Physics of Solar Flares Eduard Kontar School of Physics and Astronomy University of Glasgow, UK 5th Solar Orbiter Workshop, September 10-14, 2012, Bruges, Belgium

Solar flares and accelerated particles Global energetics and flare basics Spatial distribution of energetic particles Energy release and particle acceleration Particle transport and escape

Solar flares: basics Solar flares are rapid localised brightening in the lower atmosphere. More prominent in X-rays, UV/EUV and radio…. but can be seen from radio to 100 MeV X-rays radio waves Particles 1AU Figure from Krucker et al, 2007

Solar flares and accelerated particles

Solar flares and accelerated particles From Emslie et al., 2004, 2005 Free magnetic energy ~2 1032 ergs

“Standard” model of a solar flare/CME Energy release/acceleration Solar corona T ~ 106 K => 0.1 keV per particle Flaring region T ~ 4x107 K => 3 keV per particle Flare volume 1027 cm3 => (104 km)3 Plasma density 1010 cm-3 Photons up to > 100 MeV Number of energetic electrons 1036 per second Electron energies >10 MeV Proton energies >100 MeV Large solar flare releases about 1032 ergs (about half energy in energetic electrons) 1 megaton of TNT is equal to about 4 x 1022 ergs. Figure from Temmer et al, 2009

X-rays and flare accelerated electrons Observed X-rays Unknown electron distribution Emission cross-sections Thin-target case: For the electron spectrum F(E)~E-δ , bremsstrahlung (free-free emission)

X-ray spectrum of solar flares Thermal X-rays Solar Orbiter/STIX energy-range Gamma-ray lines Non-thermal X-rays July 23, 2002 flare Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spectrum

Compton scattering in pictures Primary Observed flux Reflected flux Direct flux Reflected Observed

Energy release and particle acceleration

Location of energy release Plasma density 1010 cm-3 Flare volume 1027 cm3 (104 km)3 6-10 keV => Number of electrons:1037 => All electrons will be evacuated from the volume within 1 second! 14-16 keV Sui et al, 2004 Do we observe quasi-2D magnetic reconnection? Standard flare model picture (Shibata, 1996)

Above the loop-top X-ray sources Separations between HXR and SXH sources From Krucker & Lin, 2008

Energy release inside the loop?   (Xu et al, 2008, Kontar etal, 2011,Guo et al,2012)

Local re-acceleration scenario Are particles accelerated within the loop? Vlahos et al 1998, Turkmani et al, 2005, Hood et al, 2008, Browning et al 2008 Simulations by Gordovskyy & Browning, 2011

From X-rays to electrons Spatial distribution of the energetic electrons and transport

X-ray emission from typical flares Soft X-ray coronal source HXR chromospheric footpoints Footpoints Coronal Source

Not always … “Cold flare” “Thermally dominated flare” Veronig et al, 2005, Xu et al, 2008, Fleishman et al, 2011

Foot-point structure and radiation Aschwanden et al, 2002 Higher energy sources appear lower in the chromosphere (consistent with simple collisional transport) Saint-Hilaire, P. et al 2010, Battaglia etal , 2011

Foot-point structure and radiation Battaglia & Kontar, 2011, Sait-Helier et al, 2008, Martínez Oliveros, et al 2012;

Note shift Gamma rays and ions Imaging of the 2.223 MeV neutroncapture line (blue contours) and the HXR electron bremsstrahlung (red contours) of the flare on October 28, 2003. The underlying image is from TRACE at 195 Å. The X-ray and γ-ray imaging shown here used exactly the same selection of detector arrays and imaging procedure. Note the apparent loop-top source in the hard X-ray contours Hurford et al 2006. Note shift

Particle escape and propagation From flare to in-situ Particle escape and propagation

Flares and accelerated particles How and where electrons are escaping ? Time DecametricType III bursts Interplanetary type III bursts Acceleration site Frequency, MHz Hard X-rays upward beams Earth's orbit 10MHz 20-30kHz 240MHz Sun 0.15R 1.5R Sun Sun 1AU Plasma frequency radio range <= plasma frequency

From X-rays to electrons g X-ray spectra from RHESSI Electron spectra at 1AU from WIND d From Krucker et al 2007

From X-rays to electrons Flare From the analysis of 16 “scatter-free” events (Lin, 1985; Krucker et al, 2007) : Although there is correlation between the total number of electrons at the Sun (thick-target model estimate) the spectral indices do not match either thick-target or thin-target models. electrons X-rays X-rays WIND RHESSI Acceleration or transport effects?

Conclusions Remote observations in combination with in-situ will be a powerful tool to diagnose the solar flares. Spatially resolved electron spectra help to understand the physics of electron transport/acceleration. Location of SO might help to reduce propagation effects. Combination of various instruments greatly improves our understanding of solar flare physics. Sunspots sketched by Richard Carrington on Sept. 1, 1859.