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Accumulation et évolution des hélicités magnétiques relative, non potentielle et mêlée, lors des phénomènes actifs de l'atmosphère solaire

Abstract : The spectacular phenomena that are solar eruptions can impact human infrastructures. As we become more and more sensitive to solar activity because of the technological evolution of our society, solar forecasting is a growing economic stake. My thesis is part of the HeliSol research project of the Agence Nationale de la Rercherche. The project aims to establish deterministic criteria allowing reliable predictions of solar eruptions. My thesis focuses on a promising quantity related to the geometry of the magnetic field: magnetic helicity. Despite some previous theoretical, analytical, and observational results, placing it at the core of various processes related to solar activity, magnetic helicity still remains largely unfathomed. More precisely, my work is focused on the study of the properties of a decomposition of magnetic helicity: the non-potential magnetic helicity (linked to the magnetic field carrying electric currents) and the volume-threading magnetic helicity (containing the rest of the information).First, I focused on the link between the eruptivity of a magnetic system and its helicities. Using magnetohydrodynamic (MHD) numerical simulations, I showed that the ratio between non-potential helicity and total helicity has a threshold beyond which an eruption systematically occurrs. Magnetic data obtained from observations allowed me to confirm that the helicity ratio is a significant marker of the eruptivity of different active regions. These analyses are based on the production of 3D reconstructions of the coronal magnetic field generated from observational measurements of the 2D distribution of the magnetic field at the level of the Sun’s « surface ». Nevertheless, as I could demonstrate, the ratio cannot be correctly determined when the reconstructed magnetic field does not respect the solenoidal property.I used analytical and numerical approaches to study the properties of the helicities that constitutes the above-mentioned ratio. I proposed a theoretical formalism to quantify the non-additivity of magnetic helicity. I also derived for the first time an analytical expression for the temporal variation of non-potential and volume-threading helicities. This allowed me to show that, unlike total helicity, non-potential and volume-threading helicities are not conserved. Their evolution is dictated not only by fluxes across boundaries, but also by a transfer from one form of helicity to another. With the help of numerical simulations, I also demonstrated that this helicity transfer has a major role in the dynamics of magnetic helicities during solar eruptions.In conclusion, the highlighted magnetic helicity ratio could be used to develop new prediction tools for solar eruptions. More generally, my work provides a better understanding of the evolution of the magnetic configurations responsible for the active events of the solar corona. These researches could also serve as the basis for future theoretical studies on the link between magnetic helicities and the trigger of solar eruptions.
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Submitted on : Friday, July 2, 2021 - 3:03:12 PM
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  • HAL Id : tel-03124289, version 2

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Luis Linan. Accumulation et évolution des hélicités magnétiques relative, non potentielle et mêlée, lors des phénomènes actifs de l'atmosphère solaire. Astrophysique stellaire et solaire [astro-ph.SR]. Université Paris sciences et lettres, 2020. Français. ⟨NNT : 2020UPSLO008⟩. ⟨tel-03124289v2⟩

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