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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 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 QuickTime™ and a decompressor are needed to see this picture. Radio White light 7 Time Moving Footprints QuickTime™ and a decompressor are needed to see this picture. 8 Sigmoid model QuickTime™ and a decompressor are needed to see this picture. Moore et. al, 2001 9 Data of Sigmoid RHESSI data QuickTime™ and a decompressor are needed to see this picture. attribute Moore et. al, 2001 10 QuickTime™ and a YUV420 codec decompressor are needed to see this picture. 11 12 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. 13 Nuclear power? The corona doesn’t have the temperature or density, unless… QuickTime™ and a decompressor are needed to see this picture. 14 No, magnetic energy EB = VB2/(2o) 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 15 Outline • Overview and Flare Observations • Physical Processes on the Sun – Standard Two-ribbon Model – Magnetic Reconnection – Particle Acceleration • Stellar Comparison • Summary 16 Two-Ribbon Flare Model “Magnetic event” (reconnection) Martins & Kuin, 1990 17 Two-Ribbon Flare Model Flow and impaction of current sheet Gyrosynchrotron radio emission Brehmsstrahlung hard X-ray & optical emission Martins & Kuin, 1990 18 Two-Ribbon Flare Model Chromospheric evaporation & condensation Gyrosynchrotron radio emission Blue-shifted UV (≈100s km/s) Red-shifted optical (≈10s km/s) Martins & Kuin, 1990 19 Two-Ribbon Flare Model Soft X-ray Corona become optically thick Optical Martins & Kuin, 1990 20 Two-Ribbon Flare Model Post flare emission (quiescent) Gyrosynchrotron radio emission Optical Martins & Kuin, 1990 21 This model explains… • • • • the relationship to CMEs the Neupert effect Sunquakes Radio observations 22 Outline • Overview and Flare Observations • Physical Processes on the Sun – Standard Two-ribbon Model – Magnetic Reconnection – Particle Acceleration • Stellar Comparison • Summary 23 Magnetic Reconnection B-field lines • Material flows in vinflow • v x B gives current into the page QuickTime™ and a decompressor are needed to see this picture. • called a ‘current sheet’ or ‘neutral sheet’ •current dissipation heats the plasma Sweet-Parker 24 (1958,1957) Magnetic Reconnection vout B-field lines vinflow Pressure is higher in the reconnection region, so flows out the ends QuickTime™ and a decompressor are needed to see this picture. Sweet-Parker (1958,1957) 25 Petschek mechanism QuickTime™ and a decompressor are needed to see this picture. Priest & Forbes, 2002 26 Reconnection Inflow QuickTime™ and a YUV420 codec decompressor are needed to see this picture. 27 Narukage & Shibata, 2006 Outline • Overview and Flare Observations • Physical Processes on the Sun – Standard Two-ribbon Model – Magnetic Reconnection – Particle Acceleration • Stellar Comparison • Summary 28 Particle acceleration 1. DC from E-fields ~ 103 Vm-1 during reconnection 2. MHD shocks - accelerate more particles more slowly - can explain the main phase 3. Highly turbulent environment may give rise to stochastic acceleration - ie fastmode Alfven-waves. 29 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. 30 Displaced ion and electron beams Hurford et al.,2006 2003, Oct 28th flare 4th with measured gamma rays all showing displacement between - and hard X-rays. This is first to show both footprints 31 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. 32 Soft X-Rays Hard X-Rays Gamma Rays Gamma-ray movie QuickTime™ and a YUV420 codec decompressor are needed to see this picture. 33 New model for particle acceleration Fletcher & Hudson, 2008 (RHESSI Nugget #68, Feb 4th, 2008) 34 Outline • Overview and Flare Observations • Physical Processes on the Sun – Standard Two-ribbon Model – Magnetic Reconnection – Particle Acceleration • Stellar Comparison • Summary 35 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? 36 Stellar Flares Osten et al., 2005 37 Big stellar flares QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. Hawley & Pettersen, 1991 38 Flare - quiet Data courtesy of Marcel Agüeros 39 X-ray/microwave ratio QuickTime™ and a decompressor are needed to see this picture. Benz & Gudel, 1994 40 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. 41 Solar vs. Stellar Aschwanden, 2007 42 Magnetic loop lengths L/R Mullan et al.,2006 V-I 43 EUVE Flare rates Audard et al., 2000 44 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. 45 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. 46 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 47 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! 48 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. 49 The End 50 Radio Flares Osten et al.,2005 51 X-ray flares 52 Su, Gan, & Li, 2006 Stats of RHESSI flares QuickTime™ and a decompressor are needed to see this picture. 53 Stats con’t QuickTime™ and a decompressor are needed to see this picture. Su, Gan, & Li, 2006 54 Stats con’td Su, Gan, & Li, 2006 QuickTime™ and a decompressor are needed to see this picture. 55 Stats con’td Su, Gan, & Li, 2006 QuickTime™ and a decompressor are needed to see this picture. 56 Statistical motivation for Avalanche Charbonneau et al., 2001 QuickTime™ and a decompressor are needed to see this picture. 57 Downward motion of centroid 6-12keV *0.5 25-50keV Sui, Homan, & Dennis, 2004 58 Disproving nano-flare heating Aschwanden, 2008 59 Reconnection & X-ray flux Jing et al., 2005 60 Correlations Jing et al., 2005 61 CMEs 62 Jing et al., 2005 Neupert Effect Veronig et al., 2005 63 Gamma-ray spectroscopy Smith et al., 2003 64 Movie-time QuickTime™ and a YUV420 codec decompressor are needed to see this picture. 65 66 Loop lengths in active stars Mullan et al.,2006 67 Avalanche Such models, although a priori far removed from the physics of magnetic reconnection and magnetohydrodynamical evolution of coronal structures, nonetheless reproduce quite well the observed statistical distribution of flare characteristics. - Belanger, Vincent, & Charbonneau, 2007 68 The model QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. 69 Model results QuickTime™ and a decompressor are needed to see this picture. Charbonneau et al., 2001 70 Model results QuickTime™ and a decompressor are needed to see this picture. Charbonneau et al., 2001 71 SOC-Cascades of Loops QuickTime™ and a decompressor are needed to see this picture. Hughes, et al.,2003 72 Cassak et. al, 2008 73 Peak values during the flares QuickTime™ and a decompressor are needed to see this picture. Benz & Gudel, 1994 74 Sunquakes QuickTime™ and a Cinepak decompressor are needed to see this picture. 75 More complicated reconnection • Petschek is just a special case of almost-uniform reconnection • There are also non-uniform models with separatrix jets. • In some cases, the sheet tears, and enters the regime of impulsive bursty reconnection • The 3D models are very complicated 3D MHD. 76 More sigmoid QuickTime™ and a decompressor are needed to see this picture. Moore et. al, 2001 77 Types of Magnetic Emission Flaring: strong, impulsive emission that decays rapidly (minutes to hours), both line and continuum flux may be detected (L ≈ 10-3 - 102 Lbol) Liebert et al. (1999) Quiescent: steady emission that persists over long periods, typically line flux only in optical (L ≈ 10-6 - 10-3 Lbol) 78 Twisted field lines QuickTime™ and a decompressor are needed to see this picture. Priest & Forbes, 2002 79 Solar to stellar - scaling laws Aschwanden, 2007 80 Veronig et al., 2005 81