Coronal Hard Xray emission

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Coronal Hard X-ray emission

• Laura Ann Bone
• February 2005

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The standard model
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Coronal hard X-ray Sources

• First observed during the 1970s in occulted
  events.
• E.g. Frost and Dennis (1971), Hudson (1978).
• Often associated with a variety of
  multiwavelength phenomena particularly microwave
  emission.

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Coronal HXR flares

• Several flares studied
• Typically show densities greater than 1011cm-3.
  Column densities greater than 1020 cm-3 .
• All but highest energy electrons are stopped
  collisionally in the corona.

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14/15th April 2002

• M3.2 flare
• Photon spectrum fitted by thermal plus broken
  power law (non-thermal).

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Some nice picture
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Some nice pictures (continued)
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20 July 2002

• X3.3 flare on eastern limb
• Originates in active region over eastern limb,
  occulted by 10'' or 11000km
• Hard X-ray source extends to 20-30'' over limb
  which when added to occultation height gives a
  source height of up to 30000km above photosphere.
  

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Some even nicer pictures
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Coronal HXR flares

• Typically one to two orders of magnitude denser
  than normal flares.
• Consistent with a coronal thick target
  interpretation
• Loop column density N is capable of stopping
  electrons with Egt50KeV (by
• )
• In flares with steep spectral index almost all
  electrons stopped in corona, no footpoint
  emission
• 
  

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Coronal HXR flares

• Increase in column density is attributed to
  evaporation of chromospheric matter by electron
  beams or thermal conduction.
• Beam driven evaporation gives a value for N
  substantially less than the observed value (up to
  two orders of magnitude).

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Coronal HXR flares

• Alternatively conductive evaporation gives N in
  excellent agreement with observed values.
• Therefore high N is probably a result of
  conductive evaporation.

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Coronal HXR flares

• Plotting of GOES flux reveals, in each case,
  preflare emission that would be capable of
  substantially increasing column density and
  Eloop.
• Preflare phase is in need of further
  investigation.

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Coronal HXR flares
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Coronal HXR flares

• Column density is already high at onset of
  impulsive phase
• Preflare emission imaged by Nobeyama
  radioheliograph is consistent with emission in a
  set of nested loops in 14th/15th April flares.
• In 20 July event emission in RHESSI is rising
  before onset of satellite night, GOES rises
  steadily throughout.

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Coronal HXR flares
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Summary

• Coronal hard X-ray emission is caused by electron
  beams dumping most of their energy before they
  reach the chromosphere.
• In flares with steep spectral indexes almost no
  footpoint emission is observed.
• Densities are consistent with conductive, rather
  than beam driven evaporation.
• Need to think of flare as a 3-d process to gain
  better understanding of these events.