One of the cornerstone effects in spintronics is spin pumping by dynamical magnetization that is steadily precessing (around, for example, the
We study the impact of compressibility on twodimensional turbulent flows, such as those modeling astrophysical disks. We demonstrate that the direction of cascade undergoes continuous transition as the Mach number
 NSFPAR ID:
 10472560
 Publisher / Repository:
 IOP Publishing
 Date Published:
 Journal Name:
 New Journal of Physics
 Volume:
 25
 Issue:
 11
 ISSN:
 13672630
 Format(s):
 Medium: X Size: Article No. 113005
 Size(s):
 ["Article No. 113005"]
 Sponsoring Org:
 National Science Foundation
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Abstract z axis) with frequencyω _{0}due to absorption of lowpower microwaves of frequencyω _{0}under the resonance conditions and in the absence of any applied bias voltage. The twodecadesold ‘standard model’ of this effect, based on the scattering theory of adiabatic quantum pumping, predicts that component of spin current vector ${I}^{{S}_{z}}$ is timeindependent while $({I}^{{S}_{x}}(t),{I}^{{S}_{y}}(t),{I}^{{S}_{z}})\propto {\omega}_{0}$ and ${I}^{{S}_{x}}(t)$ oscillate harmonically in time with a single frequency ${I}^{{S}_{y}}(t)$ω _{0}whereas pumped charge current is zero in the same adiabatic $I\equiv 0$ limit. Here we employ more general approaches than the ‘standard model’, namely the timedependent nonequilibrium Green’s function (NEGF) and the Floquet NEGF, to predict unforeseen features of spin pumping: namely precessing localized magnetic moments within a ferromagnetic metal (FM) or antiferromagnetic metal (AFM), whose conduction electrons are exposed to spin–orbit coupling (SOC) of either intrinsic or proximity origin, will pump both spin $\propto {\omega}_{0}$ and charge ${I}^{{S}_{\alpha}}(t)$I (t ) currents. All four of these functions harmonically oscillate in time at both even and odd integer multiples of the driving frequency $N{\omega}_{0}$ω _{0}. The cutoff order of such high harmonics increases with SOC strength, reaching in the onedimensional FM or AFM models chosen for demonstration. A higher cutoff ${N}_{\mathrm{m}\mathrm{a}\mathrm{x}}\simeq 11$ can be achieved in realistic twodimensional (2D) FM models defined on a honeycomb lattice, and we provide a prescription of how to realize them using 2D magnets and their heterostructures. ${N}_{\mathrm{m}\mathrm{a}\mathrm{x}}\simeq 25$ 
Abstract The Parker Solar Probe (PSP) entered a region of subAlfvénic solar wind during encounter 8, and we present the first detailed analysis of lowfrequency turbulence properties in this novel region. The magnetic field and flow velocity vectors were highly aligned during this interval. By constructing spectrograms of the normalized magnetic helicity, crosshelicity, and residual energy, we find that PSP observed primarily Alfvénic fluctuations, a consequence of the highly fieldaligned flow that renders quasi2D fluctuations unobservable to PSP. We extend Taylor’s hypothesis to sub and superAlfvénic flows. Spectra for the fluctuating forward and backward Elsässer variables (
^{±}, respectively) are presented, showing thatz ^{+}modes dominatez ^{−}by an order of magnitude or more, and thez ^{+}spectrum is a power law in frequency (parallel wavenumber)z f ^{−3/2}( ) compared to the convex ${k}_{\parallel}^{3/2}$ ^{−}spectrum withz f ^{−3/2}( ) at low frequencies, flattening around a transition frequency (at which the nonlinear and Alfvén timescales are balanced) to ${k}_{\parallel}^{3/2}$f ^{−1.25}at higher frequencies. The observed spectra are well fitted using a spectral theory for nearly incompressible magnetohydrodynamics assuming a wavenumber anisotropy , that the ${k}_{\perp}\sim {k}_{\parallel}^{3/4}$ ^{+}fluctuations experience primarily nonlinear interactions, and that the minorityz ^{−}fluctuations experience both nonlinear and Alfvénic interactions withz ^{+}fluctuations. The density spectrum is a power law that resembles neither thez ^{±}spectra nor the compressible magnetic field spectrum, suggesting that these are advected entropic rather than magnetosonic modes and not due to the parametric decay instability. Spectra in the neighboring modestly superAlfvénic intervals are similar.z 
Abstract Parker Solar Probe (PSP) observed subAlfvénic solar wind intervals during encounters 8–14, and lowfrequency magnetohydrodynamic (MHD) turbulence in these regions may differ from that in superAlfvénic wind. We apply a new mode decomposition analysis to the subAlfvénic 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 backwardpropagating slow MS modes. The mode decomposition provides phase information (frequency and wavenumber
k ) for each mode. Entropy density fluctuations have a wavenumber anisotropy ofk _{∥}≫k _{⊥}, whereas slowmode density fluctuations havek _{⊥}>k _{∥}. Magnetic field fluctuations are primarily magnetic island modes (δ B ^{i}) with anO (1) smaller contribution from unidirectionally propagating Alfvén waves (δ B ^{A+}) giving a variance anisotropy of . Incompressible magnetic fluctuations dominate compressible contributions from fast and slow MS modes. The magnetic island spectrum is Kolmogorovlike $\u3008{\delta {B}^{i}}^{2}\u3009/\u3008{\delta {B}^{A}}^{2}\u3009=4.1$ in perpendicular wavenumber, and the unidirectional Alfvén wave spectra are ${k}_{\perp}^{1.6}$ and ${k}_{\parallel}^{1.6}$ . Fast MS modes propagate at essentially the Alfvén speed with anticorrelated transverse velocity and magnetic field fluctuations and are almost exclusively magnetic due to ${k}_{\perp}^{1.5}$β _{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. 
Abstract We measure the thermal electron energization in 1D and 2D particleincell simulations of quasiperpendicular, lowbeta (
β _{p}= 0.25) collisionless ion–electron shocks with mass ratiom _{i}/m _{e}= 200, fast Mach number –4, and upstream magnetic field angle ${\mathcal{M}}_{\mathrm{ms}}=1$θ _{Bn}= 55°–85° from the shock normal . It is known that shock electron heating is described by an ambipolar, $\stackrel{\u02c6}{\mathit{n}}$ parallel electric potential jump, ΔB ϕ _{∥}, that scales roughly linearly with the electron temperature jump. Our simulations have –0.2 in units of the preshock ions’ bulk kinetic energy, in agreement with prior measurements and simulations. Different ways to measure $\mathrm{\Delta}{\varphi}_{\parallel}/(0.5{m}_{\mathrm{i}}{{u}_{\mathrm{sh}}}^{2})\sim 0.1$ϕ _{∥}, including the use of de Hoffmann–Teller frame fields, agree to tensofpercent accuracy. Neglecting offdiagonal electron pressure tensor terms can lead to a systematic underestimate ofϕ _{∥}in our lowβ _{p}shocks. We further focus on twoθ _{Bn}= 65° shocks: a ( ${\mathcal{M}}_{\mathrm{s}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}4$ ) case with a long, 30 ${\mathcal{M}}_{\mathrm{A}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}1.8$d _{i}precursor of whistler waves along , and a $\stackrel{\u02c6}{\mathit{n}}$ ( ${\mathcal{M}}_{\mathrm{s}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}7$ ) case with a shorter, 5 ${\mathcal{M}}_{\mathrm{A}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}3.2$d _{i}precursor of whistlers oblique to both and $\stackrel{\u02c6}{\mathit{n}}$ ;B d _{i}is the ion skin depth. Within the precursors,ϕ _{∥}has a secular rise toward the shock along multiple whistler wavelengths and also has localized spikes within magnetic troughs. In a 1D simulation of the , ${\mathcal{M}}_{\mathrm{s}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}4$θ _{Bn}= 65° case,ϕ _{∥}shows a weak dependence on the electron plasmatocyclotron frequency ratioω _{pe}/Ω_{ce}, andϕ _{∥}decreases by a factor of 2 asm _{i}/m _{e}is raised to the true proton–electron value of 1836. 
Abstract While it is well known that cosmic rays (CRs) can gain energy from turbulence via secondorder 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
becomes comparable to the turbulent cascade time. Magnetohydrodynamic simulations of stirred compressive turbulence in a gasCR fluid with diffusive CR transport show clear imprints of CRinduced damping, saturating at ${t}_{\mathrm{damp}}\sim \rho {v}^{2}/{\stackrel{\u0307}{E}}_{\mathrm{CR}}\propto {E}_{\mathrm{CR}}^{1}$ , where ${\stackrel{\u0307}{E}}_{\mathrm{CR}}\sim \tilde{\u03f5}$ is the turbulent energy input rate. In that case, almost all of the energy in largescale 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 smallscale turbulence largely solenoidal, with implications for thermal instability and proposed resonant scattering of $\tilde{\u03f5}$E ≳ 300 GeV CRs by fast modes. When CR transport is streaming dominated, CRs also damp largescale motions, with kinetic energy reduced by up to 1 order of magnitude in realisticE _{CR}∼E _{g}scenarios, but turbulence (with a reduced amplitude) still cascades down to small scales with the same power spectrum. Such largescale damping implies that turbulent velocities obtained from the observed velocity dispersion may significantly underestimate turbulent forcing rates, i.e., . $\tilde{\u03f5}\gg \rho {v}^{3}/L$