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 two-dimensional turbulent flows, such as those modeling astrophysical disks. We demonstrate that the direction of cascade undergoes continuous transition as the Mach number
- NSF-PAR ID:
- 10472560
- Publisher / Repository:
- IOP Publishing
- Date Published:
- Journal Name:
- New Journal of Physics
- Volume:
- 25
- Issue:
- 11
- ISSN:
- 1367-2630
- 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 0due to absorption of low-power microwaves of frequencyω 0under the resonance conditions and in the absence of any applied bias voltage. The two-decades-old ‘standard model’ of this effect, based on the scattering theory of adiabatic quantum pumping, predicts that component of spin current vector is time-independent while and oscillate harmonically in time with a single frequencyω 0whereas pumped charge current is zero in the same adiabatic limit. Here we employ more general approaches than the ‘standard model’, namely the time-dependent 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 and chargeI (t ) currents. All four of these functions harmonically oscillate in time at both even and odd integer multiples of the driving frequencyω 0. The cutoff order of such high harmonics increases with SOC strength, reaching in the one-dimensional FM or AFM models chosen for demonstration. A higher cutoff can be achieved in realistic two-dimensional (2D) FM models defined on a honeycomb lattice, and we provide a prescription of how to realize them using 2D magnets and their heterostructures. -
Abstract The Parker Solar Probe (PSP) entered a region of sub-Alfvénic solar wind during encounter 8, and we present the first detailed analysis of low-frequency 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, cross-helicity, and residual energy, we find that PSP observed primarily Alfvénic fluctuations, a consequence of the highly field-aligned flow that renders quasi-2D fluctuations unobservable to PSP. We extend Taylor’s hypothesis to sub- and super-Alfvénic 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 −spectrum withz f −3/2( ) at low frequencies, flattening around a transition frequency (at which the nonlinear and Alfvén timescales are balanced) tof −1.25at higher frequencies. The observed spectra are well fitted using a spectral theory for nearly incompressible magnetohydrodynamics assuming a wavenumber anisotropy , that the +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 super-Alfvénic intervals are similar.z -
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 wavenumber
k ) 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 (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 Kolmogorov-like in perpendicular wavenumber, and the unidirectional Alfvén wave spectra are and . 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. -
Abstract We measure the thermal electron energization in 1D and 2D particle-in-cell simulations of quasi-perpendicular, low-beta (
β p= 0.25) collisionless ion–electron shocks with mass ratiom i/m e= 200, fast Mach number –4, and upstream magnetic field angleθ Bn= 55°–85° from the shock normal . It is known that shock electron heating is described by an ambipolar, -parallel electric potential jump, ΔB ϕ ∥, that scales roughly linearly with the electron temperature jump. Our simulations have –0.2 in units of the pre-shock ions’ bulk kinetic energy, in agreement with prior measurements and simulations. Different ways to measureϕ ∥, including the use of de Hoffmann–Teller frame fields, agree to tens-of-percent accuracy. Neglecting off-diagonal electron pressure tensor terms can lead to a systematic underestimate ofϕ ∥in our low-β pshocks. We further focus on twoθ Bn= 65° shocks: a ( ) case with a long, 30d iprecursor of whistler waves along , and a ( ) case with a shorter, 5d iprecursor of whistlers oblique to both and ;B d iis 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 ,θ Bn= 65° case,ϕ ∥shows a weak dependence on the electron plasma-to-cyclotron frequency ratioω pe/Ωce, andϕ ∥decreases by a factor of 2 asm i/m eis 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 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
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 , where 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., .