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1. 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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2. Reconciling cosmic ray transport theory with phenomenological models motivated by Milky-Way data

   https://doi.org/10.1093/mnras/stac1240  

   Kempski, Philipp ; Quataert, Eliot ( May 2022 , Monthly Notices of the Royal Astronomical Society)

   Phenomenological models of cosmic ray (CR) transport in the Milky Way can reproduce a wide range of observations assuming that CRs scatter off of magnetic-field fluctuations with spectrum ∝ k−δ and δ ∼ [1.4, 1.67]. We study the extent to which such models can be reconciled with current microphysical theories of CR transport, specifically self-confinement due to the streaming instability and/or extrinsic turbulence due to a cascade of magnetohydrodynamic (MHD) fast modes. We first review why it is that on their own neither theory is compatible with observations. We then highlight that CR transport is a strong function of local plasma conditions in the multiphase interstellar medium, and may be diffusive due to turbulence in some regions and streaming due to self-confinement in others. A multiphase combination of scattering mechanisms can in principle reproduce the main trends in the proton spectrum and the boron-to-carbon ratio. However, models with a combination of scattering by self-excited waves and fast-mode turbulence require significant fine-tuning due to fast-mode damping, unlike phenomenological models that assume undamped Kolmogorov turbulence. The assumption that fast modes follow a weak cascade is also not well justified theoretically, as the weak cascade is suppressed by wave steepening and weak-shock dissipation even in subsonic turbulence. These issues suggest that there may be a significant theoretical gap in our understanding of MHD turbulence. We discuss a few topics at the frontier of MHD turbulence theory that bear on this (possible) gap and that may be relevant for CR scattering.

    

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3. Magnetic Properties and Growth‐Induced Anisotropy in Yttrium Thulium Iron Garnet Thin Films

   https://doi.org/10.1002/aelm.202100452  

   Rosenberg, Ethan R. ; Litzius, Kai ; Shaw, Justin M. ; Riley, Grant A. ; Beach, Geoffrey S. D. ; Nembach, Hans T. ; Ross, Caroline A. ( July 2021 , Advanced Electronic Materials)

   Rare‐earth iron garnets (REIG) have recently become the materials platform of choice for spintronic studies on ferrimagnetic insulators. However, thus far the materials studied have mainly been REIG with a single rare earth species such as thulium, yttrium, or terbium iron garnets. In this study, magnetometry, ferromagnetic resonance, and magneto‐optical Kerr effect imaging is used to explore the continuous variation of magnetic properties as a function of composition for Y_xTm_3−xiron garnet (Y_xTm_3−xIG) thin films grown by pulsed laser deposition on gadolinium gallium garnet substrates. It is reported that the tunability of the magnetic anisotropy energy, with full control achieved over the type of anisotropy (from perpendicular, to isotropic, to an in‐plane easy axis) on the same substrate. In addition, a nonmonotonic composition‐dependent anisotropy term is reported, which is ascribed to growth‐induced anisotropy similar to what is reported in garnet thin films grown by liquid‐phase epitaxy. Ferromagnetic resonance shows linear variation of the damping and the g‐factor across the composition range, consistent with prior theoretical work. Domain imaging reveals differences in reversal modes, remanant states, and domain sizes in Y_xTm_3−xiron‐garnet thin films as a function of anisotropy.

    

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4. Demonstration of dynamic thermal compensation for parametric instability suppression in Advanced LIGO

   https://doi.org/10.1088/1361-6382/ab8be9  

   Hardwick, T. ; Hamedan, V. J. ; Blair, C. ; Green, A. C. ; Vander-Hyde, D. ( September 2020 , Classical and Quantum Gravity)

   Advanced LIGO and other ground-based interferometric gravitational-wave detectors use high laser power to minimize shot noise and suspended optics to reduce seismic noise coupling. This can result in an opto-mechanical coupling which can become unstable and saturate the interferometer control systems. The severity of these parametric instabilities scales with circulating laser power and first hindered LIGO operations in 2014. Static thermal tuning and active electrostatic damping have previously been used to control parametric instabilities at lower powers but are insufficient as power is increased. Here we report the first demonstration of dynamic thermal compensation to avoid parametric instability in an Advanced LIGO detector. Annular ring heaters that compensate central heating are used to tune the optical mode away from multiple problematic mirror resonance frequencies. We develop a single-cavity approximation model to simulate the optical beat note frequency during the central heating and ring heating transient. An experiment of dynamic ring heater tuning at the LIGO Livingston detector was carried out at 170 kW circulating power and, in agreement with our model, the third order optical beat note is controlled to avoid instability of the 15 and 15.5 kHz mechanical modes. We project that dynamic thermal compensation with ring heater input conditioning can be used in parallel with acoustic mode dampers to control the optical mode transient and avoid parametric instability of these modes up to Advanced LIGO’s design circulating power of 750  kW. The experiment also demonstrates the use of three mode interaction monitoring as a sensor of the cavity geometry, used to maintain theg-factor product tog₁g₂= 0.829 ± 0.004.

    

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5. On the Effect of Surface Friction and Upward Radiation of Energy on Equatorial Waves

   https://doi.org/10.1175/JAS-D-21-0199.1  

   Lin, Jonathan ; Emanuel, Kerry ( March 2022 , Journal of the Atmospheric Sciences)

   In theoretical models of tropical dynamics, the effects of both surface friction and upward wave radiation through interaction with the stratosphere are oft-ignored, as they greatly complicate mathematical analysis. In this study, we relax the rigid-lid assumption and impose surface drag, which allows the barotropic mode to be excited in equatorial waves. In particular, a previously developed set of linear, strict quasi-equilibrium tropospheric equations is coupled with a dry, passive stratosphere, and surface drag is added to the troposphere momentum equations. Theoretical and numerical model analysis is performed on the model in the limits of an inviscid surface coupled to a stratosphere, as well as a frictional surface under a rigid lid. This study confirms and extends previous research that shows the presence of a stratosphere strongly shifts the growth rates of fast-propagating equatorial waves to larger scales, reddening the equatorial power spectrum. The growth rates of modes that are slowly propagating and highly interactive with cloud radiation are shown to be negligibly affected by the presence of a stratosphere. Surface friction in this model framework acts as purely a damping mechanism and couples the baroclinic mode to the barotropic mode, increasing the poleward extent of the equatorial waves. Numerical solutions of the coupled troposphere–stratosphere model with surface friction show that the stratosphere stratification controls the extent of tropospheric trapping of the barotropic mode, and thus the poleward extent of the wave. The superposition of phase-shifted barotropic and first baroclinic modes is also shown to lead to an eastward vertical tilt in the dynamical fields of Kelvin wave–like modes.

    

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