Oblique Propagation Research Articles

Hybrid Simulation of Proton Cyclotron Waves Upstream of Mars Generated by Pickup Ion Beam Distribution

AbstractLinear instability analysis as well as a corresponding two‐dimensional hybrid simulation are performed to examine the excitation of the proton cyclotron waves observed upstream of Mars. The waves are believed to be excited by the pickup ions produced from the ionization of the Martian hydrogen exosphere. And previous statistical analysis of wave observations suggested that the waves are mostly related to pickup ion beam velocity distributions. While earlier linear instability analysis of pickup ion beam distributions has mainly been focused on the parallel unstable modes, our analysis reveals that the maximum growth rate occurs at very oblique propagation. The corresponding hybrid simulation confirms the linear analysis results and further demonstrates that the pickup ions are scattered toward an isotropic shell velocity distribution by the waves excited. Interestingly, the waves at oblique propagation gradually damp out and the system is eventually dominated by waves of quasi‐parallel propagation.

Oblique Propagation and Refraction of Gravity Waves Over the Andes Observed by GLORIA and ALIMA During the SouthTRAC Campaign

AbstractGravity waves (GW) carry energy and momentum from the troposphere to the middle atmosphere and have a strong influence on the circulation there. Global atmospheric models cannot fully resolve GWs, and therefore rely on highly simplified GW parametrizations that, among other limitations, account for vertical wave propagation only and neglect refraction. This is a major source of uncertainty in models, and leads to well‐known problems, such as the late break‐up of polar vortex due to the “missing” GW drag around 60°S. To investigate these phenomena, GW observations over Southern Andes were performed during SouthTRAC aircraft campaign. This paper presents measurements from a SouthTRAC flight on 21 September 2019, including 3‐D tomographic temperature data of the infrared limb imager GLORIA (8–15 km altitude) and temperature profiles of the ALIMA lidar (20–80 km altitude). GLORIA observations revealed multiple overlapping waves of different wavelengths. 3‐D wave vectors were determined from the GLORIA data and used to initialize a GW ray‐tracer. The ray‐traced GW parameters were compared with ALIMA observations, showing good agreement between the instruments and direct evidence of oblique (partly meridional) GW propagation. ALIMA data analysis confirmed that most waves at 25–40 km altitudes were indeed orographic GWs, including waves seemingly upstream of the Andes. We directly observed horizontal GW refraction, which has not been achieved before SouthTRAC. Refraction and oblique propagation caused significant meridional transport of horizontal momentum as well as horizontal momentum exchange between waves and the background flow all along the wave paths, not just in wave excitation and breaking regions.

Parametric analysis of pitch angle scattering and losses of relativistic electrons by oblique EMIC waves

This study analyzes the effects of electromagnetic ion cyclotron (EMIC) waves on relativistic electron scattering and losses in the Earth’s outer radiation belt. EMIC emissions are commonly observed in the inner magnetosphere and are known to reach high amplitudes, causing significant pitch angle changes in primarily >1 MeV electrons via cyclotron resonance interactions. We run test-particle simulations of electrons streaming through helium band waves with different amplitudes and wave normal angles and assess the sensitivity of advective and diffusive scattering behaviors to these two parameters, including the possibility of very oblique propagation. The numerical analysis confirms the importance of harmonic resonances for oblique waves, and the very oblique waves are observed to efficiently scatter both co-streaming and counter-streaming electrons. However, strong finite Larmor radius effects limit the scattering efficiency at high pitch angles. Recently discussed force-bunching effects and associated strong positive advection at low pitch angles are, surprisingly, shown to cause no decrease in the phase space density of precipitating electrons, and it is demonstrated that the transport of electrons into the loss cone balances out the scattering out of the loss cone. In the case of high-amplitude obliquely propagating waves, weak but non-negligible losses are detected well below the minimum resonance energy, and we identify them as the result of non-linear fractional resonances. Simulations and theoretical analysis suggest that these resonances might contribute to subrelativistic electron precipitation but are likely to be overshadowed by non-resonant effects.

Effects of the super-powerful tropospheric western Pacific phenomenon of September–October 2018 on the ionosphere over China: results from oblique sounding

Abstract. Doppler measurements at oblique propagation paths from the city of Harbin, the People's Republic of China (PRC), to 10 high-frequency (HF) radio broadcast stations in the PRC, Japan, Mongolia, and the Republic of Korea captured the response in the ionosphere to the activity of the super typhoon, Typhoon Kong-rey, from 30 September to 6 October 2018. The Harbin Engineering University coherent software-defined radio system generates the database containing the complex amplitudes of the radio signals that have been acquired along 14 propagation paths since 2018. The complex amplitudes are used for calculating the temporal dependences of the Doppler spectra and signal amplitudes, and the Doppler spectra are used to plot the Doppler shift as a function of time, fD(t), for all rays. The scientific objectives of this study are to reveal the possible perturbations caused by the activity of Typhoon Kong-rey and to estimate the magnitudes of wave parameters of the ionospheric plasma and radio signals. The amplitudes, fDa, of the Doppler shift variations were observed to noticeably increase (factor of ∼2–3) on 1–2 and 5–6 October 2018, while the 20–120 min periods, T, of the Doppler shift variations suggest that the wavelike disturbances in the ionosphere are caused by atmospheric gravity waves. The periods and amplitudes of quasi-sinusoidal variations in the Doppler shift, which have been determined for all propagation paths, may be used to estimate the amplitudes, δNa, of quasi-sinusoidal variations in the electron density. Thus, T≈20 min and fDa≈0.1 Hz yield δNa≈0.4 %, whereas T≈30 min and fDa≈0.2 Hz give δNa≈1.2 %. If T≈60 min and fDa≈0.5 Hz, then δNa≈6 %. The periods T are found to change within the 15–120 min limits, and the Doppler shift amplitudes, fDa, show variability within the 0.05–0.4 Hz limits.

ECMI resonance in AKR revisited: Hyperbolic resonance, harmonics, and wave–wave interaction

Recapitulation of the resonance condition for the fundamental and higher electron cyclotron harmonics in the electron cyclotron maser instability (ECMI) enables radiation below and confirms the possibility of radiation in a narrow band above harmonics n > 1. Near n = 1 resonance on the confined lower X-mode branch, amplification is supported by the decrease in phase and group speeds. Confined slow large-amplitude quasi-electrostatic X-modes non-linearly modulate the plasma to form cavitons until self-trapped inside them at a further increasing wavenumber. They undergo wave–wave interaction, enabling escape into free space in the second harmonic band below n = 2. At a sufficiently large parallel wavenumber (oblique propagation), the fundamental resonance n = 1 is hyperbolic, a possibility so far missed but vital for an effective ECMI in the upward current region. Here, the resonance hyperbola favorably fits the loss-cone boundary, the presumably important ECMI upward-current source-electron distribution, to stimulate ECMI growth at available auroral electron energies.

The Influence of Small Variations of Plasma Density on Conditions of Propagation of Electromagnetic Waves of the Whistle Range through the Morning Ionosphere

The problem of the effect of plasma-density disturbances caused by infrasonic waves on the propagation and reflection of whistler electromagnetic waves incident on the morning ionosphere from above is considered. The influence of the parameters of an infrasonic wave on the coefficient of reflection of a whistler wave from the ionosphere from above in the general case of oblique propagation is studied. The strongest changes in the reflection coefficient of whistler waves are associated with concentration perturbations at heights of the order of 80–100 km, where the rate of decay of propagating electromagnetic radiation modes increases by more than an order of magnitude within a region that is quite local in height (less than 10–15 km). The features of the parametric effect of plasma density fluctuations in an infrasonic wave on the field of a whistler wave that reached the Earth’s surface are analyzed. At close values of the horizontal wavenumbers of the whistler and infrasonic waves, the field of the whistler wave near the Earth’s surface can increase by several times. The results obtained are important for understanding the relationship between magnetospheric wave processes of different nature. The study of the modulation of the coefficient of reflection of whistler waves from the ionosphere by infrasonic waves from above is relevant for explaining the operating modes of a plasma magnetospheric maser.

Oblique propagation of arbitrary amplitude ion acoustic solitary waves in anisotropic electron positron ion plasma

The oblique propagation of arbitrary ion acoustic solitary waves (IASWs) in magnetized electron-positron-ion plasmas is investigated by employing Sagdeev pseudopotential approach. Ions are assumed to be adiabatic having anisotropic thermal pressure. Electrons and positrons are considered to be isothermal, following Maxwellian distribution. In terms of electrostatic potential, Sagdeev potential function is obtained and analyzed numerically in the context of relevant plasma configuration parameters. The existence region of solitary pulses is defined accurately. It is investigated how several plasma configuration parameters, such as the positron concentration, parallel, and perpendicular ion pressure affect soliton characteristics.

Magnetohydrodynamic Kelvin–Helmholtz instabilities of supersonic shear layers with finite interface thickness and heat flux in anisotropic space plasmas

ABSTRACT The linear magnetohydrodynamic Kelvin–Helmholtz instability (KHI) in an anisotropic plasma is studied. The governing equations obtained as the 16 moments of Boltzmann–Vlasov kinetic equations, including the heat flow, are applied. In the case of tangential discontinuity between the supersonic flows along the magnetic field, the calculated growth rates as functions of the anisotropic plasma properties allow us to conclude that quasi-transverse modes grow faster. Then, dispersion equations for the KHI of quasi-transverse modes are derived, considering the finite width of the transition zone with different velocity profiles. For these modes, when the role of heat flow is not important, the plasma parameters are controlled so that the fundamental plasma instabilities (firehose and mirror) do not affect the KHI. The problem is solved analytically, which will be helpful in verifying numerical simulations. In contrast to the tangential discontinuity, the finite width of the transition layer confines KHI excitation as the wavenumber increases. In the general case of oblique propagation (when heat flux complicates the problem), the boundary value problem is solved to determine the spectral eigenvalues. In particular, it is observed that the fundamental plasma instabilities that arise in the transition zone between flows with a finite width can modify and considerably enhance the KHI.

Oblique propagation and temperature effects on the resonant right-hand ion beam instability

The resonant right-hand instability (RHI) is often the dominant mode driven by reflected ions upstream of Earth’s quasi-parallel bow shock. In the tradition of Peter Gary, this paper further explores the right-hand instability using numerical solutions of the plasma dispersion relation and non-linear kinetic simulations, with parameters inspired by observations from NASA’s Magnetospheric Multiscale (MMS) mission. Agreement is found between the ion distributions in the particle-in-cell simulations and Magnetospheric Multiscale spacecraft data, which show the gyrophase bunching characteristic of the instability. The non-linear structures created by right-hand instability tend to be stronger when the plasma beta is lower. These structures have sizes of around 100 to 200 ion inertial lengths perpendicular to the magnetic field, presenting planet-sized disturbances to the magnetosphere. 2d and 3D hybrid particle-in-cell simulations show that modes with a range of propagation angles oblique to the magnetic field are excited, providing a ground to understand previous statistical studies of observed foreshock waves.

Estimation and Separation of Ionospheric Narrow-Band Unknown O and X Waves

The geomagnetic field splits an electromagnetic wave into two characteristic waves, termed the ordinary (O) and extraordinary (X) waves. For the narrow-band unknown high-frequency (HF) signals, we develop a model for the oblique propagation of the characteristic waves. The two waves travel through similar paths in the ionosphere and incident on a small aperture array with the same direction of arrival (DOA) and perpendicular polarizations. In a short period, there is only amplitude ratio and phase difference between the two waveforms, resulting in an elliptically polarized resultant (R) wave. Based on the maximum likelihood estimation, DOA and elliptically polarized parameters of the R, O, and X waves are estimated using the antenna’s vector effective length and the geomagnetic field. The Cramér–Rao Bounds are also presented. In addition, the O and X waveforms are separated based on polarization differences. Simulation and experiment validate the model and performance of estimators. DOA estimator for the R wave outperforms the estimator for the characteristic waves and the estimator for matching the energy of orthogonal polarizations. One of the separated characteristic waves presents smoother and shallower fading than the R wave.

Oblique wave propagation through composite permeable porous structures

Oblique wave propagation through composite permeable porous structures

Oblique Quasi-kink Modes in Solar Coronal Slabs Embedded in an Asymmetric Magnetic Environment: Resonant Damping, Phase and Group Diagrams

There has been considerable interest in magnetoacoustic waves in static, straight, field-aligned, 1D equilibria where the exteriors of a magnetic slab are different between the two sides. We focus on trapped, transverse fundamental, oblique quasi-kink modes in pressureless setups, where the density varies continuously from a uniform interior (with density ρ i) to a uniform exterior on either side (with density ρ L or ρ R), assuming ρ L ≤ ρ R ≤ ρ i. The continuous structuring and oblique propagation make our study new, relative to pertinent studies, and lead to wave damping via the Alfvén resonance. We compute resonantly damped quasi-kink modes as resistive eigenmodes, and isolate the effects of system asymmetry by varying ρ i/ρ R from the “Fully Symmetric” (ρ i/ρ R = ρ i/ρ L) to the “Fully Asymmetric” limit (ρ i/ρ R = 1). We find that the damping rates possess a nonmonotonic ρ i/ρ R-dependence as a result of the difference between the two Alfvén continua, and resonant absorption occurs only in one continuum when ρ i/ρ R is below some threshold. We also find that the system asymmetry results in two qualitatively different regimes for the phase and group diagrams. The phase and group trajectories lie essentially on the same side (different sides) relative to the equilibrium magnetic field when the configuration is not far from a “Fully Asymmetric” (“Fully Symmetric”) one. Our numerical results are understood by making analytical progress in the thin-boundary limit, and discussed for imaging observations of axial standing modes and impulsively excited wavetrains.

Study of Low-Frequency Electromagnetic Ion-Cyclotron Wave for Ring Distribution in Magnetosphere of Saturn

Magnetic cyclotron waves were discovered by the Cassini-Huygens spacecraft in Saturn's atmospheric torus’ magnetic layer. They are left-handed and propagate at a minor angle to the ambient magnetic field in most areas because their frequency is close to the frequency of the aqua ions. The ion cyclotron instability caused by Saturn's neutral cloud ions helps explain their formation. They can be classified as n = 2 mode fluctuations because of the ion-ring distribution. We planned the characteristics of these waves in advance of starting this project. Our dispersion growth rates are evaluated using kinetic method analysis as well. The results were calculated and explained for the exemplary values of the magnetosphere parameters suitable for Saturn. Another potential free energy source for ion cyclotrons is temperature anisotropy. Instead of the standard Maxwell distribution, a ring distribution is employed in this study. The focus of this research is EMIC waves’ oblique propagation in the magnetic field, which changes their temperature anisotropy, ion energy density, and propagation angle. The interaction of relativistic particles with ion cyclotron waves is also included in this extension. EMIC wave size decreases with the increasing density of particles, as shown by a numerical study. A comparison of planetary studies based on data from space plasma environments and magnetospheric systems produced these results. HIGHLIGHTS Temperature anisotropy - free energy source for Ion Cyclotron waves EMIC wave size decreases with the increasing density of particles Saturn's neutral cloud ions helps the formation of ion cyclotron instability GRAPHICAL ABSTRACT

In Situ Observations of Lower Hybrid Drift Waves at the Inner Edge of the Ring Current by the Van Allen Probe‐A

AbstractIn the magnetosphere of the Earth, lower hybrid drift (LHD) waves effectively cause plasma diffusion and couple well to both electrons and ions. LHD waves have the characteristics of linear polarization and highly oblique propagation with frequencies near the lower hybrid resonance frequency, which is similar to the characteristics of high‐frequency magnetosonic (MS) waves. The existence of diamagnetic drift at the inner edge of the ring current provides free energy for the excitation of LHD waves, while the free energy to generate MS waves are provided by proton ring distribution. Here, we report on a typical case of LHD waves which was excited by LHD instability. Different from the global distribution of high‐frequency MS waves, statistical results indicate that most LHD waves have a uniform distribution over all MLTs except the noon sector inside the plasmapause. Besides, the occurrence of LHD waves reaches a maximum in the dusk sector outside the plasmapause.

On the use of high-frequency surface wave oceanographic research radars as bistatic single-frequency oblique ionospheric sounders

Abstract. We demonstrate that bistatic reception of high-frequency oceanographic radars can be used as single-frequency oblique ionospheric sounders. We develop methods that are agnostic of the software-defined radio system to estimate the group range from the bistatic observations. The group range observations are used to estimate the virtual height and equivalent vertical frequency at the midpoint of the oblique propagation path. Uncertainty estimates of the virtual height and equivalent vertical frequency are presented. We apply this analysis to observations collected from two experiments run at two locations in different years, but utilizing similar software-defined radio data collection systems. In the first experiment, 10 d of data were collected in March 2016 at a site located in Maryland, USA, while the second experiment collected 20 d of data in October 2020 at a site located in South Carolina, USA. In both experiments, three Coastal Oceanographic Dynamics and Applications Radars (CODARs) located along the Virginia and North Carolina coast of the US were bistatically observed at 4.53718 MHz. The virtual height and equivalent virtual frequency were estimated in both experiments and compared with contemporaneous observations from a vertical incident digisonde–ionosonde at Wallops Island, VA, USA. We find good agreement between the oblique CODAR-derived and WP937 digisonde virtual heights. Variations in the virtual height from the CODAR observations and the digisonde are found to be nearly in phase with each other. We conclude from this investigation that observations of oceanographic radar can be used as single-frequency oblique incidence sounders. We discuss applications with respect to investigations of traveling ionospheric disturbances, studies of day-to-day ionospheric variability, and using these observations in data assimilation.

Oblique Propagation of Low-Frequency Solitary Waves in a Magnetized Polarized Complex Plasma With Adiabatically Trapped Nonthermal Ions

The oblique propagation of low-frequency solitary waves is considered in a polarized magnetized complex plasma with adiabatically trapped nonthermalions. A Cairns–Gurevich distribution is adopted, and the polarization force associated with our plasma model is applied. In addition, the appropriate modified Korteweg–de Vries (mK-dV) equation, in which the Cairns–Gurevich polarization force and external magnetic field are taken into account, is established. Our numerical investigation applied to space dusty plasma reveals that the amplitude and the width of the dust acoustic (DA) solitary structure are significantly modified by the effects of the magnitude of the external static magnetic field <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\omega _{c}$ </tex-math></inline-formula> as well as the obliquity <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$l_{z}$ </tex-math></inline-formula> and polarization force. In particular, we have found that for given values of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$l_{z}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\omega _{c}$ </tex-math></inline-formula> , the DA soliton profile becomes more profound due to the presence of Cairns–Gurevich polarization force. For the sake of completeness, we have also analyzed both the effects of Cairns–Gurevich polarization force and the external magnetic field on DA soliton energy. In particular, we have shown that the energy quantities are shifted, for any value of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$l_{z}$ </tex-math></inline-formula> , toward large values, in the presence of polarization force.

First-principles method for nonlinear light propagation at oblique incidence.

We have developed a computational method to describe the nonlinear light propagation of an intense and ultrashort pulse at oblique incidence on a flat surface. In the method, coupled equations of macroscopic light propagation and microscopic electron dynamics are simultaneously solved using a multiscale modeling. The microscopic electronic motion is described by first-principles time-dependent density functional theory. The macroscopic Maxwell equations that describe oblique light propagation are transformed into one-dimensional wave equations. As an illustration of the method, light propagation at oblique incidence on a silicon thin film is presented.

Kink oscillations in a coronal loop arcade with finite plasma-β: effect of oblique propagation

ABSTRACT Kink oscillations of a curved coronal slab with finite plasma-β, simulating a loop arcade, are examined. Perpendicular propagation, i.e. propagation along the arcade axis (ky &amp;gt; 0) is taken into account. Two surface modes, labelled as faster and slower mode, are found to exist in the model. In the zero-β limit, the faster mode is a vertically polarized kink mode and the slower mode produces bending motions polarized along the arcade axis, provided $k_y^{-1}$ is of the order of or larger than the slab thickness a. Otherwise, if $k_y^{-1}$ is much less than a, the faster mode results in periodic displacement of a loop arcade along its axis and the slower mode has mixed properties. The phase speeds of both modes are very similar when $k_y^{-1}\sim a$, and they tend to the external and internal Alfvén speeds when ky → 0. As the internal plasma-β becomes finite and grows, the phase speed of the faster mode increases and that of the slower mode decreases. When βi &amp;gt; 0, these modes are a superposition of vertical kink motions and those that are oriented along the arcade axis, both supplemented with the significant cross-averaged density perturbations. It seems promising to use the obtained results for interpreting quasi-periodic pulsations, in terms of kink oscillations of flaring high-β loops, provided the developed theory is applicable to the torroidal single loop model when choosing an appropriate ky.

Oblique propagation of ion-acoustic solitary waves in magnetized electron-positron-ion plasma with Cairns distribution

The propagation of ion-acoustic (IA) solitary waves (SWs) is investigated in a magnetized electron-positron-ion (EPI) plasma with Cairns distributed electrons and positrons. The Korteweg-de Vries (KdV) and modified KdV (mKdV) equations are derived for the potential by employing the reductive perturbation technique (RPT) and its solitary wave (SW) solutions are analyzed. The effects of relevant plasma parameters (viz., nonthermality parameter β, positron concentration , ion thermality δ and magnetic field strength Ω) on the characteristics of IA solitary structures are discussed in detail.

Oblique propagation of the squeezed states of s(p)-polarized light through non-Hermitian multilayered structures.

Employing a second-quantization of the electromagnetic field in the presence of media with both gain and loss, we investigate the propagation of the squeezed coherent state of light through a dispersive non-Hermitian multilayered structure, in particular at a discrete set of frequencies for which this structure is PT-symmetric. We detail and generalize this study to cover various angles of incidence and s- and p-polarizations to reveal how dispersion, gain/loss-induced noises in such multilayered structures affect nonclassical properties of the incident light, such as squeezing and sub-Poissonian statistics. Varying the loss layers' coefficient, we demonstrate a squeezed coherent state, when transmits through the structure whose gain and loss layers have unidentical bulk permittivities, retains its nonclassical features to some extent. Our results show by increasing the number of unit cells and incident angle, the quantum features of the transmitted state for both polarizations degrade.