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1. The Kelvin–Helmholtz Instability at the Boundary of Relativistic Magnetized Jets

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

   Chow, Anthony; Davelaar, Jordy; Rowan, Michael E; Sironi, Lorenzo (July 2023, The Astrophysical Journal Letters)

   Abstract We study the linear stability of a planar interface separating two fluids in relative motion, focusing on conditions appropriate for the boundaries of relativistic jets. The jet is magnetically dominated, whereas the ambient wind is gas-pressure-dominated. We derive the most general form of the dispersion relation and provide an analytical approximation of its solution for an ambient sound speed much smaller than the jet Alfvén speedv_A, as appropriate for realistic systems. The stability properties are chiefly determined by the angleψbetween the wavevector and the jet magnetic field. Forψ=π/2, magnetic tension plays no role, and our solution resembles the one of a gas-pressure-dominated jet. Here, only sub-Alfvénic jets are unstable ( $0<M_e\equiv \left(v/v_{\mathrm{A}}\right)\cos \theta <1$ , wherevis the shear velocity andθthe angle between the velocity and the wavevector). Forψ= 0, the free energy in the velocity shear needs to overcome the magnetic tension, and only super-Alfvénic jets are unstable ( $1<M_e<\sqrt{{(1+{\mathrm{\Gamma }}_w^2)/[1+(v_{\mathrm{A}}/c)}^2{\mathrm{\Gamma }}_w^2]}$ , with Γ_wthe wind adiabatic index). Our results have important implications for the propagation and emission of relativistic magnetized jets. 

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2. Model of Super-Alfvénic MHD Turbulence and Statistics of Polarization

   https://doi.org/10.3847/1538-4357/ae40ae  

   Lazarian, A; Pogosyan, D; Hu, Yue (March 2026, The Astrophysical Journal)

   Abstract MHD turbulence driven at velocities higher than the Alfvén velocity, i.e., super-Alfvénic turbulence, is widely spread in astrophysical environments, including galaxy clusters and molecular clouds. For statistical studies of such turbulence, we explore the utility of the polarization angle structure functions $D^{\varphi }\left(R\right)=\left({\sin }^2({\varphi }_1-{\varphi }_2)\right)$ , whereϕdenotes the polarization angle measured at points separated by a projected distanceRon the plane of the sky. A. Lazarian et al. showed that in the case of super-Alfvénic turbulence the spectral slope ofD^ϕ(R) differs from that of the underlying magnetic fluctuations, limiting its applicability for field strength estimation with known techniques. In this work, we provide an analytical framework that explains the modification of theD^ϕ(R) spectral slope in super-Alfvénic turbulence and validate our predictions with numerical simulations. We demonstrate that for super-Alfvénic turbulence the structure functionD^ϕ(R) gets shallower with the increase ofM_A. Our study makesD^ϕ(R) a valuable diagnostic of super-Alfvénic turbulence and opens a way to obtainM_Afrom observations. We also explore numerically the structure function of the polarization degree and the spectrum of the polarization directions, the latter being the Fourier transform ofD^ϕ. We discuss the implications of our findings for turbulence and magnetic field studies in the intracluster and interstellar media. 

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3. Characterization of Turbulent Fluctuations in the Sub-Alfvénic Solar Wind

   https://doi.org/10.3847/1538-4357/ad34ab  

   Zank, G P; Zhao, L-L; Adhikari, L; Telloni, D; Baruwal, Prashant; Baruwal, Prashrit; Zhu, Xingyu; Nakanotani, M; Pitňa, A; Kasper, J C; et al (April 2024, The Astrophysical Journal)

   Abstract Parker Solar Probe (PSP) observed sub-Alfvénic solar wind intervals during encounters 8–14, and low-frequency magnetohydrodynamic (MHD) turbulence in these regions may differ from that in super-Alfvénic wind. We apply a new mode decomposition analysis to the sub-Alfvénic flow observed by PSP on 2021 April 28, identifying and characterizing entropy, magnetic islands, forward and backward Alfvén waves, including weakly/nonpropagating Alfvén vortices, forward and backward fast and slow magnetosonic (MS) modes. Density fluctuations are primarily and almost equally entropy- and backward-propagating slow MS modes. The mode decomposition provides phase information (frequency and wavenumberk) for each mode. Entropy density fluctuations have a wavenumber anisotropy ofk_∥≫k_⊥, whereas slow-mode density fluctuations havek_⊥>k_∥. Magnetic field fluctuations are primarily magnetic island modes (δBⁱ) with anO(1) smaller contribution from unidirectionally propagating Alfvén waves (δB^A+) giving a variance anisotropy of $〈{\delta B^i}^2〉/〈{\delta B^A}^2〉=4.1$ . Incompressible magnetic fluctuations dominate compressible contributions from fast and slow MS modes. The magnetic island spectrum is Kolmogorov-like $k_{\perp }^{-1.6}$ in perpendicular wavenumber, and the unidirectional Alfvén wave spectra are $k_{\parallel }^{-1.6}$ and $k_{\perp }^{-1.5}$ . Fast MS modes propagate at essentially the Alfvén speed with anticorrelated transverse velocity and magnetic field fluctuations and are almost exclusively magnetic due toβ_p≪ 1. Transverse velocity fluctuations are the dominant velocity component in fast MS modes, and longitudinal fluctuations dominate in slow modes. Mode decomposition is an effective tool in identifying the basic building blocks of MHD turbulence and provides detailed phase information about each of the modes. 

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4. Evolution of Magnetohydrodynamic Turbulence in the Expanding Solar Wind: Residual Energy and Intermittency

   https://doi.org/10.3847/1538-4357/ad9f38  

   Shi, Chen; Sioulas, Nikos; Huang, Zesen; Velli, Marco; Tenerani, Anna; Réville, Victor (January 2025, The Astrophysical Journal)

   We conduct 3D magnetohydrodynamic simulations of decaying turbulence in the context of the solar wind. To account for the spherical expansion of the solar wind, we implement the expanding box model. The initial turbulence comprises uncorrelated counterpropagating Alfvén waves and exhibits an isotropic power spectrum. Our findings reveal the consistent generation of negative residual energy whenever nonlinear interactions are present, independent of the normalized cross helicityσ_cand compressibility. The spherical expansion facilitates this process. The resulting residual energy is primarily distributed in the perpendicular direction, withS₂(b) − S₂(u) ∝ l_⊥or equivalently $-E_r\propto k_{\perp }^{-2}$ . HereS₂(b) andS₂(u) are second-order structure functions of magnetic field and velocity respectively. In most runs,S₂(b) develops a scaling relation $S_2\left(\mathit{b}\right)\propto l_{\perp }^{1/2}$ ( $E_b\propto k_{\perp }^{-3/2}$ ). In contrast,S₂(u) is consistently shallower thanS₂(b), which aligns with in situ observations of the solar wind. We observe that the higher-order statistics of the turbulence, which act as a proxy for intermittency, depend on the initialσ_cand are strongly affected by the expansion effect. Generally, the intermittency is more pronounced when the expansion effect is present. Finally, we find that in our simulations, although the negative residual energy and intermittency grow simultaneously as the turbulence evolves, the causal relation between them seems to be weak, possibly because they are generated on different scales. 

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5. Suppression of Collisionless Magnetic Reconnection in the High Ion β, Strong Guide Field Limit

   https://doi.org/10.3847/1538-4357/ad9274  

   Giai, Carlos A; Haggerty, Colby C; Shay, Michael A; Cassak, Paul A; Davis, Zachary K (December 2024, The Astrophysical Journal)

   Abstract In magnetic reconnection, the ion bulk outflow speed and ion heating have been shown to be set by the available reconnecting magnetic energy, i.e., the energy stored in the reconnecting magnetic field (B_r). However, recent simulations, observations, and theoretical works have shown that the released magnetic energy is inhibited by upstream ion plasma betaβ_i—the relative ion thermal pressure normalized to magnetic pressure based on the reconnecting field—for antiparallel magnetic field configurations. Using kinetic theory and hybrid particle-in-cell simulations, we investigate the effects ofβ_ion guide field reconnection. While previous works have suggested that guide field reconnection is uninfluenced byβ_i, we demonstrate that the reconnection process is modified and the outflow is reduced for sufficiently large ${\beta }_i>(B_r^2+B_g^2)/B_r^2$ . We develop a theoretical framework that shows that this reduction is consistent with an enhanced exhaust pressure gradient, which reduces the outflow speed as $v_{\mathrm{out}}\propto 1/\sqrt{{\beta }_i}$ . These results apply to systems in which guide field reconnection is embedded in hot plasmas, such as reconnection at the boundary of eddies in fully developed turbulence like the solar wind or the magnetosheath as well as downstream of shocks such as the heliosheath or the mergers of galaxy clusters. 

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