Pulsations d’intensité de longue période : signature de la stratification et de la fréquence du chauffage dans les boucles coronales solaires

Abstract : Long-period EUV intensity pulsations (periods from 3 to 16 hours) have been found recently to be very common in thesolar corona and especially in coronal loops. The heating mechanism(s) of solar coronal loops that generate million-degreeplasma and maintain it confined at this temperature remain unknown. These intensity pulsations (extreme ultraviolet)provide new constraints for loops models and thus to better understand coronal loops dynamics and heating. The centraltopic of this thesis is to explore the possible physical explanations for this phenomenon.First, I used a detection code, initially developed for SoHO/EIT images, on the SDO/AIA archive. I detected thousandsof events in the six years of data, half of them corresponding to active regions and about the half of whom are identifiedas corresponding to coronal loops. I selected three cases of long-period intensity pulsation events in loops, with a cleardetection signal and allowing to scan different periods.Second, using the six coronal channels of AIA, I made a detailed study of the thermal structure of these loops. I usedboth differential emission measure (DEM) reconstructions and an analysis of the time-lags between the intensities in thesix channels. The temperature and the density are found to be periodic with a time delay between these two physicalparameters of the plasma. This behavior is characteristic of evaporation and condensation cycles of the plasma and itallowed me to connect these intensity pulsations to thermal non-equilibrium (TNE), a well-know phenomenon in numericalsimulations and for structures such as prominences and coronal rain. Moreover, an analysis based only on the shape ofpower spectra allowed to confirm this conclusion. TNE happens when the heating is highly-stratified (mainly concentratedat low altitudes) and quasi-constant. Unambiguous identification of TNE in coronal loops has thus important implicationsfor understanding coronal heating.Third, I aimed at reproducing the observed intensity pulsations by simulations and at determining the intrinsicproperties of coronal loops that favor these particular cycles of evolution. I made extrapolations of the magnetic fieldfor the three regions studied to determine the loops geometry. These geometries have been then used as inputs for 1Dhydrodynamic simulations. I conducted a parameter space study that revealed that the TNE cycles occurrence is sensitiveto a combination of the loop geometry and heating parameters (asymmetry and heating power). This allows me to explainwhy these pulsations are encountered in some loops but not in all. I studied one simulation in particular, matching theobserved characteristics of the plasma evolution. I derived the corresponding AIA synthetic intensities which reproducedthe main characteristics of the observed pulsations. This model allows me to explain the observed pulsations as evaporationand condensation cycles.
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Clara Froment. Pulsations d’intensité de longue période : signature de la stratification et de la fréquence du chauffage dans les boucles coronales solaires. Astrophysique stellaire et solaire [astro-ph.SR]. Université Paris-Saclay, 2016. Français. ⟨NNT : 2016SACLS235⟩. ⟨tel-01402981⟩

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