Proposal submitted to ISSI  / Final report – March 2020

Solar coronal loops are the building blocks of the solar corona. These dynamic structures are shaped by the magnetic field that expands into the solar atmosphere. They can be observed in X-ray and extreme ultraviolet (EUV), revealing the high plasma temperature of the corona. However, it is still a matter of debate how the magnetic energy is dissipated to heat the plasma up to millions of degrees. In order to properly differentiate between heating mechanisms, the location and frequency of the energy deposition must be properly constrained. Achieving this, in turn, allows us to understand the heating and cooling phases, and the observed intensity variability.

The recent discovery of long-period EUV pulsations in coronal loops provides a major observational constraint for heating theories. This phenomenon, with periods between 2 and 16 hours (Auchère et al. 2014), can be found in at least half of the observed active regions, in particular in loops. The leading interpretation of these pulsations is that of evaporation and condensation cycles, resulting from a quasi-steady and highly-stratified heating (Froment et al. 2015, 2017). Such thermal cycles have long been predicted by numerical simulations, in which loops are in a state of thermal non-equilibrium (TNE). The thermal instability mechanism (runaway cooling and recombination) is thought to be the main driver of the cooling phase of the cycle, which can result in the generation of coronal rain and prominences. Understanding the characteristics of these thermal cycles is essential to understand the circulation of mass and energy in the solar corona.

The team will aim at determining the observational characteristics of the evaporation and condensation cycles, and elucidate the link with the spatial and temporal properties of the heating. The proposed team consists of experts in the leading theories behind coronal heating and coronal rain, experts in numerical simulations and forward modeling, and experts in multi-wavelength observations and Fourier analysis. The results from the team will help determine key observables needed to properly differentiate between coronal heating theories. Furthermore, impact in future instrument design for solar missions is expected, thanks to the strong involvement of the team members in currently planned ESA and NASA solar missions.