  Coronal plasma in the cores of solar active regions is impulsively heated to
more than 5 MK. The nature and location of the magnetic energy source
responsible for such impulsive heating is poorly understood. Using observations
of seven active regions from the Solar Dynamics Observatory, we found that a
majority of coronal loops hosting hot plasma have at least one footpoint rooted
in regions of interacting mixed magnetic polarity at the solar surface. In
cases when co-temporal observations from the Interface Region Imaging
Spectrograph space mission are available, we found spectroscopic evidence for
magnetic reconnection at the base of the hot coronal loops. Our analysis
suggests that interactions of magnetic patches of opposite polarity at the
solar surface and the associated energy release during reconnection are key to
impulsive coronal heating.
