  EUV observations of warm coronal loops suggest that they are bundles of
unresolved strands that are heated impulsively to high temperatures by
nanoflares. The plasma would then have the observed properties (e.g., excess
density compared to static equilibrium) when it cools into the 1-2 MK range. If
this interpretation is correct, then very hot emission should be present
outside of proper flares. It is predicted to be vey faint, however. A critical
element for proving or refuting this hypothesis is the existence of hot, very
faint plasmas which should be at amounts predicted by impulsive heating. We
report on the first comprehensive spectroscopic study of hot plasmas in active
regions. Data from the EIS spectrometer on Hinode were used to construct
emission measure distributions in quiescent active regions in the 1-5 MK
temperature range. The distributions are flat or slowly increasing up to
approximately 3 MK and then fall off rapidly at higher temperatures. We show
that active region models based on impulsive heating can reproduce the observed
EM distributions relatively well. Our results provide strong new evidence that
coronal heating is impulsive in nature.
