Wave Filamentation Research Articles

Poynting Fluxes, Field‐Aligned Current Densities, and the Efficiency of the Io‐Jupiter Electrodynamic Interaction

AbstractJuno's highly inclined orbits provide opportunities to sample high‐latitude magnetic field lines connected to the orbit of Io, among the other Galilean satellites. Its payload offers both remote‐sensing and in‐situ measurements of the Io‐Jupiter interaction. These are at discrete points along Io's footprint tail and at least one event (12th perijove) was confirmed to be on a flux tube Alfvénically connected to Io, allowing for an investigation of how the interaction evolves down‐tail. Here we present Alfvén Poynting fluxes and field‐aligned current densities along field lines connected to Io and its orbit. We explore their dependence as a function of down‐tail distance and show the expected decay as seen in UV brightness and electron energy fluxes. We show that the Alfvén Poynting and electron energy fluxes are highly correlated and related by an efficiency that is fully consistent with acceleration from Alfvén wave filamentation via a turbulent cascade process.

Competition among the two-plasmon decay of backscattered light, filamentation of the electron-plasma wave and side stimulated Raman scattering

Abstract Competition among the two-plasmon decay (TPD) of backscattered light of stimulated Raman scattering (SRS), filamentation of the electron-plasma wave (EPW) and forward side SRS is investigated by two-dimensional particle-in-cell simulations. Our previous work [K. Q. Pan et al., Nucl. Fusion 58, 096035 (2018)] showed that in a plasma with the density near 1/10 of the critical density, the backscattered light would excite the TPD, which results in suppression of the backward SRS. However, this work further shows that when the laser intensity is so high ( $>{10}^{16}$ W/cm2) that the backward SRS cannot be totally suppressed, filamentation of the EPW and forward side SRS will be excited. Then the TPD of the backscattered light only occurs in the early stage and is suppressed in the latter stage. Electron distribution functions further show that trapped-particle-modulation instability should be responsible for filamentation of the EPW. This research can promote the understanding of hot-electron generation and SRS saturation in inertial confinement fusion experiments.

Langmuir wave filamentation in the kinetic regime. II. Weak and strong pumping of nonlinear electron plasma waves as the route to filamentation

We consider two kinds of pumped Langmuir waves (LWs) in the kinetic regime, kλD≳0.2, where k is the LW wavenumber and λD is the Debye length, driven to finite amplitude by a coherent external potential whose amplitude is either weak or strong. These dynamically prepared nonlinear LWs develop a transverse (filamentation) instability whose nonlinear evolution destroys the LW's transverse coherence. Instability growth rates in the weakly pumped regime are the same as those of Bernstein-Greene-Kruskal modes considered in Part I (D. A. Silantyev et al., Phys. Plasmas 24, 042104 (2017)), while strongly pumped LWs have higher filamentation grow rates.

Nonlinear effects associated with fast magnetosonic waves and turbulent magnetic amplification in laboratory and astrophysical plasmas

This paper presents the spatio-temporal evolution of magnetic field due to the nonlinear coupling between fast magnetosonic wave (FMSW) and low frequency slow Alfven wave (SAW). The dynamical equations of finite frequency FMSW and SAW in the presence of ponderomotive force of FMSW (pump wave) has been presented. Numerical simulation has been carried out for the nonlinear coupled equations of finite frequency FMSW and SAW. A systematic scan of the nonlinear behavior/evolution of the pump FMSW has been done for one of the set of parameters chosen in this paper, using the coupled dynamical equations. Filamentation of fast magnetosonic wave has been considered to be responsible for the magnetic turbulence during the laser plasma interaction. The results show that the formation and growth of localized structures depend on the background magnetic field but the order of amplification does not get affected by the magnitude of the background magnetic field. In this paper, we have shown the relevance of our model for two different parameters used in laboratory and astrophysical phenomenon. We have used one set of parameters pertaining to experimental observations in the study of fast ignition of laser fusion and hence studied the turbulent structures in stellar environment. The other set corresponds to the study of magnetic field amplification in the clumpy medium surrounding the supernova remnant Cassiopeia A. The results indicate considerable randomness in the spatial structure of the magnetic field profile in both the cases and gives a sufficient indication of turbulence. The turbulent spectra have been studied and the break point has been found around k which is consistent with the observations in both the cases. The nonlinear wave-wave interaction presented in this paper may be important in understanding the turbulence in the laboratory as well as the astrophysical phenomenon.This paper presents the spatio-temporal evolution of magnetic field due to the nonlinear coupling between fast magnetosonic wave (FMSW) and low frequency slow Alfven wave (SAW). The dynamical equations of finite frequency FMSW and SAW in the presence of ponderomotive force of FMSW (pump wave) has been presented. Numerical simulation has been carried out for the nonlinear coupled equations of finite frequency FMSW and SAW. A systematic scan of the nonlinear behavior/evolution of the pump FMSW has been done for one of the set of parameters chosen in this paper, using the coupled dynamical equations. Filamentation of fast magnetosonic wave has been considered to be responsible for the magnetic turbulence during the laser plasma interaction. The results show that the formation and growth of localized structures depend on the background magnetic field but the order of amplification does not get affected by the magnitude of the background magnetic field. In this paper, we have shown the relevance of our model f...

Nonlinear evolution of the filamentation instability and chaos in laser–plasma interaction

AbstractFilamentation is one of the most common nonlinear phenomena taking place in the laser–plasma interaction that splits the laser beam into high-intensity spikes. The present study deals with the nonlinear evolution of filamentation instability in laser–plasma interaction and the development of chaos in contrast to linear growth as reported by Kaw et al. in 1973. We have considered a non-uniform perturbation superimposed on plane-wave pump such that due to non-uniformity of the perturbation a finite intensity gradient arises and gives rise to ponderomotive force. This causes filamentation of wave, which has been studied presently using numerical methods as well as analytical tools. The results reveal that the intensity of perturbation gets localized and delocalized with the distance of propagation. The numerical simulation results also reveal that the intensity of perturbation route from ordered to chaotic behavior depending upon the pump laser and perturbation parameters. To study the chaotic behavior, Lyapunov exponents has also been calculated. The semi-analytical method is also developed to have an insight into some of the features of simulation like the formation of localized structures.

Nonlinear magnetic field enhancement and turbulence in laboratory and astrophysical plasmas

The transient response of nonlinear coupling between relatively high frequency magnetosonic wave (HMSW) and low frequency magnetosonic wave (LMSW) in the overdense plasma has been presented in this paper. Along with this, the adiabatic response of HMSW also has been studied for low value of magnetic field in the underdense plasma. The ponderomotive force of relatively high frequency and high power pump magnetosonic wave is expected to excite the low frequency magnetosonic wave. The dynamical equation of LMSW in the presence of ponderomotive force of HMSW (pump wave) has been derived for this purpose. Using this coupled system of dynamical equations, the nonlinear behavior of the pump HMSW was studied, and the resulting turbulent spectrum has been presented. Numerical simulation has been carried out for dimensionless nonlinear coupled equations of HMSW and LMSW, and the results show quite complex localized structures that grow with time. The ensemble averaged power spectrum has also been studied which shows that the spectral index follows an approximate scaling of the order of ∼k−1.67 at larger scales and scaling of the order of ∼k−3.4 at smaller scales. The results indicate considerable randomness in the spatial structure of the magnetic field profile which gives sufficient indication of turbulence. In this context, filamentation of high frequency magnetosonic wave has been considered to be responsible for magnetic turbulence during the laser plasma interaction. The results of the present paper are found relevant with two different experimental observations. Such a nonlinear interaction is quite important in understanding turbulence in the astrophysical phenomenon.

Spatio-temporal evolution of magnetosonic wave in the laser plasma interaction

This paper presents a theoretical model for the transient response of nonlinear coupling between magnetosonic wave and ion acoustic wave in the overdense plasma. Filamentation of magnetosonic wave has been considered to be responsible for magnetic turbulence during the laser plasma interaction. The ion acoustic wave gets excited due to the ponderomotive force exerted by magnetosonic wave and this ion acoustic wave in turn generates perturbation in the background density in the form of spatial density harmonics. Numerical simulation has been carried out for dimensionless coupled equations of magnetosonic wave and ion acoustic wave; and the results show quite complex localized structures that grow with time. The power spectrum has also been studied which shows that the spectral index follows an approximate scaling of the order of ∼k−2.4 at smaller scales. The data obtained from numerical simulation are used in semi analytical model to better understand the mechanism of nonlinear evolution of magnetosonic wa...

Filamentation of magnetosonic wave and generation of magnetic turbulence in laser plasma interaction

This paper presents a theoretical model for the magnetic turbulence in laser plasma interaction due to the nonlinear coupling of magnetosonic wave with ion acoustic wave in overdense plasma. For this study, dynamical equations of magnetosonic waves and the ion acoustic waves have been developed in the presence of ponderomotive force due to the pump magnetosonic wave. Slowly converging and diverging behavior has been studied semi-analytically, this results in the formation of filaments of the magnetosonic wave. Numerical simulation has also been carried out to study nonlinear stage. From the results, it has been found that the localized structures become quite complex in nature. Further, power spectrum has been studied. Results show that the spectral index follows (∼k−2.0) scaling at smaller scale. Relevance of the present investigation has been shown with the experimental observation.

Self-organized coherent bursts of stimulated Raman scattering and speckle interaction in multi-speckled laser beams

Nonlinear physics governing the kinetic behavior of stimulated Raman scattering (SRS) in multi-speckled laser beams has been identified in the trapping regime over a wide range of kλD values (here k is the wave number of the electron plasma waves and λD is the Debye length) in homogeneous and inhomogeneous plasmas. Hot electrons from intense speckles, both forward and side-loss hot electrons produced during SRS daughter electron plasma wave bowing and filamentation, seed and enhance the growth of SRS in neighboring speckles by reducing Landau damping. Trapping-enhanced speckle interaction through transport of hot electrons, backscatter, and sidescatter SRS light waves enable the system of speckles to self-organize and exhibit coherent, sub-ps SRS bursts with more than 100% instantaneous reflectivity, resulting in an SRS transverse coherence width much larger than a speckle width and a SRS spectrum that peaks outside the incident laser cone. SRS reflectivity is found to saturate above a threshold laser intensity at a level of reflectivity that depends on kλD: higher kλD leads to lower SRS and the reflectivity scales as ∼(kλD)−4. As kλD and Landau damping increase, speckle interaction via sidescattered light and side-loss hot electrons decreases and the occurrence of self-organized events becomes infrequent, leading to the reduction of time-averaged SRS reflectivity. It is found that the inclusion of a moderately strong magnetic field in the laser direction can effectively control SRS by suppressing transverse speckle interaction via hot electron transport.

Comparative study of the power transferred from satellite-magnetosphere interactions to auroral emissions

Io's interaction with the Jovian magnetosphere generates a power of about 1012 W which propagates as Alfvn waves along the magnetic field lines and is partly transferred to electrons, resulting in intense auroral emissions. A recent study of the power transmission along the Io flux tube and of the electron acceleration at high latitudes showed that the power of the observed emissions is well explained by assuming filamentation of the Alfvn waves in the torus and the acceleration of the electrons at high latitude. At Jupiter, UV footprints related to Europa and Ganymede have also been observed. At Saturn recent observations revealed a weak UV footprint of Enceladus. We apply the Io interaction model to the Europa and Enceladus interactions. We show that the Alfvn wave filamentation leads to a precipitating electron power consistent with the power of the observed UV footprints.

Alfvén wave filamentation and dispersive phase mixing in a high-density channel: Landau fluid and hybrid simulations

Abstract. The propagation of dispersive Alfvén waves in a low-beta collisionless plasma with a high-density channel aligned with the ambient magnetic field, is studied in three space dimensions. A fluid model retaining linear Landau damping and finite Larmor radius corrections is used, together with a hybrid particle-in-cell simulation aimed to validate the predictions of this Landau-fluid model. It is shown that when the density enhancement is moderate (depending on the pump wavelength and the plasma parameters), the wave energy concentrates into a filament whose transverse size is prescribed by the dimension of the channel. In contrast, in the case of a stronger density perturbation, the early formation of a magnetic filament is followed by the onset of thin helical ribbons and the development of strong gradients. This "dispersive phase mixing" provides a mechanism permitting dissipation processes (not included in the present model) to act and heat the plasma.

Alfvén wave filamentation and particle acceleration in solar wind and magnetosphere

We present numerical simulations of Alfvén waves in steady state, leading to the formation of intense magnetic filaments when the nonlinearity arises due to the ponderomotive effects and Joule heating. When the plain Alfvén wave is perturbed by a transverse perturbation and the magnitude of the pump Alfvén wave changes, chaotic filamentary structures of magnetic field in kinetic Alfvén wave (KAW) filamentation and quasiperiodic in inertial Alfvén wave (IAW) filamentation have been observed. At higher KAW pump wave amplitude, the spectra approaches near the Kolmogorov k−5/3 scaling. The spectral index becomes k−3 scaling in IAW. Relevance of these studies in solar wind and magnetosphere for KAW and solar corona and auroral region for IAW has also been pointed out. Particle heating in these small‐scale filamentary structures has also been calculated by using velocity space diffusion coefficient in Fokker‐Planck equation.

Alfvén Wave Filamentation: from Hall-MHD to Kinetic Theory

The process of Alfvén wave filamentation, by which the energy of a quasi-monochromatic wave packet concentrates into thin field-aligned tubes, is reviewed. Most of the properties of this phenomenon are captured by envelope equations where it is associated with transverse wave collapse. Saturation and possible instability of the resulting structures are analyzed by means of three-dimensional simulations of Hall-MHD equations. To address the case of a collisionless plasma, a long-wave (KDNLS) equation is first obtained from the Vlasov-Maxwell system by a reductive perturbative expansion, thus allowing one to easily derive a dissipative non-linear Schrödinger equation that accounts for the effect of Landau damping of the ion acoustic waves. Filamentation criteria are then obtained, showing that the instability is in particular favored by a large electron-ion temperature ratio. The nonlinear regime displays a complex dynamics of focusing and defocusing magnetic filaments.

Point vortex dynamics within a background vorticity patch

Recent experiments and simulations have observed that the interaction of strong vortices with a low vorticity background can strongly affect the dynamics of both vortices and background. This paper considers an idealized model of this interaction. The background is treated as a patch of uniform vorticity with a nearly circular shape. The strong vortices are treated as point vortices, and their total circulation λ is assumed to be small compared to that of the background. It is found that Kelvin waves on the boundary of the background patch can be driven to large amplitude by the strong vortices, eventually resulting in wave breaking and filamentation. A multiscale analysis finds that the wave-breaking time scales as λ−1 ln λ−1, in agreement with contour dynamics simulations.

Alfvén Wave Filamentation Beyond the Envelope Formalism

Alfvén wave filamentation is shown to persist in the context of a long-wavelength asymptotics where the wave amplitude is larger than permitted by the envelope formalism. Furthermore, as predicted in this framework, reducing the dispersion transforms the collapsing structures from foci to gradient sheets where the wave amplitude remains moderate.

Coupling between nonlinear Alfvén waves and reduced magnetohydrodynamics for compressible fluids

A reduced description of magnetohydrodynamic (MHD) turbulence in a plasma permeated by a strong uniform magnetic field is derived using a reductive perturbative expansion method. This leads to a coupled system of equations for the velocity and magnetic fields in the planes perpendicular to the ambient field, described by the usual reduced MHD equations [Zank and W. H. Matthaeus, J. Plasma Phys. 48, 85–100 (1992)] and for parallel propagating Alfvén waves, which on the characteristic time and length scale of the transverse flow, undergo cumulative nonlinear effects described by a generalized Derivative Nonlinear Schrödinger equation (DNLS). The cases β far from unity and β≈1 are addressed separately. In the latter case, the nonlinear Alfvén waves drive a mean longitudinal velocity whose magnitude is comparable to that of the transverse components. A modulational analysis for a quasi-monochromatic plane Alfvén wave at β far from unity illustrates the effect of the transverse MHD flow on the Alfvén wave filamentation.

Plasma Heating by Alfven Wave Filamentation: A Relevant Mechanism in the Solar Corona and the Interstellar Medium

A mechanism for small-scale generation in magnetically dominated, homogeneous space plasmas is proposed, resulting from transverse collapse (filamentation) of weakly nonlinear, circularly polarized Alfven waves. It is argued that waves whose frequencies exceed a magnitude of about 10 Hz can significantly contribute to the heating of open coronal holes and to the acceleration of the fast solar wind. In coronal loops, this frequency lower bound reduces to 1 Hz. The importance of Alfven wave filamentation is also demonstrated in the warm ionized phase of the interstellar medium.

Filamentation instability of upper hybrid wave in magnetoplasmas

A four-wave parametric coupling leading to the filamentation of a large amplitude upper hybrid wave and the generation of magnetic field-aligned density striation in collisional magnetoplasmas is studied. The dispersion relation is derived including the nonlinear effects of ponderomotive force, thermal pressure force, and beating currents. It is analyzed to obtain the threshold field and the growth rate which are applied to the high latitude ionospheric modifications by powerful high frequency waves.

Laser beam filamentation in a plasma with a steep density profile

A numerical analysis is presented of the electrostrictive filamentation of an electromagnetic wave in the presence of a reflected wave. Maxwell's equations are solved in the steadystate simultaneously with one fluid hydrodynamics and a simplified equation of state to obtain a self-consistent solution. The threshold for filamentation is shown to be often below the laser intensity necessary to modify the plasma density profile, however, even weak profile modification substantially affects the filamentation.

Interaction of electromagnetic waves in the ionosphere

Time evolution of filamentation of an electromagnetic wave in the F region of the ionosphere has been studied numerically. It is seen that a plane electromagnetic beam of uniform power flux becomes unstable and gets filamented in the presence of transverse density fluctuations caused by a high power electromagnetic Gaussian beam. The computations are performed using Gaussian beams of 10, 30 and 50 μw/m2 power flux, to produce the nonuniform plasma fluctuations in the medium. The intensity of the plane wave is, however, fixed at 30 μw/m2. The filamentation of the plane wave develops in the entire region of the 100 km altitude range between 200 and 300 km used in the computations. The amplitude of the filaments increases with height and the growth is more rapid than an exponential variation. The plasma imitates the wave structure and the fluctuations in the electron temperature and density are field aligned. The relative density fluctuations associated with filaments lie in the range 0.1 to 1.5% and depend on the intensity of the incident Gaussian beam.