarios of reconnection • Open-field configuration (also generates coronal mass ejection (CME)) • Close-field configuration • Magnetic energy decreases with time Credit: Priest & Frobes

  8. Case I: Eruptive Solar Flares (cont.) Structure of flares: left: early phase, strong reconnection right: weak reconnection Credit: Priest & Frobes

  9. Case I: Eruptive Solar Flares (cont.) Observational verification of solar flare model

  10. Case II: Compact Solar Flares • Localized, simple loops; Impulsive energy release • Emergence of magnetic loops from convection zone lifts current sheet and causes turbulence • Turbulences increase resistivity and Lindquist number; therefore induce magnetic reconnection Credit: Priest & Frobes

  11. Summary • Solar flares release 1031 ergs in103-104 sec, which requires magnetic reconnection to explain the phenomenon • Petschek Model, utilizing a shock front, successfully predicts: • The entangled magnetic field lines store energy and induce reconnection • For eruptive flares, rising current sheet dominates • For compact flares, emergence of magnetic loops dominates

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