1. 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

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

3. 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

4. Solar flares and accelerated particles

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

6. “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 cm => (104 km)3
Plasma density cm-3
Photons up to > 100 MeV
Number of energetic electrons per second
Electron energies >10 MeV
Proton energies >100 MeV
Large solar flare releases about 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

7. 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)

8. 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

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

10. Energy release and particle acceleration

11. Location of energy release
Plasma density cm-3
Flare volume cm (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)

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

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

14. 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

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

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

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

18. 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

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

20. Note shift Gamma rays and ions
Imaging of the MeV neutroncapture line (blue contours) and the HXR electron
bremsstrahlung (red contours) of the flare on October 28, 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

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

22. 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

23. From X-rays to electronsg
X-ray spectra from RHESSI
Electron spectra at 1AU from WIND d
From Krucker et al 2007

24. 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?

25. 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.