Comparative Analysis of the Various Generalized Ohm's Law Terms in Magnetosheath Turbulence as Observed by Magnetospheric Multiscale

<p><span>Electric fields (<strong>E</strong>) play a fundamental role in facilitating the exchange of energy between the electromagnetic fields and the changed particles within a plasma. </span>Decomposing <strong>E</strong> into the contributions from the different terms in generalized Ohm's law, therefore, provides key insight into both the nonlinear and dissipative dynamics across the full range of scales within a plasma. Using the unique, high‐resolution, multi‐spacecraft measurements of three intervals in Earth's magnetosheath from the Magnetospheric Multiscale mission, the influence of the magnetohydrodynamic, Hall, electron pressure, and electron inertia terms from Ohm's law, as well as the impact of a finite electron mass, on the turbulent electric field<strong> </strong>spectrum are examined observationally for the first time. The magnetohydrodynamic, Hall, and electron pressure terms are the dominant contributions to <strong>E</strong> over the accessible length scales, which extend to scales smaller than the electron gyroradius at the greatest extent, with the Hall and electron pressure terms dominating at sub‐ion scales. The strength of the non‐ideal electron pressure contribution is stronger than expected from linear kinetic Alfvén waves and a partial anti‐alignment with the Hall electric field is present, linked to the relative importance of electron diamagnetic currents within the turbulence. The relative contributions of linear and nonlinear electric fields scale with the turbulent fluctuation amplitude, with nonlinear contributions playing the dominant role in shaping <strong>E</strong> for the intervals examined in this study. Overall, the sum of the Ohm's law terms and measured <strong>E</strong> agree to within ∼ 20% across the observable scales. The results both confirm a number of general expectations about the behavior of <strong>E</strong> within turbulent plasmas, as well as highlight additional features that may help to disentangle the complex dynamics of turbulent plasmas and should be explored further theoretically.</p>

Topological mechanics of knots and tangles

Knots play a fundamental role in the dynamics of biological and physical systems, from DNA to turbulent plasmas, as well as in climbing, weaving, sailing, and surgery. Despite having been studied for centuries, the subtle interplay between topology and mechanics in elastic knots remains poorly understood. Here, we combined optomechanical experiments with theory and simulations to analyze knotted fibers that change their color under mechanical deformations. Exploiting an analogy with long-range ferromagnetic spin systems, we identified simple topological counting rules to predict the relative mechanical stability of knots and tangles, in agreement with simulations and experiments for commonly used climbing and sailing bends. Our results highlight the importance of twist and writhe in unknotting processes, providing guidance for the control of systems with complex entanglements.