Low-frequency Plasma Waves Research Articles

Low-frequency waves in a high-beta collisionless plasma: polarization, compressibility and helicity

This paper considers the linear theory of waves near and below the ion cyclotron frequency in an isothermal electron-ion Vlasov plasma which is isotropic, homogeneous and magnetized. Numerical solutions of the full dispersion equation for the magnetosonic/whistler and Alfvén/ion cyclotron modes at βi = 1·0 are presented, and the polarizations, compressibilities, helicities, ion Alfvén ratios and ion cross-helicities are exhibited and compared. At sufficiently large βi and θ, the angle of propagation with respect to the magnetic field, the real part of the polarization of the Alfvén/ion cyclotron wave changes sign, so that, for such parameters, this mode is no longer left-hand polarized. The Alfvén/ion cyclotron mode becomes more compressive as the wavenumber ulereases, whereas the magnetosonic/whistler becomes more compressive with increasing θ, At oblique propagation, the helicity of both modes approaches zero in the long-wavelength limit; in contrast, the ion cross-helicity is of order unity for the Alfvén/ion cyclotron wave and decreases as θ increases for the magnetosonic/whistler mode.

Extremely-low-frequency plasma waves in the environment of comet Halley

The low-frequency plasma wave detector (APV-N) carried by the Vega 1 and Vega 2 spacecraft during their encounter with comet Halley registered an enhanced plasma-wave intensity identified with the inbound crossing of a quasi-perpendicular (Vega 1) or quasi-parallel (Vega 2) cometary bow shock. We also describe the characteristics of the plasma waves excited in the process of anomalously fast ionization of the cometary atmosphere in the inner coma.

Instability of low-frequency waves in a cold plasma mixed with hot electrons

The instability of low-frequency waves in a cold plasma mixed with hot electrons is investigated using the first-order CGL equations for electrons. It is assumed that in an equilibrium state the electrons consist of two components, cold electrons and hot electrons with bi-Maxwellians. It is shown that low-frequency waves with right-hand polarization can be generated by the hot electron temperature anisotropy and the existence of cold electrons.

Alfvén-ion cyclotron instability in the presence of high-frequency turbulence

The dynamics of low-frequency electromagnetic waves in a turbulent homogeneous collision-less plasma is studied. Using this formalism, the stabilization of left-hand circularly polarized Alfvén-ion cyclotron instability due to Langmuir turbulence is analysed. The analysis is carried out using standard techniques of weak-turbulence theory. It is found that the electromagnetic wave dispersion properties change significantly in the presence of turbulence.

Observations of the electric fields that accelerate auroral particles

Electric fields accelerate electrons and ions in the auroral zone at altitudes below 8000 km to produce several distinctive particle distributions. The electric field of electrostatic shocks and double layers produces the inverted-V precipitating electron and up-flowing ion beams. Electrostatic ion cyclotron waves heat ion beams. The electric field in low frequency plasma waves and electrostatic shocks produces ion conics and field-aligned or counterstreaming electrons. Relationships between electric fields and particle distributions are illustrated with data from the S3-3 satellite.

Low-frequency plasma waves near Saturn

Voyager planetary radio astronomy observations of low frequency emissions detected around the time of closest approach to Saturn and near the outbound ring plane crossing are presented. Near the ring plane an electron density of between 5 and 20 electrons cm−3 at distances of ∼6Rs is estimated.

Effect of magnetic field errors on low-frequency waves in a pure electron plasma

It is demonstrated how static magnetic field errors can resonantly drive low-frequency electrostatic waves in a pure electron plasma column. The driving mechanism is identified as a charge bunching that results from the field error. It is found that the wave amplitude increases linearly with time.

Nonlinear ion-acoustic waves and solitons in a magnetized plasma

A unified formulation is presented to study the nonlinear low-frequency electrostatic waves in a magnetized low-b plasma. It is found that there exist three types of nonlinear waves; (i) nonlinear ion-cyclotron periodic waves with a wave speed Vp≳Cs (ion-acoustic velocity); (ii) nonlinear ion-acoustic periodic waves with Vp<Cs cosq; and (iii) ion-acoustic solitons with Cs cosq<Vp<Cs, where q is the angle between the wave vector and the magnetic field.

Valid approximations of the dispersion equation

The problem of deriving accurate long-wavelength approximations of a particular dispersion equation for electromagnetic waves in a magnetized plasma is examined. Because the dispersion equation includes dependence upon external parameters, the approximation obtained by straightforward expansion is invalid in certain cases. An accurate approximation is derived in these cases by a modified expansion technique. Although only one specific practical problem is studied here, this example involves and exhibits general techniques that have broad application in related problems. Some features of long-wavelength low-frequency waves in a magnetized high-temperature plasma are illustrated.

Effects of Low Frequency Plasma Waves on Counter-Streaming Electron Beam Instability

Experimental investigations are made of a high frequency wave at one half the electron cyclotron frequency fee/2 excited by counter streaming electron beams in a magnetoplasma. It is found that the time averaged amplitude of the high frequency wave is decreased by modulating the beam externally at a certain lower frequency. The result is explained qualitatively by considering that the low frequency wave causes a periodic change in the growth rate of the high frequency wave.

Experimental Study of Excitation of Low Frequency Plasma Waves by Amplitude-Modulated Microwave

Effective excitation of plasma waves by an amplitude-modulated microwave (4.4 GHz, 1 kW) is presented in a highly ionized magnetoplasma. The electric field of the excited waves which is proportional to the square of the microwave field has the maximum value of 30 V/cm. The ponderomotive force may be mainly responsible for the excitation. The excited three waves seem to be, in the experimental frequency range at least, a drift wave, a transition mode wave from the shear Alfvén to the electron-cyclotron waves and a low frequency electro-acoustic wave.

Parametric amplification of Alfven waves

The present analysis has been made to investigate the effects of finite resistivity on the amplification of low frequency plasma waves, e.g. an Alfven wave due to a time dependent modulating magnetic field. It is found that the bandwidth of frequencies for which amplification occurs increases due to finite resistivity. The degree of modulation should exceed a critical value for amplification to occur. The growth rate decreases due to damping. The problem has been solved analytically.

Absolute parametric instability of low-frequency waves in a nonuniform anisotropic plasma

Absolute parametric instability of low-frequency waves in a nonuniform anisotropic plasma

Parametric instability of very-low-frequency waves in the upper ionosphere

The parametric instability of quasimonochromatic VLF waves in the upper ionosphere is considered. It is shown that the process of scattering of the initial signal into a low-frequency plasma wave as well as the process of deeay into a low-frequency plasma wave and an ioncyclotron wave take place very efficiently in an isothermal (T e ,-, T i) plasma. The parametric interaction of two whistlers and an ion-cyclotron wave are also considered. Such processes may be responsible for excitation of the lower hybrid resonance in the ionosphere and magnetosphere of the earth.

Parametric excitation of drift waves by ion-ion hybrid fields

The behaviour of low-frequency drift waves in a two-species plasma under the influence of a large-amplitude spatially uniform electric field oscillating at the ion-ion hybrid frequency and applied across an ambient static magnetic field has been studied. The threshold powers for the parametric excitation of these drift waves are calculated and the characteristics of the dispersion relation near threshold and well above threshold are examined. At large applied powers (much above threshold), interesting modifications of the natural drift modes are found.

The effect of negative ions on the plasma-sheath boundary†

A stable positive ion sheath can form on a negative surface exposed to an electron-positive ion plasma only when the ions have an adequate initial component of velocity directed into the sheath at the boundary. It is known that if the plasma contains negative ions these behave in this connection like a second electron group, usually of low mean energy. This result is generalized to apply to conditions including those in the negative end of a cold-cathode discharge in an electronegative gas. It is pointed out that the directed ion velocity at the boundary on elementary theory is not in general equal to the limiting speed of low-frequency ion waves in the electronegative plasma. The equation is raised as to whether the Landau damping of the waves has any counterpart in the properties of the sheath-plasma boundary.

Low-Frequency Beam-Plasma Interactions in a Finite-Sized Plasma

Results are presented for an experimental and theoretical investigation of low-frequency waves in a beam-generated plasma. Waves are excited at frequencies immediately above the ion-plasma frequency by sinusoidally modulating the electron beam current. The study is aimed at obtaining ion heating at the lower-hybrid resonant frequency. An electron beam of 400-1000 V energy and a few milliamperes of current is used to generate plasmas with densities in the range of 109 cm−3 in a magnetic field of a few hundred gauss. Hydrogen, argon, deuterium, and neon gases are used. The rf electric field is probed as a function of r, z, and frequency, and energetic ions are detected by a gridded probe. Two or three resonances are found in the probe response above ωpi, and are shown to be multiple half-wavelength resonances of standing plasma waveguide axisymmetric modes. A normal-mode analysis is used to calculate the probe responses and is found to correctly predict the effects of varying beam voltage, plasma density, ion mass, and dc axial magnetic field. The excited wave may be viewed as an electrostatic wave propagating at an angle very close to 90 deg from the magnetic field.

Propagation of Strong-Field Modulated Electromagnetic Waves in Plasmas

The distortion of the wave form of a modulated plane electromagnetic wave propagating in an anisotropic plasma has been experimentally investigated over a range of field strengths of the wave and plasma properties. By using right-hand circularly polarized waves, the effective frequency is [Formula: see text] (where ω is the r.f. radian frequency and ωb the cyclotron frequency) and hence the results are also applicable to the propagation of low-frequency waves in an isotropic plasma. Severe "overmodulation" of the wave form transmitted through the plasma is found in the regime [Formula: see text] where ν is the effective collision frequency for momentum transfer. The distortion of the wave form is found to increase with depth of modulation of the incident wave and decrease with increasing modulation frequency.The "demodulation" is in qualitative agreement with theory for an unmodulated wave with a non-Maxwellian (Druyvesteyn) velocity distribution for the electrons. Many of the effects of the modulation frequencies can also be qualitatively predicted by considering the variation of electron temperature in the presence of the strong-field modulated wave. A theory is developed for large changes in electron temperature induced by the incident field which shows that marked distortion of the modulation is possible.

Thermal Conductivity and Low Frequency Waves in Collisional Plasmas

Thermal fluctuations and heat flow terms are retained in the two-fluid equations which are used to derive a localized dispersion relation for low-frequency electrostatic waves in a nonuniform collisional plasma. A slab plasma immersed in a uniform magnetic field is assumed. The Nyquist analysis shows two unstable waves: the collisional drift instability and a new overstability of the entropy wave. The collisional drift wave is most easily excited for parallel wavenumbers given by k‖2 = 0.15 α2(me/mi)1/2 where α ≡ |≇ In n| (all quantities in cgs units). The stability condition at this k| is B/k⊥ < 104 (min/κTe|α|)1/2 G-cm. These are in markedly improved agreement with the experimental data. Heat transport reduces the maximum growth rate, especially at high density. A small equilibrium temperature gradient tends to stabilize the drift waves in the large magnetic field region. The entropywave instability propagates along ion drift direction and is found in the range 0.4 ≤ d¯ ≤ 2 with d¯ ≈ 0.15 (k⊥κ2Ti/Ωi2mi)2νiiνeime/k‖κ2Te. The maximum growth rate of entropy wave is roughly k‖(κTi/mi)1/2(k⊥κ2Te/miΩi2)(mi/me)1/4, and occurs around d¯ = 0.6 ∼ 1The ion thermal conductivity and temperature gradient are not sigificant for both instabilities.

Anomalous Collisionless Damping of Alf vén Waves in Inhomogeneous Plasmas

It is shown that ALFVÉN waves in a collisionless plasma, permeated by an inhomogeneous magnetic field, are affected by a strong anomalous damping. This anomalous collisionless damping seems to correspond to collision-free LANDAU damping in homogeneous magnetic fields in a similar way as the anomalous viscous and OHMic damping of ALFVÉN waves propagating in inhomogeneous magnetic fields 1 corresponds to the ordinary collisional viscous and OHMic damping of ALFVÉN waves propagating in homogeneous magnetic fields. It is found that the predicted collision-free anomalous damping will obey a law of the form exp{ — (t/τ)2} where the damping time τ depends on the wave-length, the temperature of the plasma, and the magnetic field gradient. Starting from a simplified BOLTZMANN-VLASOV equation, valid for low frequency plasma waves 2, a LAPLACE transform is carried out with regard to the time dependence and an integral equation is derived for the mean plasma velocity. By the faltung theorem this integral equation can be FOURIER transformed into a differential equation for which a W.K.B. solution can be obtained. The inverse transforms can be carried out by making a simple analytic approximation for the W.K.B. solution which is valid over a large range of values. The predicted anomalous damping of ALFVÉN waves is so strong that it may be useful for the purpose of heating a plasma up to high temperatures within a short time.