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 thes e flarelike events is detailed in the first paper in this series. We determ ine 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 ta u(decay), and calculate the radiative loss time tau(loss), the conductive l oss time tau(cond), and the thermal energy E-th. The findings are as follow s: (1) EUV nanoflares in the energy range of 10(24)-10(26) ergs represent m iniature 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 somewha t smaller spatial scales (l approximate to 2-20 Mm). (2) The cooling time t au(decay) is compatible with the radiative cooling time tau(rad), but the c onductive cooling timescale tau(cond) is about an order of magnitude shorte r, 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 o f 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 diffe rences in the detection and discrimination of EUV microflares, as well as t o different model assumptions in the calculation of the electron density. B esides the insufficient power of nanoflares to heat the corona, we find als o 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, a nd HXR.