Decay of electrostatic ion cyclotron waves

A 50‐MHz Doppler radar interferometer and a 138‐MHz Doppler radar were operated from Kennedy Space Center to study 3‐m and 1‐m plasma waves associated with two shaped‐charged barium releases from Wallops Island, Virginia, on May 13, 1986. During the first release, interferometer and Doppler power spectral studies showed the existence of short‐lived (<2 s) coherent 3‐m and 1‐m waves centered near 30 Hz. The 3‐m waves were located about 100 km above the release on a magnetic field line in contact with the first release. No radar backscatter was observed by the radars during the second release. The coherent 30‐Hz radar echoes associated with the first barium release are very similar to some natural occurring auroral radar echoes that are believed to be generated by resistive current‐driven electrostatic ion cyclotron waves. A detailed linear dispersion analysis of electrostatic ion cyclotron (EIC) waves showed that only 3‐m O+ EIC waves were unstable for field‐aligned electron drifts greater than 0.7υthe at the altitude of 510 km in a multispecies (O+, NO+, or similarly O2+) ionospheric plasma. We interpret the 30‐Hz waves seen by the two radars far above the release as strong electrostatic ion cyclotron waves generated by intense field‐aligned currents associated with the barium stream acting like an MHD generator coupled to the ionosphere.

Emissions that appear to have been electrostatic ion cyclotron (EIC) waves have been observed at low altitude in the diffuse aurora by a sounding rocket payload. The rocket was launched from Poker Flat, Alaska, at ∼2030 MLT. The flight successively traversed ∼70 km of the diffuse aurora, a dark region, and a quiet 40 kR auroral arc. In the diffuse aurora, peaks were observed in the power spectrum of the electric field at frequencies near the hydrogen and oxygen ion cyclotron frequencies. Doppler shift and polarization analyses have been performed using EIC wave spectrum parameters derived from linear theory. Both analyses indicated that these emissions had properties consistent with those expected for H+ and O+ EIC waves. Taken together, the two analyses indicated that both emission bands were due to waves propagating both up and down the field line and eastward parallel to the poleward boundary of the diffuse aurora. The large local cold plasma density and resulting large Landau damping require that the source be local. Free energy for the waves was apparently available in the 5 µA/m² downward parallel current density which was inferred from the magnetometer data. The presence of the waves indicates that this current was being carried by less than 2% of the plasma, presumably in the form of a field aligned beam of electrons with energies of a few eV.

Theoretical and experimental studies of low-frequency electrostatic waves in plasmas containing negatively charged dust grains are described. The presence of charged dust is shown to modify the properties of ion-acoustic waves and electrostatic ion-cyclotron waves through the quasineutrality condition even though the dust grains do not participate in the wave dynamics. If the dust dynamics is included in the analysis, new “dust modes” appear—dust acoustic and dust cyclotron modes. The results of laboratory experiments dealing with dust ion acoustic (DIA) waves and electrostatic dust ion cyclotron (EDIC) waves are shown. These modes are more easily excited in a plasma containing negatively charged dust. Finally, observations of dust acoustic (DA) waves are presented and measurements of the dispersion relation are compared with one obtained from fluid theory.

The excitation of electrostatic ion cyclotron (EIC) waves is studied in a single-ended Q machine in a two-ion component plasma (Ca/sup +/ and K/sup +/). Over a large range of relative concentrations of Ca/sup +/ and K/sup +/ ions, two modes are excited with frequencies greater than the respective cyclotron frequencies of the ions. The results are discussed in terms of a fluid theory of electrostatic ion cyclotron waves in a two-ion component plasma.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Emissions that have properties consistent with electrostatic ion cyclotron (EIC) waves have been observed at low altitude in the diffuse aurora by a sounding rocket payload. Peaks were observed in the power spectrum of the electric field near the hydrogen and oxygen ion cyclotron frequencies. Doppler shift and polarization analyses have been performed using EIC wave parameters derived from linear theory. Both analyses indicated that these emissions had properties consistent with those expected for H+ and O+ EIC waves. The two analyses indicated that both emission bands were due to waves propagating eastward parallel to the poleward boundary of the diffuse aurora. The large local cold plasma density and resulting Landau damping require that the source be local. Magnetometer data indicated the presence of a downward parallel current density of 5μA/m². Sufficient free energy for the waves was available from this current, although the waves were observed frequently at altitudes where the ion‐neutral collision frequency exceeded the oxygen cyclotron frequency.

We have studied the instability of electrostatic ion-cyclotron waves in collisional magnetized two-ion component plasma (light positive K + ions and heavy positive Cs + ions). An ion beam propagating through collisional magnetized plasma containing electrons and two positive ion components drives electrostatic ion cyclotron (EIC) waves to instability via Cerenkov interaction. Analytical expressions & numerical calculations have been carried out for the frequency and growth rate of ion cyclotron waves for two EIC wave modes for existing experimental parameters, and it is found that the unstable mode frequency does not depend on electron collision frequency, while the growth rate is increased linearly with the electron collision frequency. Moreover, as the light ion concentration is increased, the frequency of the heavy ion mode moves closer to its gyrofrequency. Similarly, the frequency of the light ion mode approaches the light ion cyclotron frequency as the heavy ion concentration is increased. It is also found that the normalized unstable mode frequencies remains unchanged with electron collision frequencies, while the growth rate is increased linearly with the electron collision frequencies. In addition, the unstable mode frequencies are found to be dependent on the magnetic field strengths.

The coupling of electrostatic ion cyclotron and ion acoustic waves is examined in three component magnetized plasma consisting of electrons, protons, and alpha particles. In the theoretical model relevant to solar wind plasma, electrons are assumed to be superthermal with kappa distribution and protons as well as alpha particles follow the fluid dynamical equations. A general linear dispersion relation is derived for such a plasma system which is analyzed both analytically and numerically. For parallel propagation, electrostatic ion cyclotron (proton and helium cyclotron) and ion acoustic (slow and fast) modes are decoupled. For oblique propagation, coupling between the cyclotron and acoustic modes occurs. Furthermore, when the angle of propagation is increased, the separation between acoustic and cyclotron modes increases which is an indication of weaker coupling at large angle of propagation. For perpendicular propagation, only cyclotron modes are observed. The effect of various parameters such as number density and temperature of alpha particles and superthermality on dispersion characteristics is examined in details. The coupling between various modes occurs for small values of wavenumber.

The effects of neutral‐particle collisions on current‐driven electrostatic ion cyclotron (EIC) waves are studied in a Q machine with a cesium (Cs+) plasma. We find that even when νin ≈ 0.3Ωci, EIC waves of substantial amplitude (Δn/n of several percent) can be excited.

Associated with the large heat conduction in the solar wind is a skewing of the ion and electron distribution functions. It is shown that this collisional skewing of the electron distribution function can linearly excite collisionless ion-acoustic, electrostatic ion cyclotron, magnetoacoustic, and ion cyclotron waves in the steady-state solar wind even though the net equilibrium current parallel to B is zero. The initial growth rates for these unstable waves are derived, and the effectiveness of the wave-particle interactions in heating the ions and in altering the thermal and electrical conductivities is discussed.

Results are presented of a particle simulation study of the electrostatic ion-cyclotron (EIC) instability driven by a parallel ion beam. The results of this simulation study demonstrate the nonlinear consequences of nonresonant EIC waves destabilized by an ion beam parallel to the magnetic field for the case of a large beam velocity. As a consequence of the instability, it is shown that the beam ions are heated strongly in the perpendicular direction and suffer a strong anomalous friction via EIC waves which leads to the beam slowing down. Simulation results indicate that the anomalous slowing down of beam ions by EIC waves is much larger than that from the classical electron-drag, and the perpendicular collision frequency measured from perpendicular beam heating is as large as that from Bohm diffusion. It is concluded that the ion beam driven EIC wave is a viable mechanism for the transfer of ion parallel beam energy to the ion perpendicular energy.

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.

We show the existence of a twisted electrostatic ion-cyclotron (ESIC) wave carrying orbital angular momentum (OAM) in a magnetized dusty plasma. For our purposes, we derive a 3D wave equation for the coupled ESIC and dust ion-acoustic (DIA) waves from the hydrodynamic equations that are composed of the continuity and momentum equations, together with Poisson's equation. The 3D wave equation reveals the formation of a braided or twisted ESIC wave structure carrying OAM. The braided or twisted ESIC wave structure can trap and transport plasma particles in magnetoplasmas, such as those in Saturn's F-ring and in the forthcoming magnetized dusty plasma experiments.

The seasonal and solar cycle variations of the vertical distribution of the occurrence probability of auroral kilometric radiation (AKR) sources and of upflowing ion (UFI) events have been investigated based on 13 years of plasma wave data and 8 years of energetic ion data observed by the Akebono satellite. In the summer polar region, the peak of the vertical distribution of the occurrence probability of AKR sources, identified by an intensity larger than −150 dB W/m2 Hz, is at an altitude higher than 5000 km with a value of 10% in the solar maximum period and at an altitude of 5000–6000 km with a value of 40% in the solar minimum period. In the winter polar region, the peak altitudes and values are 4000–5000 km and 50% in the solar maximum period, and 4000 km and 60% in the solar minimum period, respectively. Based on the analysis of wave polarization data, intense R‐X mode AKR is responsible for these variations. Furthermore, it has been also revealed that the vertical distribution of the occurrence probability of UFI events, identified by an ion velocity distribution function with a peak above 1 keV, also shows similar seasonal and solar cycle variations as the AKR sources. In both seasonal and solar cycle variations, the AKR and UFI occurrence probabilities decrease with increasing solar EUV ionization in the ionosphere. Therefore, ionospheric seasonal and solar cycle variations are inferred to be responsible for the AKR and UFI variations. As for the control mechanism, upwelling plasma from the ionosphere to the exosphere, up to an altitude of about 5000 km, are expected to change the temperature and density of the background plasma and control unstable conditions of plasma waves like ion acoustic waves and electrostatic ion cyclotron (EIC) waves, which generate the auroral particle acceleration regions.

Mesoscale models for the plasmasphere refilling reveal that the expansion of the ionospheric plasma into empty flux tubes during the early stage refilling produces highly supersonic counterstreaming ion beams. Such beams are found to meet the conditions for the instability of the electrostatic ion cyclotron (eic) waves in the equatorial plasmasphere. Since the spatial scale of the eic wave instability is determined by the ion Larmor radius and the corresponding timescale is determined by the ion cyclotron period, it is unresolvable by the crude spatial and temporal resolutions afforded by the mesoscale models. Thus the eic waves and the associated microprocesses are left out by the existing mesoscale refilling models based on the fluid and semikinetic approaches. I propose here an algorithm for incorporating the effects of eic waves on mesoscale plasma flow; the algorithm is based on the critical conditions for the eic instability. When the relative drift of the counterstreamig ion beams exceeds a critical value, depending on the beam temperatures, the excess momentum and energy are dissipated into thermal motion of the ions, resulting in ion heating in the degree of freedom perpendicular to the ambient magnetic field at the expense of the parallel ion drift. This leads to the creation of two distinct ion populations consisting of field‐aligned counterstreaming ions and perpendicularly heated warm anisotropic ions trapped in the flux tube. The latter ions are created during the first few hours of the refilling process primarily in the equatorial region, but eventually they spread out all over the flux tube. Highly anisotropic ions are seen near the equator by virtue of being locally created and trapped there. Estimates of the Coulomb collision time show that the warm anisotropic population (WAP) has a lifetime ranging from about 10 hours in the topside ionosphere to several days in the equatorial region. Thus once initially created during early stage refilling, WAP ions become a common feature of the outer plasmasphere. The coexisting multiple ion populations, consisting of anisotropic warm ions and the field‐aligned counterstreaming flows, have been observed from DE 1 on flux tubes undergoing refilling.

Small‐scale regions of large electric fields have been observed above the auroral zone by the S3‐3 satellite. The data from five such electrostatic shocks have been examined in as great a detail as is available. The three higher altitude shocks (all above 5700‐km altitude) were associated with upward‐going ion beams, indicating that the potential associated with the shock closed below the satellite to give rise to the parallel electric field required for the acceleration of the ion beam. Electrostatic ion cyclotron waves were found in all these cases adjacent to the shock and extending throughout the upward‐going ion beam region. The lack of noticeable Doppler shift in the electrostatic ion cyclotron waves in association with large convective drift velocities indicates that the wavelength of the electrostatic ion cyclotron wave can be several kilometers and the potential difference within the wave can be the order of 100 V. Regions having anisotropic electron distributions with the signature of parallel electric field acceleration close to but above the satellite were found on the other side of the shock from the ion beam region in two of these examples. Both these regions were associated with V shaped VLF hiss. In contrast, lower altitude shocks were associated with VLF saucers and conical ion distributions. The association of the lower altitude shocks with VLF saucers indicates that the low‐altitude shock region is a permanent feature on the scale of tens of seconds and that the low‐altitude shock region extends along the magnetic field. Since they occur in different current regions, the low‐altitude shocks described here are not merely the low altitude extension of the kind of higher altitude shocks associated with ion beams. Electrostatic shocks are always embedded in broader regions of low frequency turbulence. The shock itself is a complex low‐frequency structure that, if interpreted as a Doppler shift effect, implies a complicated small scale spatial structure. The important theoretical question of whether the parallel electric field inside the electrostatic shock exists over a large region of space parallel to the magnetic field or is confined to a narrow double layer or similar structure is not answered. The data are consistent with both possibilities.