Chromospheric Evaporation

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Chromospheric Evaporation

• Electrons are accelerated during impulsive phase
  of solar flares.
• These electrons supply the necessary energy to
  heat the chromosphere and cause it to expand.
• This process gives rise to the gt1 MK emission
  seen in post-flare loops.

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Cause and Effect

• Evidence for the non-thermal electrons is visible
  from HXR emission (gt20 keV).
• RHESSI images and spectra can determine
  properties of driving electron beam.
• The response of the chromosphere is evident from
  Doppler shifts in EUV emission lines.
• CDS can determine these velocities across a wide
  range of temperatures.

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Previous Works

• Theoretical
• - Neupert (1968)
• - Antiochos Sturrock (1978)
• - Fisher et al. (1984,
• 1985 a, b, c)
• - Mariska, Emslie Li (1989)
• - Abbett Hawley (1999)
• - Allred et al. (2005)

• Observational
• - Antonucci Dennis (1983)
• - Canfield et al. (1987)
• - Zarro Lemen (1988)
• - Czaykowska et al. (1999)
• - Teriaca et al. (2003)
• - Brosius Phillips (2004)

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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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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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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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Velocity against Temperature
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Summary

• First observations of chromospheric evaporation
  from HXR footpoints using RHESSI and SOHO/CDS
  (directly comparable with theory).
• Observational evidence for both gentle and
  explosive evaporation.
• References
• Milligan et al. (2006, ApJL, 638, 117)
• Milligan et al. (2006, ApJL, Accepted)

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Need for Imaging Spectroscopy

• Until now I have investigated only single source
  flares
• Examples such as this require the individual
  spectra from each footpoint

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Some key remaining questions

• Is evaporation observed in all flares with HXR
  emission?
• Can we distinguish between thermal and
  non-thermal heating processes?
• How is the flares energy budget divided ?
• What role do ions play?
• Does the flux of electrons account for all the
  material observed in post-flare loop?
• In the future, performing joint observations
  between RHESSI and Solar-B/EIS will help to
  answer many of these questions.

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Advantages of EIS over CDS
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CDS emission lines
Stationary Fe XIX
Stationary blueshifted Fe XIX
Fe XII
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RHESSI Lightcurves
C9.1
M2.2
14
RHESSI Spectra
C9.1
M2.2
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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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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