RHESSI Observations of Super-Hot (T > 30 MK) Plasma in Large Solar Flares

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RHESSI Observations ofSuper-Hot (T gt 30 MK)
Plasmain Large Solar Flares
Dr.

• Amir Caspi1 Robert P. Lin1
• 1 Department of Physics Space Sciences
  Laboratory,
• University of California, Berkeley, CA 94720

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Motivation

• Super-hot (T gt 30MK)
• plasma first measured in
• 1981 (Lin et al.)
• Ge detectors offer the
• necessary resolution
• Little is still known about
• the origins or evolution of
• such high temperatures
• RHESSI provides unprecedented observations
  (spectra and images) to explore the origins of
  super-hot flare plasmas

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Overview

• Breadth statistical survey of 37 large flares
• 25 M flares (Feb-Sep 2002)
• 12 X flares (Jul 2002-2004)
• Snapshot of each flare maximum RHESSI
  temperature versus other parameters
• Depth case study
• 2002-Jul-23 X4.8 flare
• Time evolution of flare parameters

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Flare Temperature Statistics
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Flare Temperature Statistics
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Flare Temperature Statistics
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Case study 2002 Jul 23 X4.8 flare

• X-class flare, shows super-hot temperature in
  statistical survey
• Well-observed throughout rise, peak, and decay
• Much studied (entire issue of ApJL) with many
  observations - provides extensive context
• Carefully calibrated for precise spectroscopy
• Evolution of spectra and images observed
  throughout the flare

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Case Study 2002 Jul 23 X4.8 flare
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Case Study 2002 Jul 23 X4.8 flare
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Case Study 2002 Jul 23 X4.8 flare
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Case Study 2002 Jul 23 X4.8 flare
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Case Study 2002 Jul 23 X4.8 flare
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2002 Jul 23 Preimpulsive phase

• Dominated by HXR coronal source
• Little to no footpoint emission throughout this
  phase, suggesting negligible chromospheric
  evaporation
• Coronal source is predominantly non-thermal, but
  analysis of Fe Fe/Ni lines shows thermal plasma
  must be present at temperatures of at least 25
  MK and densities of at least 1010-1011 cm-3
• Super-hot plasma exists at high densities from
  the start of detectable emission
• Unlikely to result from chromospheric evaporation

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Possible origins for super-hot plasma
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Summary

• Bimodal temperature distribution persists
  throughout the flare
• Super-hot flares (predominantly X-class) require
  strong coronal fields
• Require more sensitive observations to observe
  super-hot formation

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EXTRA SLIDES
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Flare Temperature Statistics
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Methodology

• For a given flare, we perform the following
  analysis
• Obtain spectra (all grids excl. 2 7) in 20-sec
  intervals during impulsive rise and gradual decay
  phases
• Fit each interval with isothermal continuum,
  power-law non-thermal continuum, 2 Gaussian lines
  (Fe Fe/Ni complexes)
• Image CLEAN w/ grids 3-9 (excl. 7) in 6.18-7.18
  keV energy band (Fe line complex,
  thermally-dominated), 60-sec durations
• Approximate thermal source volume based on area
  enclosed by 50 CLEAN contour
  (contour at 50 of peak pixel flux)
• From fit parameters and imaging, computer
  non-thermal energy flux, number of thermal
  electrons
• Compare with estimated evaporation based on heat
  input from observed non-thermal energy flux