Yohkoh statistical studies

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Yohkoh statistical studies

• Michal Tomczak
• Astronomical Institute,
• University of Wroclaw, Poland

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Introduction

• Yohkoh Hard X-ray Telescope gave for the first
  time an opportunity for massive investigation of
  spatial distribution of hard X-ray emission in
  solar flares the mission-long database (Oct 1,
  1991 Dec 14, 2001) contains 3071 events.

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Observational constraints

• to obtain a valuable hard X-ray image about
  100200 cts/SC are needed ? flux limit.
• to distinguish different sources a flare size
  should be larger than spatial resolution of the
  instrument (we cannot resolve events having h lt
  89 x 103 km) ? size limit.
• to distinguish coronal and footpoint sources a
  contamination of their photons should be omitted
  event should be seen on-side ? geometry limit.

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Selection criteria (Masuda 1994)

• peak count rate in the M2band exceeding 10 cts
  s-1 SC-1 at least one image is available in the
  channel which records the radiation of purely
  non-thermal electrons.
• heliocentric longitude exceeding 80º.

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Comparison between surveys
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What have the surveys taught us about coronal
sources?

• They are a common feature of solar flares (M
  7/10 P 15/18 TC 45/117).
• During the impulsive phase they are usually
  fainter than footpoint sources, a disproportion
  becomes more important at bursts maxima and for
  higher photon energies.
• Their photon energy spectra are usually softer
  than the spectra of footpoint sources (an
  exception the above-the-loop-top sources).

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L
M1
Masuda 1994
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August 18, 1998
Petrosian et al. 2002
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Petrosian et al. 2002
Tomczak Ciborski 2006
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Masuda 1994
Footpoint sources
Loop-top sources
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Petrosian et al. 2002
?FT 4.9 1.5 ?LT 6.2 1.5
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HXR imaging constraints

• The finite dynamic range of the HXT estimated to
  be about 1 decade (Sakao 1994).
• The generation of spurious sources by the
  reconstruction routines ? a false photometry of
  real sources.
• Weak sources suppression in the presence of
  strong sources (Alexander Metcalf 1997) ? the
  reconstruction routines treat them as a
  statistically insignificant.

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Alexander Metcalf 1997, ApJ, 489, 442
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To study weak sources in the presence of strong
sources is our case! HXR imaging constraints
limit a possibility of investigation of coronal
sources. Moreover, our results can be even false
due to the light curve mimicking that of the
footpoints.
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What can we do?

• To wait for modern instruments with a better
  dynamic range as well as for clever
  reconstructions routines.
• To investigate only examples in which the coronal
  sources dominate ? we obtain only a partial
  picture.
• To eliminate somehow stronger footpoint sources

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Behind-the-limb flares

• We use the solar limb as a screen which
  occults the lower part of the flaring structure
  (footpoint sources) and leaves emission of the
  higher part only (coronal sources)
• in this way we can separate coronal sources of
  all type our choice do not favour any particular
  physical mechanism,
• - we loose a possibility of the comparison with
  the footpoint sources.

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The behind-the-limb configuration has been used
in many different way

• for HXRs (e.g. Frost Dennis 1971, stereo-scopic
  papers of Kane).
• Yohkoh Bragg Crystal Spectrometer diagnostics of
  bright SXR loop-top kernels (e.g. Khan et al.
  1995, Mariska et al. 1996, Mariska McTiernan
  1999).
• BCS diagnostics of X-ray plasma ejections
  (Tomczak 2005).

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How can we find out such events?

• Compare the GOES list of flares to the Ha list
  from the SGD events present in the first one and
  absent in the second one can be behind-the-limb
  type ? a prompt selection.
• Check manually soft X-ray images of the
  prompt-selected flare ? impulsive SXR
  brightenings should not be seen
• Check a time of the limb passage for the active
  region in which the prompt-selected flare has
  occurred ? ?(t) extrapolation

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Tomczak (2001), AA, 366, 294

• 14 behind-the-limb flares that occurred between
  19911994 has been selected.
• In this case to investigate time variation of
  coronal sources we need not actually HXR images!
  ? our temporal resolution becomes better.

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• Additional 16 partially occulted flares observed
  by Yohkoh in years 19971999 are preparing
  (Tomczak Sokolnicki).

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Masuda flare

• Masuda et al.1994, Nature, 371, 495 about 300
  citations in the Smithsonian/NASA Astrophysics
  Data System.
• Tomczak Ciborski (2006) an additional argument
  confirming how unusual this event is.

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Explanation

• Different converging field geometry events from
  branch A occurred in more converged loops than
  the events from branch B the more converged
  loops from branch A correspond to flux tubes that
  are less helically twisted the less converged
  loops from branch B are more twisted.
• Anomalous electron scattering absent for events
  from branch A, present for events from branch B.