Left: piecewise dependence of the optically thin radiation losses per unit mass L(ρ, T) = χρT^β on temperature, where the specific values of the parameters χ and β are determined from the CHIANTI atomic database v. 8.0.7 for the plasma concentration 10¹⁶ m⁻³ and vary with the temperature interval considered. Right: variation of τ₁ (red) and τ₂ (blue) determined by Eq. (6) with temperature, for the radiative cooling shown in the left-hand panel and a heating model with the density and temperature power indices a = −0.5 and b = −3, respectively. The green dashed lines indicate the SUMER observational channels 6.3 MK and 8.9 MK.