I am pleased to invite you to my PhD defense, which will take place on Tuesday, 9 December 2025 at 09:30 at Institut d’Astrophysique Spatiale (IAS), Bâtiment 121, Université Paris-Saclay, Orsay.
The defense will also be streamed live on YouTube. After the defense, a light reception will be offered. If you plan to join, please fill out this short two-question form.
Thesis title:
“Evolution of transient structures in the solar corona and of their composition: a multi-instrument study with Solar Orbiter”
Supervisors:
- Éric Buchlin (IAS, Université Paris-Saclay)
- Miho Janvier (IAS, Université Paris-Saclay / ESA)
Jury:
- Laurent Verstraete — Professor, Université Paris-Saclay (IAS) — President
- Louise Harra — Professor, ETH Zürich (PMOD/WRC) — Reviewer
- Étienne Pariat — CNRS Research Scientist (French-Spanish Laboratory for Astrophysics in Canarias) — Reviewer
- David Brooks — Professor, George Mason University / MSSL–UCL / JAXA — Examiner
- Alessandra Giunta — Associate Professor, Università di Catania — Examiner
Abstract:
Plasma composition is a powerful diagnostic for understanding the solar corona and for linking remote observations to in-situ solar-wind measurements. Above the transition region, elemental abundances differ from photospheric values due to the First Ionization Potential (FIP) effect: low-FIP elements (FIP < 10 eV) are typically enhanced by factors of ∼3–4 in the corona and solar wind, while high-FIP elements remain near photospheric levels.
A proposed explanation for this bias is the ponderomotive force, which arises near the top of the chromosphere as a result of gradients in Alfvén-wave energy density. This thesis focuses on mid-FIP elements (FIP ≈ 10 eV), especially sulfur (10.36 eV), whose fractionation can mimic low- or high-FIP behavior depending on magnetic topology and the depth of wave penetration into the chromosphere.
Using space-borne spectroscopy with Solar Orbiter/SPICE, I perform one of the first remote-sensing measurements of the sulfur FIP bias across distinct solar structures and compare the results with predictions of the ponderomotive-force model. To enable this, I developed SAFFRON, an open-source Python tool tailored for SPICE Level-2 data that provides cosmic-ray removal, adaptive binning, constrained multi-Gaussian fitting, uncertainty propagation, and computation of FIP-bias maps.
Applied to coronal-hole plumes (bright, open structures rooted in coronal holes and contributing to the fast wind), sulfur enrichment is measured in the transition region at plume footpoints, consistent with models where plume formation facilitates mid-FIP fractionation. In active regions, however, sulfur diagnostics become density-sensitive around ∼0.1 MK, increasing uncertainties. I quantify this effect and outline strategies, incorporating independent density constraints, to retrieve reliable composition in dense environments.
Together, these results improve our understanding of coronal composition variability and establish mid-FIP diagnostics as indicators of transition-region activity and chromospheric Alfvén waves. The upcoming high-latitude phases of Solar Orbiter will be essential to extend this work to polar coronal holes. Coordinated multi-instrument campaigns combining ground-based spectropolarimetry (e.g., DKIST, GST) with EUV spectroscopy (SPICE, Hinode/EIS, Solar-C/EUVST) will further constrain the physical processes that shape elemental fractionation in the solar atmosphere.
(Transmis par Slimane Mzerguat)