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1. Quantifying the Agyrotropy of Proton and Electron Heating in Turbulent Plasmas

   https://doi.org/10.3847/1538-4357/acb25a  

   Yang, Yan; Pecora, Francesco; Matthaeus, William H.; Roy, Sohom; Cuesta, Manuel Enrique; Chasapis, Alexandros; Parashar, Tulasi; Bandyopadhyay, Riddhi; Gershman, D. J.; Giles, B. L.; et al (February 2023, The Astrophysical Journal)

   Abstract An important aspect of energy dissipation in weakly collisional plasmas is that of energy partitioning between different species (e.g., protons and electrons) and between different energy channels. Here we analyse pressure–strain interaction to quantify the fractions of isotropic compressive, gyrotropic, and nongyrotropic heating for each species. An analysis of kinetic turbulence simulations is compared and contrasted with corresponding observational results from Magnetospheric Multiscale Mission data in the magnetosheath. In assessing how protons and electrons respond to different ingredients of the pressure–strain interaction, we find that compressive heating is stronger than incompressive heating in the magnetosheath for both electrons and protons, while incompressive heating is stronger in kinetic plasma turbulence simulations. Concerning incompressive heating, the gyrotropic contribution for electrons is dominant over the nongyrotropic contribution, while for protons nongyrotropic heating is enhanced in both simulations and observations. Variations with plasma β are also discussed, and protons tend to gain more heating with increasing β . 

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2. Pressure–Strain Interaction as the Energy Dissipation Estimate in Collisionless Plasma

   https://doi.org/10.3847/1538-4357/ac5d3e  

   Yang, Y.; Matthaeus, W.; Roy, S.; Roytershteyn, V.; Parashar, T.; Bandyopadhyay, R.; Wan, Minping (April 2022, The Astrophysical journal)

   The dissipative mechanism in weakly collisional plasma is a topic that pervades decades of studies without a consensus solution. We compare several energy dissipation estimates based on energy transfer processes in plasma turbulence and provide justification for the pressure–strain interaction as a direct estimate of the energy dissipation rate. The global and scale-by-scale energy balances are examined in 2.5D and 3D kinetic simulations. We show that the global internal energy increase and the temperature enhancement of each species are directly tracked by the pressure–strain interaction. The incompressive part of the pressure–strain interaction dominates over its compressive part in all simulations considered. The scale-by-scale energy balance is quantified by scale filtered Vlasov–Maxwell equations, a kinetic plasma approach, and the lag dependent von Kármán–Howarth equation, an approach based on fluid models. We find that the energy balance is exactly satisfied across all scales, but the lack of a well-defined inertial range influences the distribution of the energy budget among different terms in the inertial range. Therefore, the widespread use of the Yaglom relation in estimating the dissipation rate is questionable in some cases, especially when the scale separation in the system is not clearly defined. In contrast, the pressure– strain interaction balances exactly the dissipation rate at kinetic scales regardless of the scale separation 

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3. Turbulent Magnetogenesis in a Collisionless Plasma

   https://doi.org/10.3847/2041-8213/ac36cf  

   Pucci, F.; Viviani, M.; Valentini, F.; Lapenta, G.; Matthaeus, W. H.; Servidio, S. (November 2021, The Astrophysical journal)

   We demonstrate an efficient mechanism for generating magnetic fields in turbulent, collisionless plasmas. By using fully kinetic, particle-in-cell simulations of an initially nonmagnetized plasma, we inspect the genesis of magnetization, in a nonlinear regime. The complex motion is initiated via a Taylor–Green vortex, and the plasma locally develops strong electron temperature anisotropy, due to the strain tensor of the turbulent flow. Subsequently, in a domino effect, the anisotropy triggers a Weibel instability, localized in space. In such active wave–particle interaction regions, the seed magnetic field grows exponentially and spreads to larger scales due to the interaction with the underlying stirring motion. Such a self-feeding process might explain magnetogenesis in a variety of astrophysical plasmas, wherever turbulence is present. 

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4. A magnetic analog of pressure–strain interaction

   https://doi.org/10.1063/5.0241603  

   Barbhuiya, M Hasan; Cassak, P A (December 2024, Physics of Plasmas)

   We study the evolution equation for magnetic energy density for a non-relativistic magnetized plasma in the (Lagrangian) reference frame comoving with the electron bulk velocity. Analyzing the terms that arise due to the ideal electric field, namely, perpendicular electron compression and magnetic field line bending, we recast them to reveal a quantity with a functional form analogous to the often-studied pressure–strain interaction term that describes one piece of internal energy density evolution of the species in a plasma, except with the species pressure tensor replaced by the magnetic stress tensor. We dub it the “magnetic stress–strain interaction.” We discuss decompositions of the magnetic stress–strain interaction analogous to those used for pressure–strain interaction. These analogies facilitate the interpretation of the evolution of the various forms of energy in magnetized plasmas and should be useful for a wide array of applications, including magnetic reconnection, turbulence, collisionless shocks, and wave–particle interactions. We display and analyze all the terms that can change magnetic energy density in the Lagrangian reference frame of the electrons using a particle-in-cell simulation of magnetic reconnection. 

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5. Scale filtering analysis of kinetic reconnection and its associated turbulence

   https://doi.org/10.1063/5.0185132  

   Adhikari, Subash; Yang, Yan; Matthaeus, William H; Cassak, Paul A; Parashar, Tulasi N; Shay, Michael A (February 2024, Physics of Plasmas)

   Previously, using an incompressible von Kármán–Howarth formalism, the behavior of cross-scale energy transfer in magnetic reconnection and turbulence was found to be essentially identical to each other, independent of an external magnetic (guide) field, in the inertial and energy-containing ranges [Adhikari et al., Phys. Plasmas 30, 082904 (2023)]. However, this description did not account for the energy transfer in the dissipation range for kinetic plasmas. In this Letter, we adopt a scale-filtering approach to investigate this previously unaccounted-for energy transfer channel in reconnection. Using kinetic particle-in-cell simulations of antiparallel and component reconnection, we show that the pressure–strain interaction becomes important at scales smaller than the ion inertial length, where the nonlinear energy transfer term drops off. Also, the presence of a guide field makes a significant difference in the morphology of the scale-filtered energy transfer. These results are consistent with kinetic turbulence simulations, suggesting that the pressure strain interaction is the dominant energy transfer channel between electron scales and ion scales. 

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