RHESSI Microflares

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RHESSI Microflares

• Steven Christe1,2, Säm Krucker2,
• Iain Hannah3, R. P. Lin1,2
• 1Physics Department, University of California at
  Berkeley
• 2Space Sciences Lab, University of California at
  Berkeley
• 3Physics Department, University of Glasgow,
  Scotland, UK

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RHESSI Microflare Observations

• Provides unique sensitivity in the 3 to15 keV
  energy range.
• Effective area 100 times larger then HXIS (SMM)
  at 10 keV.
• An energy resolution of 1 keV can give a better
  handle on how to interpret spectra.
• RHESSI can provide new information on low-level
  energy releases.

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Dataset Overview

• Period considered January 27-30, 2004May 1-5,
  2004
• These periods were chosen because of extremely
  low solar activity.
• Number of Events 230
• GOES Range A0.01 - C
• Flaring rate 2.5 per hour

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Flare examples
Size - S
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Flare examples
Size - XS
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Flare examples
Size - XSS
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8-13 and 13-20 keV Peak Rate

• We see a clear correlation since larger flares
  have more emission in all channels.
• The spread in the correlation gives information
  on the steepness of the flare spectra.

down to 3 keV 300x smaller
define peak rate photons s-1 cm-2 keV-1
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8-13 and 13-20 keV Peak Rate
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Distribution of g and Temp.

• Superhot components with Tgt20 MK do occur though
  usually only associated with large flares (Hudson
  Nitta 1996).
• For small bursts, a typical temperature is 10 MK.
• Therefore emission is most likely nonthermal.

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4-7 and 7-10 keV

• Interpretation of emission is unclear.

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Full Spectra Results

• Finer energy bins can be used on the spectra of
  larger flares (gt 1000 total counts, 25 of
  events).
• These larger flares show only thermal emission
  below 10 keV and confirm ratio estimations.

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Conclusions

• The transition from thermal to nonthermal must
  occur between 7 - 13 keV for small flares if
  small flare are not superhot.
• Interpreting thermal and nonthermal emission in
  the smallest flares will be difficult.
• Using these results we find the smallest events
  have energies of 1027 ergs (1023 ergs if the
  cutoff is set to 25 keV).

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RHESSI Microflares

• Steven Christe1,2, Säm Krucker2,
• Iain Hannah3, R. P. Lin1,2
• 1Physics Department, University of California at
  Berkeley
• 2Space Sciences Lab, University of California at
  Berkeley
• 3Physics Department, University of Glasgow,
  Scotland, UK

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The End.
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Flare Search Criteria

• Events were selected if a 5s increase was
  detected between adjacent time bins. Time bins
  from 4 s. to 3 mins. were considered.
• Periods considered May 1-5 and Jan 27-30, 2004
• Non-solar event were individually rejected.
• Each event is also associated with a post or
  pre-event background.

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Discussion of energy?

• Thermal energy
• If we assume a source size at the limit of RHESSI
  resolution then Eth 1e27 erg
• Nonthermal energy
• A maximum value of energy for the smallest flares
  - if the cutoff is set to 7 keV 1e27 erg
• If the low energy cutoff is set to 25 keV 1e23
  erg