  Impulsive nanoflares are expected to transiently heat the plasma confined in
coronal loops to temperatures of the order of 10 MK. Such hot plasma is hardly
detected in quiet and active regions, outside flares. During rapid and short
heat pulses in rarified loops the plasma can be highly out of equilibrium of
ionization. Here we investigate the effects of the non-equilibrium of
ionization (NEI) on the detection of hot plasma in coronal loops.
Time-dependent loop hydrodynamic simulations are specifically devoted to this
task, including saturated thermal conduction, and coupled to the detailed
solution of the equations of ionization rate for several abundant elements. In
our simulations, initially cool and rarified magnetic flux tubes are heated to
10 MK by nanoflares deposited either at the footpoints or at the loop apex. We
test for different pulse durations, and find that, due to NEI effects, the loop
plasma may never be detected at temperatures above ~5 MK for heat pulses
shorter than about 1 min. We discuss some implications in the framework of
multi-stranded nanoflare-heated coronal loops.
