Characteristic thermal misbalance time τ_M (11) for typical coronal densities and temperatures and a fixed heating model a = 0.5 and b = −3.5, for which both acoustic and thermal modes are stable over the entire intervals of plasma densities and temperatures considered (see Fig. 1). The colour scheme is adjusted so that the values of τ_M from about 10 min to 100 min are shown in green. The blue symbols illustrate typical combinations of coronal plasma density and temperature in hot and dense loops in active regions (star), warm and less dense quiescent loops (circle), and polar plumes (box).