Perpendicular Wave Research Articles

TRAPPING CAPABILITY OF MICROLENS 2D ARRAY BY ACOUSTIC MODULATION

In this article, the microlens 2D array created in acoustic-optical medium by the ultrasonic wave modulation is proposed. The expression described the refractive index induced by cross-interference of two perpendicular ultrasonic waves is approximately derived. By simulation, the 2D array of the Graded–refractive index lenses are appeared in the thin layer with certain strain-acoutstic constant and thickness. The dependence of focal length and the radius of lens, i.e. its numerical aperture (AN) on thickness and strain constant of layer, and ultrasonic wave intensity are simulated and trapping capability of optical tweezer array is discussed.

Entanglement of helicity and energy in kinetic Alfvén wave/whistler turbulence

The role of magnetic helicity is investigated in kinetic Alfvén wave and oblique whistler turbulence in presence of a relatively intense external magnetic fieldb0e∥. In this situation, turbulence is strongly anisotropic and the fluid equations describing both regimes are the reduced electron magnetohydrodynamics (REMHD) whose derivation, originally made from the gyrokinetic theory, is also obtained here from compressible Hall magnetohydrodynamics (MHD). We use the asymptotic equations derived by Galtier and Bhattacharjee (2003Phys. Plasmas10, 3065–3076) to study the REMHD dynamics in the weak turbulence regime. The analysis is focused on the magnetic helicity equation for which we obtain the exact solutions: they correspond to the entanglement relation,n + ñ= −6, wherenandñare the power law indices of the perpendicular (tob0) wave number magnetic energy and helicity spectra, respectively. Therefore, the spectra derived in the past from the energy equation only, namelyn= −2.5 andñ= −3.5, are not the unique solutions to this problem but rather characterize the direct energy cascade. The solutionñ= −3 is a limit imposed by the locality condition; it is also the constant helicity flux solution obtained heuristically. The results obtained offer a new paradigm to understand solar wind turbulence at sub-ion scales where it is often observed that −3 <n< −2.5.

The trapping of equatorial magnetosonic waves in the Earth's outer plasmasphere

We investigate the excitation and propagation of equatorial magnetosonic waves observed by the Van Allen Probes and describe evidence for a trapping mechanism for magnetosonic waves in the Earth's plasmasphere. Intense equatorial magnetosonic waves were observed inside the plasmasphere in association with a pronounced proton ring distribution, which provides free energy for wave excitation. Instability analysis along the inbound orbit demonstrates that broadband magnetosonic waves can be excited over a localized spatial region near the plasmapause. The waves can subsequently propagate into the inner plasmasphere and remain trapped over a limited radial extent, consistent with the predictions of near-perpendicular propagation. By performing a similar analysis on another observed magnetosonic wave event, we demonstrate that magnetosonic waves can also be trapped within local density structures. We suggest that perpendicular wave propagation is important for explaining the presence of magnetosonic waves in the Earth's plasmasphere at locations away from the generation region.

Investigation of storm time magnetotail and ion injection using three‐dimensional global hybrid simulation

AbstractDynamics of the near‐Earth magnetotail associated with substorms during a period of extended southward interplanetary magnetic field is studied using a three‐dimensional (3‐D) global hybrid simulation model that includes both the dayside and nightside magnetosphere, for the first time, with physics from the ion kinetic to the global Alfvénic convection scales. It is found that the dayside reconnection leads to the penetration of the dawn‐dusk electric field through the magnetopause and thus a thinning of the plasma sheet, followed by the magnetotail reconnection with 3‐D, multiple flux ropes. Ion kinetic physics is found to play important roles in the magnetotail dynamics, which leads to the following results: (1) Hall electric fields in the thin current layer cause a systematic dawnward ion drift motion and thus a dawn‐dusk asymmetry of the plasma sheet with a higher (lower) density on the dawnside (duskside). Correspondingly, more reconnection occurs on the duskside. Bidirectional fast ions are generated due to acceleration in reconnection, and more high‐speed earthward flow injections are found on the duskside than the dawnside. Such finding of the dawn‐dusk asymmetry is consistent with recent satellite observations. (2) The injected ions undergo the magnetic gradient and curvature drift in the dipole‐like field, forming a ring current. (3) Ion particle distributions reveal multiple populations/beams at various distances in the tail. (4) Dipolarization of the tail magnetic field takes place due to the pileup of the injected magnetic fluxes and thermal pressure of injected ions, where the fast earthward flow is stopped. Oscillation of the dipolarization front is developed at the fast‐flow braking, predominantly on the dawnside. (5) Kinetic compressional wave turbulence is present around the dipolarization front. The cross‐tail currents break into small‐scale structures with k⟂ρi∼1, where k⟂ is the perpendicular wave number. A sharp dip of magnetic field strength is seen just in front of the sharp rise of the magnetic field at the dipolarization front, mainly on the duskside. (6) A shear flow‐type instability is found on the duskside flank of the ring current plasma, whereas a kinetic ballooning instability appears on the dawnside. (7) Shear Alfvén waves and compressional waves are generated from the tail reconnection, and they evolve into kinetic Alfvén waves in the dipole‐like field region. Correspondingly, multiple field‐aligned current filaments are generated above the auroral ionosphere.

EXCITATION OF KINETIC ALFVÉN WAVES BY FAST ELECTRON BEAMS

Energetic electron beams, which are ubiquitous in a large variety of active phenomena in space and astrophysical plasmas, are one of the most important sources that drive plasma instabilities. In this paper, taking account of the return-current effect of fast electron beams, kinetic Alfven wave (KAW) instability driven by a fast electron beam is investigated in a finite-beta plasma of Q < beta < 1 (where beta is the kinetic-to-magnetic pressure ratio and Q = m(e)/m(i) is the mass ratio of electrons to ions). The results show that the kinetic resonant interaction of beam electrons is the driving source for KAW instability, unlike the case driven by a fast ion beam, where both the kinetic resonant interaction of beam ions and the return-current are the driving source for the KAW instability. KAW instability has a nonzero growth rate in the range of the perpendicular wave number, 0 < k(perpendicular to) < k(perpendicular to)(u), and the maximum growth rate, gamma(m), occurs between 0.5k(perpendicular to)(u) < k(perpendicular to)(m) < 0.8k(perpendicular to)(u). Both the maximal growing perpendicular wave number k(perpendicular to)(m) and the maximal growth rate gamma(m) depend sensitively on the velocity of electron beam upsilon(b), and the most favorable beam velocity occurs between 8 upsilon(A) < upsilon(b) < 10 upsilon(A). On the other hand, the excited KAWs are weakly dispersive with k(perpendicular to) rho(i) < 1 and have the maximum growth rate at relatively low perpendicular wave numbers in the range 0.3 < k(perpendicular to)(m) rho(i) < 0.6 for a beam velocity upsilon(b) < 10 upsilon(A). A possible application to the upward electron beams in the terrestrial magnetosphere is briefly discussed.

Linear mode conversion of Langmuir/z mode waves to radiation: Averaged energy conversion efficiencies, polarization, and applications to Earth's continuum radiation

AbstractLinear mode conversion (LMC) is the linear transfer of energy from one wave mode to another in a density gradient. It is relevant to planetary continuum radiation, type II and III radio bursts, and ionospheric radio emissions. This paper analyzes LMC by calculating angle‐averaged energy (ε) and power (εp) conversion efficiencies in both 2‐D and 3‐D for Langmuir/z mode waves (including upper hybrid waves for perpendicular wave vectors) converting to free‐space radiation in turbulent plasmas. The averages are over the distributions of the incoming Langmuir/z mode wave vectors k, density scale lengths L, and angles α and δ, where α is the angle between k and the background magnetic field B0 and δ is the angle between the density gradient ∇N0 and B0. The results show that the averaged and unaveraged conversion efficiencies are dependent on γβ, where γ is the adiabatic index and β is related to the electron temperature Te by β = Te/mec2. The averaged energy conversion efficiencies are proportional to γβ in 2‐D and to (γβ)3/2 in 3‐D, whereas the power conversion efficiencies are proportional to (γβ)1/2 in 2‐D and γβ in 3‐D. The special case of a perpendicular density gradient (δ≈90°) is considered and used to predict the conversion efficiencies of terrestrial continuum radiation (TCR) in three known source regions: the plasmapause, magnetopause, and the plasma sheet. The observed energy conversion efficiencies are estimated and are found to be consistent with the 2‐D and 3‐D predicted efficiencies; importantly, these results imply that LMC is a possible generation mechanism for TCR. The polarization of TCR is also predicted: TCR should be produced primarily in the o mode at the plasmapause and in both the o and x modes at the magnetopause and plasma sheet. These predictions are consistent with previous independent predictions and observations.

Analytical solutions for oscillations in a harbor with a hyperbolic-cosine squared bottom

Based on the linear shallow water approximation, longitudinal oscillations in a rectangular harbor with a hyperbolic-cosine squared bottom induced by incident perpendicular waves are analytically investigated, which could be described by combining the associated Legendre functions of the first and second kinds. The effects of topographic parameters on the resonant spectrum and response are examined in detail. When the width of the harbor is of the same order magnitude as wavelengths, transverse oscillations may exist due to the wave refraction. Analytic solutions for transverse oscillations within a harbor of hyperbolic-cosine squared bottom are derived. These oscillations are typically standing edge waves. The transverse eigenfrequency is found to be related not only to the width of the harbor, but also to the varying water depth parameters.

Theory for helical turbulence under fast rotation.

Recent numerical simulations have shown the strong impact of helicity on homogeneous rotating hydrodynamic turbulence. The main effect can be summarized through the law n+ñ=-4, where n and ñ are the power law indices of the one-dimensional energy and helicity spectra, respectively. We investigate this rotating turbulence problem in the small Rossby number limit by using the asymptotic weak turbulence theory derived previously. We show that the empirical law is an exact solution of the helicity equation where the power law indices correspond to perpendicular (to the rotation axis) wave number spectra. It is proposed that when the cascade towards small scales tends to be dominated by the helicity flux the solution tends to ñ=-2, whereas it is ñ=-3/2 when the energy flux dominates. The latter is compatible with the solution previously observed numerically and derived theoretically in the weak turbulence regime when only the energy equation is used, whereas the former solution is constrained by a locality condition.

Mode conversion and absorption of fast waves at high ion cyclotron harmonics in inhomogeneous magnetic fields

The propagation and absorption of high harmonic fast waves is of interest for non-inductive current drives in fusion experiments. The fast wave can be coupled with the ion Bernstein wave that propagates in the high magnetic field side of an ion cyclotron harmonic resonance layer. This coupling and the absorption are analyzed using the hot plasma dispersion relation and a wave equation that was converted from an approximate dispersion relation for the case where λi=k⊥2ρi2/2≳1 (where k⊥ is the perpendicular wave number and ρi is the ion Larmor radius). It is found that both reflection and conversion may occur near the harmonic resonance layer but that they decrease rapidly, giving rise to a sharp increase in the absorption as the parallel wave number increases.

Oblique wave attack on rubble mound breakwaters

Stability formulae for armour layers of rubble mound breakwaters are usually being applied assuming perpendicular wave attack. Often the effects of oblique waves are neglected. This is however a conservative assumption since the stability of armour slopes generally increases for oblique waves. New wave basin tests have been performed to assess the effects of oblique waves on the stability of rock slopes and the stability of cube armoured rubble mound breakwaters. The physical model tests were focussed on wave directions between perpendicular (0°) and parallel (90°). The test programme included tests with long-crested waves and tests with short-crested waves. The results show that for rock slopes the influence of oblique waves is larger for long-crested waves. Based on the test results a design guideline is provided to account for effects of oblique waves on the stability of rock slopes, armour layers with a double layer of cubes, and armour layers with a single layer of cubes.

Nonlinear excitation of geodesic acoustic mode by collisionless trapped electron mode

Excitation of geodesic acoustic mode (GAM) by collisionless trapped electron mode (CTEM) is studied within the framework of nonlinear gyrokinetic theory. The dispersion relation describing GAM excitation by drift wave turbulence valid for arbitrary k⊥ρi is derived. Here, k⊥ and ρi are, respectively, perpendicular wave vector and thermal ion Larmor radius. It is found that the parametric decay process of CTEM into a GAM and a CTEM sideband is unstable in both small and large k⊥ρi regimes due to, respectively, ion and trapped electron nonlinearity. For moderate k⊥ρi, however, there exists a negligible-GAM-excitation state where ion and trapped electron nonlinearities cancel each other.

The diffracted field and its gradient near the edge of a thin screen.

The secondary edge source line integral formulation for diffraction modeling uses edge sources with a 1/r dependency, in addition to a directivity function. At first sight, this might seem to contradict the expected 1/r behavior for the gradient near the edge of a thin screen. An analysis is presented for the special case of perpendicular plane wave incidence onto a thin screen, which shows that the secondary edge source formulation does indeed lead to the expected behavior close to the edge of an ideal wedge.

STEREO and Wind observations of intense cyclotron harmonic waves at the Earth's bow shock and inside the magnetosheath

We present the first observations of electron cyclotron harmonic waves at the Earth's bow shock from STEREO and Wind burst waveform captures. These waves are observed at magnetic field gradients at a variety of shock geometries ranging from quasi‐parallel to nearly perpendicular along with whistler mode waves, ion acoustic waves, and electrostatic solitary waves. Large amplitude cyclotron harmonic waveforms are also observed in the magnetosheath in association with magnetic field gradients convected past the bow shock. Amplitudes of the cyclotron harmonic waves range from a few tens to more than 500 mV/m peak‐peak. A comparison between the short (15 m) and long (100 m) Wind spin plane antennas shows a similar response at low harmonics and a stronger response on the short antenna at higher harmonics. This indicates that wavelengths are not significantly larger than 100 m, consistent with the electron cyclotron radius. Waveforms are broadband and polarizations are distinctively comma‐shaped with significant power both perpendicular and parallel to the magnetic field. Harmonics tend to be more prominent in the perpendicular directions. These observations indicate that the waves consist of a combination of perpendicular Bernstein waves and field‐aligned waves without harmonics. A likely source is the electron cyclotron drift instability which is a coupling between Bernstein and ion acoustic waves. These waves are the most common type of high‐frequency wave seen by STEREO during bow shock crossings and magnetosheath traversals and our observations suggest that they are an important component of the high‐frequency turbulent spectrum in these regions.

Convective amplification of electromagnetic ion cyclotron waves from ring‐distribution protons in the inner magnetosphere

AbstractThe growth of electromagnetic ion cyclotron (EMIC) waves due to a ring distribution of hydrogen ions is examined. Though these distributions are more commonly implicated in the generation of equatorial noise, their potential for exciting EMIC waves is considered here. It is shown that since the ring distribution is nonmonotonic in perpendicular velocity, the amplification achieved by this instability is greater than bi‐Maxwellian distributions for typical observed anisotropies, because the waves can maintain resonance over a much longer part of its trajectory. For ring speeds (Vr) close to the Alfvén speed (VA), the growth rate is largest at parallel propagation but decreases less rapidly toward oblique angles compared with a bi‐Maxwellian and can have a second peak for shorter wavelengths. The ring distribution parallel thermal speed is deduced from the observation of equatorial noise because a statistical survey of this parameter is not available. A ring density to background plasma density of a few percent is sufficient to achieve moderate gain. The gain is also shown to increase with L shell since the perpendicular wave number does not rise as rapidly along the raypath as compared with smaller L. The analysis suggests that EMIC wave activity should be closely associated with equatorial noise.

Propulsion of swimming microrobots inspired by metachronal waves in ciliates: from biology to material specifications

The quest for swimming microrobots originates from possible applications in medicine, especially involving navigation in bodily fluids. Swimming microorganisms have become a source of inspiration because their propulsion mechanisms are effective in the low-Reynolds number regime. In this study, we address a propulsion mechanism inspired by metachronal waves, i.e. the spontaneous coordination of cilia leading to the fast swimming of ciliates. We analyse the biological mechanism (referring to its particular embodiment in Paramecium caudatum), and we investigate the contribution of its main features to the swimming performance, through a three-dimensional finite-elements model, in order to develop a simplified, yet effective artificial design. We propose a bioinspired propulsion mechanism for a swimming microrobot based on a continuous cylindrical electroactive surface exhibiting perpendicular wave deformations travelling longitudinally along its main axis. The simplified propulsion mechanism is conceived specifically for microrobots that embed a micro-actuation system capable of executing the bioinspired propulsion (self-propelled microrobots). Among the available electroactive polymers, we select polypyrrole as the possible actuation material and we assess it for this particular embodiment. The results are used to appoint target performance specifications for the development of improved or new electroactive materials to attain metachronal-waves-like propulsion.

Developing serpent-type wave generators to create solitary wave simulations with BEM

Developing serpent-type wave generators to generate solitary waves in a 3D-basin was investigated in this study. Based on the Lagrangian description with time-marching procedures and finite differences of the time derivative, a 3D multiple directional wave basin with multidirectional piston wave generators was developed to simulate ocean waves by using BEM with quadrilateral elements, and to simulate wave-caused problems with fully nonlinear water surface conditions. The simulations of perpendicular solitary waves were conducted in the first instance to verify this scheme. Furthermore, the comparison of the waveform variations confirms that the estimation of 3D solitary waves is a feasible scheme.

Microwave Doppler reflectometer system in the Experimental Advanced Superconducting Tokamak

A Doppler reflectometer system has recently been installed in the Experimental Advanced Superconducting (EAST) Tokamak. It includes two separated systems, one for Q-band (33-50 GHz) and the other for V-band (50-75 GHz). The optical system consists of a flat mirror and a parabolic mirror which are optimized to improve the spectral resolution. A synthesizer is used as the source and a 20 MHz single band frequency modulator is used to get a differential frequency for heterodyne detection. Ray tracing simulations are used to calculate the scattering location and the perpendicular wave number. In EAST last experimental campaign, the Doppler shifted signals have been obtained and the radial profiles of the perpendicular propagation velocity during L-mode and H-mode are calculated.

Global hybrid simulation of mode conversion at the dayside magnetopause

Mode conversion at the magnetopause has been suggested to lead to the generation of kinetic Alfvén waves (KAWs) and effective mass transport from the solar wind into the magnetosphere. To investigate the mode conversion process in the dynamic dayside system, a 3‒D global hybrid simulation associated with a quasi‒parallel shock under a radial interplanetary magnetic field (IMF) is carried out, in which the foreshock compressional pulses are self‒consistently generated by the interaction between the solar wind and the geomagnetic field. The results show that as the compressional pulses propagate from the magnetosheath to the magnetopause, short‒wavelength structures of k⊥ρi∼0.5–1 with enhanced parallel electric field E∥ are excited in the subsolar magnetopause boundary layer (MPBL), where k⊥≃kx is the perpendicular waves number nearly along the GSE x direction, and ρi is the ion Larmor radius. The wave‒phase relationship between the magnetic field and the density changes from in‒phase in the magnetosheath to antiphase in the short‒wavelength MPBL perturbations. The wave polarization is predominantly compressive in the magnetosheath, whereas strong transverse wave powers appear abruptly around the MPBL. The mode conversion from the compressional pulses to KAWs is identified around the predicted Alfvén resonance surface in the MPBL. As these KAWs evolve, KAW modes dominated by azimuthal wave number with kyρi∼1 are also generated. The KAWs in the MPBL are identified by the sharp increases in E∥ and the electromagnetic field polarization relations of Alfvén mode, as well as a spectral analysis. The KAW perturbations propagate poleward into the cusps along the MPBL. Due to the differential flow convection speeds at various latitudes, the KAW packets expand along the north‒south direction, while they may also merge with newly‒formed KAWs due to newly‒arrived compressional waves.

Kinetic Alfvén wave instability driven by field‐aligned currents in a low‐β plasma

Kinetic Alfvén waves (KAWs) can play an important role in the energization of plasma particles and the formation of filamentous structures, which commonly are encountered and frequently accompanied by field‐aligned currents in various magneto‐plasmas, such as laboratory, auroral, and coronal plasmas. Based on a low‐frequency kinetic dispersion equation in frequency ω&lt;ωci (the ion cyclotron frequency), KAW instability driven by a field‐aligned current, which is carried by the field‐aligned drift of electrons at velocity VD, is investigated in a low‐β plasma of β&lt;Q≪1, where β is the kinetic‐to‐magnetic pressure ratio and Q (≡me/mi≪1) is the mass ratio of electrons to ions. An instability condition and the corresponding growth rate are obtained, which depends on the plasma β parameter as well as the drift velocity VD. The results show that the KAW instability occurs in the perpendicular wave number range of , in which the growth rate reaches its maximum at for fixed VD and β. As VD increases, this growing wave number range widens and the growth rate increases, but the maximal‐growing wave number decreases. On the other hand, as the plasma βparameter decreases, the growing wave number range also widens, and the maximal‐growing wave number and growth rate both increase. These results have potential importance in understanding the physics of the electric current dissipation and plasma active phenomena since the field‐aligned current is one of the most active factors in space and astrophysical plasmas.

Lower hybrid drift instability in a current sheet with anisotropic temperature

The effect of the temperature anisotropy on the lower hybrid drift instability (LHDI) in a current sheet is investigated using local kinetic theory. It is found that the ratio of the perpendicular to parallel electron temperatures can significantly affect the instability. In fact, a critical value exists , such that when the LHD waves are unstable if the perpendicular wave vector is between two threshold values, and when the LHD mode is stable for any . It is also found that increases and the unstable LHD regime shrinks as the parallel wave vector increases. That is, sufficiently low perpendicular electron temperature can stabilize the LHDI, especially that of short parallel wavelength.