1 / 80

Physical Processes in Solar and Stellar Flares

Physical Processes in Solar and Stellar Flares. Eric Hilton. General Exam March 17th, 2008. Outline. Overview and Flare Observations Physical Processes on the Sun Standard Two-ribbon Model Magnetic Reconnection Particle Acceleration Stellar Comparison Summary. The Sun.

naiara
Download Presentation

Physical Processes in Solar and Stellar Flares

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Physical Processes in Solar and Stellar Flares Eric Hilton General Exam March 17th, 2008

  2. Outline • Overview and Flare Observations • Physical Processes on the Sun • Standard Two-ribbon Model • Magnetic Reconnection • Particle Acceleration • Stellar Comparison • Summary

  3. The Sun

  4. Magnetic loops TRACE image ~ 109 cm Footprints

  5. Flare basics • Flares are the sudden release of energy, leading to increased emission in most wavelength regimes lasting for minutes to hours.

  6. Light curves Kane et al., 1985 X-rays Radio White light Time

  7. Moving Footprints

  8. Sigmoid model Moore et. al, 2001

  9. Data of Sigmoid RHESSI data attribute Moore et. al, 2001

  10. Where does the energy come from? A typical Solar flare emits about 1032 ergs total. The typical size is L ~ 3x109 cm, H ~ 2x109 cm, leading to V ~ 2x1028 cm3 Thermal energy? In the chromosphere, the column density, col is ~0.01 g/cm2 and T~ 1x104 K. In the corona, it’s 3x10-6 g/cm2, 3x106 K Eth ≈ 3 colkTL2/mH ≈ 2x1029 ergs for chromosphere ≈ 2x1028 ergs for corona. Not cutting it.

  11. Nuclear power? The corona doesn’t have the temperature or density, unless…

  12. No, magnetic energy EB = VB2/(2o) so , for B = 300-1000 G, you’re at 1x1032-33 erg. Now, how is the energy released quickly enough? t ~ L2o ~ 5x1011 seconds for diffusion, way too long So, do it quickly in a current sheet

  13. Outline • Overview and Flare Observations • Physical Processes on the Sun • Standard Two-ribbon Model • Magnetic Reconnection • Particle Acceleration • Stellar Comparison • Summary

  14. Two-Ribbon Flare Model “Magnetic event” (reconnection) Martins & Kuin, 1990

  15. Two-Ribbon Flare Model Flow and impaction of current sheet Gyrosynchrotron radio emission Brehmsstrahlung hard X-ray & optical emission Martins & Kuin, 1990

  16. Two-Ribbon Flare Model Gyrosynchrotron radio emission Chromospheric evaporation & condensation Blue-shifted UV (≈100s km/s) Red-shifted optical (≈10s km/s) Martins & Kuin, 1990

  17. Two-Ribbon Flare Model Soft X-ray Corona become optically thick Optical Martins & Kuin, 1990

  18. Two-Ribbon Flare Model Post flare emission (quiescent) Gyrosynchrotron radio emission Optical Martins & Kuin, 1990

  19. This model explains… • the relationship to CMEs • the Neupert effect • Sunquakes • Radio observations

  20. Outline • Overview and Flare Observations • Physical Processes on the Sun • Standard Two-ribbon Model • Magnetic Reconnection • Particle Acceleration • Stellar Comparison • Summary

  21. Magnetic Reconnection B-field lines • Material flows in • v x B gives current into the page • called a ‘current sheet’ or ‘neutral sheet’ • current dissipation heats the plasma vinflow Sweet-Parker (1958,1957)

  22. Magnetic Reconnection vout B-field lines Pressure is higher in the reconnection region, so flows out the ends vinflow Sweet-Parker (1958,1957)

  23. Petschek mechanism Priest & Forbes, 2002

  24. Reconnection Inflow Narukage & Shibata, 2006

  25. Outline • Overview and Flare Observations • Physical Processes on the Sun • Standard Two-ribbon Model • Magnetic Reconnection • Particle Acceleration • Stellar Comparison • Summary

  26. Particle acceleration • DC from E-fields ~ 103 Vm-1 during reconnection • MHD shocks - accelerate more particles more slowly - can explain the main phase • Highly turbulent environment may give rise to stochastic acceleration - ie fast-mode Alfven-waves.

  27. Ion beam • 2.223 MeV is a neutron capture line - ions collide with atmosphere, producing fast neutrons. • These neutrons thermalize for ~100 sec before being captured by Hydrogen. • Hydrogen is turned into Deuterium, releasing a -ray • Time profiles (with 100 sec delay) suggest beams happen at same time.

  28. Displaced ion and electron beams 2003, Oct 28th flare 4th with measured gamma rays - all showing displacement between - and hard X-rays. This is first to show both footprints Hurford et al.,2006

  29. Ion and electron beam displacement • Possible displacement caused by drift of electrons and ions with different sign of charge. This effect is 2 orders of magnitude too small. • Currently, it’s not known why there is displacement.

  30. Gamma-ray movie Soft X-RaysHard X-RaysGamma Rays

  31. New model for particle acceleration Fletcher & Hudson, 2008 (RHESSI Nugget #68, Feb 4th, 2008)

  32. Outline • Overview and Flare Observations • Physical Processes on the Sun • Standard Two-ribbon Model • Magnetic Reconnection • Particle Acceleration • Stellar Comparison • Summary

  33. Stellar comparison • When we look at a star, we lose all spatial resolution, lots of photons, and continuous monitoring. • We can’t observe hard X-rays, and only observe limited soft X-rays • We gain new regimes of temperature, magnetic field generation and configuration, plasma density, etc. • We can adopt the Solar analogy, but is it valid? What observations can we make?

  34. Osten et al., 2005 Stellar Flares

  35. Big stellar flares Hawley & Pettersen, 1991

  36. Flare - quiet Data courtesy of Marcel Agüeros

  37. X-ray/microwave ratio Benz & Gudel, 1994

  38. The Sun is not a Flare Star! • Although some parts of the analogy clearly hold, we would not see flares on the Sun if it were further away. • Are the flares we see fundamentally different? • We are biased to detecting only the largest flares, so must be cautious about extrapolating to rates of smaller flares.

  39. Solar vs. Stellar Aschwanden, 2007

  40. Magnetic loop lengths L/R Mullan et al.,2006 V-I

  41. EUVE Flare rates Audard et al., 2000

  42. My Thesis • I will make hundreds of hours of new observations of M dwarfs to determine flare rates • I am creating model galaxy simulations to predict flare rates on a Galactic scale that includes spectral type and activity level. We can ‘observe’ this model to predict what LSST will see.

  43. Summary of Solar Flares • Magnetic loops become entangled by motions of the footprints, storing magnetic energy • This energy is released through rapid magnetic reconnection that accelerates particles. • Flares emit in all wavelength regimes. • The general theory is well-established, but the details continue to be very complex.

  44. The Sun, in closing “Coronal dynamics remains an active research area. Details of the eruption process including how magnetic energy is stored, how eruptions onset, and how the stored energy is converted to other forms are still open questions.” - Cassak, Mullan, & Shay published March 3rd, 2008

  45. Summary of Stellar Flares • Many aspects of the Solar model seem to be true on stars as well. • Observations have revealed inconsistencies that have not yet been resolved. • Flares are the coolest!

  46. Thanks • Thanks to my committee, esp. Mihalis for coming all the way from Ireland on St. Patrick’s Day. • Thanks to my fellow grad students for feedback on my practice talk.

  47. The End

  48. Radio Flares Osten et al.,2005

More Related