What Percentage Of The Brightest Coronal Loops Are Rooted In Mixed-polarity Magnetic Flux?

We have previously shown (Tiwari et al. 2017, ApJ Letters, 843, L20) that the heating in active region (AR) coronal loops depends systematically on their photospheric magnetic setting. There, we found that the brightest and hottest loops of ARs are the ones connecting sunspot umbra/penumbra at one end to (a) penumbra, (b) unipolar plage, or (c) mixed-polarity plage on the other end. The coolest loops are the ones that connect sunspot umbra at both ends. In this work we study the brightest loops during 24 hours in the core of the active region that was observed by Hi-C 2.1. These loops have neither foot in sunspot umbra or penumbra, but in plage. We investigate what percentage of the brightest coronal loops (in SDO/AIA Fe XVIII emission) have mixed-polarity magnetic flux at least at one of their feet, and so the heating could be driven by magnetic flux cancellation. We confirm the footpoint locations of loops via non-force-free field extrapolations (using SDO/HMI magnetograms) and find that ∼40% of the loops have both feet in unipolar flux, and ∼60% of the loops have at least one foot in mixed-polarity flux. The loops having mixed-polarity foot-point flux are ∼15% longer lived on average than the ones with both feet unipolar, but their peak-intensity averages do not show any significant difference. While the presence of mixed-polarity magnetic flux at least at one foot in majority of loops strongly supports the cancellation idea, the absence of mixed-polarity magnetic flux (to the detection limit of HMI) in about 40% of the loops suggests cancellation may not be necessary for heating coronal loops, but rather might enhance heating by some factor. We will further discuss some points that support, and some points that challenge, the flux cancellation idea of coronal heating.