Electrostatic Ion Cyclotron Mode Research Articles

Plasma–neutral coupling allows electrostatic ion cyclotron waves to propagate below ion cyclotron frequency

The effect of ion–neutral collisions on the propagation characteristics of electrostatic ion cyclotron (EIC) waves in a partially ionized plasma is investigated. The dispersion relation of EIC waves is derived using a fluid model taking neutral dynamics into account. The propagation properties of EIC modes, including the damping factor, are examined for various ionization degrees and collision frequencies, which determine the momentum transferred from ions to neutral particles. It is found that the motion of neutral particles driven by plasma–neutral coupling leads to an increase in the effective ion mass, and consequently, EIC waves can propagate even below the ion cyclotron frequency. In a hot neutral gas, the gas-thermal mode can also propagate as well as the EIC mode. The possibility of observing in the laboratory and the Earth's ionosphere is discussed.

NSEE Yielding Electron Temperature Measurements at the Arecibo Observatory

AbstractNarrowband stimulated electromagnetic emissions (NSEE), a component of radio emissions created during high‐frequency (HF) radiowave ionospheric modification experiments, occur within 1 kHz of the HF pump frequency. NSEE was observed and studied for the first time at the HAARP (High Frequency Active Auroral Research Program) facility in Alaska (Norin et al., 2009, https://doi.org/10.1103/PhysRevLett.102.065003; Bernhardt et al., 2010, https://doi.org/10.1103/PhysRevLett.104.165004). Magnetized stimulated Brillouin scatter (MSBS) is a component of NSEE, which was also first observed at HAARP (Norin et al., 2009, https://doi.org/10.1103/PhysRevLett.102.065003; Bernhardt et al., 2010, https://doi.org/10.1103/PhysRevLett.104.165004) and later at European Incoherent Scatter Scientific Association (Fu et al., 2015, https://doi.org/10.5194/angeo-33-983-2015). Ion‐acoustic and electrostatic ion cyclotron modes are the daughter products of MSBS and can be used for the determination of the electron temperature and of the presence of minor ion species, respectively, in the HF‐modified ionosphere. Here we present the first observations of the MSBS process at magnetic midlatitudes, excited during radio wave ionospheric modification experiments at the Arecibo Observatory. The NSEE observations, in combination with a theoretical model and the wave matching conditions, are used to estimate background ionospheric parameters. A qualitative comparison of the MSBS component of the NSEE spectrum with the thermal ion line measured by incoherent scatter radar is presented.

Electrostatic ion-cyclotron waves in a plasma with heavy negative ions

Results of a laboratory study of electrostatic ion-cyclotron (EIC) waves in a plasma containing K + (39 amu) positive ions, electrons and C 7F 14 − (350 amu) negative ions are presented. Excitation of the fundamental and higher harmonic light and heavy ion EIC modes was observed. The presence of heavy negative ions in the plasma has a significant effect on the excitation of the light ion EIC modes. The results may be relevant to the understanding of plasma wave properties in plasmas containing negative ions, such as those found in the Earth's ionosphere, the solar system, and, in particular, near Saturn's moon Titan, where an abundance of heavy negative ion species has recently been discovered [Coates, et al., 2007. Discovery of heavy negative ions in Titan's ionosphere. Geophys. Res. Lett. 34, L22103].

Generation of “spiky” potential structures associated with multiharmonic electrostatic ion-cyclotron waves

The production of coherent, “spiky” electrostatic potential structures, similar to the spiky electric-field structures that have been observed in the Earth’s auroral region, is investigated in a magnetized Q machine plasma. These structures are associated with coherent multiharmonic electrostatic ion-cyclotron (EIC) waves in a current-free plasma having a localized region of ion flow shear (ion flow parallel to B with a gradient in the direction transverse to B). A multiharmonic EIC spectrum is detected in the region of ion flow shear when broadband electrostatic white noise is applied to the plasma column from an antenna. The time series of the potential fluctuations exhibit spiky structures with repetition rates near the ion-cyclotron frequency. Spiky structures can arise from linear combinations of phase-locked multiharmonic EIC waves when the amplitudes of the harmonic components are comparable to that of the fundamental EIC mode.

Velocity-shear-induced ion-cyclotron turbulence: Laboratory identification and space applications

Laboratory measurements are reported that identify a new class of plasma oscillation driven by the inhomogeneity in wave energy density caused by transverse-velocity shear [Ganguli et al., Phys. Fluids 31, 823 (1988)]. The experiments concentrate on a branch of oscillation in the ion-cyclotron range of frequencies that results from the coupling of the magnetic-field-aligned current and the inhomogeneous dc electric field localized in a layer thicker than the ion gyroradius. The observed transition between the well-known current-driven electrostatic ion-cyclotron mode and this inhomogeneous energy–density-driven mode is related to the ratio of the azimuthal and axial Doppler shifts. The mode characteristics associated with the two instabilities have significantly different properties. For conditions of large transverse-velocity shear, turbulence is generated with a broadband, spiky spectrum around the ion-cyclotron frequency at small values of the magnetic-field-aligned current. The experimental identification is reinforced with numerical results from a nonlocal eigenvalue condition.

The significance of off-resonance terms for ion-cyclotron modes at lower harmonics in weakly-ionized plasmas with Te/Ti

A method of approximate analytical evaluation of ionic off-resonances terms in the dispersion equation for ion-cyclotron waves, appropriate for lower-order harmonics and for Td/Ti<or=10, is presented. The significance of these terms for the quasi-perpendicular electrostatic ion-cyclotron modes and instabilities in the longwave range, in a weakly-ionized Maxwellian plasma with one and two ion species and with electrons drifting along magnetic lines of forces, is demonstrated. It is shown that the inclusion of the off-resonance terms alters the forms of spectra, lowers the threshold drifts and augments the critical wavelengths at which the instabilities set in. Moreover, for some values of Te/Ti of the order of excitation of harmonics is reversed, as the second harmonic is found to have a smaller threshold drift than the first one.

Laboratory simulations of suprauroral mechanisms leading to perpendicular ion heating and conic formation

Laboratory experiments are presented simulating aspects of perpendicular ion heating and conic formation that are observed or hypothesized to occur in the terrestrial ionosphere and magnetosphere. Previous laboratory observations of ion conics in the presence of the current‐driven electrostatic ion cyclotron wave are reviewed. Field‐aligned ion beams, accompanied by beam‐generated electrostatic ion cyclotron modes, resulted in perpendicular energization of beam ions and also the heating of background plasma ions. Antenna‐launched broadband and narrow‐band lower hybrid waves produced considerable perpendicular ion heating and non‐Maxwellian “tail” formation. Laboratory results are discussed in light of in situ measurements by the S3‐3 satellite and the MARIE sounding rocket.

Low‐frequency oscillations associated with a polar cap auroral arc: Local spectral density estimation

Enhanced low‐frequency electrostatic oscillations measured from a rocket in the high‐latitude upper F region during the crossing of a polar cap auroral arc are analyzed using local spectral analysis techniques. The turbulent spectra were reduced from measurements made by Langmuir probes whose orientation remained approximately perpendicular to B throughout the flight. The increased low‐frequency turbulence appears at an altitude of 400 km, coincident with electrostatic analyzer indications of increased energetic particle flux. Langmuir probe measurements simultaneously show an increasing temperature and generally decreasing, although irregular, electron density profiles. Electrostatic wave data are analyzed using cross‐correlation analysis to determine phase shift; local wave number approximations provide an estimate of the plasma wave spectral density, S(k, ω). Theoretical expectations for threshold excitation and growth rates of possible instabilities in a two‐component high‐latitude F region ionosphere are investigated. The most likely source for the measured oscillations appears to be excitation of a collisionless electrostatic ion cyclotron mode.

Parametric instabilities associated with intense electron cyclotron heating in the MTX tokamak

The paper presents a survey of possible parametric instabilities in the upcoming Microwave Tokamak Experiment (MTX) at the Lawrence Livermore National Laboratory in which an intense pulsed freeelectron laser will be used to carry out electron cyclotron heating in the Alcator C tokamak. Potentially the most dangerous instability is stimulated Brillouin backscatter by electrostatic ion cyclotron modes or quasi-modes for ordinary-mode heating at the fundamental harmonic or extraordinary-mode heating at the second electron cyclotron harmonic. Large reflectances may occur for coherent microwave sources. In addition, extraordinary waves at the amplitudes considered here can be non-linearly absorbed by parametric decay processes at the resonance layers where the pump frequency equals the second electron cyclotron harmonic and where it equals twice the local upper hybrid frequency. Finite-bandwidth effects can significantly inhibit parametric instabilities. Density fluctuations induced by drift wave turbulence at the plasma surface, with amplitudes as observed in earlier Alcator C experiments, are expected to spread the electron cyclotron wave packets but may be of insufficient amplitude to suppress Brillouin backscatter instability.

Parametric decay during lower hybrid heating on a tokamak

Injection of radio-frequency power at twice the lower hybrid frequency results in parametric decay of the pump wave into electrostatic ion-cyclotron modes. The location of the conversion point is inferred from the data.

Characteristic Oscillations in a Cylindrical Plasma with the Ion Larmor Radius

A charge neutral cylindrical plasma with the radius of the order of the ion Larmor radius is considered, and eigen frequencies are calculated for an arbitrary normalized density ω p i 2 /ω c i 2 of up to 10 3 . It is found that there exist the electrostatic ion cyclotron modes with frequencies between ω c i and 2 ω c i .

Quasilinear theory of suprathermal ions in mirror machines in the presence of radio-frequency fluctuations

A theory of fast-ion generation in the 2XIIB mirror experiment due to a single resonant electrostatic ion-cyclotron mode is presented. Assuming weak radial gradients of the magnetic induction, which appear to be consistent with experimental data, it is shown that ions with small parallel excursions can be accelerated into the 40 keV region. Allowance for a periodic temporal variation of the amplitude of the mode results in stochastic motion of those ions whose range of parallel excursions form a limited but significant fraction of the plasma length. The diffusion coefficients of this class of ions are computed in the quasilinear approximation. The corresponding diffusion times are in reasonable accord with those from the experiment.

Characteristic Oscillations in Sheet Plasmas

Electrostatic eigenmodes in a sheet plasma of thickness of the order of the ion Larmor radius ρ i have been studied numerically. The frequency range considered is in the ion cyclotron regime, where the ion Bernstein mode and the electrostatic ion cyclotron mode can be excited. The method is based on an integral equation in the wavenumber space derived from the Vlasov-Poisson system of equations and on the direct computation of the integral equation. Eigenfrequencies and eigenfunctions are studied systematically by varying the plasma thickness, the plasma density, the electron-to-ion temperature ratio and the parallel wavenumber. Unique features for the case of the ideal sheet plasma, where the plasma thickness is equal to ρ i are disclosed and explained by a simple physical model, whereas for a plasma of thickness 5ρ i the eigenfrequencies are nearly equal to those of a uniform plasma. Results are compared with experimental results.

Lower hybrid acceleration and ion evolution in the suprauroral region

It is shown that ions can be accelerated perpendicularly to the magnetic field lines by resonant interactions with lower hybrid modes. Taking into account the effects of the magnetic field inhomogeneity, we demonstrate that the accelerated portion of the ions of ionospheric origin in the suprauroral region of the magnetosphere can evolve into conic distributions and propagate upwards along the field lines. Thus these ions can reach the region where they can be strongly energized along the field lines by the electrostatic shocks. We can argue that the ion distribution resulting from the combination of these accelerated ions with the background ions can depart significantly from a thermal distribution and lead to the excitation of electrostatic ion cyclotron modes.

Upper-Hybrid Wave Collapse

It is shown that the upper-hybrid mode can collapse into thin magnetic-field--aligned filaments as a result of coupling to the electrostatic ion cyclotron mode.

Ion heating by strong electrostatic ion cyclotron turbulence

A theory of the ion heating due to electrostatic ion cyclotron (EIC) waves in the auroral zone is presented. Due to the slowly convecting nature of the EIC mode, quasi‐linear plateau formation cannot stabilize the waves, and growth occurs until the nonlinear mechanisms of ion resonance broadening and electron trapping provide saturation. The large amplitude and coherent nature of the resulting wave imply that quasi‐linear theory provides only a lower limit to the ion heating. An upper bound on the heating rate is derived using a time‐average model of ion dynamics in the coherent waves. The effects of ion heating in the presence of the magnetic gradient force and parallel electric fields are considered, with the result that perpendicular energies over 100 eV are easily attainable from a 1 eV source plasma. Perpendicular heating in the absence of a parallel electric field yields conical ion distributions, which in the presence of an electric field become field‐aligned beams.

Magnetic field‐aligned potential drops due to electrostatic ion cyclotron turbulence

Measurements of DC and AC electric fields and plasma densities on the S3‐3 satellite have shown that electrostatic ion cyclotron (EIC) turbulence is associated with significant DC electric fields parallel to auroral field lines. The EIC mode has been identified in regions of strong DC electric fields and magnetic field aligned currents by AC electric field and density fluctuations with frequency components between the local hydrogen gyrofrequency and its harmonics, and by wave vector polarization primarily perpendicular to the magnetic field at wavelengths of an ion gyroradius. The anomalous resistivity has been computed for measured AC electric fields of 50 mV/m in the EIC mode, yielding ∼1 mV/m DC parallel electric fields for measured currents of ∼10 µA/m². This theoretical result implies that several kV potential drops will occur if the anomalous resistivity region extends over several thousand kilometers along auroral field lines. This is adequate to accelerate auroral particles to observed energies and consistent with the scale height over which parallel electric fields and EIC turbulence are statistically observed on S3‐3.

Experimental and theoretical investigation of waves and resonances of a bounded hot electron, cold ion magnetized plasma

A plasma column is produced by diffusion along a steady magnetic field by a helical microwave discharge source located at one end of a linear machine. Waves in the plasma are electrostatically excited by a split-cylindrical, slow-wave structure. Movable well-balanced RF double probes are used to measure RF electric field amplitude as a function of frequency. This frequency domain includes the Trivelpiece-Gould (T-G) mode (fLH<f<fpe), surface wave mode, electrostatic ion cyclotron mode (fci<f<fLH) and ion-acoustic mode (f<or approximately=fci). Experimentally observed resonances and dispersion curves of the bounded plasma system are in good agreement with theory for ion-acoustic waves, surface waves and T-G mode.

Parametric excitation by magnetosonic waves in a multispecies plasma

A magnetosonic pump wave propagating in a plasma with two ion species decays into two electrostatic ion-cyclotron modes at a threshold significantly less than those needed to excite parametric or modified two-stream instabilities in a single-species plasma when Te/Ti &amp;gt; 1. The pump wave frequency must somewhat exceed the sum of the ion gyrofrequencies.

Dispersion Measurements of Electrostatic Ion Waves in a Uniform Magnetic Field

The phase and group velocities of electrostatic ion waves in a uniform magnetic field have been studied experimentally, both as a function of propagation angle (θ) with respect to the field, and as a function of wave frequency. For ω ≫ ωci, the ion cyclotron frequency, the phase velocity is found to be identical to the ion acoustic velocity (KTe/M)1/2. For ω ≳ ωci, the electrostatic ion cyclotron mode has been observed. The slow mode (ω &amp;lt; ωci cos θ) has been observed to have a constant phase velocity (KTe/M)1/2 cos θ, in apparent contradiction to theory. The frequency gap (ohgr;ci cos θ &amp;lt; ω &amp;lt; ωci), where waves theoretically do not propagate, has been demonstrated.