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1. Magnetogenesis in a Collisionless Plasma: From Weibel Instability to Turbulent Dynamo

   https://doi.org/10.3847/1538-4357/ad0b0f  

   Zhou, Muni ; Zhdankin, Vladimir ; Kunz, Matthew W. ; Loureiro, Nuno F. ; Uzdensky, Dmitri A. ( December 2023 , The Astrophysical Journal)

   We report on a first-principles numerical and theoretical study of plasma dynamo in a fully kinetic framework. By applying an external mechanical force to an initially unmagnetized plasma, we develop a self-consistent treatment of the generation of “seed” magnetic fields, the formation of turbulence, and the inductive amplification of fields by the fluctuation dynamo. Driven large-scale motions in an unmagnetized, weakly collisional plasma are subject to strong phase mixing, which leads to the development of thermal pressure anisotropy. This anisotropy triggers the Weibel instability, which produces filamentary “seed” magnetic fields on plasma-kinetic scales. The plasma is thereby magnetized, enabling efficient stretching and folding of the fields by the plasma motions and the development of Larmor-scale kinetic instabilities such as the firehose and mirror. The scattering of particles off the associated microscale magnetic fluctuations provides an effective viscosity, regulating the field morphology and turbulence. During this process, the seed field is further amplified by the fluctuation dynamo until energy equipartition with the turbulent flow is reached. By demonstrating that equipartition magnetic fields can be generated from an initially unmagnetized plasma through large-scale turbulent flows, this work has important implications for the origin and amplification of magnetic fields in the intracluster and intergalactic mediums.

    

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2. Insensitivity of a turbulent laser-plasma dynamo to initial conditions

   https://doi.org/10.1063/5.0084345  

   Bott, A. F. ; Chen, L. ; Tzeferacos, P. ; Palmer, C. A. ; Bell, A. R. ; Bingham, R. ; Birkel, A. ; Froula, D. H. ; Katz, J. ; Kunz, M. W. ; et al ( July 2022 , Matter and Radiation at Extremes)

   It has recently been demonstrated experimentally that a turbulent plasma created by the collision of two inhomogeneous, asymmetric, weakly magnetized, laser-produced plasma jets can generate strong stochastic magnetic fields via the small-scale turbulent dynamo mechanism, provided the magnetic Reynolds number of the plasma is sufficiently large. In this paper, we compare such a plasma with one arising from two pre-magnetized plasma jets whose creation is identical save for the addition of a strong external magnetic field imposed by a pulsed magnetic field generator. We investigate the differences between the two turbulent systems using a Thomson-scattering diagnostic, x-ray self-emission imaging, and proton radiography. The Thomson-scattering spectra and x-ray images suggest that the external magnetic field has a limited effect on the plasma dynamics in the experiment. Although the external magnetic field induces collimation of the flows in the colliding plasma jets and although the initial strengths of the magnetic fields arising from the interaction between the colliding jets are significantly larger as a result of the external field, the energies and morphologies of the stochastic magnetic fields post-amplification are indistinguishable. We conclude that, for turbulent laser-plasmas with supercritical magnetic Reynolds numbers, the dynamo-amplified magnetic fields are determined by the turbulent dynamics rather than the seed fields or modest changes in the initial flow dynamics of the plasma, a finding consistent with theoretical expectations and simulations of turbulent dynamos. 

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3. Insensitivity of a turbulent laser-plasma dynamo to initial conditions

   Bott, A. F. ; Chen, L. ; Tzeferacos, P. ; Palmer, C. A. ; Bell, A. R. ; Bingham, R. ; Birkel, A. ; Froula, D. H. ; Katz, J. ; Kunz, M. W. ; et al ( May 2022 , Matter and radiation at extremes)

   It has recently been demonstrated experimentally that a turbulent plasma created by the collision of two inhomogeneous, asymmetric, weakly magnetized, laser-produced plasma jets can generate strong stochastic magnetic fields via the small-scale turbulent dynamo mechanism, provided the magnetic Reynolds number of the plasma is sufficiently large. In this paper, we compare such a plasma with one arising from two pre-magnetized plasma jets whose creation is identical save for the addition of a strong external magnetic field imposed by a pulsed magnetic field generator. We investigate the differences between the two turbulent systems using a Thomson-scattering diagnostic, x-ray selfemission imaging, and proton radiography. The Thomson-scattering spectra and x-ray images suggest that the external magnetic field has a limited effect on the plasma dynamics in the experiment. Although the external magnetic field induces collimation of the flows in the colliding plasma jets and although the initial strengths of the magnetic fields arising from the interaction between the colliding jets are significantly larger as a result of the external field, the energies and morphologies of the stochastic magnetic fields post-amplification are indistinguishable. We conclude that, for turbulent laser-plasmas with supercritical magnetic Reynolds numbers, the dynamo-amplified magnetic fields are determined by the turbulent dynamics rather than the seed fields or modest changes in the initial flow dynamics of the plasma, a finding consistent with theoretical expectations and simulations of turbulent dynamos. https://doi.org/10.1063/5.0084345 

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4. Critical magnetic Reynolds number of the turbulent dynamo in collisionless plasmas

   https://doi.org/10.1093/mnras/stad3967  

   Achikanath Chirakkara, Radhika ; Seta, Amit ; Federrath, Christoph ; Kunz, Matthew W. ( December 2023 , Monthly Notices of the Royal Astronomical Society)

   The intracluster medium of galaxy clusters is an extremely hot and diffuse, nearly collisionless plasma, which hosts dynamically important magnetic fields of ∼μG strength. Seed magnetic fields of much weaker strength of astrophysical or primordial origin can be present in the intracluster medium. In collisional plasmas, which can be approximated in the magnetohydrodynamical (MHD) limit, the turbulent dynamo mechanism can amplify weak seed fields to strong dynamical levels efficiently by converting turbulent kinetic energy into magnetic energy. However, the viability of this mechanism in weakly collisional or completely collisionless plasma is much less understood. In this study, we explore the properties of the collisionless turbulent dynamo using three-dimensional hybrid-kinetic particle-in-cell simulations. We explore the properties of the collisionless turbulent dynamo in the kinematic regime for different values of the magnetic Reynolds number, Rm, initial magnetic-to-kinetic energy ratio, (Emag/Ekin)i, and initial Larmor ratio, (rLarmor/Lbox)i, i.e. the ratio of the Larmor radius to the size of the turbulent system. We find that in the ‘un-magnetized’ regime, (rLarmor/Lbox)i > 1, the critical magnetic Reynolds number for the dynamo action Rmcrit ≈ 107 ± 3. In the ‘magnetized’ regime, (rLarmor/Lbox)i ≲ 1, we find a marginally higher Rmcrit = 124 ± 8. We find that the growth rate of the magnetic energy does not depend on the strength of the seed magnetic field when the initial magnetization is fixed. We also study the distribution and evolution of the pressure anisotropy in the collisionless plasma and compare our results with the MHD turbulent dynamo.

    

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5. The Ca ii H and K Rotation–Activity Relation in 53 Mid-to-late-type M Dwarfs

   https://doi.org/10.3847/1538-4357/ac5cbf  

   Boudreaux, Thomas M. ; Newton, Elisabeth R. ; Mondrik, Nicholas ; Charbonneau, David ; Irwin, Jonathan ( April 2022 , The Astrophysical Journal)

   In the canonical theory of stellar magnetic dynamo, the tachocline in partially convective stars serves to arrange small-scale fields, generated by stochastic movement of plasma into a coherent large-scale field. Mid-to-late-type M dwarfs, which are fully convective, show more magnetic activity than classical magnetic dynamo theory predicts. However, mid-to-late-type M dwarfs show tight correlations between rotation and magnetic activity, consistent with elements of classical dynamo theory. We use data from the Magellan Inamori Kyocera Echelle Spectrograph to detail the relation between CaiiH and K flux and rotation period for these low-mass stars. We measure$R_{\mathrm{HK}}^{\prime }$values for 53 spectroscopically identified M dwarfs selected from the MEarth survey; these stars span spectral classes from M5.0 to M3.5 and have rotation periods ranging from hours to months. We present the rotation–activity relationship as traced through these data. We find power-law and saturated regimes consistent to within 1σof previously published results and observe a mass dependence in$R_{\mathrm{HK}}^{\prime }$.

    

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