Ion Cyclotron Fluctuations Research Articles

Coexistence of Weak and Strong Wave Turbulence in Incompressible Hall Magnetohydrodynamics

We report a numerical investigation of three dimensional, incompressible, Hall magnetohydrodynamic turbulence with a relatively strong mean magnetic field. Using helicity decomposition and cross-bicoherence analysis, we observe that the resonant three--wave coupling is substantial among ion cyclotron and whistler waves. A detailed study of the degree of non-linearity of these two populations shows that the ion cyclotron component experiences a transition from weak to strong wave turbulence going from large to small scales, while the whistler fluctuations display a weak wave turbulence character for all scales. This non-trivial coexistence of the two regimes with the two populations of waves gives rise to anomalous anisotropy and scaling properties. The weak and strong wave turbulence components can be distinguished rather efficiently using spatio-temporal Fourier transforms. The analysis shows that while resonant triadic interactions survive the highly non-linear bath of ion cyclotron fluctuations at large scales for which the degree of non-linearity is low for both populations of waves, whistler waves tend to be killed by the non-linear cross-coupling at smaller scales where the ion cyclotron component is in the strong wave turbulent regime. Such situation may have far-reaching implications for the physics of magnetized turbulence in many astrophysical and space plasmas where different waves coexist and compete to transfer non-linearly energy across scales.

Relativistic electron scattering by electromagnetic ion cyclotron fluctuations: Test particle simulations

Relativistic electron scattering by electromagnetic ion cyclotron (EMIC) fluctuations is studied using test particle computations coupled to the results of a hybrid simulation code. The enhanced EMIC fluctuations are derived from a 1‐D, self‐consistent hybrid simulation model and are due to the growth of the Alfvén cyclotron instability driven by the ion temperature anisotropy, Ti⊥ > Ti∥ (where the subscripts ⊥ and ∥ refer to directions perpendicular and parallel to the background magnetic field, respectively), in a magnetized, homogeneous, collisionless plasma with a single ion species. The test particle computations follow the motion of relativistic test electrons in the input EMIC fluctuations. The time evolution of the mean square pitch angle change of the test electrons is calculated and used to determine the pitch angle diffusion coefficient. Finally, the results are compared with quasi‐linear diffusion theory. The diffusion coefficients given by the test particle computations agree with the ones from quasi‐linear theory very well except for large‐amplitude waves (δB/B0 ≥ 0.03 in the case presented, where δB is the wave magnetic field amplitude and B0 is the background magnetic field) when the weak turbulence approximation in quasi‐linear theory breaks down. Quasi‐linear theory overestimates the pitch angle diffusion coefficient for large‐amplitude waves and may, consequently, overestimate the pitch angle diffusion of relativistic electrons in the radiation belts at high L values.

On the development and evolution of nonlinear ion acoustic wave packets

Abstract. A simple model of ion fluctuations (ion acoustic and ion cyclotron fluctuations for example) driven by an electron current which leads to intermittent fluctuations when the linear growth rate exceeds the wave packet dispersion rate is analized. The normalized fluctuation amplitude eφ0/T can be much larger than the mass ratio (me/mi) level predicted by the conventional quasilinear theory or Manheimer's theory (see references in this document), and where φ0 represents the amplitude of the main peak of the ion fluctuations. Although the ion motion is linear, intermittency is produced by the strong nonlinear electron response, which causes the electron momentum input to the ion fluctuations to be spatially localized. We treat the 1-D case because it is especially simple from an intuitive and analytical point of view, but it is readily apparent and one can put forward the conjecture that the effect occurs in a three dimensional magnetized plasma. The 1-D analysis, as shown in this manuscript will clearly help identify the subtle difference between turbulence as conventionally understood and intermittency as it occurs in space and laboratory plasmas. Keywords. Meteorology and atmospheric dynamics (Turbulence) – Ionosphere (Wave-particles interactions) – Space plasma physics (Waves and instabilities)

Effects of Alfven lon Cyclotron Modes on Tandem Mirror Energy Transport.

An energy transport model taking into account the effects of Alfven ion cyclotron fluctuations is presented to explain the hot-ion-mode experiments of tandem mirror GAMMA 10. The energy transport equations for the electron and ion including the ion temperature relaxation due to the quasilinear effects of unstable Alfven ion cyclotron modes are derived based on a point model and are solved numerically for the steady state. It is shown that the numerical results obtained from the present model can well explain the experimental results in the hot-ion-mode of GAMMA 10.

Ion cyclotron modes in a two‐ion‐component plasma with transverse‐velocity shear

Electrostatic ion cyclotron fluctuations driven by combination of a magnetic‐field‐aligned electron current and a localized transverse electric field in two‐ion‐component plasmas are investigated through the use of nonlocal linear theory for a collisionless plasma. The emphasis is on the stability properties of these modes as a function of relative concentration of plasma components. For various characteristic parameters, such as the spatial dimension of the inhomogeneous electric field profile, magnitude of the E × B velocity, magnitude of the field‐aligned current, and temperature ratio τ = Ti/Te, it is shown that in contrast to the homogeneous case of the current‐driven ion cyclotron mode, the ion cyclotron modes in the presence of a transverse‐velocity shear can be unstable in a significantly wider range of relative concentration ratios. Moreover, positive growth rates are possible for relatively small and even zero values of field‐aligned current in the inhomogeneous electric field case. The mode characteristics in the two cases differ significantly. For example, in the inhomogeneous case, the frequency spectrum is broadband including frequencies much smaller than ion gyrofrequency. The relevance of these results to ionospheric observations is discussed.

Electrostatic ion-cyclotron wave experiments in the WVU Q machine

The influence of transverse, localized, DC electric fields (TLEs) on the current-driven electrostatic ion-cyclotron (CDEIC) instability is being investigated in a Q machine. A small (diameter approximately 10 ion gyroradii) segmented disk electrode is being used to excite the mode in a narrow electron-current channel along which exists a radial electric field between regions that magnetically map to the different circular segments (separated by a radial gap of approximately 3 ion gyroradii). Experiments aimed at demonstrating a TLE dependence in the threshold current for mode excitation are described. A comparison of observed and predicted mode frequencies over a range of magnetic field strengths is presented for the benchmark case of no applied transverse electric field. When the electric field is present, ion-cyclotron fluctuations are observed for cases in which the current is below the CDEIC instability threshold. >

Observations on period-doubling phenomena in a basic plasma experiment

Observations on period-doubling phenomena by exciting lower hybrid and ion cyclotron fluctuations in the presence of rf power in a plasma experiment are reported. The Feigenbaum number δ is calculated for the first two bifurcations and found to be 4.138. Nonlinear nature of the ion cyclotron oscillations gives rise to the interaction between modes that ultimately generate higher harmonics of the initial frequencies.

Nonlinear scattering of whistlers by electrostatic fluctuations

It is found that nonlinear coupling of whistler waves with ion-cyclotron fluctuations can lead to spiky whistler electric-field envelopes. Analytical results for the latter, which are solutions with discontinuous derivatives, are given. Possible application of our results in solar-wind plasmas is discussed.

Operation of the Tandem-Mirror Plasma Experiment with Skew Neutral-Beam Injection

Tandem-mirror plasma with sloshing-ion end plugs has been produced. Ion-velocity distribution functions are peaked at 47\ifmmode^\circ\else\textdegree\fi{} to the magnetic field lines. Low-level ion-cyclotron fluctuations were detected at 1.8 times the plug-midplane frequency and with properties predicted by theory; they did not degrade plasma confinement in the plugs or central cell.

Observation of Alfvén Ion-Cyclotron Fluctuations in the End-Cell Plasma in the Tandem Mirror Experiment

Ion-cyclotron fluctuations in the tandem mirror experiment have wave characteristics consistent with the Alfv\'en ion-cyclotron instability. The oscillations are coherent, have small azimuthal mode numbers, and have frequencies as low as 12% below the diamagnetically depressed, minimum ion-cyclotron frequency in the end cell. Polarization is in the direction of ion gyration, and azimuthal propagation is in either the electron or the ion diamagnetic direction, with the former prevailing.

Thermal fluctuations and the diffuse electrostatic emissions

Persistent, weak‐banded electrostatic emissions with µV/m amplitudes have been observed in the magnetosphere at frequencies between the electron gyrofrequency ωce and upper hybrid frequency ωuh when ωce < ωuh. The suggestion that these low‐level emissions are thermal fluctuations is quantitatively explored by using a simple hot‐cold model for dayside electron distributions with density and temperature ratios nc/nh ≫ 1 and Th/Tc ≫ 1. We conclude that for typical dayside conditions the observed waves are weakly damped Bernstein‐Harris modes whose spectral density 〈E²(f)〉 ∼ 10−16V²/m²‐Hz and polarization perpendicular to the ambient magnetic field can be accounted for by the theory. The electrostatic power spectrum of the waves scales as ωce²Th/Tc3/2 for a Maxwellian hot component but may be enhanced considerably for the case of marginally stable or nonconvective modes. Wavelengths scale as the cold electron gyroradius ρc so that significant attenuation of the measured spectrum may occur when measurements are made with antennas of characteristic lengths l ∼ ρc. Representative fluctuation spectrums are computed for dayside sychronous orbit by using typical hot‐cold plasma parameters and hypothetical dipole antenna lengths of 50 and 100 m. It is suggested that the formalism developed here may be applicable to weak electron cyclotron emissions at Jupiter and Saturn and may be extended to lower‐frequency ion cyclotron fluctuations by modifying the theory to include ion contributions.

Tandem mirror confinement in the presence of ion cyclotron fluctuations

A theoretical model is developed for the effect of an electrostatic ion-cyclotron wave on central-cell ion confinement in a tandem mirror. Ion heating occurs in the central cell where the wave and local ion-cyclotron frequencies are equal. Coulomb pitch-angle collisions then scatter these ions into the loss region of velocity space. The wave heating is calculated analytically from the single-particle equations of motion, and the effect of collisions is modelled numerically by using a Monte-Carlo technique. A formula is obtained for the degradation of the ion-confinement time as a function of the RF wave amplitude. This formula reproduces the experimental results from the Livermore Tandem Mirror Experiment (TMX) as they are presented in a companion paper. The effect of the wave on density and temperature profiles is also calculated.

Prompt striations in ionospheric barium clouds due to a velocity space instability

The linear theory of electrostatic instabilities of the drift loss‐cone type has been developed for a velocity distribution appropriate to the shaped‐charge barium injection experiment Buaro. The dispersion relation for the instability in a Vlasov plasma has been solved numerically. The results show a low‐frequency instability that we interpret as the source of prompt striations observed in Buaro and cyclotron instabilities that appear to correspond to observed ion cyclotron fluctuations. These instabilities have much faster growth rates than drift instabilities of fluid theories and therefore provide a better description of the observations.