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S. Christe (Physics Dept., UC, Berkeley), S. Krucker (Space Science Lab, UC, Berkeley), R.P. Lin (Physics Dept., UC, Berkeley), I. Hannah (Physics Dept., U. Glasgow, Scotland, UK)
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provides uniquely high sensitivity in the 3-15 keV energy range with an effective area from 14 to 130 times larger then previous solar instruments. We present a microflare frequency distribution derived from RHESSI observations. Times were chosen such that activity is very low (GOES B Class background) and includes a 24 hour period when no active regions were present on the Sun. Microflares were found through searching for 5\sigma increases in count rate, summing over all detectors, between two adjacent time bins in the 3-10 keV energy band. Count rates were binned from 4 seconds to 3 minutes, increasing by 4 seconds increments, and the search repeated in order to detect flares of various time scales. Each microflare was individually checked in order to reject non-solar events. Few microflares are seen to occur below the time scale of minutes. The observed occurence rate is one microflare every ~ 8 minutes above a threshold flux of ~ 0.4 cm-2 s-1 keV-1 over the 5-10 keV energy range. Microflares were also found when no active regions were present on the Solar disk. This work was supported by NASA contract NAS5-98033.
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Bulletin of the American Astronomical Society, 36 #2
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