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1. Spectrum of kinetic-Alfvén-wave turbulence: intermittency or tearing mediation?

   https://doi.org/10.1093/mnras/stad2231  

   Zhou, Muni; Liu, Zhuo; Loureiro, Nuno_F (July 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We investigate the spectral properties of the electromagnetic fluctuations of sub-ion scale turbulence in weakly collisional, low-beta plasmas using a two-field isothermal gyrofluid model. The numerical results strongly support a description of the turbulence as a critically balanced Kolmogorov-like cascade of kinetic Alfvén wave fluctuations, as amended by previous studies to include intermittency effects. The measured universal index of the energy spectra from systems with different flux-unfreezing mechanisms excludes the role of tearing mediation in determining the spectra. The fluctuations remain isotropic in the plane perpendicular to the strong background magnetic fields as they cascade to smaller scales, which explains the absence of tearing mediation. The calculation of high-order, multipoint structure functions of magnetic fluctuations suggests that the intermittent structures have a quasi-2D, sheet-type morphology. These results are useful for explaining recent observations of the spectrum and structure of magnetic and density fluctuations in the solar wind at sub-proton scales, and are relevant for modelling the energy dissipation in a broad range of astrophysical systems. 

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2. Investigating Turbulence Effects on Magnetic Reconnection Rates through 3D Resistive Magnetohydrodynamic Simulations

   https://doi.org/10.3847/1538-4357/addc62  

   Vicentin, Giovani H; Kowal, Grzegorz; de_Gouveia_Dal_Pino, Elisabete M; Lazarian, Alex (July 2025, The Astrophysical Journal)

   Abstract We investigate the impact of turbulence on magnetic reconnection through high-resolution 3D magnetohydrodynamic (MHD) simulations, spanning Lundquist numbers fromS= 10³to 10⁶. Building on the A. Lazarian & E. T. Vishniac theory, which asserts reconnection rate independence from ohmic resistivity, we introduce small-scale perturbations untilt= 0.1t_A. Even after the perturbations cease, turbulence persists, resulting in sustained high reconnection rates ofV_rec/V_A∼ 0.03–0.08. These rates exceed those generated by resistive tearing modes (plasmoid chain) in 2D and 3D MHD simulations by factors of 5–6. Our findings match observations in solar phenomena and previous 3D MHD global simulations of solar flares, accretion flows, and relativistic jets. The simulations show a steady-state fast reconnection rate compatible with the full development of turbulence in the system, demonstrating the robustness of the process in turbulent environments. We confirm reconnection rate independence from the Lundquist number, supporting Lazarian and Vishniac’s theory of fast turbulent reconnection. Additionally, we find a mild dependence ofV_recon the plasma–βparameter, decreasing from 0.036 to 0.028 (in Alfvén units) asβincreases from 2.0 to 64.0 for simulations with a Lundquist number of 10⁵. Lastly, we explore the magnetic Prandtl number’s (Pr_m=ν/η) influence on the reconnection rate and find it negligible during the turbulent regime across the range tested, from Pr_m= 1 to 60. 

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3. Relativistic Alfvén Turbulence at Kinetic Scales

   https://doi.org/10.3847/1538-4357/ad2e02  

   Vega, Cristian; Boldyrev, Stanislav; Roytershteyn, Vadim (April 2024, The Astrophysical Journal)

   Abstract In a strongly magnetized, magnetically dominated relativistic plasma, Alfvénic turbulence can extend to scales much smaller than the particle inertial scales. It leads to an energy cascade somewhat analogous to inertial- or kinetic-Alfvén turbulent cascades existing in nonrelativistic space and astrophysical plasmas. Based on phenomenological modeling and particle-in-cell numerical simulations, we propose that the energy spectrum of such relativistic kinetic-scale Alfvénic turbulence is close tok⁻³or slightly steeper than that due to intermittency corrections or Landau damping. We note the analogy of this spectrum with the Kraichnan spectrum corresponding to the enstrophy cascade in 2D incompressible fluid turbulence. Such turbulence strongly energizes particles in the direction parallel to the background magnetic field, leading to nearly one-dimensional particle momentum distributions. We find that these distributions have universal log-normal statistics. 

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4. On the Effect of Driving Turbulent-like Fluctuations on a Harris Current Sheet Configuration and the Formation of Plasmoids

   https://doi.org/10.3847/1538-4357/ad5e73  

   Agudelo_Rueda, Jeffersson_A; Liu, Yi-Hsin; Germaschewski, Kai; Hesse, Michael; Bessho, Naoki (August 2024, The Astrophysical Journal)

   Abstract Energy dissipation in collisionless plasmas is one of the most outstanding open questions in plasma physics. Magnetic reconnection and turbulence are two phenomena that can produce the conditions for energy dissipation. These two phenomena are closely related to each other in a wide range of plasmas. Turbulent fluctuations can emerge in critical regions of reconnection events, and magnetic reconnection can occur as a product of the turbulent cascade. In this study, we perform 2D particle-in-cell simulations of a reconnecting Harris current sheet in the presence of turbulent fluctuations to explore the effect of turbulence on the reconnection process in collisionless nonrelativistic pair plasmas. We find that the presence of a turbulent field can affect the onset and evolution of magnetic reconnection. Moreover, we observe the existence of a scale-dependent amplitude of magnetic field fluctuations above which these fluctuations are able to disrupt the growing of magnetic islands. These fluctuations provide thermal energy to the particles within the current sheet and preferential perpendicular thermal energy to the background population. 

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5. Scaling of Magnetic Reconnection Electron Bulk Heating in the High-Alfvén-speed and Low-β Regime of Earth’s Magnetotail

   https://doi.org/10.3847/1538-4357/acdf44  

   Øieroset, M; Phan, T D; Oka, M; Drake, J F; Fuselier, S A; Gershman, D J; Maheshwari, K; Giles, B L; Zhang, Q; Guo, F; et al (August 2023, The Astrophysical Journal)

   Abstract We have surveyed 21 reconnection exhaust events observed by Magnetospheric MultiScale in the low-plasma-βand high-Alfvén-speed regime of the Earth’s magnetotail to investigate the scaling of electron bulk heating produced by reconnection. The ranges of inflow Alfvén speed and inflow electronβ_ecovered by this study are 800–4000 km s⁻¹and 0.001–0.1, respectively, and the observed heating ranges from a few hundred electronvolts to several kiloelectronvolts. We find that the temperature change in the reconnection exhaust relative to the inflow, ΔT_e, is correlated with the inflow Alfvén speed,V_Ax,in, based on the reconnecting magnetic field and the inflow plasma density. Furthermore, ΔT_eis linearly proportional to the inflowing magnetic energy per particle, ${m_{\mathrm{i}}{V}_{\mathrm{Ax},\mathrm{in}}}^2$, and the best fit to the data produces the empirical relation ΔT_e= 0.020 ${m_{\mathrm{i}}{V}_{\mathrm{Ax},\mathrm{in}}}^2$, i.e., the electron temperature increase is on average ∼2% of the inflowing magnetic energy per particle. This magnetotail study extends a previous magnetopause reconnection study by two orders of magnitude in both magnetic energy and electronβ, to a regime that is comparable to the solar corona. The validity of the empirical relation over such a large combined magnetopause–magnetotail plasma parameter range ofV_A∼ 10–4000 km s⁻¹andβ_e∼ 0.001–10 suggests that one can predict the magnitude of the bulk electron heating by reconnection in a variety of contexts from the simple knowledge of a single parameter: the Alfvén speed of the ambient plasma. 

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