Estimations of τ_M (the black contours, in minutes) for varying heating models (power indices a and b) in warm and less dense quiescent loops (left), polar plumes (middle), and hot and dense loops (right). For specific combinations of the plasma density and temperature for these cases see Sect. 4. The colour schemes in all panels show the ratio of τ_M to typical periods τ of slow waves observed in those structures. The red solid lines show the values of a and b, above which the thermal mode becomes unstable. The red dashed lines indicate the intervals for which the thermal mode instability growth time is up to 5 times longer than the corresponding acoustic period τ.