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1. Compressible two-dimensional turbulence: cascade reversal and sensitivity to imposed magnetic field

   https://doi.org/10.1088/1367-2630/ad0172  

   Fouxon, Itzhak ; Kritsuk, Alexei G. ; Mond, Michael ( November 2023 , New Journal of Physics)

   We study the impact of compressibility on two-dimensional turbulent flows, such as those modeling astrophysical disks. We demonstrate that the direction of cascade undergoes continuous transition as the Mach numberMaincreases, from inverse atMa = 0, to direct at$Ma=\mathrm{\infty }$. Thus, at$Ma\sim 1$comparable amounts of energy flow from the pumping scale to large and small scales, in accord with previous data. For supersonic turbulence with$Ma\gg 1$the cascade is direct, as in three dimensions, which results in multifractal density field. For that regime ($Ma\gg 1$) we derive a Kolmogorov-type law for potential forcing and obtain an explicit expression for the third order correlation tensor of the velocity. We further show that all third order structure functions are zero up to first order in the inertial range scales, which is in sharp contrast with incompressible turbulence where the third order structure function, that describes the energy flux associated with the energy cascade is non-zero. The properties of compressible turbulence have significant implications on the amplification of magnetic fields in conducting fluids. We thus demonstrate that imposing external magnetic field on compressible flows of conducting fluids allows to manipulate the flow producing possibly large changes even at small Mach numbers. Thus Zeldovich’s antidynamo theorem, by which atMa = 0 the magnetic field is zero in the steady state, must be used with caution. Real flows have finiteMaand, however small it is, for large enough values ofI, the magnetic flux through the disk, the magnetic field changes the flow appreciably, or rearranges it completely. This renders the limitMa → 0 singular for non-zero values ofI. Of particular interest is the effect of the density multifractality, at$Ma\gg 1$which is relevant for astrophysical disks. We demonstrate that in that regime, in the presence of non-zeroIthe magnetic field energy is enhanced by a large factor as compared to its estimates based on the mean field. Finally, based on the insights described above, we propose a novel two-dimensional Burgers’ turbulence, whose three-dimensional counterpart is used for studies of the large-scale structure of the Universe, as a model for supersonic two-dimensional magnetohydrodynamic flows.

    

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2. 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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3. Cosmic-Ray Drag and Damping of Compressive Turbulence

   https://doi.org/10.3847/1538-4357/aceef9  

   Bustard, Chad ; Oh, S. Peng ( September 2023 , The Astrophysical Journal)

   While it is well known that cosmic rays (CRs) can gain energy from turbulence via second-order Fermi acceleration, how this energy transfer affects the turbulent cascade remains largely unexplored. Here, we show that damping and steepening of the compressive turbulent power spectrum are expected once the damping time$t_{\mathrm{damp}}\sim \rho v^2/{\dot{E}}_{\mathrm{CR}}\propto E_{\mathrm{CR}}^{-1}$becomes comparable to the turbulent cascade time. Magnetohydrodynamic simulations of stirred compressive turbulence in a gas-CR fluid with diffusive CR transport show clear imprints of CR-induced damping, saturating at${\dot{E}}_{\mathrm{CR}}\sim \widetilde{ϵ}$, where$\widetilde{ϵ}$is the turbulent energy input rate. In that case, almost all of the energy in large-scale motions is absorbed by CRs and does not cascade down to grid scale. Through a Hodge–Helmholtz decomposition, we confirm that purely compressive forcing can generate significant solenoidal motions, and we find preferential CR damping of the compressive component in simulations with diffusion and streaming, rendering small-scale turbulence largely solenoidal, with implications for thermal instability and proposed resonant scattering ofE≳ 300 GeV CRs by fast modes. When CR transport is streaming dominated, CRs also damp large-scale motions, with kinetic energy reduced by up to 1 order of magnitude in realisticE_CR∼E_gscenarios, but turbulence (with a reduced amplitude) still cascades down to small scales with the same power spectrum. Such large-scale damping implies that turbulent velocities obtained from the observed velocity dispersion may significantly underestimate turbulent forcing rates, i.e.,$\widetilde{ϵ}\gg \rho v^3/L$.

    

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4. 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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5. Magnetic field-tuned Fermi liquid in a Kondo insulator

   https://doi.org/10.1038/s41467-019-13421-w  

   Kushwaha, Satya K. ; Chan, Mun K. ; Park, Joonbum ; Thomas, S. M. ; Bauer, Eric D. ; Thompson, J. D. ; Ronning, F. ; Rosa, Priscila F. S. ; Harrison, Neil ( December 2019 , Nature Communications)

   Kondo insulators are expected to transform into metals under a sufficiently strong magnetic field. The closure of the insulating gap stems from the coupling of a magnetic field to the electron spin, yet the required strength of the magnetic field–typically of order 100 T–means that very little is known about this insulator-metal transition. Here we show that Ce$${}_{3}$$${}_3$Bi$${}_{4}$$${}_4$Pd$${}_{3}$$${}_3$, owing to its fortuitously small gap, provides an ideal Kondo insulator for this investigation. A metallic Fermi liquid state is established above a critical magnetic field of only$${B}_{{\rm{c}}}\approx$$$B_c\approx$11 T. A peak in the strength of electronic correlations near$${B}_{{\rm{c}}}$$$B_c$, which is evident in transport and susceptibility measurements, suggests that Ce$${}_{3}$$${}_3$Bi$${}_{4}$$${}_4$Pd$${}_{3}$$${}_3$may exhibit quantum criticality analogous to that reported in Kondo insulators under pressure. Metamagnetism and the breakdown of the Kondo coupling are also discussed.

    

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