PhD Thesis and Habilitation
From 2001 to 2004 I have prepared a PhD thesis in Solar physics, under the supervision of Jean-Claude Vial and Marco Velli. The title is « Signatures and models of small-scale turbulent coronal heating ». The defense took place at IAS in Orsay (Univ. Paris-Sud), on December 17th, 2004. The thesis is available here (PDF file, 12MB), and on the Thèses-en-Ligne server. It is mainly in French, with a 3-page summary and the papers in English.
In solar physics, the complexity and small scales generated by magnetohydrodynamic (MHD) turbulence suggest to tackle the problem of the heating of the corona using statistics. We use therefore spectra of the fields, distributions of probability of structures or events, and structure functions, to analyze observations and numerical simulations, and to detect common signatures of turbulence, intermittency, and small-scale heating.
Our numerical simulations model a coronal magnetic loop, which is excited by the motions of the photosphere, and in which non-linearly interacting Alfvén waves propagate. As we need statistics, we need to simplify these interactions: we have chosen to model these interactions by cellular automata on one hand, and then by shell-models on the other hand. The results of these loop models are consistent with observations, and allow to understand some observational effects. Furthermore, signatures of intermittency can be found in the shell-model-based model, which includes a better representation of the non-linear terms of MHD than the cellular automata model. The analysis of these models and of their parametric behavior gives some information on the heating mechanisms in the corona and some clues about the interpretation of observations.
We also analyze the intensity and velocity fields observed in 1996 on the quiet Sun with the SoHO/SUMER spectrograph. The statistics of these fields (mainly the intensity field, as the velocity field is regrettably too noisy) acquaint us with the turbulent nature of the corona and with its intermittency.
The discrepancy between the distributions of events observed by different authors in the corona leads us finally to interest ourselves to the different possible definitions of an event. We give a comprehensive set of such definitions and we compare them, using lowly and highly intermittent signals.
I have defended my Habilitation thesis on 24 October 2014 at IAS in Orsay (Univ. Paris-Sud). The manuscript is available on the Thèses-en-Ligne server. It is mainly in French, with the included papers in English.
The variability of the heliosphere and in particular ``space weather'' are the result of a chain of processes involving the solar corona and wind. Turbulence plays a fundamental role in these phenomena, but it makes them (as well as their stellar analogues) very complex, with a wide range of scales and a large number of physical mechanisms. The corona can thus be heated to more than a million kelvins by the dissipation of small-scale structures that are created by turbulence. Observations allowed us to investigate properties of turbulence and heating events, and the thermal structure of the transition region between the thereby heated corona and the cooler lower layers of the solar atmosphere. I have modeled turbulent heating in the case of magnetically closed (coronal loops) and open (coronal holes) regions. In coronal loops, I have demonstrated the existence of a feedback between heating and cooling processes, and I have obtained signatures of turbulent heating. In coronal holes, we have shown that this heating could accelerate the wind, and we have also obtained spectra of anisotropic MHD and Hall-MHD turbulence. At the interface between both these types of coronal regions, we have determined the proportion of the mass flux in a coronal jet that effectively contributes to the wind. The magnetic field also maintains filaments of cool material in the hot corona, and their eruption can strongly disturb the heliosphere; we have observed one of these eruptions in detail, and we are developing a code to detect these structures automatically, before their eruption.
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Contact: Éric Buchlin
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