Evidence for highly stratified and quasi-steady heating of solar coronal loops
|Institut d'Astrophysique Spatiale|
|Auteur(s) supplémentaire(s)||F. Auchère, G. Aulanier, Z. Mikíc, K. Bocchialini, E. Buchlin, J. Solomon|
|Institution(s) supplémentaire(s)||LESIA; Predictive Science, Inc., San Diego, USA|
In solar coronal loops, thermal non-equilibrium (TNE) is a phenomenon that could occur when the heating is both highly-stratified (i.e. mainly concentrated near solar coronal loops footpoints) and quasi-constant. It is resulting in a particular response of the plasma: evaporation and condensation cycles with periodic evolution of the temperature and the density.
Unambiguous observational identification of TNE would thus permit to strongly constrain heating scenarios. Up to now, while TNE is the standard interpretation of coronal rain, it was not believed to happen commonly in warm coronal loops.
Auchère et al 2014 report the detection of long-period intensity pulsations (periods of several hours) with SoHO/EIT. This phenomenon appears to be very common in coronal loops.
Three intensity pulsation events that have been studied in detail with data from SDO/AIA, show strong evidence for TNE in warm loops (Froment et al 2015). For each active region studied, only one loop bundle undergoes this kind of pulsation. These loops appear to have the same cooling/heating behavior as the rest of the active region but show a different response of the plasma.
To understand why these particular loops are showing pulsations, we investigate a particular magnetic topology that could favor this particular plasma response. The loops geometry from the magnetic field extrapolations is also used as input for 1D hydrodynamic simulations. We conduct a parameter space study and conclude on the high sensitivity of TNE to both loops geometry and heating intensity/geometry. We finally compare the properties of simulated loops with the properties of the ones studied with SDO/AIA.
These new simulations further strenghten the interpretation of the observed pulsations as signatures of TNE. This implies that the heating in active regions must be highly stratified and that the frequency of the heating events must be shorter than the typical cooling time.
We can not discriminate the heating mechanism(s) involved but conclude on their main spatial localization in loop bundles and on the timescale of the heating.