The Cooling of Coronal Plasmas. Iii. Enthalpy Transfer as a Mechanism for Energy Loss

The cooling of impulsively heated coronal loops is examined with emphasis on the phase when optically thin radiation is dominant over conduction. It is shown that this regime cannot be described as purely "radiative cooling" because an enthalpy flux to the transition region plays an important and, on occasions, a dominant role. The scaling between coronal temperature and density (T ∝ nδ) during such cooling is reconsidered. The parameter δ is determined by the relative importance of the coronal radiative losses to the enthalpy flux to the transition region, which in turn powers the transition region radiation. It is seen that δ is in the region of 2 for short loops, while gravitational stratification reduces δ to below 2 and values of 1 occur for very long, tenuous loops. This can be understood by noting that for given transition region parameters (and hence required inward enthalpy flux), stratification reduces the coronal losses. It is thus appropriate to refer to this stage of coronal evolution as "radiative-enthalpy cooling."