Thick Target Coronal HXR Sources

1. Thick Target Coronal HXR Sources Astrid M. Veronig Institute of Physics/IGAM, University of Graz, Austria

2. General scenario • Footpoint HXR sources: Thick-target bremsstrahlung from electron beams collisionally stopped in the “dense“ chromosphere (full energy loss) • Coronal HXR sources: Thermal bremsstrahlung from hot plasma or Thin-target bremsstrahlung from electron beams in a tenuous plasma (negligible energy loss, electron distribution unchanged) + trapping If the column density is high enough to collisionally stop an electron beam withinthe corona thick-target coronal HXR sources

3. Thick-target coronal HXR sourcesObservational evidence • Basic (necessary) evidence: - HXR images in which emission is predominantly from the corona (without footpoints being occulted) - “Nonthermal behavior“ (power-law spectra, spiky time profiles) • Kosugi et al. (1994): Yohkoh/HXT • Lin et al. (2003), Krucker et al. (2003): RHESSI observations of the pre-impulsive phase of the 23rd July 2002 X-class flare • Kosugi et al. (1994): Yohkoh/HXT • Lin et al. (2003), Krucker et al. (2003): RHESSI observations of the pre-impulsive phase of the 23rd July 2002 X-class flare

4. 23rd July 2002 X4.8 flarePre-impulsive nonthermal coronal HXR source Broken power-lawspectrum ~ 5 ~ 7 Lin et al. (2003) Krucker et al. (2003) Säm Krucker

5. 23rd July 2002 X4.8 flareImpulsive phase thermal coronal HXR source thermal (coronal LT) + power-law (FPs) Emslie et al. (2003) Krucker et al. (2003) Säm Krucker

6. Thick-target coronal HXR sourcesObservational evidence • Basic (necessary) evidence: - HXR images in which emission is predominantly from the corona (without footpoints being occulted) - “Nonthermal behavior“ (power-law spectra, spiky time profiles) • Kosugi et al. (1994): Yohkoh/HXT • Lin et al. (2003), Krucker et al. (2003): RHESSI observations of the pre-impulsive phase of the 23rd July 2002 X-class flare • Veronig & Brown (2004): RHESSI observations of 2 homologous M-class flares (14/15 & 15 Apr 2002, same events as in Sui et al.) with HXR emission predominantly from the loop • + • Derivation of beam spectral characteristics and thermal plasma parameters to test coronal thick-target hypothesis • Kosugi et al. (1994): Yohkoh/HXT • Lin et al. (2003), Krucker et al. (2003): RHESSI observations of the pre-impulsive phase of the 23rd July 2002 X-class flare • Veronig & Brown (2004): RHESSI observations of 2 homologous M-class flares (14/15 & 15 Apr 2002, same events as in Sui et al.) with HXR emission predominantly from the loop • + • Derivation of beam spectral characteristics and thermal plasma parameters to test coronal thick-target hypothesis

7. 14/15 Apr 2002 M3.2 FlareRHESSI Lightcurves

8. 14/15 Apr 2002 M3.2 FlareRHESSI images HXR emission predominantly from loop top (vs footpoints) Images: 6 – 12 keV Contours: 25 – 50 keV Veronig & Brown (2004) Movie link

9. 14/15 Apr 2002 M3.2 Flare RHESSI spectra Sequence of spatially integrated RHESSI spectra Spectra: isothermal + powerlaw Light curves: fast time variations Images: emission from loop (top)  Nonthermal emission from loop (top) Movie link

10. 14/15 Apr 2002 M3.2 Flare Electron beam characteristics > ~ very steep spectra:  7

11. 14/15 Apr 2002 M3.2 Flare Hot loop plasma parameters I A = 2  1017 cm2 L = 45  108 cm V = AL ~ 1027 cm3 EM = n2V n N = n L/2

12. 14/15 Apr 2002 M3.2 Flare Hot loop plasma parameters II High column densities: Npeak  (35)1020 cm2 Electrons with energy E <Eloop are stopped above TR. 2535 keV < Eloop < 4560 keV

13. 14/15 Apr 2002 M3.2 Flare Theoretical: Footpoint vs loop emission Ratio footpoint to total emission for photon energy  = 25 keV as function of electron spectral index  for thick-target HXR emission

14. 14/15 Apr 2002 M3.2 Flare Summary of main characteristics • Loop is so dense • Electron beam spectra are so steep • Most of the electrons are stopped within the loop: Appearance of thick target HXR loop (top) • Beam is very efficient in heating the loop. Ergo: Efficient chromospheric evaporation by heat conduction from hot loop top But why is n (and N) so high at the very beginning?

15. 14/15 Apr 2002 M3.2 Flare Preflare 18 Å T(t) RHESSI Obs 0.54 Å EM(t) Preflare Flare RHESSI Obs

16. 14/15 Apr 2002 M3.2 Flare NoRH flare and preflare images No RHESSI observations of preflare available but NoRH Image: preflare (23:41 UT) Contours: flare (00:02 UT) Image: preflare (23:41 UT) Contours: flare (00:02 UT) Nearby set of loops! Chromospheric evaporation during preflare already fills the loops Veronig et al. 2005 More detailed analysis in Bone et al. (2006)

17. 16 Apr 2002 M2.5 flareAnother thick-target loop candidate

18. 16 Apr 2002 M2.5 flareRHESSI & TRACE imaging TRACE 195 TRACE running diff RHESSI One of Sui et al. flares

19. 16 Apr 2002 M2.5 flareRHESSI 20-50 keV HXR image sequence Again: HXR emission predominantly from loop top (vs footpoints)

20. 16 Apr 2002 M2.5 flareImages and spectra during peak

21. 16 Apr 2002 M2.5 flareImages and spectra during peak • steep spectra •  (LT, FP) ~ 0.5  smaller than in Battaglia & Benz (2006) sample thin-target LT, thick-target FPs: expected  = 2 FP2 LT FP1

22. 16 Apr 2002 M2.5 flareHot loop parameters Again: High column densities, steep HXR spectra

23. 9 Sep 2002 M2.3 Flare Maybe another thick-target loop candidate Ji et al. 2004 Peak 1 Peak 1 25 – 50 keV Peak 1 1 12 – 25 keV

24. 9 Sep 2002 M2.3 Flare GOES, RHESSI & H lightcurves H excitation: nonthermal (beam) & thermal (heat flux) Thick-target coronal flare: T is expected to change with beam flux (~HXRs) & increasing column density (~EM ~ SXRs) ? Ji et al. 2004 GOES RHESSI 12-25 keV RHESSI 25-50 keV H ribbons H 1.3 Å kernels @ HXR FPs

25. Intermediate thick-thin target in corona(instead of pure thick-target in corona) _____ _____ FP spectrum LT spectrum Dense region at loop apex (or extended part of coronal loop)  intermediate thick-thin target to traversing electron beam. Purely collisional. Wheatland & Melrose 1995 LT  1 (thick) FP  1 (thick)  = Eloop Eloop  (Nloop)1/2 LT  +1 (thin)

26. Intermediate thick-thin target in corona(instead of pure thick-target in corona) Testable model predictions (Wheatland & Melrose 1995): • Electrons with energies E < Eloop are stopped within corona. • LT and FP spectra are broken power-laws, break energy  = Eloop. Spatially integrated spectra have single power-law. • X-ray spectral index of LT source at photon energies  < Eloop is thesame as that of FP sources for  > Eloop:  = 1 (thick target). • X-ray spectral index of LT source at photon energies  > Eloop:  = +1 (thin target). • At photon energies  < Eloop the flux from the LT sources should dominate, at  > Eloop the flux from the FP sources should dominate.  To be checked for candidate flares with good count statistics!

27. Once more: 23rd July 2002 X4.8 flarePre-impulsive nonthermal coronal HXR source Broken power-lawspectrum ~ 5 ~ 7 Lin et al. (2003) Krucker et al. (2003)

28. Once more: 23rd July 2002 X4.8 flarePre-impulsive nonthermal coronal HXR source  ~ 2 Lin et al. (2003) T(t) EM(t) If thick-thin target transition in corona, then we epxect for LT: 1)  ~ 2 2) break~ Eloop 3) break increases in time asEloop (t)  N(t)1/2  EM(t)1/4 (t) ? Ebreak(t)

29. Part 2Particle acceleration in a collapsing magnetic trap(from an observational point of view)

30. Collapsing magnetic trap = System of moving magnetic field lines expelled from the reconnection region Encloses the region between the current sheet and the Fast Oblique Collisionless Shock (FOCS) above magnetic obstacle (MO) In this trap, pre-accelerated (e.g. by DC electric field) particles can be further accelerated and heated trap Somov & Kosugi 1997

31. Collapsing magnetic trap2 main processes of particle acceleration – Decrease of the field line length provides first-order Fermi acceleration (Somov & Kosugi 1997, Bogachev & Somov 2005) Compression of the magnetic field lines provides betatron acceleration (Faraday’s law) (Brown & Hoyng 1975, Somov & Bogachev 2003, Karlický & Kosugi 2004) B. Somov

32. Collapsing magnetic trapResults • The highest energy that an electron can acquire in the collapsing magnetic trap is the same for Fermi and betatron mechanism • However, trap with dominant betatron acceleration confines particles better betatron much more efficient in production of HXR coronal sources Bogachev & Somov (2005)

33. Collapsing magnetic trapResults ~ 5 ~ 7 • Formation of double power-law spectra in collapsing trap with background plasma (S. Bogachev, priv. comm. ) RHESSI observations Magnetic trap modeling

34. Altitude decrease of LT source in early flare phase RHESSI observations Movie 3 Nov 03, X3: H + RHESSI 10–15 keV Movie 3 Nov 03, X3: RHESSI 10–15 keV 10–15 keV Movie 24 Oct 03, M9: RHESSI 6–12 keV 3 Nov 2003 X2.7 flare (Joshi et al. 2006)

35. Altitude decrease of LT source in early flare phase RHESSI observations Height structure of LT source: Higher energies are located above lower energies 15 Apr 2002 M1.2 flare (Sui et al. 2004)

36. Altitude decrease of LT source in early flare phase Collapsing magnetic trap modeling Time evolution of emission centroid (for thermal bremsstrahlung) Simple model of the bottom of magnetic collapsing trap, betatron heating (M. Karlický in Veronig et al. 2006, see also Karlický 2006) Can account for the early LT altitude decrease In case of thermal X-ray emission also for the observed height structuring