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Time Variability of the Quiet Sun Observed with TRACE. I. Instrumental Effects, Event Detection, and Discrimination of EUV Nanoflares View all abstracts by submitter

Markus J Aschwanden   Submitted: 1999-10-06 21:02

The {sl Transition and Coronal Explorer (TRACE)} observed a ``Quiet Sun'' region on 1999 Feb 17 from 01: 30 UT to 10: 00 UT with full resolution (0.5'' pixel size), high cadence (125 s), and deep exposures (65 s and 46 s) in the 171 ang and 195 ang wavelengths. We start our investigation of the time variability of ``Quiet Sun'' images with a detailed analysis of instrumental and nonsolar effects, such as orbital temperature variations, filtering of particle radiation spikes, spacecraft pointing jitter, and solar rotation tracking. We quantify the magnitude of various noise components (photon Poisson statistics, data digitization, data compression, readout noise) and establish an upper limit for the data noise level, above which temporal variability can safely be attributed to solar origin. We develop a pattern recognition code which extracts spatio-temporal events with significant variability, yielding a total of 3131 events in 171 ang and 904 events in 195 ang . We classify all 904 events detected in 195 ang according to flare-like characteristics and establish a numerical flare criterion based on temporal, spatial, and dynamic cross-correla- cross-correlation coefficients between the two observed temperatures (0.9 MK and 1.4 MK). This numerical criterion matches the visual flare classification in 83% and can be used for automated flare search. Using this flare discrimination criterion we find that only 35% (and 25%) of the events detected in 171 (and 195) ang represent flare-like events. The discrimination of flare events leads to a frequency distribution of peak fluxes, N(Delta F) propto Delta F-1.83pm0.07 at 195 ang, that is significantly flatter than the distribut- distribution of all events. A sensitive discrimination criterion of flare events is therefore important for microflare statistics and for conclusions on their occurrence rate and efficiency for coronal heating.

Authors: Aschwanden,M.J., Nightingale,R., Tarbell,T. S., and Wolfson,C.J.

Publication Status: ApJ 535, 1027-1046, (2000)
Last Modified: 2000-08-31 07:47
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