More Like this

1. Bridging hybrid- and full-kinetic models with Landau-fluid electrons: I. 2D magnetic reconnection

   https://doi.org/10.1051/0004-6361/202140279  

   Finelli, F.; Cerri, S. S.; Califano, F.; Pucci, F.; Laveder, D.; Lapenta, G.; Passot, T. (September 2021, Astronomy & Astrophysics)

   null (Ed.)

   Context. Magnetic reconnection plays a fundamental role in plasma dynamics under many different conditions, from space and astrophysical environments to laboratory devices. High-resolution in situ measurements from space missions allow naturally occurring reconnection processes to be studied in great detail. Alongside direct measurements, numerical simulations play a key role in the investigation of the fundamental physics underlying magnetic reconnection, also providing a testing ground for current models and theory. The choice of an adequate plasma model to be employed in numerical simulations, while also compromising with computational cost, is crucial for efficiently addressing the problem under study. Aims. We consider a new plasma model that includes a refined electron response within the “hybrid-kinetic framework” (fully kinetic protons and fluid electrons). The extent to which this new model can reproduce a full-kinetic description of 2D reconnection, with particular focus on its robustness during the nonlinear stage, is evaluated. Methods. We perform 2D simulations of magnetic reconnection with moderate guide field by means of three different plasma models: (i) a hybrid-Vlasov-Maxwell model with isotropic, isothermal electrons, (ii) a hybrid-Vlasov-Landau-fluid (HVLF) model where an anisotropic electron fluid is equipped with a Landau-fluid closure, and (iii) a full-kinetic model. Results. When compared to the full-kinetic case, the HVLF model effectively reproduces the main features of magnetic reconnection, as well as several aspects of the associated electron microphysics and its feedback onto proton dynamics. This includes the global evolution of magnetic reconnection and the local physics occurring within the so-called electron-diffusion region, as well as the evolution of species’ pressure anisotropy. In particular, anisotropy-driven instabilities (such as fire-hose, mirror, and cyclotron instabilities) play a relevant role in regulating electrons’ anisotropy during the nonlinear stage of magnetic reconnection. As expected, the HVLF model captures all these features, except for the electron-cyclotron instability. 

   more » « less
2. The kinetic analog of the pressure–strain interaction

   https://doi.org/10.1063/5.0231200  

   Conley, S A; Juno, J; TenBarge, J M; Barbhuiya, M H; Cassak, P A; Howes, G G; Lichko, E (December 2024, Physics of Plasmas)

   Energy transport in weakly collisional plasma systems is often studied with fluid models and diagnostics. However, the applicability of fluid models is limited when collisions are weak or absent, and using a fluid approach can obscure kinetic processes that provide key insights into the physics of energy transport. Kinetic diagnostics retain all of the information in 3D-3V phase space and thereby reach beyond the insights of fluid models to elucidate the mechanisms responsible for collisionless energy transport. In this work, we derive the Kinetic Pressure–Strain (KPS): a kinetic analog of the pressure–strain interaction, which is the channel between flow energy density and internal energy density in fluid models. Through two case studies of electron Landau damping, we demonstrate that the KPS diagnostic can elucidate kinetic mechanisms that are responsible for energy transport in this channel, just as the related field–particle correlation is known to identify kinetic mechanisms of transport between electromagnetic field energy density and kinetic energy density in particle flows. In addition, we show that resonant electrons play a major role in transferring energy between fluid flows and internal energy during the process of Landau damping. 

   more » « less
3. Combining finite element space-discretizations with symplectic time-marching schemes for linear Hamiltonian systems

   https://doi.org/10.3389/fams.2023.1165371  

   Cockburn, Bernardo; Du, Shukai; Sánchez, Manuel A. (April 2023, Frontiers in Applied Mathematics and Statistics)

   We provide a short introduction to the devising of a special type of methods for numerically approximating the solution of Hamiltonian partial differential equations. These methods use Galerkin space-discretizations which result in a system of ODEs displaying a discrete version of the Hamiltonian structure of the original system. The resulting system of ODEs is then discretized by a symplectic time-marching method. This combination results in high-order accurate, fully discrete methods which can preserve the invariants of the Hamiltonian defining the ODE system. We restrict our attention to linear Hamiltonian systems, as the main results can be obtained easily and directly, and are applicable to many Hamiltonian systems of practical interest including acoustics, elastodynamics, and electromagnetism. After a brief description of the Hamiltonian systems of our interest, we provide a brief introduction to symplectic time-marching methods for linear systems of ODEs which does not require any background on the subject. We describe then the case in which finite-difference space-discretizations are used and focus on the popular Yee scheme (1966) for electromagnetism. Finally, we consider the case of finite-element space discretizations. The emphasis is placed on the conservation properties of the fully discrete schemes. We end by describing ongoing work. 

   more » « less
4. On the Inefficiency of Moist Geostrophic Turbulence: A Theory for the Energetic Output under Subsaturated Conditions

   https://doi.org/10.1175/JAS-D-24-0179.1  

   Brown, Marguerite; Pauluis, Olivier; Gerber, Edwin P (November 2025, Journal of the Atmospheric Sciences)

   Abstract The equator-to-pole temperature gradient has traditionally been understood as the primary driver of the midlatitude storm tracks, which derive their kinetic energy in the process of transporting sensible heat down the gradient. Latent heat, however, accounts for an estimated 30%–60% of the meridional energy transport, a portion which is likely to increase in a warmer world. The contribution of latent heat to the energetics is complicated in that it is inefficient: Only a fraction of the transported latent heat is converted into kinetic energy. Currently, there is no complete theory to explain the relationship between meridional energy transport and kinetic energy generation by midlatitudes eddies. We use a two-layer moist quasigeostrophic model to develop a theory of how the energetic output of the midlatitude atmosphere depends on the relative humidity structure. By varying the surface evaporation rate, we show that the system only reaches the maximum possible energetic output in the saturated limit, producing substantially less kinetic energy at lower evaporation rates. We quantify this reduction in kinetic energy production in terms of a moist conversion efficiency. Using a moist energetic framework, we identify that precipitation dissipation and the diffusion of moisture in subsaturated regions account for the reduction in energetic output. We then show that the moist conversion efficiency can be diagnosed from the distribution of humidity. Significance StatementThe impact of humidity on the strength of midlatitude storms is not well understood. Humidity will increase as the planet warms, but it is unclear whether storms will become stronger or weaker as a result. We use an idealized computer model to learn how humidity will impact the strength of storms. We focus on the effect of evaporation at the planet’s surface, with simulations ranging from a completely dry atmosphere to one with rain everywhere. In between these two limits, it is raining in only part of the atmosphere, and storms are much weaker than in the case with rain everywhere. We discuss how to connect these results to more complex models and real-world data. 

   more » « less
5. On the derivation of the wave kinetic equation for NLS

   https://doi.org/10.1017/fmp.2021.6  

   Deng, Yu; Hani, Zaher (January 2021, Forum of Mathematics, Pi)

   Abstract A fundamental question in wave turbulence theory is to understand how the wave kinetic equation describes the long-time dynamics of its associated nonlinear dispersive equation. Formal derivations in the physics literature, dating back to the work of Peierls in 1928, suggest that such a kinetic description should hold (for well-prepared random data) at a large kinetic time scale $$T_{\mathrm {kin}} \gg 1$$ and in a limiting regime where the size L of the domain goes to infinity and the strength $$\alpha $$ of the nonlinearity goes to $$0$$ (weak nonlinearity). For the cubic nonlinear Schrödinger equation, $$T_{\mathrm {kin}}=O\left (\alpha ^{-2}\right )$$ and $$\alpha $$ is related to the conserved mass $$\lambda $$ of the solution via $$\alpha =\lambda ^2 L^{-d}$$ . In this paper, we study the rigorous justification of this monumental statement and show that the answer seems to depend on the particular scaling law in which the $$(\alpha , L)$$ limit is taken, in a spirit similar to how the Boltzmann–Grad scaling law is imposed in the derivation of Boltzmann’s equation. In particular, there appear to be two favourable scaling laws: when $$\alpha $$ approaches $$0$$ like $$L^{-\varepsilon +}$$ or like $$L^{-1-\frac {\varepsilon }{2}+}$$ (for arbitrary small $$\varepsilon $$ ), we exhibit the wave kinetic equation up to time scales $$O(T_{\mathrm {kin}}L^{-\varepsilon })$$ , by showing that the relevant Feynman-diagram expansions converge absolutely (as a sum over paired trees). For the other scaling laws, we justify the onset of the kinetic description at time scales $$T_*\ll T_{\mathrm {kin}}$$ and identify specific interactions that become very large for times beyond $$T_*$$ . In particular, the relevant tree expansion diverges absolutely there. In light of those interactions, extending the kinetic description beyond $$T_*$$ toward $$T_{\mathrm {kin}}$$ for such scaling laws seems to require new methods and ideas. 

   more » « less