Soutenance de thèse de Vincent David (LPP) le 21/9/2023 à 14h
Annonce transmise par Guillaume Aulanier, Président du PNST (LPP)
Titre: Multiscale solar wind turbulence: from theory to observations.
Date: 21 Septembre 2023, 14:00
Lieu: Amphitheater Gay-Lussac, Ecole Polytechnique, Palaiseau.
The solar wind is a turbulent plasma that can be measured in situ by spacecraft such as Voyager/NASA, THEMIS/ESA, or PSP/NASA. Measurements reveal magnetic field fluctuations over a wide range of frequencies, with a change in slope around 1 Hz, indicating a transition from the single-fluid MHD behavior of the plasma to a state where ions and electrons have distinct dynamics. A second transition is observed around 50 Hz, beyond which the magnetic spectrum becomes steeper, marking a change in physics where the inertia effects of electrons become significant. The study of this turbulence is closely linked to understanding the origin of local heating, characterized by a slow decrease in ion temperature with increasing heliospheric distance. This decrease is interpreted as a signature of heating resulting from the transfer of energy from large to small scales by turbulence. The objective of this thesis is to study solar wind turbulence from MHD scales to electron inertial scales.
In the first part, we use the Zeroth law of turbulence to measure energy dissipation at MHD scales. This law states that energy dissipation per unit mass approaches a non-zero limit, known as anomalous dissipation, as viscosity/resistivity decreases. A local form of Kolmogorov’s exact law is used with THEMIS and PSP data to show that heating calculated using anomalous dissipation can be significantly higher than the average heating predicted by the exact MHD law. Furthermore, the application of anomalous dissipation proves the Zeroth law in a simplified MHD model. Its application to Voyager 2 data reveals that heating generated by shocks near Jupiter is dominant compared to that from turbulent fluctuations.
In the second part, we focus on sub-MHD scales (frequencies between 1 and 50 Hz). In situ measurements show a monofractal behavior of magnetic fluctuations, whereas at MHD scales a (standard) multifractal behavior is observed. To study this difference, high-resolution 3D direct numerical simulations of the electron reduced MHD equations are conducted in weak and strong wave turbulence regimes. These simulations reveal that only weak turbulence can reproduce the monofractality. Combined with recent work, this result suggests that at electron scales, the solar wind is in a regime of weak kinetic Alfvén wave turbulence without collisions.
Finally, a theory of (weak) wave turbulence for inertial electron MHD in the presence of a strong external magnetic field is developed. Exact solutions (Kolmogorov—Zakharov spectrum) are provided, and it is shown that the cascade is direct. The importance of considering electron mass in this regime is highlighted. Remarkably, these equations are identical (up to a constant) to those describing inertial wave turbulence in rapidly rotating non-ionized fluids. This connection underscores the importance of laboratory investigations to study turbulence at these scales, which are currently challenging to access by satellites.
These studies provide a comprehensive understanding of the turbulent behavior of the solar wind from observational, numerical, and theoretical perspectives.