Evidence for chromospheric heating in the late phase of solar flares

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Evidence for chromospheric heating in the late
phase of solar flares

• David Alexander
• Lockheed Martin Solar and Astrophysics Lab.

Collaborators Anja Czaykowska MPI
für extraterrestrische Physik Bart De
Pontieu LMSAL
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Summary of Presentation
SOHO/CDS

• Chromospheric evaporation revisited

• Summary of flare

• Coronal Diagnostic Spectrometer

• Results of data analysis

• Implications for chromospheric heating

• Conclusions

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Chromospheric Evaporation models

• Non-thermal energy deposition of energetic
  particles accelerated in flare
• Brown (1973) Hirayama (1974) Nagai Emslie
  (1984) Fisher, Canfield McClymont (1985)
  Mariska, Emslie Li (1989)
• Thermal energy is transported
• to chromosphere via thermal
• conduction fronts of related shocks
• Brown (1974) Hirayama (1974)
• Antiochos Sturrock (1978)
• Forbes, Malherbe Priest (1989)
• Yokoyama Shibata (1997)

from Cargill Priest (1983)
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Chromospheric heating non-thermal

• Fisher et al., 1985a,b made distinction between
  gentle and explosive evaporation
• Gentle evaporation ? Velocities lt 100 km/s
• Upflow velocities depend crucially on total flux
  of electrons.
• Fisher et al. (1985a,b) F (E/Ec)-d d 4
  Ec 20 keV
• Mariska et al. (1989) F (E/Ec)-d d 6 Ec
  15 keV

f 109 ergs/cm2/s ? Vupflow lt 30 km/s f 1010
ergs/cm2/s ? Vupflow 130 km/s
f total incident electron energy flux
f1010 ergs/cm2/s ? Vupflow ? 200 km/s
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QUICK LOOK AT THE FLARE
BBSO Ha
MDI
Sunspot plage
expanding ribbons
EIT FeXII
CDS OV
0.25 MK
1.5 MK
CDS FeXVI
CDS FeXIX
2.0 MK
8.0 MK
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CDS DOPPLERGRAMS

• distinctive pattern of
• redshifts and blueshifts
• blueshifts confined to
• leading edges of arcade
• redshifts predominate
• towards neutral line

Interesting differences near sunspot
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Velocity profiles

• Spatial profiles (a) show
• transition from blue- to
• red-shift.
• Line profiles (b) show broad
• lines but resolvable shifts
• Different locations along
• ribbon show similar behaviour

Velocity discrimination OV Dv 5-10
km/s FeXVI Dv 10-20 km/s FeXIX Dv 30 km/s
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Location of upflow regions

• Upflows at leading
• edge of Ha ribbon
• Ridge of upflowing
• plasma moves with
• Ha ribbon
• Upflow regions
• become downflow
• regions as ribbons
• move outwards

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Continued heating in late gradual phase
The CDS observations provide direct evidence for
the presence of continuing energisation presumably
due to ongoing reconnection
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Hard X-ray Observations

• The ratio of the counts in the two
• medium energy bands HXT M2/M1
• yields a photon spectral index of
• g4 during the initial decay phase of
• the flare.
• All channels show a count rate
• below background levels by about
• 1700 UT, some 40 minutes prior to
• the first CDS observations.

Yohkoh HXT
Background level in HXT L channel is 1.25
cts/s/SC or 80 cts/s summed over all
detectors. Thus, a background subtracted signal
strength of 26 cts/s will produce a 2s detection
in the integrated HXT L channel.
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Hard X-ray production from a non-thermal
electron beam
Assume that chromosphere acts like a thick-target
to a beam of electrons with energy distribution
FAE-d ?
Mariska, Emslie Li (1989)
Convolve photon spectrum with HXT response
function to get count rate in HXT L channel
Alexander Metcalf (1999)
h(e) is the transmission efficiency of the HXT
filter, G(e,p) is the pulse height distribution
of the detector s(e) is the probability that an
incoming photon will escape with an energy e.
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Chromospheric heating non-thermal

• Fisher et al., 1985a,b made distinction between
  gentle and explosive evaporation
• Gentle evaporation ? Velocities lt 100 km/s
• Upflow velocities depend crucially on total flux
  of electrons.
• Fisher et al. (1985a,b) F (E/Ec)-d d 4
  Ec 20 keV
• Mariska et al. (1989) F (E/Ec)-d d 6 Ec
  15 keV

f 109 ergs/cm2/s ? Vupflow lt 30 km/s f
1010 ergs/cm2/s ? Vupflow 130 km/s
f total incident electron energy flux
f1010 ergs/cm2/s ? Vupflow ? 200 km/s
Observed upflows ? 109 ? f ? 1010 ergs/cm2/s
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Simulated HXR emission
Single footpoint Ec 20 keV N(EltEc)E-2
20 footpoints Ec 20 keV N(EltEc)E-2
HXT L flux (cts/s)
f 1010
f 109
2s detection
3 4 5 6 7
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3 4 5 6 7
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Spectral Index d
Expected HXT L channel count rates as a function
of spectral index
Single footpoint means S1017 cm2 ? 1 CDS pixel
Electron fluxes necessary to produce observed
upflow velocities would also generate detectable
hard X-ray signatures
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Chromospheric Heating conduction fronts (I)

• Electrons are heated as they diffuse through
• the conduction front.
• Fronts stand in front of slow-mode shocks
• For efficient heating the thermal thickness
• of the slow shock must exceed the height
• of the flare loop ( 5 x 104 km)

T 10 MK, n ? 2x1010 cm-3, v ? 50km/s, cp
2.07x108 cm2s-2K-1 ?
w 9 x 104 km
Forbes Malherbe (1986) Forbes, Malherbe
Priest (1989)
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Chromospheric Heating conduction fronts (II)
w 9 x 104 km
Velocities Forbes et al. predict very small
evaporative flows v ? 5 km/s Recent numerical
reconnection model of Yokoyama Shibata (1997)
includes conduction and yields evaporative
upflows with speeds 0.2 - 0.3 x the local sound
speed v ? 40 km/s
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Chromospheric Heating conduction fronts (II)
w 9 x 104 km
Velocities Forbes et al. predict very small
evaporative flows v ? 5 km/s Recent numerical
reconnection model of Yokoyama Shibata (1997)
includes conduction and yields evaporative
upflows with speeds 0.2 - 0.3 x the local sound
speed v ? 40 km/s
Thus, our observations suggest that conduction
front heating of the chromosphere dominates at
this stage of the flare. This agrees well with
the conclusions of Falchi, Qiu Cauzzi (1997)
who detected 20-30 km/s downflows at the outer
edge of Ha ribbons in the decay phase of an M2.6
flare.
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Conclusions

• Reconnection is an ongoing process throughout
  the entire
• duration of a solar flare.
• The dominant consequences of that reconnection
• transition smoothly(?) from energetic particle
  production
• to shock and conduction front formation. cf.
  Wülser et al (1994)

Relative strength of thermal/non-thermal heating
with time