The radiative response of solar loop plasma subject to transient heating

In Bradshaw & Mason (2003) we carried out a hydrodynamic simulation of a cooling solar loop and investigated the nonequilibrium response of the population of CVII ions to the changing conditions in the plasma. We also compared equilibrium and nonequilibrium calculations of the total plasma emissivity. In this paper we present two simulations of a solar loop subject to a transient heating process delivering energy on the nanoflare scale at its apex. One simulation treats the ion populations and the energy radiated from the loop plasma entirely as though the system were in equilibrium and the other simulation performs a full nonequilibrium treatment by coupling the time-dependent ion populations to the hydrodynamic equations through the radiative energy loss. Our radiative model accounts for the 15 most abundant elements of the solar atmosphere including C, O, Ne, Mg, Si and Fe. We find some pronounced differences between the populations of certain transition region ions and the corresponding plasma emissivity curves in the equilibrium and nonequilibrium simulations. Though the apex heating event is relatively weak in comparison to energy released on the microflare and flare scales, nonetheless a significant amount of energy reaches the loop footpoint region to heat the plasma there and we find a nonequilibrium spike in emissivity. However, more surprisingly we find considerable differences between some of the coronal ions in the equilibrium and nonequilibrium simulations, with important consequences for the plasma emissivity curves. In particular, we find that the total plasma emissivity calculated assuming equilibrium conditions is up to a factor of 5 lower than the nonequilibrium emissivity and this is due almost entirely to the response of the coronal Fe ions. Finally, we suggest possible observational signatures of nonequilibrium ionisation and ways in which one might identify it. This is important because an invalid assumption of equilibrium ion populations may well lead one to incorrect conclusions about the properties of the plasma in both a broad-band and narrow-band/emission line based analysis.