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Session 70 - Solar Flares and Ejections.
Display session, Thursday, June 13
Tripp Commons,

[70.14] Evidence for a Cutoff in the Frequency Distribution of Solar Flares from Small Active Regions

T. A. Kucera (Applied Research Corp.), B. R. Dennis (NASA/GSFC), R. A. Schwartz (Hughs/STX), D. Shaw (NOAA/SEL)

We present evidence that active regions with small-sunspot areas have an upper limit to the energy of the flares they produce. This result is consistent with predictions of the avalanche model of Lu et al. (1993).

The results were obtained using data from the Hard X-ray Burst Spectrometer on the Solar Maximum Mission to study differences in the frequency distributions of solar flares as grouped by active region characteristics. Active region parameters considered were longitudinal extent, total sunspot area, Mt.\ Wilson Class, and McIntosh Class.

We find that there are fewer high count-rate flares (\ga 10^4 counts s^-1) from regions with small sunspot areas (0-500 micro-hemispheres) than would be expected from a power-law extrapolation from the frequency distribution of flares with peak-rates >50 counts s^-1. This is not found in the distribution of flares produced by regions with large sunspot areas (600-3600 micro-hemispheres). Using our analysis of the data and the predictions of the avalanche model, we calculated a limit to the energy of a flare which can be produced by an active region with given sunspot area.

There are no statistically significant differences between the frequency distributions of flares with peak count rates \ga 10^3 counts s^-1 grouped according to the other region characteristics studied.

We also find that in all cases, large, complex regions appear to produce a lower percentage of low-energy events than do smaller, simpler regions. It is possible that this effect is the result of biases against observations of low count-rate flares and determination of their locations.

Program listing for Thursday