The Flow-chart Loop: Temperature, Density, and Cooling Observables Supporting Nanoflare Coronal Heating Models
We have tested three controversial properties for a target loop observed with the Atmospheric Imaging Assembly: (1) overdense loops; (2) long-lifetime loops; and (3) multithermal loops. Our loop is overdense by a factor of about 10 compared to results expected from steady uniform heating models. If this were the only inconsistency, our loop could still be modeled as a single strand, but the density mismatch would imply that the heating must be impulsive. Moving on to the second observable, however, we find that the loop lifetime is at least an order of magnitude greater than the predicted cooling time. This implies that the loop cannot be composed of a single flux tube, even if the heating were dynamic, and must be multi-stranded. Finally, differential emission measure analysis shows that the cross-field temperature of the target loop is multithermal in the early and middle phases of its lifetime, but effectively isothermal before it fades from view. If these multithermal cooling results are found to be widespread, our results could resolve the original coronal loop controversy of "isothermal" versus "multithermal" cross-field temperatures. That is, the cross-field temperature is not always "multithermal" nor is it always "isothermal," but might change as the loop cools. We find that the existence and evolution of this loop is consistent with predictions of nanoflare heating.