Coronal Trapping of Energetic Flare Particles: Yohkoh/HXT Observations

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Coronal Trapping of Energetic Flare Particles
Yohkoh/HXT Observations

• T. Metcalf
• D. Alexander
• Lockheed Martin Solar and Astrophysics Laboratory

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Introduction

• Trapping has been seen in many flares using
  observations of time delays.
• Time delays can be ambiguous is it a trap or is
  it, e.g. a two-step acceleration process or is
  the time dependence of the spectra reflecting a
  spectrally varying time dependence of the
  acceleration process?
• We observe the time independent spectral
  signature of electron trapping in the corona,
  independent of the temporal properties of the
  acceleration process.

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Trapping Time

• In the weak diffusion limit, the trapping time
  for an electron of energy E is proportional to
  the energy to the 3/2 power

• Hence, higher energy electrons stay in the trap
  longer.
• The spectrum of particles in the trap changes
  with time as the lower energy electrons leak out
  first.
• For a continuous injection of particles, the trap
  spectrum evolves until it is 3/2 powers harder
  than the injected spectrum.

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Trap Spectrum Impulsive
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Trap Spectrum Continuous
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Trapping Signatures
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Method

• To observe the spectral signature of trapping, we
  must have a measure of both the injected spectrum
  and the spectrum of the particles in the trap.
• Here we make the assumption that the electron
  spectrum observed at the footpoints is a measure
  of the injected spectrum.
• We compare the spectrum observed in the corona to
  the spectrum observed at the footpoints.

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Flare Images
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Light Curves
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Spectral Analysis
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Trap Models

• To fully explain the observations, the coronal
  trap must be thick-thin, i.e. thick to low energy
  electrons and thin to high energy electrons with
  the transition about 25 keV (Wheatland
  Melrose).
• This implies a trap density of

• Alternatively, the trap itself may be short
  lived. This collapsing trap model would not
  require such high densities in the coronal HXR
  source (Somov Kosugi).
• For a trap collapse time of order 10s, electrons
  below 25 keV preferentially escape the trap.
  This is only marginally consistent with the HXT
  data.
• HESSI will be able to discriminate between
  various models with is superior spectral
  resolution.

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Conclusions

• Three of Six flares studied are consistent with a
  coronal trap in the weak diffusion limit..
• Of the remaining three, one is thermal, one has a
  broken power law spectrum, and one may be
  consistent with a trap in the strong diffusion
  limit.
• Both the trapped and the precipitating
  populations are consistent with a single electron
  spectrum injected into the corona.
• There must be a magnetic geometry conducive to
  trapping in the corona.
• To make this observation, HXR imaging
  spectroscopy is required. The HXT spectral
  resolution is marginal, but HESSI will allow a
  more detailed study.