Background Density Fluctuations Research Articles

Electrostatic Wave Decay in the Randomly Inhomogeneous Solar Wind

Despite a few space observations where Langmuir and ion acoustic waves are expected to participate in the mechanism of electrostatic decay, this is to date believed to be the main and fastest nonlinear wave process in the solar wind. However, in such a plasma where random density fluctuations are ubiquitous, the question of whether nonlinear wave processes play a significant role in Langmuir wave turbulence generated by electron beams associated with type III solar radio bursts remains still open. This paper provides several answers by studying, owing to two-dimensional challenging particle-in-cell simulations, the dynamics and the properties of the ion acoustic waves excited by such Langmuir wave turbulence and the role they play in the electrostatic decay. The impact on this process of plasma background density fluctuations and electron-to-ion temperature ratio is studied. Moreover, it is shown that, for a typical solar wind plasma with an average level of density fluctuations of a few percent of the ambient density and a temperature ratio of the order of 1, nonlinear induced scattering off ions occurs, with small intensity low-frequency quasi-modes and only in localized plasma regions where density is depleted or weakly perturbed by low-frequency turbulence.

Second Harmonic Electromagnetic Wave Emissions from a Turbulent Plasma with Random Density Fluctuations

In the solar wind, electromagnetic waves at the harmonic plasma frequency 2ω p can be generated as a result of coalescence between forward- and backward-propagating Langmuir waves. A new approach to calculate their radiation efficiency in plasmas with external background density fluctuations is developed. The evolution of Langmuir wave turbulence is studied by solving numerically the Zakharov equations in a two-dimensional randomly inhomogeneous plasma. Then, the dynamics of the nonlinear electric currents modulated at frequencies close to 2ω p are calculated, as well as their radiation into harmonic electromagnetic waves. In the frame of this non-self-consistent approach where all transformations of Langmuir waves on density inhomogeneities are taken into account, the electromagnetic wave radiation rate (emissivity) is determined numerically as well as analytically, providing in both cases similar results. Moreover, scaling laws of the harmonic wave emissivity as a function of the ratio of the light velocity to the electron plasma thermal velocity are found. It is also shown how the emissivity depends on the average level of density fluctuations and on the isotropic/anisotropic character of the Langmuir waves’ and density fluctuations’ spectra.

Nonlinear evolution of dispersive Alfvén waves and the effect of magnetic islands in the solar wind plasmas

Solar wind is often known as a source of energizing Earth's magnetosphere and it is a highly complex system displaying various phenomena. Alfvén waves are believed to play a crucial role in its dynamics. The dispersive Alfvén waves (DAWs) may explain various plasma processes, for example, the transfer of energy over different length scales, particle energization, etc., and it is one of the promising sources in the context of magnetic reconnection. We develop a model based upon the two-fluid approximation to study the Alfvén waves, which becomes dispersive on account of the finite frequency correction of the wave, propagating in the medium with a pre-existing chain of magnetic islands and under the impact of the background density fluctuations arising from the ponderomotive nonlinearity of the wave. In the present paper, we study how the dispersive Alfvén waves (DAWs) contribute in the two significant space phenomena, i.e., turbulence and magnetic reconnection. We show that DAWs are important in the generation of localized (coherent) structures and the formation of current sheets. A strong and weak DAW is studied using numerical simulation and semi-analytical approaches in the vicinity of magnetic reconnection sites, i.e., reconnection induced magnetic islands. For the case of strong DAWs, the power spectral density of the fluctuations has been obtained for studying the behavior of the generated turbulence in the solar wind plasmas at 1 AU. The consistency of these results with the observational findings is also reported.

Electromagnetic Wave Emissions from a Turbulent Plasma with Density Fluctuations

Abstract In the solar wind, Langmuir turbulence can generate electromagnetic waves at the fundamental plasma frequency ω p . This process can likely result from either linear wave transformations on the ambient random density inhomogeneities or resonant three-wave interactions involving Langmuir waves and ion acoustic oscillations. In the presence of sufficiently intense plasma density fluctuations of scales much larger than the Langmuir wavelengths, the first mechanism may be more efficient than the second one. A new approach to calculate the electromagnetic wave emissions by Langmuir wave turbulence in plasmas with background density fluctuations is developed. The evolution of the Langmuir turbulence is studied by numerically solving the Zakharov equations in such a two-dimensional plasma. The dynamics of the spatial distributions of the electric currents with frequencies close to ω p is calculated, as well as their emission into electromagnetic waves. The efficiency of this radiation is determined as a function of the level of the Langmuir turbulence, the characteristics of the density fluctuations, the background plasma temperature, the position of the satellite receiver, and the durations of the source’s emissions and spacecraft’s observations. The results obtained by the theoretical modeling and numerical simulations are successfully compared with space observations of electromagnetic waves radiated during Type III solar radio bursts.

Investigation of the geodesic acoustic mode in EAST H-mode operation using Doppler backscattering systems

The geodesic acoustic mode (GAM) has been observed for the first time in EAST H-mode operation using Doppler backscattering (DBS) systems. The poloidal and toroidal symmetries of the radial electric field (Er) fluctuation (m = 0 and n = 0) have also been demonstrated. Using the multichannel DBS system, the GAM frequencies at different radial locations have been investigated, with observations showing that H-mode GAM acts as a radial coherent eigenmode, rather than a continuum of frequencies in L-mode. The intermittency of GAM is observed in both L-mode and H-mode. The interplay between the background density fluctuations and the GAM intensity is revealed. The bicoherence analysis is also used to verify the presence of the nonlinear interaction between the GAM and background turbulence.

Fine Structures of Solar Radio Type III Bursts and Their Possible Relationship with Coronal Density Turbulence

Abstract Solar radio type III bursts are believed to be the most sensitive signatures of near-relativistic electron beam propagation in the corona. A solar radio type IIIb-III pair burst with fine frequency structures, observed by the Low Frequency Array (LOFAR) with high temporal (∼10 ms) and spectral (12.5 kHz) resolutions at 30–80 MHz, is presented. The observations show that the type III burst consists of many striae, which have a frequency scale of about 0.1 MHz in both the fundamental (plasma) and the harmonic (double plasma) emission. We investigate the effects of background density fluctuations based on the observation of striae structure to estimate the density perturbation in the solar corona. It is found that the spectral index of the density fluctuation spectrum is about −1.7, and the characteristic spatial scale of the density perturbation is around 700 km. This spectral index is very close to a Kolmogorov turbulence spectral index of −5/3, consistent with a turbulent cascade. This fact indicates that the coronal turbulence may play the important role of modulating the time structures of solar radio type III bursts, and the fine structure of radio type III bursts could provide a useful and unique tool to diagnose the turbulence in the solar corona.

Turbulent amplification of magnetic field in laser plasma interaction and astrophysical plasmas

The investigation of the nonlinear evolution of magnetosonic wave (MSW) in the presence of density fluctuations at the background has been presented in this paper. The propagation of a single beam or counter propagation of beams is assumed to change the background density accordingly. The model equation for MSW has been obtained by considering the effect of modified plasma density in the background, along with the nonlinear ponderomotive force. The equation so found has been numerically solved to study its effect on the localization of MSW. From the results, the localized and filamentary structures of the MSW can be observed. The effect of variation of the amplitude of density perturbation has been studied on the amplification of magnetic field. To get better insight of these structures, a semi-analytical model with paraxial approximation has been studied. The effect of background density fluctuations on the resulting turbulent spectrum has been evaluated. The results show that the turbulent spectrum gets flattened towards smaller scales as the counter propagation of beams takes place as compared to the single beam propagation. The nonlinear interaction presented here may be important in interpreting the phenomenon of turbulence and magnetic field amplification due to mergers and jets in central galaxy.

Effect of background density fluctuations on the localized structures of inertial Alfvén wave and turbulent spectrum

In the present paper, we have investigated the nonlinear evolution of three dimensionally (3D) propagating inertial Alfvén wave (IAW) in the presence of background density fluctuations for low β plasma. The ponderomotive force, due to IAW and modified background plasma density, has been taken into account in order to develop the model equation. The governing equation has been solved numerically as well as semi-analytically to study the effect of background fluctuations on the evolution of IAW. Results reveal that the IAW gets localized and breaks up into intense localized structures. These localized structures are found to be influenced by the presence of background density fluctuations. To get the physical insight of this proposed interaction, a semi-analytical model based on the paraxial approximation has been developed. Resulting turbulent spectrum has also been studied for different levels of background fluctuations. As a result, it has been found that background density fluctuations affect the nonlinear evolution of IAW which may contribute towards the heating and acceleration of the plasma particles. The significance of the present work is pointed out with the observation received by the spacecraft FAST and Hawkeye1 in the auroral region.

Study of localized structures of kinetic Alfvén wave and generation of turbulence

Localization of kinetic Alfvén waves (KAW) due to ponderomotive nonlinearity can be regarded as an important mechanism for heating the space plasmas. The present paper investigates the effect of background density fluctuations on the formation of large amplitude localized structures and turbulent spectrum of KAW applicable to magnetopause. The dynamical equations are derived, taking into account the ponderomotive nonlinearity of the KAW as well as the background fluctuations which are in the form of ion acoustic waves. The system is studied numerically as well as semi-analytically. The results reveal that the presence of density fluctuations affects the formation of localized structures. These fluctuations affecting the localization of KAW may also affect heating and acceleration of plasma. Respective turbulent scaling for the different amplitude of background fluctuations has also been studied. The relevance of the numerical results has been discussed with the THEMIS observations near the magnetopause [C. Chaston et al., Geophys. Res. Lett. 35, L17S08 (2008)].

Inertial Alfvén wave localization and turbulent spectrum

AbstractThe localization of pump inertial Alfvén wave (IAW) in low β plasmas (β ≪ me/ mi) has been investigated by developing a model based on weak IAW and finite amplitude background density fluctuations. When IAW is perturbed by these fluctuations which are in the form of magnetosonic and ion acoustic waves, its phase velocity gets modified and IAW breaks up into localized structures of very high intensity. Numerical simulation has been carried out to analyze the localized structures and magnetic fluctuation spectrum of pump IAW. We have also developed a simplified model to understand the physical insight into evolution pattern of IAW. Results obtained from simulation reveal that turbulent spectrum up to k⊥λe ≈ 1 follows Kolmogorov power law. Furthermore, at shorter wavelengths, dispersion starts and spectrum steepens with power law index ∼ − 3.8. Energy transport to smaller length scales through this mechanism may be accountable for the observed parallel electron heating in low β plasmas. Results obtained from the numerical simulation are consistent with the observations of various spacecraft like FAST and Hawkeye.

Effect of turbulent density-fluctuations on wave-particle interactions and solar flare X-ray spectra

The aim of this paper is to demonstrate the effect of turbulent background density fluctuations on flare-accelerated electron transport in the solar corona. Using the quasi-linear approximation, we numerically simulated the propagation of a beam of accelerated electrons from the solar corona to the chromosphere, including the self-consistent response of the inhomogeneous background plasma in the form of Langmuir waves. We calculated the X-ray spectrum from these simulations using the bremsstrahlung cross-section and fitted the footpoint spectrum using the collisional "thick-target" model, a standard approach adopted in observational studies. We find that the interaction of the Langmuir waves with the background electron density gradient shifts the waves to a higher phase velocity where they then resonate with higher velocity electrons. The consequence is that some of the electrons are shifted to higher energies, producing more high-energy X-rays than expected if the density inhomogeneity is not considered. We find that the level of energy gain is strongly dependent on the initial electron beam density at higher energy and the magnitude of the density gradient in the background plasma. The most significant gains are for steep (soft) spectra that initially had few electrons at higher energies. If the X-ray spectrum of the simulated footpoint emission are fitted with the standard "thick-target" model (as is routinely done with RHESSI observations) some simulation scenarios produce more than an order-of-magnitude overestimate of the number of electrons $>50$keV in the source coronal distribution.

SOLAR WIND DENSITY TURBULENCE AND SOLAR FLARE ELECTRON TRANSPORT FROM THE SUN TO THE EARTH

Solar flare accelerated electron beams propagating away from the Sun can interact with the turbulent interplanetary media, producing plasma waves and type III radio emission. These electron beams are detected near the Earth with a double power-law energy spectrum. We simulate electron beam propagation from the Sun to the Earth in the weak turbulent regime taking into account the self-consistent generation of plasma waves and subsequent wave interaction with density fluctuations from low frequency MHD turbulence. The rate at which plasma waves are induced by an unstable electron beam is reduced by background density fluctuations, most acutely when fluctuations have large amplitudes or small wavelengths. This suppression of plasma waves alters the wave distribution which changes the electron beam transport. Assuming a 5/3 Kolmogorov-type power density spectrum of fluctuations often observed near the Earth, we investigate the corresponding energy spectrum of the electron beam after it has propagated 1 AU. We find a direct correlation between the spectrum of the double power-law below the break energy and the turbulent intensity of the background plasma. For an initial spectral index of 3.5, we find a range of spectra below the break energy between 1.6-2.1, with higher levels of turbulence corresponding to higher spectral indices.

Numerical simulations of stochastically excited sound waves in a random medium

In turbulent acoustic media such as the solar envelope, both wave sources and the propagation characteristics (back- ground density, refractive index, dissipation, etc.) are stochastic quantities. By means of numerical simulation of the Euler equations, we study two cases in a homogeneous stochastic medium in which the background density fluctuations and wave sources are 1) correlated and 2) uncorrelated. We find that in the uncorrelated case, the coherent (or mean) acoustic field is zero, leaving only an incoherent field. In the correlated case, the coherent field is nonzero, yielding both coherent and incoherent fields. We question the use of mean-field dispersion relations to determine frequency shifts in p-mode and f-mode spectra, since the coherent field can be non-existent or weak relative to the incoherent field. We demonstrate the importance of accounting for a stochastic wave source by showing that the two cases give very different frequency shifts.

Electron beam formation and stability

The dynamics of fast electrons streaming from the Sun through the interplanetary plasma is reviewed from the kinetic viewpoint. How can a bump appear on the tail of the electron distribution function? How can the beam survive the beam-plasma instability and propagate up to 1 AU as observed? These two questions are discussed in light of the recent data acquired in situ on electron distributions, Langmuir waves, ion acoustic waves, and background density fluctuations.