Langmuir Frequency Research Articles

Origin of the type III radiation observed near the Sun

Aims. We investigate processes associated with the generation of type III radiation using Parker Solar Probe measurements. Methods. We measured the amplitudes and phase velocities of electric and magnetic fields and their associated plasma density fluctuations. Results. 1. There are slow electrostatic waves near the Langmuir frequency and at as many as six harmonics, the number of which increases with the amplitude of the Langmuir wave. Their electrostatic nature is shown by measurements of the plasma density fluctuations. From these density fluctuations and the electric field magnitude, the k-value of the Langmuir wave is estimated to be 0.14 and kλd = 0.4. Even with the large uncertainty in this quantity (more than a factor of two), the phase velocity of the Langmuir wave was < 10 000 km/s. 2. The electromagnetic wave near the Langmuir frequency has a phase velocity lower than 50 000 km/s. 3. We cannot determine whether there are electromagnetic waves at the harmonics of the Langmuir frequency. If they existed, their magnetic field components would be below the noise level of the measurement. 4. The rapid (less than one millisecond) amplitude variations typical of the Langmuir wave and its harmonics are artifacts resulting from the addition of two waves, one of which has small frequency variations that arise because the wave travels through density irregularities. None of these results are expected in or consistent with the conventional model of the three-wave interaction of two counter-streaming Langmuir waves that coalesce to produce the type III wave. They are consistent with a new model in which electrostatic antenna waves are produced at the harmonics by radiation of the Langmuir wave, after which at least the first harmonic wave evolved through density irregularities such that its wave number decreased and it became the type III radiation.

Characterization of high-frequency waves in the Martian magnetosphere

Context. Various high-frequency waves in the vicinity of upper-hybrid and Langmuir frequencies are commonly observed in different space plasma environments. Such waves and fluctuations have been reported in the magnetosphere of the Earth, a planet with an intrinsic strong magnetic field. Mars has no intrinsic magnetic field and, instead, it possesses a weak induced magnetosphere, which is highly dynamic due to direct exposure to the solar wind. In the present paper, we investigate the presence of high-frequency plasma waves in the Martian plasma environment by making use of the high-resolution electric field data from the Mars Atmosphere and Volatile Evolution missioN (MAVEN) spacecraft. Aims. This study aims to provide conclusive observational evidence of the occurrence of high-frequency plasma waves around the electron plasma frequency in the Martian magnetosphere. We observe two distinct wave modes with frequency below and above the electron plasma frequency. The characteristics of these high-frequency waves are quantified and presented here. We discuss the generation of possible wave modes by taking into account the ambient plasma parameters in the region of observation. Methods. We have made use of the medium frequency (100 Hz–32 kHz) burst mode-calibrated electric field data from the Langmuir Probe and Waves instrument on board NASA’s MAVEN mission. Due to the weak magnetic field strength, the electron gyro-frequency is much lower than the electron plasma frequency, which implies that the upper-hybrid and Langmuir waves have comparable frequencies. A total of 19 wave events with wave activities around electron plasma frequency were identified by examining high-resolution spectrograms of the electric field. Results. These waves were observed around 5 LT when MAVEN crossed the magnetopause boundary and entered the magnetosheath region. These waves are either a broadband- or narrowband-type with distinguishable features in the frequency domain. The narrowband-type waves have spectral peak above the electron plasma frequency. However, in the case of broadband-type waves, the spectral peak always occurred below the electron plasma frequency. The broadband waves consistently show a periodic modulation of 8–14 ms. Conclusions. The high-frequency narrowband-type waves observed above the electron plasma frequency are believed to be associated with upper-hybrid or Langmuir waves. However, the physical mechanism responsible for the generation of broadband-type waves and the associated 8–14 ms modulation remain unexplained and further investigation is required.

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.

Спектральные характеристики плазменных волн ионосферы при возбуждении мощными КВ-радиоволнами на частотах излучения вблизи гирогармоник электронов и критической частоты слоя F2

We present the result of the studies into characteristics of high-latitude ionospheric F-region longitudinal plasma waves (Langmuir and ion-acoustic), caused by the impact of powerful HF radio waves of ordinary (O-mode) or extraordinary (X-mode) polarization of the EISCAT/Heating facility, Tromsø, Norway. The powerful HF radio waves on October 20, 2012 and February 26, 2013 were emitted in the direction of the magnetic zenith with a step change in the effective radiation power (ERP). The radiation frequency fH of the EISCAT/Heating facility on February 26, 2013 was close to the F2-layer critical frequency foF2 (fH/foF2~1) and exceeded the electron gyroresonance frequency fH>5fce. On October 20, 2012, the conditions fH/foF2~0.85–0.95 and fH<6fce were fulfilled. Analysis of EISCAT measurements of an incoherent scatter radar (ISR) at a frequency of 930 MHz, spatially aligned with the heating facility for radiation conditions ERP<200 MW, has shown that parametric decay instabilities are excited at the ionospheric heights where the pump frequency is close to the plasma Langmuir frequency, fH≈fPL. We have studied peculiarities of excitation of the decay parametric instabilities as a function of height in the ionosphere and pump wave polarization, the ratios between fH and foF2, and also fH and nfce.

Spectral features of ionospheric plasma waves excited by powerful HF radio waves radiated at frequencies near electron gyroharmonics and F2-layer critical frequency

We present the result of the studies into characteristics of high-latitude ionospheric F-region longitudinal plasma waves (Langmuir and ion-acoustic), caused by the impact of powerful HF radio waves of ordinary (O-mode) or extraordinary (X-mode) polarization of the EISCAT/Heating facility, Tromsø, Norway. The powerful HF radio waves on October 20, 2012 and February 26, 2013 were emitted in the direction of the magnetic zenith with a step change in the effective radiation power (ERP). The radiation frequency fH of the EISCAT/Heating facility on February 26, 2013 was close to the F2-layer critical frequency foF2 (fH/foF2~1) and exceeded the electron gyroresonance frequency fH>5fce. On October 20, 2012, the conditions fH/foF2~0.85–0.95 and fH<6fce were fulfilled. Analysis of EISCAT measurements of an incoherent scatter radar (ISR) at a frequency of 930 MHz, spatially aligned with the heating facility for radiation conditions ERP<200 MW, has shown that parametric decay instabilities are excited at the ionospheric heights where the pump frequency is close to the plasma Langmuir frequency, fH≈fPL. We have studied peculiarities of excitation of the decay parametric instabilities as a function of height in the ionosphere and pump wave polarization, the ratios between fH and foF2, and also fH and nfce.

Waves propagating parallel to the magnetic field in relativistically hot plasmas: A hydrodynamic model with the average reverse gamma factor evolution

AbstractThe high‐frequency part of the dispersion dependence of electromagnetic waves propagating parallel to the external magnetic field in the plasma is considered. The plasma is macroscopically motionless, but has relativistic temperatures , where is the electron rest mass and, is the speed of light. The analysis is based on a novel hydrodynamic model based on four equations for the recently developed material fields (Andreev, Phys. Scr., 2022, 97, 085602). These material fields are the concentration, velocity field, average reverse relativistic factor, and the flux of the reverse relativistic factor. In the nonrelativistic regime, there are three transverse waves (the ions are motionless). In this study, we trace the charge of these properties under the influence of relativistic effects. Strong thermal effects lead to a coefficient in front of the cyclotron frequency, which decreases its effective contribution of the cyclotron frequency. At , we have a decrease in the existence area of the fast magneto‐sound wave from the area of large frequencies. However, the existence area of extraordinary waves becomes larger at smaller frequencies. In the strong magnetic field limit , an additional wave appears with a frequency below the thermally decreased cyclotron frequency, where is the electron cyclotron frequency, and is the Langmuir frequency. A further increase in the temperature leads to the disappearance of fast magneto‐sound waves and to a considerable increase in the existence area of the extraordinary waves towards smaller frequencies.

High‐Resolution Observation of Heater‐Induced Daytime Plasma Fluctuations at Arecibo

AbstractIonospheric heating is known to induce density and temperature irregularities in the ionospheric plasma, which have been observed by various in‐situ and radar techniques. Analysis of incoherent scatter radar (ISR) plasma line spectra offers high resolution reconstruction of ionospheric features, but the plasma line profile is frequently difficult to reconstruct at altitudes where the heater is generating Langmuir turbulence and thus creating additional signals in the ISR spectrum. This work presents a novel, automated method based on Gabor filter image analysis to reconstruct plasma line spectra recorded with the coded long‐pulse technique. The method permits the unsupervised reconstruction of the Langmuir frequency with resolution in frequency (sub‐kHz) and altitude (∼300 m) typical of the coded long‐pulse technique even in regions directly coupling to the heater. The technique is applied to ISR data from the June 2019 Arecibo Observatory heating experiment, demonstrating the formation of field‐aligned irregularities of magnitude ∼0.5% in the Langmuir frequency during daytime conditions and spatial scale perpendicular to the magnetic field (fluctuation periodicity of several kilometers, corresponding to k ≈ 1 km−1) consistent with the patches of depletions of free electrons previously observed by in‐situ methods. Additionally, the method allows observation of the formation and evolution of the bunches over the course of several hours. Additional F‐layer irregularities were found in the ambient ionosphere in regions away from the heater coupling altitude. The automated nature of this technique offers the potential for precision analysis of Arecibo's ISR data archives for a variety of ionospheric physics applications.

Parametric studies of stream instability-induced higher harmonics in plasma ionization breakdown near an emissive dielectric surface

In this work, we comprehensively investigate the generation of higher harmonic (HH) electric fields normal to the applied rf electric field in multipactor-coexisting plasma breakdown by fully kinetic particle-in-cell (PIC) simulations and a theoretical model. Firstly, a base case at driving frequency 1 GHz, transverse rf electric field amplitude 3 MV m−1, and background gas pressure 0.2 Torr, is studied in detail. The enhanced harmonic frequency observed is around ten times the fundamental rf frequency, significantly lower than the Langmuir frequency. A theoretical model reveals that the fundamental mechanism of HHs generation is stream–plasma instability, which originates from stream-like secondary electron emission interacting with plasma. The resulting HH frequency and the growth rate of its oscillating amplitude from the theoretical model, agree well with the PIC simulations. With increasing pressure, the HH oscillations are found to be significantly reduced. This is because at higher pressure the gas ionization rate is higher, which causes a more rapidly increasing plasma density, leaving less time for the growth of instability. Furthermore, the parameter space in terms of background gas pressure and rf field amplitude within which the HHs appear is revealed. Finally, the effect of the driving rf frequency on HHs is also investigated, and it is found that the instability-induced oscillating HHs field remains when the driving frequency is increased to 2 GHz, however, it is highly reduced at higher driving frequency of 5 GHz, as oscillations at the fundamental frequency start playing a more important role.

Cherenkov FEL Reaction With Plasma-Filled Cylindrical Waveguide in FractionalD-Dimensional Space

The possibility of generating an electromagnetic (EM) wave by a free-electron laser (FEL) beam from the Cherenkov device to control the cylindrical waveguide's field attenuation filled with plasma has been investigated by analytical formalism. This new study sheds light on Cherenkov FEL (C-FEL) beam interaction with electrons of inhomogeneous warm plasma to generate an EM wave in fractional dimensional space. The new analysis of traveling and standing waves in terms of Hankel and Bessel functions paves a way for introducing controlled EM wave propagation based on fractional D-dimensional space. It has been found that the C-FEL beam excites the EM wave and enhances the propagation of the electrical field through fractional dimensional space with propagation constant depending on the Langmuir frequency. Within the plasma in the cylindrical waveguide, a TM mode emerges, which contains spatial frequencies with a faster growth rate for traveling waves than standing waves.

Reversal of laser wake phase velocity generates high-power broadband Cherenkov signal

As a femtosecond terawatt laser pulse propagates along a positive density gradient within a stratified plasma column, it drives a low-frequency electromagnetic wake wave, the period of electron fluid oscillations in the wake gradually shrinking. Phase velocity of the wake promptly exceeds the vacuum speed of light, setting in near-forward emission of terahertz Cherenkov radiation. As the wake accelerates further, the Cherenkov emission ray rotates by 180∘. Emission from a given plasma locality is sustained for a finite interval of time, in the middle of which the wake experiences a ‘reversal,’ its phase velocity becoming singular and changing sign (Zhang C J et al 2017 Phys. Rev. Lett. 119 064801) At this instant, the electromagnetic energy flows at 90∘, the emission power reaching its peak. After the reversal, the wake keeps radiating into the rear hemisphere until its phase velocity becomes subluminal. Experimentally capturing evolution of the Cherenkov signal may thus shed light onto the plasma wake dynamics. Far away from the plasma, the radiation fills an expanding, almost spherical shell, the shell thickness increasing with an increase in the observation angle. The length of the terahertz signal sampled in the wave zone thus ranges from zero (forward emission) to a few tens of picoseconds (backward emission). The signal is positively chirped, its frequency increasing from the Langmuir frequency at the foot of the column to the Langmuir frequency at the top. Theoretical estimates for the regimes involving 10 TW-class drive pulses promise a few-kW emission power; the energy conversion efficiency, from optical to terahertz, of order 10−7; and an MV m−1-scale electric field strength centimeters away from the plasma.

The electromagnetic waves propagating parallel to the magnetic field in a uniform magnetized plasma with a transverse electric field

In natural and laboratory magnetized plasmas, an equilibrium electric field may exist perpendicular to the background magnetic field. In such a situation all the plasma species experience a common drift and the equilibrium distribution functions have a common shift in velocity space. The susceptibility tensor is derived for such a situation using the characteristics and then the Lorentz transformation method is adopted to verify the results. As an application we give an analysis of the waves propagating parallel to the background magnetic field for a simple electron plus single charged ion plasma. It is predicted that the presence of a transverse equilibrium electric field can drastically change the wave propagation only if the wave frequency is close to the plasma Langmuir frequency.

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.

Radiation emission at Langmuir frequency from laser wake in longitudinally stratified plasma column

Theoretical analysis shows that a small-amplitude laser wake (an electrostatic Langmuir wave), driven in a periodically stratified, cylindrical plasma column, generates a superluminal, azimuthally polarized rotational current at a Langmuir frequency. The current radiates a radially polarized (i.e. transverse magnetic) Cherenkov signal into the plasma-free space. The opening angle of the Cherenkov emission cone is defined by the spatial period of stratification. Lifetime of the laser wake, limited by wave breaking in the inhomogeneous plasma, defines the terahertz (THz) signal length. This length ranges from a few picoseconds to hundreds of picoseconds, depending on the wake amplitude, amplitude of background density modulation, and stratification period. Monochromaticity and coherence distinguishes this THz signal from the ultrashort, uncollimated, broadband signals generated by photoionization currents in plasma filaments. The efficiency of electromagnetic energy conversion, from optical to THz, reaches its peak when the drive pulse waist size is close to the column radius. The efficiency increases with an increase in the drive pulse wavelength, and reaches the maximum when the drive pulse becomes near-critical for relativistic self-focusing. Numerical examples with sub-Joule, near-infrared terawatt drive pulses demonstrate the highest conversion efficiency of the order 10−5, with the total emitted energy of several µJ, a few-hundred MV m−1 electric field at the column surface, and an MV m−1 field 10 cm away from the source.

Nonlinear excitation of electron plasma waves trapped in a blob in the O1-mode ECRH experiments

We analyze a scenario of low-threshold parametric decay instability of the pump ordinary wave associated with excitation in a plasma blob of two trapped electron plasma waves at frequencies smaller than the local Langmuir frequency.

Single-cycle THz signal accompanying laser wake in photoionized plasmas

Photoionization of an ambient gas by a weakly relativistic, femtosecond laser pulse leaves behind the pulse a flat-top plasma column. The uniform-density core of the column is surrounded by a micron-thin shell, within which the density of plasma species falls down to zero. As the laser pulse ponderomotive force drives the wave of charge separation (the laser wake), electron fluid velocity oscillates along this boundary layer. Coupling velocity oscillations to the transverse density gradient forms an azimuthally polarized rotational current confined within the boundary layer. At each radial offset, this current oscillates with a local (THz-scale) Langmuir frequency, transforming the cylindrical electrostatic plasma wave into a radially evanescent Fourier component of an electromagnetic signal. Summing up these Fourier components yields the time-domain THz signal which is (a) non-local (i.e. not phase-locked to the Langmuir oscillation at any given location), (b) contains the entire frequency band from zero to the Langmuir frequency of the density plateau, and (c) is detectable at a distance orders of magnitude larger than the plasma column radius. A few millimeters away from the column, rapid evanescence of the higher-frequency components transforms the signal into a radially polarized, single-cycle picosecond pulse with a kV m−1-scale electric field. Detecting this pulse with existing (e.g. electro-optical) techniques may bring information on the efficiency of laser coupling to the plasma, the amplitude of wake fields, and the structure of the column boundary.

Interaction of counterstreaming rotating electron-positron beams with inhomogeneous electron-ion plasma

The interaction occurring between two counterstreaming rotating electron-positron beams and an inhomogeneous magnetized electron-ion plasma is studied with the focus of research on either positrons or electrons propagating in the direction of the magnetic field. Using the Vlasov theory along with geometrical optics, the linear eikonal equation corresponds to the gradient drift wave which is extracted in the background plasma, taking into account the beam contribution. The results reveal that the gradient drift instability is experienced where the gradients of density and temperature of electrons stand in the opposite directions, and in addition, the gradients act as destabilization effects. Regarding the beam contribution, when the electron beams propagate in the direction of the magnetic field, the parallel and perpendicular components of velocity and the Langmuir frequency of the rotating beams can induce stabilization effects on the unstable inhomogeneous configuration. However, as a considerable achievement, the mentioned stabilization effects vanish for the perpendicular velocity component lower than a certain threshold value. In addition, the destabilization effects of the characteristic parameters of the counterstreaming beams are observed as well, when the positron beams propagate in the direction of the magnetic field.

Turbulence and Microprocesses in Inhomogeneous Solar Wind Plasmas

The random density fluctuations observed in the solar wind plasma crucially influence on the Langmuir wave turbulence generated by energetic electron beams ejected during solar bursts. Those are powerful phenomena consisting of a chain of successive processes leading ultimately to strong electromagnetic emissions. The small-scale processes governing the interactions between the waves, the beams and the inhomogeneous plasmas need to be studied to explain such macroscopic phenomena. Moreover, the complexity induced by the plasma irregularities requires to find new approaches and modelling. Therefore theoretical and numerical tools were built to describe the Langmuir wave turbulence and the beam’s dynamics in inhomogeneous plasmas, in the form of a self-consistent Hamiltonian model including a fluid description for the plasma and a kinetic approach for the beam. On this basis, numerical simulations were performed in order to shed light on the impact of the density fluctuations on the beam dynamics, the electromagnetic wave radiation, the generation of Langmuir wave turbulence, the waves’ coupling and decay phenomena involving Langmuir and low frequency waves, the acceleration of beam electrons, their diffusion mechanisms, the modulation of the Langmuir waveforms and the statistical properties of the radiated fields’ distributions. The paper presents the main results obtained in the form of a review.

On the Theory of Beam-Plasma Instability in the Nonstationary Plasma

The beam instability in the plasma with density slowly varying with time has been investigated using the potential approximation. It has been shown that a frequency shift occurs in the amplified signal propagating in such an electrodynamic system; analytical relations for determining the value of this shift have been obtained. It is demonstrated that the frequency is shifted most strongly as the plasma density decreases, when the Langmuir frequency of plasma electrons approaches the signal frequency and the instability increment increases with time.

Extraordinary SEAWs under influence of the spin-spin interaction and the quantum Bohm potential

The separate spin evolution (SSE) of electrons causes the existence of the spin-electron acoustic wave. Extraordinary spin-electron acoustic waves (SEAWs) propagating perpendicular to the external magnetic field have a large contribution of the transverse electric field. Its spectrum has been studied in the quasi-classical limit at the consideration of the separate spin evolution. The spin-spin interaction and the quantum Bohm potential give contribution in the spectrum extraordinary SEAWs. This contribution is studied in this paper. Moreover, it is demonstrated that the spin-spin interaction leads to the existence of the extraordinary SEAWs if the SSE is neglected. It has been found that the SSE causes the instability of the extraordinary SEAW at the large wavelengths, but the quantum Bohm potential leads to the full stabilization of the spectrum.

Nonlinear theory for axisymmetric self-similar two-dimensional oscillations of electrons in cold plasma with constant proton background

We consider simultaneous oscillations of electrons moving both along the axis of symmetry and also in the direction perpendicular to the axis. We derive a system of three nonlinear ordinary differential equations which describe self-similar oscillations of cold electrons in a constant proton density background (np = n0 = constant). These three equations represent an exact class of solutions. For weak nonlinear conditions, the frequency spectra of electric field oscillations exhibit split frequency behavior at the Langmuir frequency ωp0 and its harmonics, as well as presence of difference frequencies at low spectral values. For strong nonlinear conditions, the spectra contain peaks at frequencies with values ωp0(n+m2), where n and m are integer numbers (positive and negative). We predict that both spectral types (weak and strong) should be observed in plasmas where axial symmetry may exist. To illustrate possible applications of our theory, we present a spectrum of electric field oscillations observed in situ in the solar wind by the WAVES experiment on the Wind spacecraft during the passage of a type III solar radio burst.