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1. Intermittency and Dissipative Structures Arising from Relativistic Magnetized Turbulence

   https://doi.org/10.3847/1538-4357/ad284c  

   Davis, Zachary; Comisso, Luca; Giannios, Dimitrios (March 2024, The Astrophysical Journal)

   Abstract Kinetic simulations of relativistic turbulence have significantly advanced our understanding of turbulent particle acceleration. Recent progress has highlighted the need for an updated acceleration theory that can account for particle acceleration within the plasma’s coherent structures. Here, we investigate how intermittency modeling connects statistical fluctuations in turbulence to regions of high-energy dissipation. This connection is established by employing a generalized She–Leveque model to characterize the exponentsζ_pfor the structure functions $S^p\propto l^{{\zeta }_p}$. The fitting of the scaling exponents provides us with a measure of the codimension of the dissipative structures, for which we subsequently determine the filling fraction. We perform our analysis for a range of magnetizationsσand relative fluctuation amplitudesδB₀/B₀. We find that increasing values ofσandδB₀/B₀allow the turbulent cascade to break sheetlike structures into smaller regions of dissipation that resemble chains of flux ropes. However, as their dissipation measure increases, the dissipative regions become less volume filling. With this work, we aim to inform future turbulent acceleration theories that incorporate particle energization from interactions with coherent structures within relativistic turbulence. 

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2. Electron and Proton Energization in 3D Reconnecting Current Sheets in Semirelativistic Plasma with Guide Magnetic Field

   https://doi.org/10.3847/2041-8213/ad2fa5  

   Werner, Gregory R.; Uzdensky, Dmitri A. (March 2024, The Astrophysical Journal Letters)

   Abstract Using 3D particle-in-cell simulation, we characterize energy conversion, as a function of guide magnetic field, in a thin current sheet in semirelativistic plasma, with relativistic electrons and subrelativistic protons. There, magnetic reconnection, the drift-kink instability (DKI), and the flux-rope kink instability all compete and interact in their nonlinear stages to convert magnetic energy to plasma energy. We compare fully 3D simulations with 2D in two different planes to isolate reconnection and DKI effects. In zero guide field, these processes yield distinct energy conversion signatures: ions gain more energy than electrons in 2Dxy(reconnection), while the opposite is true in 2Dyz(DKI), and the 3D result falls in between. The flux-rope instability, which occurs only in 3D, allows more magnetic energy to be released than in 2D, but the rate of energy conversion in 3D tends to be lower. Increasing the guide magnetic field strongly suppresses DKI, and in all cases slows and reduces the overall amount of energy conversion; it also favors electron energization through a process by which energy is first stored in the motional electric field of flux ropes before energizing particles. Understanding the evolution of the energy partition thus provides insight into the role of various plasma processes, and is important for modeling radiation from astrophysical sources such as accreting black holes and their jets. 

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3. Coherent mode and turbulence measurements with a fast camera

   https://doi.org/10.1063/5.0219330  

   Bartolo, Gustavo E; Yadav, Sonu; Fitz, Chloelle; Scime, Earl E (September 2024, Review of Scientific Instruments)

   This study employs a fast camera with frame rates up to 900,000 fps to measure the transfer of energy across spatial scales in helicon source plasmas and during flux rope mergers and the measurement of azimuthal mode structures in helicon plasmas. By extracting pixel-scale dispersion relations and power spectral density (PSD) measurements, we measure the details of turbulent wave modes and energy distribution across a broad range of spatial scales within the plasma. We confirm the presence of drift waves in helicon plasmas, as well as the existence of strong dissipation regions in the PSD at electron skin depth scales for both helicon and flux rope merger experiments. This approach overcomes many limitations of conventional probes, providing high spatial and temporal resolution, without perturbing the plasma. 

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4. The vortex gas scaling regime of baroclinic turbulence

   https://doi.org/10.1073/pnas.1916272117  

   Gallet, Basile; Ferrari, Raffaele (March 2020, Proceedings of the National Academy of Sciences)

   The mean state of the atmosphere and ocean is set through a balance between external forcing (radiation, winds, heat and freshwater fluxes) and the emergent turbulence, which transfers energy to dissipative structures. The forcing gives rise to jets in the atmosphere and currents in the ocean, which spontaneously develop turbulent eddies through the baroclinic instability. A critical step in the development of a theory of climate is to properly include the eddy-induced turbulent transport of properties like heat, moisture, and carbon. In the linear stages, baroclinic instability generates flow structures at the Rossby deformation radius, a length scale of order 1,000 km in the atmosphere and 100 km in the ocean, smaller than the planetary scale and the typical extent of ocean basins, respectively. There is, therefore, a separation of scales between the large-scale gradient of properties like temperature and the smaller eddies that advect it randomly, inducing effective diffusion. Numerical solutions show that such scale separation remains in the strongly nonlinear turbulent regime, provided there is sufficient drag at the bottom of the atmosphere and ocean. We compute the scaling laws governing the eddy-driven transport associated with baroclinic turbulence. First, we provide a theoretical underpinning for empirical scaling laws reported in previous studies, for different formulations of the bottom drag law. Second, these scaling laws are shown to provide an important first step toward an accurate local closure to predict the impact of baroclinic turbulence in setting the large-scale temperature profiles in the atmosphere and ocean. 

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5. Lagrangian subgrid-scale modeling applied to evolving firebrand particle transport

   Santos, I.; Mahato, B.; Bornhoft, B.; Jain, S.; Yaghoobian, N. (February 2023, Center for Turbulence Research Proceedings of the Summer Program 2022)

   The transport and deposition of firebrand particles is an important fire spread mechanism in wildland fires. These particles can be transported by wind over large distances and can ignite secondary fires upon landing. The transport of firebrands by wind is a complex, multiscale process that is largely controlled by interactions between the firebrand particles and the atmospheric wind. To account for the complex temporal evolution of atmospheric turbulence over large scales, the use of large-eddy simulation (LES) techniques is necessary. However, filtering of subgrid-scale (SGS) turbulence in LES hinders the accuracy of particle transport models. In this work, we employ a Lagrangian SGS model in an LES framework to investigate the effects of small-scale turbulence on the transport of mass- and size-changing firebrand particles. The impact of SGS turbulence was analyzed by comparing landing and trajectory statistics for firebrand and regular (fixed size and mass) particles under different Stokes numbers. It was found that the presence of SGS turbulence modifies the particle transport behavior, which is characterized by smaller spanwise dispersions but larger travel distances along the streamwise direction compared with particles under no SGS turbulence. As expected, the enhanced velocity field produced by the SGS model has larger influence on the statistics of firebrand particles compared with regular particles due to the time-evolving reduction in particle mass and size induced by pyrolysis. 

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