Long-period EUV intensity pulsations (periods from 2 to 16 hours) have been found recently to be very common in the solar corona and especially in coronal loops. The heating mechanism(s) of solar coronal loops that generate million-degree plasma and maintain it confined at this temperature remain unknown. These intensity pulsations provide new constraints for loops models and thus to better understand coronal loops dynamics and heating.
The detailed study of the thermal structure of several events revealed that the temperature and the density are found to be periodic with a time delay between these two physical parameters of the plasma. This behavior is characteristic of evaporation and condensation cycles that allowed us to connect long-period intensity pulsations to thermal non-equilibrium (TNE), a well-know phenomenon in numerical simulations and for structures such as prominences and coronal rain. TNE happens when the heating is highly-stratified (mainly concentrated at low altitudes) and quasi-constant.
Unambiguous identification of TNE in coronal loops has thus important implications for understanding coronal heating.
We conducted dedicated simulations to reproduce the observed intensity pulsations and to determine the intrinsic properties of coronal loops that favor these particular cycles of evolution. A parameter space study reveal that the TNE cycles occurrence is sensitive to a combination of the loop geometry and heating parameters (asymmetry and heating power). This allows me to explain why these pulsations are encountered in some loops but not in all. Moreover, the derived AIA synthetic intensities can actually reproduced the main characteristics of the observed pulsations, that confirm that TNE can explain our observations.