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 within the
corona? thick-target coronal HXR sources
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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

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23rd July 2002 X4.8 flare Pre-impulsive
nonthermal coronal HXR source
Lin et al. (2003) Krucker et al. (2003)
Säm Krucker
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23rd July 2002 X4.8 flare Impulsive phase
thermal coronal HXR source
Emslie et al. (2003) Krucker et al. (2003)
Säm Krucker
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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

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14/15 Apr 2002 M3.2 FlareRHESSI Lightcurves

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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
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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
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14/15 Apr 2002 M3.2 Flare Electron beam
characteristics
very steep spectra ? 7
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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
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14/15 Apr 2002 M3.2 Flare Hot loop plasma
parameters II
25?35 keV lt Eloop lt 45?60 keV
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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
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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?
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14/15 Apr 2002 M3.2 Flare Preflare
Preflare Flare
RHESSI Obs
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14/15 Apr 2002 M3.2 Flare NoRH flare and
preflare images
No RHESSI observations of preflare available but
NoRH Image preflare (2341 UT) Contours flare
(0002 UT)
Image preflare (2341 UT) Contours flare
(0002 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)
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16 Apr 2002 M2.5 flareAnother thick-target loop
candidate

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16 Apr 2002 M2.5 flareRHESSI TRACE imaging
TRACE 195 TRACE running diff RHESSI
One of Sui et al. flares
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16 Apr 2002 M2.5 flareRHESSI 20-50 keV HXR image
sequence
Again HXR emission predominantly from loop top
(vs footpoints)
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16 Apr 2002 M2.5 flareImages and spectra during
peak
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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
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16 Apr 2002 M2.5 flareHot loop parameters
Again High column densities, steep HXR spectra
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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
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9 Sep 2002 M2.3 Flare GOES, RHESSI H?
lightcurves
Ji et al. 2004
GOES RHESSI 12-25 keV RHESSI 25-50 keV H?
ribbons
H? ?1.3 Å kernels _at_ HXR FPs
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Intermediate thick-thin target in corona(instead
of pure thick-target in corona)
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)
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Intermediate thick-thin target in corona(instead
of pure thick-target in corona)

• Testable model predictions (Wheatland Melrose
  1995)
• Electrons with energies E lt 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 ? lt Eloop is the same as that of FP
  sources for ? gt Eloop ? ??1 (thick target).
• X-ray spectral index of LT source at photon
  energies ? gt Eloop ? ?1 (thin target).
• At photon energies ? lt Eloop the flux from the
  LT sources should dominate, at ? gt Eloop the
  flux from the FP sources should dominate.
• ? To be checked for candidate flares with good
  count statistics!

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Once more 23rd July 2002 X4.8 flare
Pre-impulsive nonthermal coronal HXR source
Lin et al. (2003) Krucker et al. (2003)
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Once more 23rd July 2002 X4.8 flare
Pre-impulsive nonthermal coronal HXR source
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 as Eloop (t) ?
  N(t)1/2 ? EM(t)1/4

?(t)
?
Ebreak(t)
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Part 2Particle acceleration in a collapsing
magnetic trap(from an observational point of
view)
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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
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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
(Faradays law) (Brown Hoyng 1975, Somov
Bogachev 2003, Karlický Kosugi 2004)
B. Somov
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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)
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Collapsing magnetic trapResults

• Formation of double power-law spectra in
  collapsing trap with background plasma (S.
  Bogachev, priv. comm. )

RHESSI observations
Magnetic trap modeling
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Altitude decrease of LT source in early flare
phase RHESSI observations
Movie 3 Nov 03, X3 H? RHESSI 1015 keV
Movie 3 Nov 03, X3 RHESSI 1015 keV
1015 keV
Movie 24 Oct 03, M9 RHESSI 612 keV
3 Nov 2003 X2.7 flare (Joshi et al. 2006)
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Altitude decrease of LT source in early flare
phase RHESSI observations
15 Apr 2002 M1.2 flare (Sui et al. 2004)
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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