Energetic particles and X-ray emission in solar flares

Transcription

1 Energetic particles and X-ray emission in solar flares Eduard Kontar School of Physics and Astronomy University of Glasgow, UK RAS discussion meeting, London, October 12, 2012

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 ~ 10 6 K => 0.1 kev per particle Flaring region T ~ 4x10 7 K => 3 kev per particle Flare volume cm 3 => (10 4 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 Figure from Temmer et al, 2009 Large solar flare releases about ergs (about half energy in energetic electrons) 1 megaton of TNT is equal to about 4 x ergs.

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 Tomblin, 1971, Bai &Ramtaty 1978 Kontar et al, 2006

10 Energy release and particle acceleration

11 Location of energy release 6-10 kev Plasma density cm -3 Flare volume (10 4 km) 3 cm3 => Number of electrons:10 37 => All electrons will be evacuated from the volume within 1 second! 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 Extended acceleration scenario Are particles accelerated within the loop? Multiple current sheets Vlahos et al 1998, Turkmani et al, 2005, Hood et al, 2008, Browning et al 2008, Gordovskyy et al, 2012 Plasma turbulence acceleration Sturrock, 1966, Melrose, 1968 Miller et al 1997, Petrosian et al, 1994; Bian et al, 2012 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 Footpoints Soft X-ray coronal source HXR chromospheric footpoints Coronal Source

17 Not always Cold flare Thermally dominated flare Veronig et al, 2005, Xu et al, 2008, Jeffrey & Kontar 2012 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) Similar results: statistical survey and individual flare analysis Saint-Hilaire, P. et al 2010, Battaglia etal, 2011 The transport of particles is complicated by return current (Zharkova Gordovskyy, 2004, 2006,), Langmuir waves (e.g. Hannah et al, 2009), Electron pitch angle scattering (e.g. Bespalov et al, 1987; Melnikov 1994) Trapping e.g. Kai etal 1966, Brown & Melrose, 1976, Fletcher 1997, etc

19 Foot-point structure and radiation Battaglia & Kontar, 2011, Sait-Hillare et al, 2008, Martínez Oliveros, et al 2012; Also see near infrared observations e.g. Xu et al, 2006 Wang et al, 2012 White Light observations, Heing 1991, Fletcher et al, 2007

20 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 Note shift

21 Particle escape and propagation From flare to in-situ

22 Flares and accelerated particles How and where electrons are escaping? Frequency, MHz Time Earth's orbit Sun 0.15R Sun 1.5R Sun Plasma frequency radio range <= plasma frequency 1AU

23 From X-rays to electrons γ X-ray spectra from RHESSI The radio diagnostics of energetic electrons, e.g. Reid et al, 2011 => Location of acceleration region Electron spectra at 1AU from WIND δ From Krucker et al 2007

24 From X-rays to electrons Flare electrons 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 (thicktarget model estimate) the spectral indices do not match either thick-target or thintarget models. X-rays X-rays WIND RHESSI Acceleration or transport effects?

25 Conclusions X-ray observations (especially in combination with radio in-situ) are a powerful tool to diagnose the solar flares. Spatially resolved electron spectra (notably with RHESSI, hopefully With STIX) help to understand the physics of electron transport/acceleration. Sunspots sketched by Richard Carrington on Sept. 1, Many aspects of solar flares is not well understood