RHESSI Microflares Steven Christe 1 2 Sm Krucker

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

RHESSI Microflare Observations Provides unique sensitivity in the 3 to 15 ke. V energy range. l Effective area ~100 times larger then HXIS (SMM) at 10 ke. V. l An energy resolution of 1 ke. V can give a better handle on how to interpret spectra. l RHESSI can provide new information on low-level energy releases. l

Dataset Overview l l l 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 : A 0. 01 - C Flaring rate : 2. 5 per hour

Flare examples Size - S

Flare examples Size - XS

Flare examples Size - XSS

8 -13 and 13 -20 ke. V 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. *define peak rate : photons s-1 cm-2 ke. V-1 CGRO/BATSE Lin et al. 2001 down to 3 ke. V 300 x smaller

8 -13 and 13 -20 ke. V Peak Rate

Distribution of g 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 ke. V • 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). l These larger flares show only thermal emission below 10 ke. V and confirm ratio estimations. l

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

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

The End.

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