Excitation Of Lower Hybrid Waves Research Articles

Electron Heat Flux Instabilities in the Inner Heliosphere: Radial Distribution and Implication on the Evolution of the Electron Velocity Distribution Function

Abstract This Letter investigates the electron heat flux instability using the radial models of the magnetic field and plasma parameters in the inner heliosphere. Our results show that both the electron acoustic wave and the oblique whistler wave are unstable in the regime with large relative drift speed (ΔV e ) between electron beam and core populations. Landau-resonant interactions of electron acoustic waves increase the electron parallel temperature that would lead to suppressing the electron acoustic instability and amplifying the growth of oblique whistler waves. Therefore, we propose that the electron heat flux can effectively drive oblique whistler waves in an anisotropic electron velocity distribution function. This study also finds that lower-hybrid waves and oblique Alfvén waves can be triggered in the solar atmosphere, and that the former instability is much stronger than the latter. Moreover, we clarify that the excitation of lower-hybrid waves mainly results from the transit-time interaction of beaming electrons with resonant velocities v ∥ ∼ ω/k ∥, where ω and k ∥ are the wave frequency and parallel wavenumber, respectively. In addition, this study shows that the instability of quasi-parallel whistler waves can dominate the regime with medium ΔV e at the heliocentric distance nearly larger than 10 times of the solar radius.

Excitation of lower hybrid wave by counterpropagating cosh Gaussian laser beams in a magnetized plasma

This paper presents a scheme for the excitation of lower hybrid wave (LHW) by beating of two counterpropagating cosh Gaussian (ChG) laser beams in a magnetized plasma. The beating wave having frequency ω=(ω1-ω2) and wave vector k→=k→1+k→2, exerts a pondermotive force on electrons and drive lower hybrid wave in both homogeneous and inhomogeneous plasma. The driver wave acts as a pump wave for the excitation of the lower hybrid wave. We study the variation of the normalized potential and power distribution with respect to normalized frequency under resonance conditions. The appearance of peaks shows the excitation of the lower hybrid waves. The decentred parameter and beamwidth profile of ChG laser beams play an important role in shifting the characteristics of peaks. In results, we also discuss the convective losses, escaping, and trapping conditions of waves in the magnetized plasma.

Excitation of lower hybrid wave by an ion beam in magnetized plasma – Reinvestigated

Study of various kinds of waves propagating through partially or fully ionized plasma has been of great interest for scientists. In this direction, theoretical and experimental studies have been carried out and results have been published in series of papers from time to time. Excitation of lower hybrid wave by an ion beam in magnetized plasma has been considered by Prakash et al. [11]. We have looked into that work and have found several major discrepancies. Therefore, the results of that paper may not be reliable.

Magneto-thermal reconnection processes, related mode momentum and formation of high energy particle populations

In the context of a two-fluid theory of magnetic reconnection, when the longitudinal electron thermal conductivity is relatively large, the perturbed electron temperature tends to become singular in the presence of a reconnected field component and an electron temperature gradient. A finite transverse thermal diffusivity removes this singularity while a finite ‘inductivity’ can remove the singularity of the relevant plasma displacement. Then (i) a new ‘magneto-thermal’ reconnection producing mode, is found with characteristic widths of the reconnection layer remaining significant even when the macroscopic distances involved are very large; (ii) the mode phase velocities can be both in the direction of the electron diamagnetic velocity as well in the opposite (ion) direction. A numerical solution of the complete set of equations has been carried out with a simplified analytical reformulation of the problem. A sequence of processes is analyzed to point out that high-energy particle populations can be produced as a result of reconnection events. These involve mode-particle resonances transferring energy of the reconnecting mode to a superthermal ion population and the excitation of lower hybrid waves that can lead to a significant superthermal electron population. The same modes excited in axisymmetric (e.g. toroidal) confinement configurations can extract angular momentum from the main body of the plasma column and thereby sustain a local ‘spontaneous rotation’ of it.

Excitation of lower hybrid wave by an ion beam in magnetized plasma

AbstractLower hybrid wave excitation in magnetized plasma by an ion beam via Cerenkov interaction is studied. The lower hybrid modes showed maximum growth rate of the instability when phase velocity of the lower hybrid mode along the magnetic field is comparable to the electron thermal velocity. We have derived the expression for the maximum growth rate and found that the growth rate of the instability increases with beam density. Moreover, the maximum growth rate of the instability scales as the one-third power of the beam density. The real part of the frequency of the unstable wave increases as almost the square root of the beam energy.

Excitation of lower hybrid waves by a gyrating ion beam in a negative ion plasma

A gyrating ion beam propagating through a magnetized plasma cylinder containing K+ positive ions, electrons, and SF6− negative ions drives electrostatic lower hybrid waves to instability via Cyclotron interaction. Numerical calculations of the unstable mode frequencies and growth rates of both the unstable positive ion and negative ion modes have been carried out for the existing negative ion plasma parameters. It is found that the unstable mode frequencies of both the modes increase, with the relative density of negative ions. In addition, the growth rates of both the unstable modes also increases with relative density of negative ions. Moreover, the growth rates of both the unstable modes scale as the one-third power of the beam density. The frequencies of both the unstable modes also increase with the magnetic fields. The real part of the unstable wave frequency increases as almost the square root of the beam energy.

Lower hybrid wave excitation and propagation in the spherical tokamak Globus-M

The problem of lower hybrid (LH) wave excitation and current drive (CD) in tokamaks with a small aspect ratio (spherical tokamaks) is discussed. It is proposed to solve this problem by exciting the waves slowed down in the poloidal rather than the toroidal direction. As a result, due to the strong poloidal inhomogeneity of the magnetic field in spherical tokamaks, even the waves with comparatively weak slowing down (N ≈ 3–5) excited by a waveguide antenna in the equatorial plane can penetrate into the dense plasma and be absorbed via the Landau mechanism. This approach was applied for modeling the LHCD experiments in the low aspect ratio tokamak Globus-M (R = 0.36 m, a0 = 0.24 m, B0 = 0.4 T, Ip = 0.25 MA, vertical elongation k = 1.6, operating frequency 2.45 GHz). The modeling was carried out using four independent codes: (i) the self-consistent antenna coupling code GRILL3D, (ii) the ray-tracing code incorporating a specially developed ray-tracing technique including some corrections necessary in strongly inhomogeneous plasma, (iii) the 2D full-wave code WAVETOP2D and (iv) the Fokker–Plank code combined with the ray-tracing code allowing simulation of the driven current density profile. The results of simulations were cross-checked and appeared to be in a good agreement. It was demonstrated that the proposed scenario provides a possibility for the efficient LHCD in a spherical tokamak.

Excitation of lower hybrid waves by a spiraling ion beam in a magnetized dusty plasma cylinder

A spiraling ion beam propagating through a magnetized dusty plasma cylinder drives electrostatic lower hybrid waves to instability via cyclotron interaction. Numerical calculations of the growth rate and unstable mode frequencies have been carried out for the Princeton Q-1 device using the experimental dusty plasma parameters [e.g., Barkan et al., Planet. Space Sci. 43, 905 (1995)]. It is found that as the density ratio δ(=nio∕neo, where ni0 is the ion plasma density and ne0 is the electron density) of negatively charged dust grains to electrons increases, the unstable mode frequency of the lower hybrid waves increases. In addition, the growth rate of the instability also increases with the density ratio δ. In other words, the presence of negatively charged dust grains can further destabilize the lower hybrid wave instability. The growth rate has the largest value for the modes where Jl(pnro) is maximum [here pn=xn∕r0, where pn is the perpendicular wave number in cm−1, r0 is the plasma radius, and xn are the zeros of the Bessel function J1(x)] i.e., whose eigenfunctions peak at the location of the beam. The growth rate scales as one third power of the beam current.

The nonlinear coupling of electromagnetic ion cyclotron and lower hybrid waves in the ring current region: the magnetic storm 1-7May 1998

Abstract. The excitation of lower hybrid waves (LHWs) is a widely discussed mechanism of interaction between plasma species in space, and is one of the unresolved questions of magnetospheric multi-ion plasmas. In this paper we present the morphology, dynamics, and level of LHW activity generated by electromagnetic ion cyclotron (EMIC) waves during the 2-7 May 1998 storm period on the global scale. The LHWs were calculated based on a newly developed self-consistent model (Khazanov et. al., 2002) that couples the system of two kinetic equations: one equation describes the ring current (RC) ion dynamic, and another equation describes the evolution of EMIC waves. It is found that the LHWs are excited by helium ions due to their mass dependent drift in the electric field of EMIC waves. The level of LHW activity is calculated assuming that the induced scattering process is the main saturation mechanism for these waves. The calculated LHWs electric fields are consistent with the observational data.

Parametric excitation of lower hybrid waves by electron plasma waves

Parametric decay of an obliquely (to the magnetic field) propagating electron plasma wave to an obliquely propagating electron plasma sideband and a lower hybrid wave is studied. This decay process is then considered as a cascade channel for electron plasma waves to broaden their downshifted frequency spectra. The dependence of the threshold pump field on the number of cascade is determined.

Role of lower hybrid turbulence in surfing acceleration at perpendicular shocks

The mechanism for excitation of lower hybrid waves by shock surfing pickup ions is proposed. It is based on the modified two‐stream instability. The stationary distribution function of the surfing particles is calculated, which can serve as a driver for the excitation of the lower hybrid waves. The instability growth rate is found, as well as conditions under which waves have enough time to grow while being convected to the shock front together with the solar wind flow. The excited waves are responsible for the velocity diffusion of the surfing particles in the direction opposite to the acceleration. These particles are detrapped from the shock front and escape from the shock surfing acceleration. The flux of escaping particles is calculated in the quasi‐linear approximation, and it is shown to decrease quickly with the mass of the accelerated particles. This result can provide a possible explanation of Voyager observations, which indicate that the injection efficiency of interstellar pickup into the acceleration process at the solar wind termination shock increases significantly for heavy ions in comparison with hydrogen.

Erratum: “Excitation of lower hybrid waves by a density modulated electron beam in a plasma cylinder” [Phys. Plasmas 5, 3161 (1998)

Erratum: “Excitation of lower hybrid waves by a density modulated electron beam in a plasma cylinder” [Phys. Plasmas <b>5</b>, 3161 (1998)

Parametric excitation of lower hybrid waves by Z‐mode waves near electron cyclotron harmonics at Tromso

Z‐mode wave converted from O‐mode heater wave is considered as the pump wave in Tromso HF heating experiments for the parametric excitation of lower hybrid decay mode together with electron Bernstein sidebands in the region above the O‐mode reflection layer. This is the process, suggested by Mishin et al. [1997], which generates superthermal electrons by the excited lower hybrid waves. These superthermal electrons produce plasma waves in the region above the O‐mode reflection layer, with frequencies much greater than the heater wave frequency, as observed by Isham et al. [1990, 1996]. Our detailed analyses of this process are carried out for two experimental cases wherein the pump wave frequencies are near the third and fifth harmonic electron cyclotron frequencies, respectively. The results show that the minimum threshold field of the instability in either case is less than 0.2 V m−1. However, because the wavelength of the instability is also short, the collisionless dampings restrict the instability occurrence to narrow wavelength and frequency ranges. For 1 V m−1 pump field intensity, the instability can be excited in less than 1 ms. Since the excited lower hybrid waves are strongly Landau damped, superthermal electrons are efficiently generated by them.

Excitation of lower hybrid waves by a density-modulated electron beam in a plasma cylinder

A density-modulated electron beam propagating through a plasma cylinder drives electrostatic lower hybrid waves to instability via Cerenkov interaction. The lower hybrid wave instability has the largest growth rate γ when the frequency and wave number of the modulation are comparable to that of the unstable wave. The growth rate of the instability increases with the modulation index Δ and is maximized for Δ=1. For Δ=0, γ turns out to be ∼0.43×107 sec−1. The growth rate scales as the one-third power of the beam density. The real frequency of the unstable wave increases as almost the square root of the beam voltage. The results of the theory are applied to explain some of the experimental observations of Chang [Phys. Rev. Lett. 35, 285 (1975)].

Laboratory study of some lightning-induced effects in the ionospheric plasma

Laboratory experiments have been conducted at MIT, using the student-built Versatile Toroidal Facility (VTF), to investigate some ionospheric plasma effects produced by lightning-induced whistler waves. Lower hybrid waves, generated by the lightning-induced whistler waves, can cause a chain of extensive plasma effects, such as the acceleration of electrons and ions and the spectral broadening of plasma waves. Two mechanisms by which whistler waves generate lower hybrid waves can be important in the ionosphere. One is the simultaneous excitation of lower hybrid waves and low-frequency mode waves by intense whistler waves. The other is the nonlinear mode conversion of whistler waves into lower hybrid waves in the presence of short-scale field-aligned density striations. The effect of lower hybrid waves on the spectra of Langmuir waves has been investigated. The results of VTF laboratory experiments show that lower hybrid waves can beat with Langmuir waves to produce frequency-upshifted and frequency-downshifted Langmuir waves, broadening the spectra of Langmuir waves. The intensity of these beat waves, however, depends upon the angle of wave propagation with respect to the background magnetic field.

The excitation of lower hybrid waves driven by the ∇B effect in the current sheet

We have investigated the excitation of electrostatic and electromagnetic lower hybrid waves in the magnetic field reversal region of the plasma sheet using a two‐fluid approach. We consider a two‐dimensional tail configuration that includes the Bx and Bz magnetic field components and examine the nonlocal properties of lower hybrid modes near the weak field region, where the field Bx(z) reverses sign. We have shown that an electrostatic lower hybrid mode is excited by the magnetic field gradient while electromagnetic modes are driven unstable by the combined effects of the magnetic field gradient and the electron fluid drift across the Bx field. We have shown that the electrostatic and electromagnetic lower hybrid modes grow in a few milliseconds and a few tens of milliseconds, respectively, the former for wavelengths shorter than the current sheet width (that is, for kyλ ≥ 1, where λ is the current sheet width) and the latter for long wavelengths (kyλ ≪ 1).

Excitation of Lower-Hybrid Wave by Beating of Two Electron-Cyclotron Waves

The first experiment is described on the excitation of lower-hybrid wave by the beating of two extraordinary electron-cyclotron waves. Electron wave frequencies were near I GHz, the lower-hybrid wave was in the frequency range 1-3 MHz. The resonance dependence of excitation of the eat wave on the magnetic field was proved.

Generation of auroral kilometric radiation in upper hybrid wave‐lower hybrid soliton interaction

Sporadic bursts of auroral kilometric radiation (AKR) associated with strong bursty electrostatic turbulence in the vicinity of the lower hybrid frequency have been frequently recorded in the AKR source region by the Viking satellite. The variation time scale of these emissions is typically 1 s, long enough for lower hybrid waves to grow to amplitudes of several hundred millivolts per meter and to evolve nonlinearly into solitons. On the basis on these observations it is suggested that formation of lower hybrid solitons may play a role in the generation of AKR. A theoretical model is proposed which is based on the direct acceleration of electrons in the combined lower hybrid soliton and upper hybrid wave fields. The solitons act as sporadic, localized antennas allowing for efficient conversion of the electrostatic energy stored in upper hybrid waves into electromagnetic radiation at a frequency above the X mode cutoff. Excitation of lower hybrid waves is due to the presence of energetic electron beams in the auroral zone found to be associated with steep plasma density gradients. Upper hybrid waves can be excited by a population of energetic electrons with loss cone distributions. The power of the electromagnetic radiation obtained is only noticeable in regions where the plasma frequency is less than the electron gyrofrequency. The theory predicts spectral power densities of the order of 10−11 to 10−9 W m−2 Hz−1 in the source region, in good agreement with the Viking observations. The sporadic nature of the radiation derives from lower hybrid soliton collapses which occur on ∼1‐s time scales.

Excitation of lower hybrid waves by a gyrating ion beam in a magnetized plasma cylinder

A long plasma cylinder with uniform density supports lower hybrid modes. In the presence of an energetic gyrating ion beam these modes are driven unstable via cyclotron harmonic interaction. A nonlocal theory of this process has been developed. It is seen that the modes with wavefunction maximum at the location of the beam possess a large growth rate. The instability frequency is close to the ion plasma frequency.

Parametric excitation of lower hybrid waves in the vicinity of the resonance cone

Parametric instability of lower hybrid and ion sound waves in the vicinity of the resonance cone arising from a point source that oscillates at a frequency slightly higher than lower hybrid resonance, is considered. Estimates of the dimension of the effective interaction region are given.