1. RHESSI Microflares Steven Christe 1,2, Säm Krucker 2, Iain Hannah 3, R. P. Lin 1,2 1 Physics Department, University of California at Berkeley 2 Space Sciences Lab, University of California at Berkeley 3 Physics Department, University of Glasgow, Scotland, UK

2. 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.

3. Dataset Overview Period considered : January 27-30, 2004 May 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

4. Flare examples Size - S

5. Flare examples Size - XS

6. Flare examples Size - XSS

7. 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. CGRO/BATSE Lin et al. 2001 down to 3 keV 300x smaller *define peak rate : photons s -1 cm -2 keV -1

8. 8-13 and 13-20 keV Peak Rate

9. 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.

10. 4-7 and 7-10 keV Interpretation of emission is unclear. Interpretation of emission is unclear.

11. 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.

12. 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 10 27 ergs (10 23 ergs if the cutoff is set to 25 keV).

13. RHESSI Microflares Steven Christe 1,2, Säm Krucker 2, Iain Hannah 3, R. P. Lin 1,2 1 Physics Department, University of California at Berkeley 2 Space Sciences Lab, University of California at Berkeley 3 Physics Department, University of Glasgow, Scotland, UK

14. The End.

15. 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.

16. 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