Nanoflares coronali: la resa dei conti? ovvero Solar Dynamics Observatory discovers thin high temperature strands in coronal active regions | F. Reale ( Dipartimento di Fisica – Universita` di Palermo )

The question of what heats the million degrees solar corona has been debated for decades. One scenario proposed since the 80s has been a finely stranded corona where each strand is heated by a rapid pulse. Unfortunately, neither such fine structure has been resolved, nor direct or conclusive evidence has been found so far, e.g., extensive superhot plasma, nanoflaring activity, so that alternative hypotheses have been proposed. Recently it has been shown that the observed difference in appearance of cool and warm coronal loops (~1 MK, ~2-3 MK, respectively) – cool loops show fine structure and warm loops appear “fuzzy” – can be explained with multi-stranded coronal loops pulse-heated up to 10 MK, where the strands are interpreted as subarcsecond via modeling. That work predicts that images of hot coronal loops (>6 MK) should again show fine structure. Here we show that the predicted effect is indeed widely observed in an active region with the Solar Dynamics Observatory, and that therefore fine-structured energy pulses play a major role in heating the active corona.