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

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.

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

Flare examples

Size - Size - SS

Flare examples

Size - XSSize - XS

Flare examples

Size - XSSSize - XSS

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.

CGROCGRO/BATSE /BATSE Lin et al. 2001Lin et al. 2001

down to down to 3 keV 3 keV 300x 300x

smallersmaller

*define peak rate : *define peak rate : photons sphotons s-1-1 cm cm-2-2 keV keV-1-1

8-13 and 13-20 keV Peak Rate

Distribution of and Temp.

• Superhot components with T>20 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.

4-7 and 7-10 keV

• Interpretation of emission is unclear.Interpretation of emission is unclear.

Full Spectra Results

Finer energy bins can be used on the spectra of larger flares (> 1000 total counts, 25% of events).

These larger flares show only thermal emission below 10 keV and confirm ratio estimations.

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).

RHESSI MicroflaresSteven 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

The End.The End.

Flare Search Criteria

Events were selected if a 5 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.