Electron Langmuir Research Articles

Electron Cyclotron Maser Emission in Solar Radio Bursts

Radio bursts are ubiquitous in the cosmic plasma. Solar radio emission mainly comes from the outer atmosphere of the sun. It is an induced radiation phenomenon generated by the interaction between energetic electrons and solar atmospheric plasma. Different dynamic spectra of solar radio bursts (SRBs) contain physical information of the plasma structure and state in the radiation source region. Therefore, the radiative mechanism of radio bursts has always been the object of research. There are two kinds of coherent radiation mechanisms related to solar radio bursts: one is the plasma radiation mechanism based on electron Langmuir frequency; the other is the electron cyclotron maser (ECM) radiation mechanism based on the electron cyclotron frequency. Although these two radiation mechanisms were proposed almost at the same time, based on the understanding of the coronal environment and the ECM mechanism at that time, the ECM radiation mechanism did encounter some difficulties in explaining SRBs. Until 1979, Wu & Lee introduced the relativistic effect and used the ECM radiation to explain the earth’s Auroral Kilometric Radiation (AKR). Since then, the ECM emission has attracted wide attention. Considering some difficulties in applying the ECM emission mechanism to SRBs, we proposed a series of modified models in recent years. Firstly, the cutoff in the energy spectrum of the power-law electrons can effectively drive the ECM instability without relying on the anisotropic distribution of electron velocity. Secondly, considering the influence of Alfvén wave perturbations which are prevalent in space and celestial plasmas, a self-consistent ECM emission mechanism excited by energetic electron beams is developed. On this basis, this paper summarizes the application of the ECM emission mechanism in traditional SRB phenomena from type I to V and microwave SRBs in recent years.

On the electric field components in the collisionless plasmoid instability

Using two-dimensional particle-in-cell simulations of collsionless plasmoid instability, the spatial and temporal variations of electric field components are investigated. The initial magnetic field is a Harris sheet which is added by a non-uniform perpendicular magnetic field (guide field). The effects of weak and medium guide field on the evolution of plasmoid instability and associated electric field are discussed with the input parameters of the solar corona with the condition $\omega _{{\rm pe}}/\omega _{{\rm ce}}>1$ . As the current sheet elongates, magnetic reconnection in an X-point transits into the plasmoid instability, in which secondary X-points are formed and the consequent magnetic islands coalesce and form larger islands. During the instability, electric field components show two types of disturbances. (1) Transient waves, being excited very early, propagate mainly perpendicular to the current sheet and become damped as the instability proceeds to the nonlinear stage. Fourier analysis reveals that electromagnetic right- and left-hand polarized modes parallel to the initial magnetic field and also the extraordinary mode perpendicular to the current sheet are excited. Additionally, electron Langmuir and slow magnetosonic waves are observed in both directions. (2) Inside the current sheet, an electric field is generated by the plasmoid-dominated reconnection and grows in amplitude as the plasmoid instability proceeds. Moreover, in our study, the effects of amplitude-varying guide field on the spatial structure of electric field components outside and inside the current sheet are discussed. This study is valuable because understanding the spatial and temporal variation of the electric field is an essential prerequisite for the issue of particle acceleration in plasmoid-dominated magnetic reconnection.

Grid-based Vlasov simulation of collisionless plasma expansion

Grid-based Vlasov simulations are carried out to re-evaluate the one-dimensional collisionless plasma expansion into vacuum. The grid-based method eliminates the inherent statistical noise in particle-based methods and allows us to extend the solution beyond the self-similar expansion region and resolve small electron timescale wave perturbations. It is shown that the expansion generates both an ion-acoustic rarefaction wave mode and an electron Langmuir wave mode that propagate into the unperturbed plasma upstream. The assumption used in the classical expansion solution that the electrons are an isothermal fluid is accurate within a quasi-neutral, self-similar expansion region but fails in both the upstream and downstream of that region due to electron timescale perturbations.

Crude oil and S500 diesel removal from seawater by polyurethane composites reinforced with palm fiber residues

In this work, we prepared PU-composites with Australian palm residues (PR) in different contents (5, 10, 15, and 20 wt%) and granulometry (28 and 35 mesh) to improve the oil (crude oil and S500 Diesel) sorption capacity. The foams were characterized by life cycle assessment (LCA), scanning electron microscopy, oil sorption, desorption, and Langmuir, Freundlich, and Temkin sorption isotherms. LCA indicated that higher PR contents decreased the foam environmental impacts than the classical residue handling, indicating that 20 wt% PR is the better environmental option, independent of the residues granulometry. The PR incorporation into PU foams resulted in smaller pore sizes, with a higher number of homogeneous open-cells. The PU composites exhibited higher oil adsorption capacity than the pristine foam. The PU sample showed maximum absorption capability of 6.1 and 6.7 g g−1 for diesel S500 and crude oil, and the composites showed increased values of ∼18 g g−1 and ∼24 g g−1. The Langmuir model presented the best fit and predicted a maximum adsorption capacity of 30.39 and 25.57 g g−1 for PU-20% PR 28 and 35 mesh, respectively. The composites presented excellent reusability with PU-20% PR (28 mesh) and PU-20% PR (35 mesh), showing removal efficiency after 16 and 9 cycles, respectively. The results classify the developed foams as excellent materials to sorb spilled crude oil in marine accidents.

Longitudinal electron waves and instability of collisional plasma formed by multiphoton ionization of gas atoms

Longitudinal collective modes in plasma produced by multiphoton ionization of inert gas atoms are studied. The high-frequency and the low-frequency modes in Xe plasma are considered in detail. It is shown that due to the Ramsauer–Townsend effect, photoelectron collisions with neutral atoms lead to a significant increase in decrement of high-frequency mode in the long-wavelength region where this mode frequency is close to the electron Langmuir one. In the region of wave numbers larger than the ratio of electron Langmuir frequency to photoelectrons average velocity, the dispersion law of high-frequency mode is close to the dispersion law of electron acoustic wave, the phase and the group velocities of which are close to photoelectrons' average velocity. In this case, the acoustic wave is weakly damped even for photoelectrons' collision frequencies comparable to electron Langmuir frequency. There is a low-frequency mode along with the high-frequency one. In the region of relatively small wave numbers, this mode describes aperiodic instability with the growth rate not exceeding half of the electron Langmuir frequency. The instability is caused by the Ramsauer–Townsend effect. As the wave number increases, the low-frequency mode transforms into a longitudinal wave, the amplitude of which increases with time. In the region of wave numbers greater than the ratio of photoelectron average velocity to electron Langmuir frequency, the low-frequency wave is strongly damped due to photoelectron collisions. Influence of the spread of photoelectrons in energy on the low-frequency mode is relatively small. In contrast, a finite width of photoelectron distribution over energy leads to significant collisionless damping of high-frequency wave in the short-wavelength region.

A Review of Recent Solar Type III Imaging Spectroscopy

Solar type III radio bursts are the most common impulsive radio signatures from the Sun, stimulated by electron beams travelling through the solar corona and solar wind. Type III burst analysis provides us with a powerful remote sensing diagnostic tool for both the electron beams and the plasma they travel through. Advanced radio telescopes like the LOw Frequency ARray (LOFAR), the Murchison Widefield Array (MWA) and the Karl G. Jansky Very Large Array (VLA) are now giving us type III imaging spectroscopy with orders of magnitude better resolution than before. In this review, the recent observational progress provided by the new observations is discussed, including how this enhanced resolution has facilitated study of type III burst fine structure. The new results require more detailed theoretical understanding of how type III bursts are produced. Consequently, recent numerical work is discussed which improves our understanding of how electron beams, Langmuir waves and radio waves evolve through the turbulent solar system plasma. Looking towards the future, some theoretical challenges are discussed that we need to overcome on our quest to understand type III bursts and the electron beams that drive them.

Collective modes of plasma formed by multiphoton ionization of rarefied gas

High-frequency collective modes in plasma with strongly anisotropic velocity distribution of photoelectrons formed by multiphoton or above-threshold ionization of gas atoms are studied. In the case of multiphoton ionization, along with the usual electromagnetic wave, there are two additional modes. In the region of large wavelengths, the higher-frequency mode is similar to the electron Langmuir wave. Its group velocity is mainly determined by the average photoelectron velocity, and Cherenkov damping is due to small velocity dispersion of photoelectron distribution. In the region of short waves, but with a small Cherenkov damping, the group and phase velocities of this wave are close to the average electron velocity. The second mode, which has lower frequency, in the region of wavelengths smaller than the ratio of average electron velocity to the plasma frequency, corresponds to quasi-potential wave. Its dispersion law is close to the linear one. In contrast, in the region of large wavelengths, this mode corresponds to aperiodic instability, the maximum growth rate of which is comparable to the plasma frequency. The distribution of photoelectrons formed during above-threshold ionization is characterized by the large number of energy peaks, which is accompanied by increasing in the number of collective modes. In particular, in plasma with photoelectron distribution which has two energy peaks, in addition to the electromagnetic mode four extra modes are possible. In the shortwave region, all four modes correspond to the waves that are damped due to Cherenkov interaction with photoelectrons. Two of these modes in the region of relatively long wavelengths are unstable. One of these unstable modes corresponds to quasi-potential wave whose amplitude aperiodically increases with time. The reason for the instabilities is the presence of counter streams of photoelectrons.

Electron–Langmuir wave resonance in three dimensions

In the one-dimensional (1D) treatment of Langmuir wave generation by a particle distribution via the kinetic beam instability, there is a one-to-one resonance between the electron speed v and the phase speed vϕ of the wave. The 1D condition for wave growth is ∂f/∂v>0 with v=vϕ, and f(v) evolves due to quasilinear relaxation toward a plateau distribution ∂f/∂v=0. We show here that none of these results apply in a 3D treatment of the problem. For a wave with wavevector k and phase speed vϕ, there is a many-to-one resonance with all electrons with v>vϕ moving obliquely to k. Although growth requires a region with ∂f/∂v>0 below a peak in f(v), the growth can be driven primarily by particles above the peak where ∂f/∂v<0, and growth can even occur at phase velocities vϕ where ∂f/∂v|v=vϕ<0. Resonance at v≫vϕ favors diffusion of the particle distribution in angle, rather than plateau formation. These properties imply that intuition based on the 1D model can be seriously misleading, with far-reaching implications for modeling phenomena such as type III solar radio bursts.

Separate spin evolution of electrostatic energy flow in a degenerate quantum plasma

We have discussed energy densities and energy flow speed in a spin polarized plasma when longitudinal waves [Spin electron acoustic wave (SEAW) and Langmuir wave] propagate through the plasma. Employing the separate spin evolution quantum hydrodynamic model, we have derived the expression for energy densities and energy flow speed. It is found that the spin polarization changes the profiles of various energy densities. Specifically, we find that the spin polarization broadens the profile of the electrostatic energy density retaining the same peak value. In the case of kinetic and quantum energy densities, the profiles become narrower with the decrease in the peak value for the former case and increase for the latter. On the other hand, in the case of Langmuir waves, the spin polarization effect is similar for electrostatic energy density but opposite to the peak values of kinetic and quantum energy densities. The corresponding profiles become narrower as in the case of SEAW. Furthermore, energy flow speed associated with the SEAW and Langmuir wave is reduced for higher values of spin polarization. It is also noted that the contribution of Bohm potential in the dispersion compensates the reduction of energy flow due to spin polarization. The results are graphically analyzed for the choice of solid state plasma parameters.

Landau damping of electrostatic modes in nonthermal plasmas

The dispersion relation (DR) and Landau damping of basic electrostatic modes propagating in an electron-ion plasma modeled by a Cairn's distribution with a spectral index α are kinetically investigated. In the high-frequency range, i.e., ω/k>vte, the DR and Landau damping of the electron Langmuir wave (LW) are obtained. In the low-frequency range where vte>ω/k>vti, the DR of ion acoustic wave (IAW) is investigated. The phase velocity and damping of both the Langmuir and the IAWs are shown to be significantly influenced by the nonthermality parameter α.

High-frequency waves in plasma formed as a result of tunnel ionization of atoms by circularly polarized radiation

New dependencies of frequency and damping decrement of high-frequency longitudinal waves on the wave vector in photoionized plasma formed by tunnel ionization of atoms in the field of circularly polarized radiation are found. Weakly damped longitudinal waves with a frequency much higher than the electron Langmuir frequency are predicted.

SUPRATHERMAL SOLAR WIND ELECTRONS AND LANGMUIR TURBULENCE

ABSTRACT The steady-state model recently put forth for the solar wind electron velocity distribution function during quiet time conditions, was originally composed of three population electrons (core, halo, and superhalo) with the core remaining nonresonant with any plasma waves while the halo and superhalo separately maintained steady-state resonance with whistler- and Langmuir-frequency range fluctuations, respectively. However, a recent paper demonstrates that whistler-range fluctuations in fact have no significant contribution. The present paper represents a consummation of the model in that a self-consistent model of the suprathermal electron population, which encompasses both the halo and the superhalo, is constructed solely on the basis of the Langmuir fluctuation spectrum. Numerical solutions to steady-state particle and wave kinetic equations are obtained on the basis of an initial trial electron distribution and Langmuir wave spectrum. Such a finding offers a self-consistent explanation for the observed steady-state electron distribution in the solar wind.

Comparisons of mapped magnetic field lines with the source path of the 7 April 1995 type III solar radio burst

AbstractIdeally, the sources of type III solar radio bursts, which are produced mainly by flare‐accelerated electron beams, trace the magnetic field lines along which the beams propagate from the Sun to interplanetary space. A recently developed 2‐D approach for large‐scale mapping of magnetic field lines between the Sun and Earth in the solar equatorial plane is applied to the sources of the 7 April 1995 type III radio burst imaged by Ulysses and Wind. The approach uses near‐Earth solar wind data and a solar wind model with intrinsic nonradial magnetic field at the source surface of the solar wind. Quantitative agreement is found between the mapped field lines, the observed path of the radio source centroids, and the field configurations inferred from solar wind suprathermal electrons observed by Wind. Moreover, the mapped field lines are consistent with Wind not observing the in situ type III electron beam, Langmuir waves, and local radio emission for this type III event.

Double Langmuir frequency radiation due to transformation processes in turbulent plasma

We investigate the transformation process of longitudinal Langmuir wave into the transverse electromagnetic wave in turbulent plasma subjected to an upper hybrid pump. The case, when upper hybrid pump wave decays into daughter and ion – sound waves is considered. The transformation of the Langmuir wave into electromagnetic one is considered as the possible mechanism of energy radiation from the plasma. It is shown that the frequency of such radiation is chosen to be near double electron Langmuir frequency 2ωpe. These results give us the possibility to explain the nature of radiation from the laboratory and cosmic plasmas (particularly, from the solar crown).

A new low-frequency backward mode in inhomogeneous plasmas

When an electromagnetic transverse wave propagates through an inhomogeneous plasma so that its electric field has a component in the direction of the background density gradient, there appears a disbalance of charge in every plasma layer, caused by the density gradient. Due to this, some additional longitudinal electric field component appears in the direction of the wave vector. This longitudinal field may couple with the usual electrostatic longitudinal perturbations like the ion acoustic, electron Langmuir, and ion plasma waves. As a result, these standard electrostatic waves are modified and in addition to this a completely new low-frequency mode appears. Some basic features of the coupling and modification of the ion acoustic wave, and properties of the new mode are discussed here, in ordinary electron-ion and in pair plasmas.

Iterative addition of parallel temperature effects to finite-difference simulation of radio-frequency wave propagation in plasmas

Accurate simulations of how radio frequency (RF) power is launched, propagates, and absorbed in a magnetically confined plasma is a computationally challenging problem that for which no comprehensive approach presently exists. The underlying physics is governed by the Vlasov–Maxwell equations, and characteristic length scales can vary by three orders of magnitude. Present algorithms are, in general, based on finding the constituative relation between the induced RF current and the RF electric field and solving the resulting set of Maxwell’s equations. These linear equations use a Fourier basis set that is not amenable to multi-scale formulations and have a large dense coefficient matrix that requires a high-communications overhead factorization technique. Here the use of operator splitting to separate the current and field calculations, and a low-overhead iterative solver leads to an algorithm that avoids these issues and has the potential to solve presently intractable problems due to its data-parallel and favorable scaling characteristics. We verify the algorithm for the iterative addition of parallel temperature effects for a 1D electron Langmuir by reproducing the solution obtained with the existing Fourier kinetic RF code aorsa (Jaeger et al., 2008).

Small-scale Langmuir wave instability in preflare chromosphere of solar active region

Necessary conditions have been investigated for the appearance of instability of high-frequency electron Langmuir waves in plasma of solar chromosphere near the foot-point of loop structure. We have considered the earliest stage of a flare process in solar active region.

ON QUIET-TIME SOLAR WIND ELECTRON DISTRIBUTIONS IN DYNAMICAL EQUILIBRIUM WITH LANGMUIR TURBULENCE

A recent series of papers put forth a self-consistent theory of an asymptotically steady-state electron distribution function and Langmuir turbulence intensity. The theory was developed in terms of the κ distribution which features Maxwellian low-energy electrons and a non-Maxwellian energetic power-law tail component. The present paper discusses a generalized κ distribution that features a Davydov-Druyvesteyn type of core component and an energetic power-law tail component. The physical motivation for such a generalization is so that the model may reflect the influence of low-energy electrons interacting with low-frequency kinetic Alfvenic turbulence as well as with high-frequency Langmuir turbulence. It is shown that such a solution and the accompanying Langmuir wave spectrum rigorously satisfy the balance requirement between the spontaneous and induced emission processes in both the particle and wave kinetic equations, and approximately satisfy the similar balance requirement between the spontaneous and induced scattering processes, which are nonlinear. In spite of the low velocity modification of the electron distribution function, it is shown that the resulting asymptotic velocity power-law index α, where fe ~ v –α is close to the average index observed during the quiet-time solar wind condition, i.e., whereas αaverage ~ 6.69, according to observation.

Excitation of electron Langmuir frequency harmonics in the solar atmosphere

An alternative mechanism for the excitation of electron Langmuir frequency harmonics as a result of the development of explosive instability in a weakly relativistic beam-plasma system in the solar atmosphere is proposed. The efficiency of the new mechanism as compared to the previously discussed ones is analyzed.

Electron kappa distribution and steady-state Langmuir turbulence

In a recent pair of papers, the present author discussed a self-consistent theory of asymptotically steady-state electron distribution function and Langmuir turbulence intensity in one [P. H. Yoon, Phys. Plasmas 18, 122303 (2011)] and three [P. H. Yoon, Phys. Plasmas 19, 012304 (2012)] dimensions. The resulting electron distribution function is a type of kappa distribution that features a non-Maxwellian energetic tail component. However, while the one-dimensional solution is rigorously correct, the three-dimensional solution, which was obtained using the cylindrical coordinate representation, contains two features that may be inconsistent for field-free plasmas. One is the assumption that the resonance condition can be approximated by ω-k·v≈ω-k∥v∥. Needless to say, this is not the most general condition. The second inconsistency is that while the electron distribution is isotropic in velocity, the Langmuir turbulence intensity depends on the wave propagation direction. While these features may not be too unrealistic in the presence of an implicit ambient magnetic field, they certainly cannot be correct if the plasma is genuinely unmagnetized. In the present paper, we rectify such shortcomings by properly reformulating the problem using a spherical coordinate system in a truly free-field plasma.