Raffaello Foldes

Transfert local d'énergie dans les fluides géophysiques et les plasmas spatiaux: La turbulence dans les fluides géophysiques et les plasmas spatiaux entre en concurrence avec les ondes internes pour transférer l'énergie à travers les échelles. Des preuves pointent vers la possibilité qu'un transfert d'énergie vers les grandes échelles, ressemblant au cas des écoulements turbulents bidimensionnels, puisse se développer dans l'atmosphère et dans les océans, sous l'effet de la rotation, éventuellement aidé par le grand rapport d'aspect du domaine. En effet, des transferts d'énergie bidirectionnels (à grande et petite échelles) ont été observés dans différents contextes naturels, dans les océans par exemple, mais aussi dans les plasmas cinétiques, dans lesquels les champs magnétiques favorisent la propagation des ondes et la reconnexion magnétique contribue à rendre la dynamique de ces flux encore plus riche. Dans ces situations, caractérisées par une forte inhomogénéité et anisotropie, les outils d'analyse standards ne peuvent fournir qu'une information partielle sur la répartition de l'énergie aux différentes échelles. Afin de parvenir à une caractérisation plus exhaustive du transfert d'énergie, dans cette thèse, nous avons utilisé la technique du filtrage spatial (SF) pour étudier l'énergétique des écoulements turbulents stratifiés, d'intérêt géophysique, et des plasmas dans le régime cinétique, ce dernier étant pertinent pour comprendre la dynamique du milieu interplanétaire. En particulier, nous avons ciblé deux phénomènes majeurs, les courants verticaux extrêmes se développant dans les écoulements géophysiques stratifiés et les événements de reconnexion observés dans les plasmas héliosphériques et magnétosphériques, toujours à l'aide de données de simulation.Après avoir encore plus affiné la technique SF, nous l'avons utilisée pour analyser un ensemble de simulations numériques directes d'écoulements stratifiés, en faisant varier le nombre de Froude, en nous concentrant sur la rétroaction du développement des vents verticaux extrêmes sur le transfert d'énergie, localement dans l'espace physique et de Fourier. Nous avons pu vérifier que les courants verticaux injectent effectivement de l'énergie, renforçant finalement la turbulence et la dissipation, et affectant les propriétés de mélange des écoulements géophysiques. La même approche a finalement été mise en œuvre sur les sorties des simulations de plasma hybride-cinétique, afin d'évaluer les effets des événements de reconnexion magnétique sur le transfert d'énergie aux échelles sub-ioniques. Notre analyse a souligné pour la première fois le rôle de la reconnexion comme déclencheur de transferts double d'énergie simultanément vers des échelles plus grandes et plus petites que les échelles associées aux événements de reconnexion observés.

Recent studies based on simulations of the Boussinesq equations indicate that stratified turbulent flows can develop large-scale intermittency in the velocity and temperature fields, as detected in the atmosphere and oceans. In particular, emerging powerful vertical drafts were found to generate local turbulence, proving necessary for stratified flows to dissipate the energy as efficiently as homogeneous isotropic turbulent flows. The existence of regions characterized by enhanced turbulence and dissipation, as observed, for instance, in the ocean, requires appropriate tools to assess how energy is transferred across the scales and at the same time locally in the physical space. After refining a classical coarse-graining procedure, here we investigate the feedback of extreme vertical velocity drafts on energy transfer and exchanges in subdomains of simulations of stably stratified flows of geophysical interest. Our analysis shows that vertical drafts are indeed able to trigger upscale and downscale energy transfers, strengthening the coupling between kinetic and potential energies at certain scales, depending on the intensity of the local vertical velocity.

The study of space plasmas at the kinetic scale has seen rapid growth in recent years due to the exponential increase in computational power and more accurate in-situ measurements. Both numerical simulations and observations have revealed a clear transition across ion scales from the magnetohydrodynamic (MHD) to the kinetic regime, characterized by different physical phenomena dominating the turbulent properties and the heating of plasmas. Several studies have associated the so-called ion break with magnetic reconnection, which is considered responsible for injecting energy into this range, thereby driving the sub-ion energy cascade.In this work, we analyze a 2D3V hybrid-Vlasov simulation of forced plasma turbulence using the space-filtering (or coarse-graining) technique, which allows for a simultaneous investigation of energy transfer properties as a function of scale, space, and time. Using this approach, we quantitatively show, for the first time, that magnetic reconnection in non-collisional plasmas is associated with dual energy transfer across ion scales, bridging the MHD and kinetic regimes. The onset of reconnection events triggers the formation of sub-ion scale turbulent fluctuations and plays a crucial role in the appearance of an inverse energy transfer regime originating at these sub-ion scales.

The properties of energy transfer in the kinetic range of plasma turbulence have fundamental implications on the turbulent heating of space and astrophysical plasmas. It was suggested that magnetic reconnection may be responsible for driving the subion scale cascade, and that this process would be characterized by a direct energy transfer toward even smaller scales (until dissipation), and a simultaneous inverse transfer of energy toward larger scales, until the ion break. Here we employ the space-filter technique on high-resolution 2D3V hybrid-Vlasov simulations of continuously driven turbulence providing quantitative evidence that magnetic reconnection is indeed able to trigger a dual energy transfer originating at subion scales.

The properties of energy transfer in the kinetic range of plasma turbulence have fundamental implications on the turbulent heating of space and astrophysical plasmas. It was recently suggested that magnetic reconnection may be responsible for driving the sub-ion scale cascade, and that this process would be characterized by a direct energy transfer towards even smaller scales (until dissipation), and a simultaneous inverse transfer of energy towards larger scales, until the ion break. Here we employ the space-filter technique on high-resolution 2D3V hybrid-Vlasov simulations of continuously driven turbulence providing for the first time quantitative evidence that magnetic reconnection is indeed able to trigger a dual energy transfer originating at sub-ion scales.

One of the goals of Space Weather studies is to achieve a better understanding of impulsive phenomena, such as Coronal Mass Ejections (CMEs), to improve our ability to forecast their propagation characteristics and mitigate the risks to our technologically driven society. The essential part of achieving this goal is to assess the performance of forecasting models. To this end, the quality and availability of suitable data are of paramount importance. In this work, we merged publicly available data of CMEs from both in-situ and remote observations in order to build a dataset of CME properties. To evaluate the accuracy of the dataset and confirm the relationship between in-situ and remote observations, we have employed the Drag-Based Model (DBM) due to its simplicity and modest consumption of computational resources. In this study, we have also explored the parameter space for the drag parameter and solar wind speed using a Monte Carlo approach to evaluate how efficiently the DBM determines the propagation of CMEs for the events in the dataset. The geoeffective CMEs selected as a result of this work are compliant with the hypothesis of DBM (isolated CME, constant solar wind speed beyond 20 R_⊙) and also yield further insight into CME features such as arrival time and arrival speed at L1 point, lift-off time, speed at 20 R_⊙ and other similar quantities. Our analysis based on the acceptance rate in the DBM inversion procedure shows that almost 50% of the CME events in the dataset are well described by DBM as they propagate in the heliosphere. The dataset includes statistical metrics for the DBM model parameters. The probability distribution functions (PDFs) for the free parameters of DBM have been derived through a Monte Carlo-like inversion procedure. Probability distribution functions obtained from this work are comparable to PDFs employed in previous works. The analysis showed that there exist two different most probable values (median values) of solar wind speed for DBM input based on slow (w_slow ≈ 386 km/s) and fast (w_fast ≈ 547 km/s) solar wind type. The most probable value for the drag parameter (γ ≈ 0.687 × 10⁻⁷ km⁻¹) in our study is somewhat higher than the values reported in previous studies. Using a data-driven approach, this procedure allows us to present a homogeneous, reliable, and robust dataset for the investigation of CME propagation. Additionally, possible CME events are identified where the DBM prediction is not valid due to model limitations and higher uncertainties in the input parameters. These events require further thorough investigation in the future.

How turbulent convective fluctuations organize to form larger-scale structures in planetary atmospheres remains a question that eludes quantitative answers. The assumption that this process is the result of an inverse cascade was suggested half a century ago in two-dimensional fluids, but its applicability to atmospheric and oceanic flows remains heavily debated, hampering our understanding of the energy balance in planetary systems. We show using direct numerical simulations with spatial resolutions of 12288 ² × 384 points that rotating and stratified flows can support a bidirectional cascade of energy, in three dimensions, with a ratio of Rossby to Froude numbers comparable to that of Earth’s atmosphere. Our results establish that, in dry atmospheres, spontaneous order can arise through an inverse cascade to the largest spatial scales.