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

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 within the corona  thick-target coronal HXR sources

 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 Thick-target coronal HXR sources Observational evidence

23rd July 2002 X4.8 flare Pre-impulsive nonthermal coronal HXR source Lin et al. (2003) Krucker et al. (2003) Broken power-law spectrum  ~ 7  ~ 5 Säm Krucker

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

Thick-target coronal HXR sources Observational 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

14/15 Apr 2002 M3.2 Flare RHESSI Lightcurves

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

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

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

EM = n 2 V  n  N = n L/2 14/15 Apr 2002 M3.2 Flare Hot loop plasma parameters I A = 2  cm 2 L = 45  10 8 cm V = AL ~ cm 3

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

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

14/15 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?

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

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

16 Apr 2002 M2.5 flare Another thick-target loop candidate

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

16 Apr 2002 M2.5 flare RHESSI keV HXR image sequence Again: HXR emission predominantly from loop top (vs footpoints)

16 Apr 2002 M2.5 flare Images and spectra during peak

FP2 FP1 LT  steep spectra   (LT, FP) ~ 0.5  smaller than in Battaglia & Benz (2006) sample thin-target LT, thick-target FPs: expected  = 2

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

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

9 Sep 2002 M2.3 Flare GOES, RHESSI & H  lightcurves GOES RHESSI keV RHESSI keV H  ribbons Ji et al H   1.3 Å HXR FPs 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) ?

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

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

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

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

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

Collapsing magnetic trap 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 Somov & Kosugi 1997

Collapsing magnetic trap 2 main processes of particle acceleration B. Somov 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) –

Collapsing magnetic trap Results  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)

 ~ 7  ~ 5 Collapsing magnetic trap Results  Formation of double power-law spectra in collapsing trap with background plasma (S. Bogachev, priv. comm. ) RHESSI observationsMagnetic trap modeling

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

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

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 Altitude decrease of LT source in early flare phase Collapsing magnetic trap modeling