Sound Modes Research Articles

Dark sound: Collective modes of the axionic dark matter condensate

We discuss the axion dark matter (DM) condensate and the consequences the interactions of dark matter would have on the spectrum of collective modes. We find that DM self-interactions change the spectrum of excitations from a quadratic to a linearlike dispersion with velocity ${v}_{s}$ which is set by the interactions, but dominated by gravity. For typical DM densities and interactions we find ${v}_{s}\ensuremath{\sim}{10}^{\ensuremath{-}12}c$. This soundlike mode corresponds to DM density oscillations just like in any other Bose liquid, hence we call it dark sound (DS). The DS mode is well defined and describes stable density oscillations at intermediate length scales $k\ensuremath{\ge}{k}_{\mathrm{min}}\ensuremath{\sim}{10}^{4}\text{ }\text{ }{\mathrm{lyr}}^{\ensuremath{-}1}$. In the extreme long-wavelength limit gravity dominates and leads to Jeans instability of the sound mode at the scale of clump formation $k\ensuremath{\le}{k}_{\mathrm{min}}$. We also discuss the possible observable consequences of the DS, including quantized DS modes inside clumps, their characteristic energy, and noise features that might facilitate the observation of DM.

Flight Experiment Validation of Altitude Measurement Performance of MOSIR on Tianwen-1 Orbiter

The MOSIR (Mars Orbiter Subsurface Investigation Radar) is one of the scientific payloads carried by the Tianwen-1 orbiter. MOSIR conducted a ground experiment in the desert near Dengkou County, northern China, before the launch of the Tianwen-1 satellite. The MOSIR prototype was suspended from a hot air balloon and flew over a flat region at an altitude of 2500–3300 m. This experiment aimed to verify the system performance and data processing. The data collected in subsurface sounding mode is performed range compression, and the altitude measurement data removes invalid data. After processing, the altitude measurement results of two operating modes are analyzed and compared with that of the Global Position System (GPS), which verifies the accuracy of the altitude measurement.

Сравнительный анализ погрешности приближения однократного рассеяния при решении одной обратной задачи в двумерном и трехмерном случаях

The inverse problem for the nonstationary radiative transfer equation is considered, which consists in finding the scattering coefficient for a given time-angular distribution of the solution to the equation at a certain point. To solve this problem, the single scattering approximation in the pulsed sounding mode is used. A comparative analysis of the error in solving the inverse problem in the single scattering approximation for two-dimensional and three-dimensional models describing the process of high-frequency acoustic sounding in a fluctuating ocean is carried out. It is shown that in the two-dimensional case the error of the approximate solution significantly exceeds the error in the three-dimensional model.

Low-Frequency Excess Vibrational Modes in Two-Dimensional Glasses.

Glasses possess more low-frequency vibrational modes than predicted by Debye theory. These excess modes are crucial for the understanding of the low temperature thermal and mechanical properties of glasses, which differ from those of crystalline solids. Recent simulational studies suggest that the density of the excess modes scales with their frequency ω as ω^{4} in two and higher dimensions. Here, we present extensive numerical studies of two-dimensional model glass formers over a large range of glass stabilities. We find that the density of the excess modes follows D_{exc}(ω)∼ω^{2} up to around the boson peak, regardless of the glass stability. The stability dependence of the overall scale of D_{exc}(ω) correlates with the stability dependence of low-frequency sound attenuation. However, we also find that, in small systems, where the first sound mode is pushed to higher frequencies, at frequencies below the first sound mode, there are excess modes with a system size independent density of states that scales as ω^{3}.

Collective modes of type-IIWeyl fermions with repulsive S-wave interaction

Three-dimensional type-II Weyl fermions possess overtilted cone-like low-energy band dispersion. Unlike the closed ellipsoidal Fermi surface for type-I Weyl fermions, the Fermi surface is an open hyperboloid for type-II Weyl fermions. We evaluate the spin and density susceptibility of type-II Weyl fermions with repulsive S-wave interaction by means of Green’s functions. We obtain the particle–hole continuum along the tilted momentum direction and perpendicular to the tilted momentum direction respectively. We find the zero sound mode in some repulsive interaction strengths by numerically solving the pole equations of the susceptibility within the random-phase approximation.

Numerical modeling of the sound field interference pattern in the presence of intense internal waves

The purpose of the paper is numerical modeling of the sound field in the presence of intense internal waves to study the interference pattern in inhomogeneous ocean waveguide. It is assumed that intense internal waves are the reason for the sound field modes coupling and horizontal refraction. The results of sound field modeling are processed to obtain acoustic intensity distributions in the frequency-time domain (interferogram) for unmoving sources. The two-dimensional Fourier transform (2D-FT) is applied to analyze the interferogram. The result of the 2D-FT can be called the Fourier-hologram (hologram). The hologram allows us to coherently accumulate the sound intensity of the interferogram in the narrow area as focal spots. It is demonstrated in the paper that the position of focal spots in the hologram depends on the distance between the source and the receiver. The relationship between the interferogram and hologram structures and the unperturbed and scattered fields parameters is obtained in the paper. The hologram spectral density consists of the two disjoint regions corresponding to the scattered and scattered fields. The filtering of these areas allows us to transmit the non-distorted information through an inhomogeneous ocean environment. The numerical simulation results of interferograms and holograms in the presence of the internal waves are considered. [This research was supported by the RFBR (19-29-06075, 19-38-90326).]

Effective field theory for hydrodynamics without boosts

We formulate the Schwinger-Keldysh effective field theory of hydrodynamics without boost symmetry. This includes a spacetime covariant formulation of classical hydrodynamics without boosts with an additional conserved particle/charge current coupled to Aristotelian background sources. We find that, up to first order in derivatives, the theory is characterised by the thermodynamic equation of state and a total of 29 independent transport coefficients, in particular, 3 hydrostatic, 9 non-hydrostatic non-dissipative, and 17 dissipative. Furthermore, we study the spectrum of linearised fluctuations around anisotropic equilibrium states with non-vanishing fluid velocity. This analysis reveals a pair of sound modes that propagate at different speeds along and opposite to the fluid flow, one charge diffusion mode, and two distinct shear modes along and perpendicular to the fluid velocity. We present these results in a new hydrodynamic frame that is linearly stable irrespective of the boost symmetry in place. This provides a unified covariant stable approach for simultaneously treating Lorentzian, Galilean, and Lifshitz fluids within an effective field theory framework and sets the stage for future studies of non-relativistic intertwined patterns of symmetry breaking.

Stability analysis of implicit semi-Lagrangian methods for numerical solution of non-hydrostatic atmospheric dynamics equations

Abstract The stability of implicit semi-Lagrangian schemes for time-integration of the non-hydrostatic atmosphere dynamics equations is analyzed in the present paper. The main reason for the instability of the considered class of schemes is the semi-Lagrangian advection of stratified thermodynamic variables coupled to the fixed point iteration method used to solve the implicit in time upstream trajectory computation problem. We identify two types of unstable modes and obtain stability conditions in terms of the scheme parameters. Stabilization of sound modes requires the use of a pressure reference profile and time off-centering. Gravity waves are stable only for an even number of fixed point method iterations. The maximum time step is determined by inverse buoyancy frequency in the case when the reference profile of the potential temperature is not used. Generally, applying time off-centering and reference profile to pressure variable is necessary for stability. Using reference profile for potential temperature and an even number of the iterations allows one to significantly increase the maximum time-step value.

A three-dimensional analytical approach for large rectangular splitter silencers in the presence of mean flow

The dissipative splitter silencer is a common means for broadband noise attenuation in flow ducts such as ventilation and gas turbine systems where many higher order modes are cut-on. To predict the effect of a splitter silencer, analytical approaches have obvious advantages over the numerical methods on computation efficiency, but the development of analytical approaches are often based on many simplifications like the infinite length assumption. This paper presents a three-dimensional analytical method to solve the sound propagation in a duct with a finite length splitter silencer in the presence of mean flow. In the developed method, the convergence problem, which is often encountered by using the existing mode-based methods for high-frequency computations of soft-wall eigenmodes, is overcome by applying the equivalent surface source concept. With very low computational cost, the present method is validated to give consistent transmission loss predictions with the numerical results and experimental measurements available in the literature. Further, the performance of a splitter silencer is studied with different higher order incident modes, and reflection versus dissipation analysis is conducted by using the developed method. It is found that the sound attenuation is dependent on the sound energy spatial distributions of the incident sound modes, and the dissipation dominates the transmission loss behavior of the splitter silencer.

SOLAR RADIATION WITHIN THE WATER COLUMN OF SMALL KARELIAN LAKES

Measurements of solar radiation fluxes were taken with minute discreteness from March 27 to April 6, 2020 in the subglacial layer of Lake Vendyurskoe and in the water column of Lake Golubaya Lamba at 0–2.9 m depths. Seasonal surveys using the RBRConcerto probe in the vertical sounding mode were carried out in these lakes and in Lake Rindozero in early summer (June) and in autumn (October). When the ice was 40 cm thick and the snow was 1–2 cm thick, the solar radiation flux in the under-ice layer of Lake Vendyurskoe reached 90–100 W/m2, and when snow covered the surface, it sharply decreased to almost zero. With the same ice and snow thickness, the flux of photosynthetic active solar radiation (PAR) reached 360–410 μmol/(m2·s) in the under-ice layer of Golubaya Lamba. For Golubaya Lamba, the values of the extinction coefficients averaged for 0–2.9 m depths were 0.47–0.57 m-1 during the spring under-ice period; the depth of 1 % irradiation (photic zone) calculated from these values was 3.2–6.7 m, meaning it can reached the bottom on some sunny days. In June 2020, PAR fluxes in the surface layer of Lake Vendyurskoe (depth 0.5 m) reached 1000 μmol/(m2·s), decreasing to zero below 4 m depth. In Golubaya Lamba, the PAR flux reached 150 μmol/(m2·s) at a depth of 4 m in summer, and 50 μmol/(m2·s) during the ice-covered period and in autumn. In Lake Rindozero, the PAR flux rapidly decreased with depth and was practically zero below 2 m in June and October 2020. The values of the extinction coefficients for the open water period varied within 0.99–1.97 m-1 in Lake Vendyurskoe, 1.60–1.88 m-1 in Lake Rindozero, 0.12–0.14 m-1 in Golubaya Lamba.

Long-Range Nematic Order in Two-Dimensional Active Matter.

Working in two space dimensions, we show that the orientational order emerging from self-propelled polar particles aligning nematically is quasi-long-ranged beyond ℓ_{r}, the scale associated to induced velocity reversals, which is typically extremely large and often cannot even be measured. Below ℓ_{r}, nematic order is long-range. We construct and study a hydrodynamic theory for this de facto phase and show that its structure and symmetries differ from conventional descriptions of active nematics. We check numerically our theoretical predictions, in particular the presence of π-symmetric propagative sound modes, and provide estimates of all scaling exponents governing long-range space-time correlations.

Direct evidence of gradient drift instability being the origin of a rotating spoke in a crossed field plasma

A plasma rotating spoke in a crossed field discharge is studied using 2D radial-azimuthal fully kinetic particle-in-cell Monte Carlo collision simulations. The kinetic model reveals the whole perturbation spectrum of the gradient drift instability in the linear stage: Simon–Hoh, lower hybrid, and ion sound modes, providing direct evidence of the spoke of gradient drift instability nature. The two-fluid dispersion relation of gradient drift instability was utilized to analyze the linear development of instabilities in the simulations. The charge separation effect was incorporated in the fluid linear theory and a super-resolution signal processing method (multiple signal classification) was applied to obtain the numerical frequency spectrum. The simulated spectrum and growth rate show excellent agreement with the theoretical dispersion relation (real frequency and imaginary frequency) in the investigated cases. The most linearly unstable mode was found to be the lower hybrid instability and the mode transition into the m = 1 macroscopic rotating structure after saturation of the linear phase is accompanied by an inverse energy cascade. In the nonlinear stage, pronounced spoke phenomena can occur when the heating of E θ × B electron flow channeled in the spoke front passage suffices to provide enhanced ionization.

Anomalous Hydrodynamics in a One-Dimensional Electronic Fluid.

We construct multimode viscous hydrodynamics for one-dimensional spinless electrons. Depending on the scale, the fluid has six (shortest lengths), four (intermediate, exponentially broad regime), or three (asymptotically long scales) hydrodynamic modes. Interaction between hydrodynamic modes leads to anomalous scaling of physical observables and waves propagating in the fluid. In the four-mode regime, all modes are ballistic and acquire Kardar-Parisi-Zhang (KPZ)-like broadening with asymmetric power-law tails. "Heads" and "tails" of the waves contribute equally to thermal conductivity, leading to ω^{-1/3} scaling of its real part. In the three-mode regime, the system is in the universality class of a classical viscous fluid [O. Narayan and S. Ramaswamy, Anomalous Heat Conduction in One-Dimensional Momentum-Conserving Systems, Phys. Rev. Lett. 89, 200601 (2002).PRLTAO0031-900710.1103/PhysRevLett.89.200601, H. Spohn, Nonlinear fluctuating hydrodynamics for anharmonic chains, J. Stat. Phys. 154, 1191 (2014).JSTPBS0022-471510.1007/s10955-014-0933-y]. Self-interaction of the sound modes results in a KPZ-like shape, while the interaction with the heat mode results in asymmetric tails. The heat mode is governed by Levy flight distribution, whose power-law tails give rise to ω^{-1/3} scaling of heat conductivity.

Dissipation and fluctuations in elongated bosonic Josephson junctions

We investigate the dynamics of bosonic atoms in elongated Josephson junctions. We find that these systems are characterized by an intrinsic coupling between the Josephson mode of macroscopic quantum tunneling and the sound modes. This coupling of Josephson and sound modes gives rise to a damped and stochastic Langevin dynamics for the Josephson degree of freedom. From a microscopic Lagrangian, we deduce and investigate the damping coefficient and the stochastic noise, which includes thermal and quantum fluctuations. Finally, we study the time evolution of relative-phase and population-imbalance fluctuations of the Josephson mode and their oscillating thermalization to equilibrium.

Asymmetric thermal transport in strained Weyl semimetals due to axial anomaly

We show that strained Weyl semimetals exhibit a directional asymmetry of the heat transfer along the strain axis. The forward/backward asymmetry of the thermal conductivity appears as a result of the axial anomaly, which leads to the emergence of unidirectionally propagating ``chiral'' sound modes. The chiral phonons modify, in a directionally asymmetric manner, the spectrum of the acoustic lattice vibrations, and both types of phonons contribute to the thermal asymmetry of the material. We estimate the magnitude of the effect and discuss its possible experimental detection.

Collective modes and superfluidity of a two-dimensional ultracold Bose gas

The collective modes of a quantum liquid shape and impact its properties profoundly, including its emergent phenomena such as superfluidity. Here we present how a two-dimensional Bose gas responds to a moving lattice potential. In particular we discuss how the induced heating rate depends on the interaction strength and the temperature. This study is motivated by the recent measurements of Sobirey {\it et al.} arXiv:2005.07607 (2020), for which we provide a quantitative understanding. Going beyond the existing measurements, we demonstrate that this probing method allows to identify first and second sound in quantum liquids. We show that the two sound modes undergo hybridization as a function of interaction strength, which we propose to detect experimentally. This gives a novel insight into the two regimes of Bose gases, defined via the hierarchy of sounds modes.

Assessment of Leachate Contamination of Groundwater at Some Waste Disposal Sites in the Southern Guinea Savannah Ecological Zone of Nigeria Using Geoelectric Processes

This study aims at determining the leachate contamination of the groundwater resource at selected domestic wastes disposal sites in Minna, Nigeria for a population about 2.1 million, to locate aquifers and hydraulically active structures by tracing the movement of contaminant plumes and seepages in ground at the selected locations. Resistivity data was collected using a terrameter (SAS4000) while the Vertical Electrical Sounding (VES) mode was deployed using the Schlumberger array to enable investigation of the depth penetration of contaminant plume. The induced polarization (IP) was used to determine the level of contaminant plume. The VES readings measured at 50m intervals along each profile line and 100m inter-profile distance, with a maximum current electrode separation of 200m and potential electrode separation of 30m. There are equal numbers of three and four layers observed on the profile, which has ten VES points. The first layer has a resistivity range between 48.4 Ωm & 428 Ωm and thickness between 0.65m & 3.83m. However, isolated resistivity area such as VES; N5 (287Ωm), N6 (295Ωm) and N8 (428Ωm) also suggested sandy/soil rich in organic matter (humus material/soil). The second and third layer is the fractured basement which has very low resistivity values for most VES (N1–48.5Ωm, N2–38.7Ωm, N3–41.6Ωm, N5–61.5Ωm, N7–49.6Ωm, N8–60.7Ωm, N9–108Ωm and N10–97.6Ωm) that indicated leachate presence and contamination, which results from increased ionic concentration. In conclusion, it was discovered that the study area had high conductivity values for some of the locations using the resitivity determination method. This indicated the presence of water within the study area. It was also concluded that the IP which indicated high concentration of metals caused the lowering of the resisitivity values at some of the locations, thus indicating the presence of metals within the study area.

Jet wake from linearized hydrodynamics

We explore how to improve the hybrid model description of the particles originating from the wake that a jet produced in a heavy ion collision leaves in the droplet of quark-gluon plasma (QGP) through which it propagates, using linearized hydrodynamics on a background Bjorken flow. Jet energy and momentum loss described by the hybrid model become currents sourcing linearized hydrodynamics. By solving the linearized hydrodynamic equations numerically, we investigate the development of the wake in the dynamically evolving droplet of QGP, study the effect of viscosity, scrutinize energy-momentum conservation, and check the validity of the linear approximation. We find that linearized hydrodynamics works better in the viscous case because diffusive modes damp the energy-momentum perturbation produced by the jet. We calculate the distribution of particles produced from the jet wake by using the Cooper-Frye prescription and find that both the transverse momentum spectrum and the distribution of particles in azimuthal angle are similar in shape in linearized hydrodynamics and in the hybrid model. Their normalizations are different because the momentum-rapidity distribution in the linearized hydrodynamics analysis is more spread out, due to sound modes. Since the Bjorken flow has no transverse expansion, we explore the effect of transverse flow by using local boosts to add it into the Cooper-Frye formula. After including the effects of transverse flow in this way, the transverse momentum spectrum becomes harder: more particles with transverse momenta bigger than 2 GeV are produced than in the hybrid model. Although we defer implementing this analysis in a jet Monte Carlo, as would be needed to make quantitative comparisons to data, we gain a qualitative sense of how the jet wake may modify jet observables by computing proxies for two example observables: the lost energy recovered in a cone of varying open angle, and the fragmentation function. We find that linearized hydrodynamics with transverse flow effects added improves the description of the jet wake in the hybrid model in just the way that comparison to data indicates is needed. Our study illuminates a path to improving the description of the wake in the hybrid model, highlighting the need to take into account the effects of both transverse flow and the broadening of the energy-momentum perturbation in spacetime rapidity on particle production.

Collisionless sound of bosonic superfluids in lower dimensions

The superfluidity of low-temperature bosons is well established in the collisional regime. In the collisionless regime, however, the presence of superfluidity is not yet fully clarified, in particular in lower spatial dimensions. Here we compare the Vlasov-Landau equation, which does not take into account the superfluid nature of the bosonic system, with the Andreev-Khalatnikov equations, which instead explicitly contain a superfluid velocity. We show that recent experimental data of the sound mode in a two-dimensional collisionless Bose gas of $^{87}$Rb atoms are in good agreement with both theories but the sound damping is better reproduced by the Andreev -Khalatnikov equations below the Berezinskii-Kosterlitz-Thouless critical temperature $T_c$ while above $T_c$ the Vlasov-Landau results are closer to the experimental ones. For one dimensional bosonic fluids, where experimental data are not yet available, we find larger differences between the sound velocities predicted by the two transport theories and, also in this case, the existence of a superfluid velocity reduces the sound damping.

Localization of ultrasound in 2D phononic crystal with randomly oriented asymmetric scatterers

A phononic crystal of aluminum rods with an asymmetric cross section in water is used for the study of Anderson localization of sound. Due to asymmetry, these scatterers may be arranged in three different configurations: a periodic 2D structure, a random structure with 2D disorder, and a random structure with 1D disorder. The last configuration where the rods are equally oriented within the columns and disoriented along the rows is fabricated for the experimental study of 1D Anderson localization in the 2D random system. An exponentially weakening transmission of the ultrasound is demonstrated for the waves propagating along the direction of disorder. In the perpendicular direction where the scatterers are ordered, sound propagates as an extended (delocalized) wave. The localization length is controlled by the degree of disorder. For weak disorder, when orientations of the rods weakly fluctuate around a given direction, Thouless’s theoretical prediction for the scaling of the Lyapunov exponent with disorder is experimentally observed for a mode within the transmission band. For the sound mode close to the band edge, anomalous scaling is confirmed.