From simulations to observations of cool plasma: What we can learn from synthetic spectra

Cool plasma of coronal rain or prominences and filaments is ubiquitous in the solar corona. Spectroscopic observations of it have been gathered for decades; however, they have proven difficult to interpret. Improved diagnostics would allow us to better disentangle the complex atmosphere at the source of these spectra and directly relate specific observables to the conditions in the atmosphere. We tried to achieve this by exploring the synthetic spectra of Mg II h&k lines in detail and analysing how well a non-adiabatic magnetohydrodynamic (MHD) simulation matches observations when comparing the synthetic with the observed spectra. A Python implemented, non-local thermodynamic equilibrium (non-LTE) radiative-transfer framework, , was recently developed specifically for condensations in the corona. We applied to create synthetic spectra from a 2.5D MHD simulation created with . The simulation shows a dynamic system of numerous cold, dense thread-like structures that we compare to observations of the (IRIS), with a focus on flare-driven coronal rain. Promweaver Promweaver MPI-AMRVAC Interface Region Imaging Spectrograph We give a detailed description of the simulation from the aspect of the synthetic spectra. We show how and why the particular spectra form. A single thread already demonstrates great complexity that can influence the shape of the observed spectral line. We further used the quartile analysis to systematically explore the evolution of observables in the observed and the synthetic spectra. This gives an overview of how the parameters change in time and that the changes seen in the simulation fit with the changes seen in observations. The results show that the simulation, in some aspects, matches particularly well to the observations of flare-driven coronal rain. Furthermore, we explored relations of parameters of the simulation and the observables. We find significant correlation between intensity and density, but not with temperature (in the studied temperature range of 5 to 20,kK). As a result, there is also a correlation of intensity and pressure. This relation allows us to further relate intensity and the vertical component of the Lorentz force. All of this brings us a step closer to a better understanding of the connection between the observed spectra and the atmosphere from which it originates.