Yan Yang

Pressure–strain interaction is an important channel for converting bulk-flow energy into heat in weakly collisionless plasmas. Its decomposition into compressive and incompressive work, how their relative importance depends on geometry, and how it relates spatially and statistically to electromagnetic work, remain unclear. Previous simulation studies of plasma turbulence and reconnection starting from weakly compressive initial data have typically found that incompressive work continues to dominate over compressive work. In contrast, many magnetospheric multiscale (MMS) mission events in the magnetosheath exhibit stronger compressive contributions. In this study, we investigate energy conversion in collisionless magnetic reconnection using fully kinetic particle-in-cell (PIC) simulations for interchange reconnection and compare the simulation results with MMS observations. The results show that electromagnetic work is highly localized near current layers, whereas pressure–strain work is distributed more broadly. The difference in their peak regions indicates a separation between where field-particle energy conversion occurs and where the bulk flow energy is converted into internal energy. These support a two-step energy conversion pathway for a Vlasov–Maxwell system and its dependence on reconnection geometries. The consistency between simulation and observation results underscores the pressure–strain diagnostics for understanding energy conversion in space plasmas.

A key principle underlying most turbulence theories is that the mean energy dissipation rate remains finite even as viscosity vanishes: the so-called zeroth law of turbulence. Although this property has been established for hydrodynamic and magnetohydrodynamic (MHD) turbulence, its validity in weakly collisional plasmas has remained uncertain, where viscous and resistive closures fail. Using fully kinetic numerical simulations, we provide confirmation that the mean dissipation rate approaches a finite, system-independent value when expressed in terms of an effective Reynolds number derived from a heuristic collisionless viscosity. This result demonstrates a finite dissipation anomaly in collisionless plasma turbulence, establishing that the zeroth law extends beyond the collisional MHD regime.

Solar wind alpha particles exhibit preferential heating and acceleration relative to protons; however, their behavior in the vicinity of turbulent coherent structures remains less understood. We report the first evidence of localized alpha particle and proton heating within coherent structures identified using the Partial Variance of Increments (PVI) method based on Parker Solar Probe observations. Our results show that high-PVI events are associated with significant, species-dependent temperature enhancements: Protons undergo a relative larger temperature increase than alpha particles. This preferential proton heating produces a localized decrease in the alpha-to-proton temperature ratio, indicating that the plasma is driven toward thermal equilibration between species. The heating is also anisotropic, being dominated by enhancements in the perpendicular temperature. These temperature signatures coincide with a pronounced reduction in the normalized alpha–proton differential flow speed and a localized minimum in the Coulomb collision age, suggesting that the relaxation is affected primarily by collisionless kinetic effects. These findings provide new insight into the intermittent energy conversion and ion thermodynamics in the solar wind.

The idea of an energy cascade in the inertial range is often invoked in turbulent space plasmas to estimate the energy dissipation rate. Laws governing the behavior of third‐order structure functions in the inertial range, so‐called third‐order laws, are among the few rigorous theoretical results quantifying cross‐scale energy transfer. The widely used third‐order‐law derived rate assumes isotropy, which fundamentally conflicts with the anisotropic nature of space plasmas. Elementary questions persist regarding how such anisotropic energy cascades can be quantified using multi‐spacecraft constellations. As the heliospheric community increasingly progresses toward multi‐spacecraft, multi‐scale constellations, such as Plasma Observatory and HelioSwarm, we revisit these crucial issues pertinent to accurately measuring the inertial‐range energy transfer. Here, we make a systematic comparison between two methods: direction‐averaging (DA) and lag polyhedral derivative ensemble (LPDE) to determine the full three‐dimensional (3D) dependence of cross‐scale energy transfer. We find that DA exhibits both polar and azimuthal dependence, but is insensitive to spacecraft configuration. By contrast, LPDE is strongly affected by spacecraft separation and tetrahedral shape, while being comparatively insensitive to the sampling trajectory. Our findings have direct implications for current and future multi‐spacecraft missions. Both DA and LPDE will provide crucial information on the nature of turbulence in space and astrophysics.

In solar wind turbulence, the energy transfer/dissipation rate is typically estimated using MHD third-order structure functions calculated using spacecraft observations. However, the inherent anisotropy of solar wind turbulence leads to significant variations in structure functions along different observational directions, thereby affecting the accuracy of energy dissipation rate estimation. An unresolved issue is how to optimise the selection of observation angles under limited directional sampling to improve estimation precision. We conduct a series of MHD turbulence simulations with different mean magnetic field strengths, B ₀ . Our analysis of the third-order structure functions reveals that the global energy dissipation rate estimated around a polar angle of θ = 60 ^∘ agrees reasonably with the exact one for 0 ≤ B ₀ / b _rms ≤ 5, where b _rms denotes the rms magnetic field fluctuation. The speciality of 60 ^∘ polar angle can be understood by the mean value theorem of integrals, since the spherical integral of the polar-angle component ( T θ ˜ ) of the divergence of Yaglom flux is zero, and T θ ˜ changes sign around 60 ^∘ . Existing theory on the energy flux vector as a function of the polar angle is assessed, and supports the speciality of the 60 ^∘ polar angle. The angular dependence of the third-order structure functions is further assessed with virtual spacecraft data analysis. The present results can be applied to measure the turbulent dissipation rates of energy in the solar wind, which are of potential importance to other areas in which turbulence takes place, such as laboratory plasmas and astrophysics.

In solar wind turbulence, the energy transfer/dissipation rate is typically estimated using MHD third-order structure functions calculated using spacecraft observations. However, the inherent anisotropy of solar wind turbulence leads to significant variations in structure functions along different observational directions, thereby affecting the accuracy of energy-dissipation rate estimation. An unresolved issue is how to optimise the selection of observation angles under limited directional sampling to improve estimation precision. We conduct a series of MHD turbulence simulations with different mean magnetic field strengths, B0. Our analysis of the third-order structure functions reveals that the global energy dissipation rate estimated around a polar angle of θ = 60◦ agrees reasonably with the exact one. The speciality of 60◦ polar angle can be understood by the Mean Value Theorem of Integrals, since the spherical integral of the polar-angle component of the divergence of Yaglom flux is zero, and this polar-angle component changes sign around 60◦. Existing theory on the energy flux vector as a function of the polar angle is assessed, and supports the speciality of 60◦ polar angle. The angular dependence of the third-order structure functions is further assessed with virtual spacecraft data analysis. The present results can be applied to measure the turbulent dissipation rates of energy in the solar wind, which are of potential importance to other areas in which turbulence takes place, such as laboratory plasmas and astrophysics.

The invariants of the velocity gradient tensor in turbulence offer a compact description of local kinematics and flow topology. For incompressible magnetohydrodynamic turbulence, analysis of the second and third invariants (Q, R) of the velocity gradient tensor clarifies how coherent structures are organized and evolve. Extending the same analysis to the magnetic field gradient tensor provides additional information on the dynamics. In this study, pseudo-spectral simulation is used to obtain the velocity and magnetic field of the turbulent flow, and analysis of the flow field is conducted through joint probability density functions (PDFs) of the invariants. Furthermore, we explore the influence of the external mean magnetic field strength, B0. The results show that when an external magnetic field is present, the Q–R joint PDF no longer maintains the familiar teardrop distribution for the velocity field, and the flow field structure tends to be two-dimensional with increasing B0. For the fluctuation magnetic field, the Q–R joint PDF takes on a “cigar” shape that becomes more elongated as B0 increases. Moreover, as the strength of the external mean magnetic field increases, the turbulence exhibits enhanced small-scale dissipation and localization, accompanied by a reduction in the effective dimensionality of the system toward a quasi-two-dimensional regime.

Separate exact scaling laws are derived for the cascades of the kinetic and magnetic energy in incompressible homogeneous isotropic magnetohydrodynamic (MHD) turbulence, and validated by numerical simulations. The third-order moments exhibit linear scaling with respect to the spatial displacement scale in both the subviscous region and in the inertial range, and are proportional to the rate of conversion of kinetic energy to magnetic energy in the subviscous region, but to the energy injection rates in the inertial range, highlighting the impact of forcing mechanisms on energy transfer within MHD turbulence. Further analysis demonstrates that the Politano and Pouquet [Phys. Rev. E 57, R21 (1998).PLEEE81063-651X10.1103/PhysRevE.57.R21] 4/5 law emerges as a specific case of our exact laws, when the antisymmetric flux term, closely associated with the nonlocal nature of energy transfer in MHD turbulence, is neglected, which is only valid in the absence of magnetic energy injection. These laws are of fundamental importance in MHD turbulence theory, and offer substantial potential to enhance the predictive capability to estimate the dissipation rates of the solar wind.