Time variability of the quiet Sun observed with TRACE.: II.: Physical parameters, temperature evolution, and energetics of extreme-ultraviolet nanoflares

We present a detailed analysis of the geometric and physical parameters of 281 EUV nanoflares, simultaneously detected with the TRACE telescope in the 171 and 195 Angstrom wavelengths. The detection and discrimination of these flarelike events is detailed in the first paper in this series. We determine the loop length l, loop width w, emission measure EM, the evolution of the electron density n(e)(t) and temperature T-e(t), the hare decay time tau(decay), and calculate the radiative loss time tau(loss), the conductive loss time tau(cond), and the thermal energy E-th. The findings are as follows: (1) EUV nanoflares in the energy range of 10(24)-10(26) ergs represent miniature versions of larger flares observed in soft X-rays (SXR) and hard X-rays (HXR), scaled to lower temperatures (T-e less than or similar to 2 MK), lower densities (n(e) less than or similar to 10(9) cm(-3)), and somewhat smaller spatial scales (l approximate to 2-20 Mm). (2) The cooling time tau(decay) is compatible with the radiative cooling time tau(rad), but the conductive cooling timescale tau(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) approximate to 10(-46)(E/10(24))(-1.8) (s(-1) cm(-2) ergs-l) matches that of SXR microflares in the energy range of 10(26)-10(29), and exceeds that of nonthermal energies of larger flares observed in HXR by a factor of 3-10 tin the energy range of 10(29)-10(32) ergs). Discrepancies of the power-law slope with other studies, which report higher values in the range of a = 2.0-2.6 (Krucker & Bent; 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 less than or similar to 10(24) ergs, 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.