Observational Evidence for Chromospheric Evaporation in Solar Flares

1
Observational Evidence for Chromospheric
Evaporation in Solar Flares

• Ryan Milligan (QUB/NASA-GSFC)
• Peter Gallagher (TCD/L3/NASA-GSFC)
• Mihalis Mathioudakis (QUB)

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Chromospheric Evaporation

• Electrons are accelerated during impulsive phase
  of solar flares
• These non-thermal electrons supply the necessary
  energy to heat the chromosphere and hence cause
  flows

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Theory (Fisher et al.1985)

• Gentle Evaporation
• Chromosphere heated directly by non-thermal
  electrons (or by thermal conduction)
• Electron flux lt 1010 ergs cm-2 s-1
• Upflow velocities lt 100 km s-1
• No downflows

• Explosive Evaporation
• Chromosphere unable to radiate energy deposited
  by non-thermal electrons
• Flux gt 3?1010 ergs cm-2 s-1
• Upflow velocities gt 200 km s-1
• Downflow velocities lt 100 km s-1

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Purpose of this study

• To search for signatures of chromospheric
  evaporation in solar flares using simultaneous
  X-ray and EUV observations
• To investigate the relationship between the flux
  of non-thermal electrons (RHESSI) and the dynamic
  response of the solar atmosphere (CDS)

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RHESSI Spectra Images

• Thermal spectrum from hot (107K) plasma within
  the loop
• Non-thermal spectrum from thick-target collisions
  between fast e- (gt20 keV) and dense chromospheric
  plasma

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SOHO/CDS

• Emission line spectra observed across wide range
  of temperatures
• He I (0.03 MK)
• O V (0.25 MK)
• Mg X (1.2 MK)
• Fe XVI (2.5 MK)
• Fe XIX (8 MK)

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RHESSI Lightcurves
C9.1
M2.2
Vertical dashed lines represent timerange over
which CDS observed blueshifts in Fe XIX. RHESSI
images and spectra were therefore formed to
correspond to these times.
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Evidence for Gentle Evaporation

• Flux 5109 ergs cm-2 s-1
• ec 20 keV, d 5.2
• He I upflow 15 km s-1
• O V upflow 15 km s-1
• Fe XIX - upflow 100 km s-1

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Evidence for Explosive Evaporation

• Flux 41010 ergs cm-2 s-1
• ec 20 keV, d 7.3
• He I downflow 35 km s-1
• O V downflow 45 km s-1
• Fe XIX upflow 230 km s-1

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Velocity vs. Temperature
Downflows
Upflows
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Summary and Future Work

• First simultaneous observations of chromospheric
  evaporation from HXR footpoints using RHESSI and
  SOHO/CDS
• Observational evidence for both gentle and
  explosive evaporation
• Extend this study to a larger sample of flares
• In future extend to joint RHESSI and Solar-B/EIS

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CDS emission lines
Stationary Fe XIX
Stationary blueshifted Fe XIX
Fe XII
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RHESSI spectra
C9.1
M2.2
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Evidence for Upflows
Doppler shifts measured relative to a stationary
component Fe XIX ? v 26015 km/s
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M2.2 Flare CDS/EIT/GOES