13 Where does the energy come from? A typical Solar flare emits about 10 32 ergs total. The typical size is L ~ 3x10 9 cm, H ~ 2x10 9 cm, leading to V ~ 2x10 28 cm 3 Thermal energy? In the chromosphere, the column density, col is ~0.01 g/cm 2 and T~ 1x10 4 K. In the corona, it’s 3x10 -6 g/cm 2, 3x10 6 K E th ≈ 3 col kTL 2 /m H ≈ 2x10 29 ergs for chromosphere ≈ 2x10 28 ergs for corona. Not cutting it.
14 Nuclear power? The corona doesn’t have the temperature or density, unless…
15 No, magnetic energy E B = VB 2 /(2 o ) so, for B = 300-1000 G, you’re at 1x10 32-33 erg. Now, how is the energy released quickly enough? t ~ L 2 o ~ 5x10 11 seconds for diffusion, way too long So, do it quickly in a current sheet
16 Outline Overview and Flare Observations Physical Processes on the Sun –Standard Two-ribbon Model –Magnetic Reconnection –Particle Acceleration Stellar Comparison Summary
20 Two-Ribbon Flare Model Soft X-ray Optical Corona become optically thick Martins & Kuin, 1990
21 Post flare emission (quiescent) Two-Ribbon Flare Model Gyrosynchrotron radio emission Optical Martins & Kuin, 1990
22 This model explains… the relationship to CMEs the Neupert effect Sunquakes Radio observations
23 Outline Overview and Flare Observations Physical Processes on the Sun –Standard Two-ribbon Model –Magnetic Reconnection –Particle Acceleration Stellar Comparison Summary
24 Magnetic Reconnection Sweet-Parker (1958,1957) B-field lines v inflow Material flows in v x B gives current into the page called a ‘current sheet’ or ‘neutral sheet’ current dissipation heats the plasma
25 Magnetic Reconnection Sweet-Parker (1958,1957) B-field lines v inflow v out Pressure is higher in the reconnection region, so flows out the ends
28 Outline Overview and Flare Observations Physical Processes on the Sun –Standard Two-ribbon Model –Magnetic Reconnection –Particle Acceleration Stellar Comparison Summary
29 Particle acceleration 1.DC from E-fields ~ 10 3 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 fast- mode Alfven-waves.
30 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.
31 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
32 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.
33 Gamma-ray movie Soft X-Rays Hard X-Rays Gamma Rays
34 New model for particle acceleration Fletcher & Hudson, 2008 (RHESSI Nugget #68, Feb 4th, 2008)
35 Outline Overview and Flare Observations Physical Processes on the Sun –Standard Two-ribbon Model –Magnetic Reconnection –Particle Acceleration Stellar Comparison Summary
36 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?
41 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.
45 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.
46 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.
47 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
48 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!
49 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.
67 Loop lengths in active stars Mullan et al.,2006
68 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
76 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.
78 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 - 10 2 L bol ) Quiescent: steady emission that persists over long periods, typically line flux only in optical (L ≈ 10 -6 - 10 -3 L bol ) Liebert et al. (1999)