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Time Variability of the Quiet Sun Observed with TRACE. II. Physical Parameters, Temperature Evolution, and Energetics of EUV Nanoflares View all abstracts by submitter

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

We present a detailed analysis of the geometric and physical parameters of 281 EUV nanoflares, simultaneously detected with the {sl TRACE} telescope in the 171 and 195 ang wavelengths. The detection and discrimination of these flare-like events is detailed in Paper I. We determine the loop length l, loop width w, emission measure EM, the evolution of the electron density n_e(t) and temperature Te(t), the flare decay time { au}decay, and calculate the radiative loss time { au}loss, the conductive loss time { au}cond { au}cond, and the thermal energy Eth. The findings are: (1) EUV nanoflares in the energy range of 1024-1026 erg represent miniature versions of larger flares observed in soft X-rays and hard X-rays, scaled to lower temperatures (Te lapprox 2 MK), lower densities (n_e lapprox 109 cm-3), and somewhat smaller spatial scales (lapprox 2-20 Mm); (2) The cooling time { au}decay is compatible with the radiative cooling time { au}rad, but the conductive cooling time scale { au}cond is about an order of magnitude shorter, suggesting repetitive heating cycles in time intervals of a few minutes; (3) The frequency distribution of thermal energies of EUV nanoflares, N(E)approx 10-46(E/1024)-1.8 [s-1 cm-2 erg-1] matches that of SXR microflares in the energy range of 1026-10^- 1026-1029, and exceeds that of nonthermal energies of larger flares observed in HXR by a factor of 3-10 (in the energy range of 1029-1032 erg. Discrepancies of the power-law slope with other studies, which report higher values in the range of aschwanden@lmsal.com: =2.3-2.6 (Krucker & Benz, or Parnell & Jupp), are attributed to methodical differences in the detection and discrimination of EUV microflares, as well as to different model assumptions in the calculation of the electron density. Besides the insufficient power of nanoflares to heat the corona, we find also other physical limits for nanoflares at energies lapprox 1024 erg, such as the area coverage limit, the heating temperature limit, the lower coronal density limit, and the chromospheric loop height limit. Based on these quantitative physical limitations, it appears that coronal heating requires other energy carriers that are not luminous in EUV, SXR, and HXR.

Authors: Aschwanden,M.J., Tarbell,T.D., Nightingale,R.W., Schrijver,C.J., Title,A.,

Publication Status: 2000, ApJ 535, 1047-1065
Last Modified: 2000-08-31 07:59
Go to main E-Print page  Time Variability of the Quiet Sun Observed with TRACE.         I. Instrumental Effects, Event Detection, and Discrimination of EUV Nanoflares  Large-Scale Activity in Solar Eruptive Events of October-November 2003 by SOHO/EIT Data   Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

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