Do EUV Nanoflares Account for Coronal Heating

Abstract

Recent observations with EUV imaging instruments such as SOHO/EIT and TRACE have shown evidence for flare-like processes at the bottom end of the energy scale, in the range of E th ≈ 1024 – 1027 erg. Here we compare these EUV nanoflares with soft X-ray microflares and hard X-ray flares across the entire energy range. From the observations we establish empirical scaling laws for the flare loop length, L(T) ~ T, the electron density, n e (T) ~ T 2, from which we derive scaling laws for the loop pressure, p(T) ~ T 3, and the thermal energy, E th ~ T 6. Extrapolating these scaling laws into the picoflare regime we find that the pressure conditions in the chromosphere constrain a height level for flare loop footpoints, which scales with h eq (T) ~ T −0.5 Based on this chromospheric pressure limit we predict a lower cutoff of flare loop sizes at L min ≲ 5 Mm and flare energies E min ≲ 1024 erg. We show evidence for such a rollover in the flare energy size distribution from recent TRACE EUV data. Based on this energy cutoff imposed by the chromospheric boundary condition we find that the energy content of the heated plasma observed in EUV, SXR, and HXR flares is insufficient (by 2–3 orders of magnitude) to account for coronal heating.