Generation Of Ion Cyclotron Waves Research Articles

Ion-cyclotron wave generation by planetary ion pickup

Ion-cyclotron waves play important roles in planetary magnetospheres and are diagnostic of the processes operating in the magnetosphere and of the composition of the plasma producing the waves. At Jupiter, Io's exosphere interacts with the corotating magnetospheric plasma. At Saturn, the neutral torus around the E ring interacts with the corotating plasma. At the unmagnetized planets, Mars and Venus, the interaction is between the solar-wind flow and the planetary exosphere. A possible analog of these processes exists in the vicinity of the Earth's polar cusp where the shocked solar-wind plasma penetrates the Earth's exosphere.

One‐dimensional hybrid simulations of obliquely propagating ion cyclotron waves: Application to ion pickup at Io

In Jupiter and Saturn's magnetospheres, ionization of neutrals produced by the satellites and rings results in populations of newborn pickup ions with T⊥ > T∥, which are unstable to the generation of electromagnetic ion cyclotron waves. Linear dispersion analysis of this anisotropy instability finds maximum growth at parallel propagation, with decreasing growth rates at wave vector angles, θ, oblique to the ambient magnetic field, B0. Observed S+ and SO+ ion cyclotron waves near the Jovian moon, Io, propagate at a variety of angles within 60° of B0. Using one‐dimensional hybrid simulation, we study the properties of the obliquely propagating waves in the Io plasma torus environment for θ ≤ 60°. We find that the maximum growth rate decreases with increasing angle from B0, as predicted by linear theory, with the growth rate at θ = 60° equal to ∼70% the growth rate at parallel propagation. At the oblique angles, cyclotron harmonics of S+ and SO+ are excited and grow larger with θ. At θ = 60°, the fundamental mode saturation amplitude is roughly four times that of the first harmonic. However, we do not expect to see these harmonics in the Io torus because thermalized O+ and S+ damps their growth. We also find that SO+ cyclotron waves may dominate over S+ if > 2.5 – 3.

Self‐consistent model of magnetospheric ring current and propagating electromagnetic ion cyclotron waves: 2. Wave‐induced ring current precipitation and thermal electron heating

This paper continues presentation and discussion of the results from our new global self‐consistent theoretical model of interacting ring current ions and propagating electromagnetic ion cyclotron waves (Khazanov et al., 2006) currently developing in NASA Marshall Space Flight Center. To study the effects of electromagnetic ion cyclotron wave propagation and refraction on the wave induced ring current precipitation and heating of the thermal plasmaspheric electrons, we simulate the May 1998 storm. The main findings after the simulation can be summarized as follows. First, the wave induced ring current precipitation exhibits quite a lot of fine structure and is highly organized by location of the plasmapause gradient. The strongest fluxes of about 4 × 106 (cm2 s sr)−1 are observed during the main and early recovery phases of the storm. The very interesting and probably more important finding is that in a number of cases the most intense precipitating fluxes are not connected to the most intense waves in simple manner. The characteristics of the wave power spectral density distribution over the wave normal angle are extremely crucial for the effectiveness of the ring current ion scattering. Second, comparison of the global proton precipitating patterns with the results from RAM (Kozyra et al., 1997a) reveals that although we observe a qualitative agreement between the localizations of the wave induced precipitations in the models, there is no quantitative agreement between the magnitudes of the fluxes. The quantitative differences are mainly due to a qualitative difference between the characteristics of the wave power spectral density distributions over the wave normal angle in RAM and in our model. Third, the heat fluxes to plasmaspheric electrons caused by Landau resonate energy absorption from electromagnetic ion cyclotron waves are observed in the postnoon‐premidnight MLT sector and can reach the magnitude of 1011 eV/(cm2 s). The Coulomb energy degradation of the RC H+ and O+ ions maximizes at about 1011 eV/(cm2 s) and typically leads to electron energy deposition rates of about 2 × 1010 eV/(cm2 s) which are observed during two periods, 32–48 hours and 76–86 hours after 1 May, 0000 UT. The theoretically derived spatial structure of the thermal electron heating caused by interaction of the ring current with the plasmasphere is strongly supported by concurrent and conjugate plasma measurements from the plasmasphere, ring current, and topside ionosphere (Gurgiolo et al., 2005). Finally, the wave induced intense electron heating has a structure of the spot‐like patches along the most enhanced density gradients in the plasmasphere boundary layer and can be a possible driver of the observed but still not explained small‐scale structures of enhanced emissions in the stable auroral red arcs.

Interrelation of the activity of ion-cyclotron waves in the earth’s magnetosphere with parameters of the solar wind

Long series of continuous observations of ioncyclotron waves in 0.2–5.0 Hz range in the Earth’s magnetosphere detected on the Earth surface in the form of geomagnetic pulsations Pc1 were the basis for studies of cyclic variations of their activity. Comparison of the peculiarities of the cyclic variation of Pc1with the data of direct measurements of the solarwind parameters allowed one to obtain new results related to the role of the interplanetary medium in the processes of generation of ion-cyclotron waves in the Earth’s magnetosphere.

Shear-flow-driven ion cyclotron instabilities of magnetic field-aligned flow of inhomogeneous plasma

The effect of magnetic field-aligned flow with a transverse flow velocity gradient (velocity shear) on the generation of electrostatic ion cyclotron waves is studied analytically by using kinetic formalism. It is shown that flow shear not only modifies the frequency and growth rate of a known current-driven electrostatic ion cyclotron instability (CDEIC), but is the source of the development of specific kinetic and hydrodynamic shear-flow-driven ion cyclotron instabilities at the levels of current along the ambient magnetic field, which is subcritical for the development of the CDEIC instability.

Pc 1 waves and associated unstable distributions of magnetospheric protons observed during a solar wind pressure pulse

We present observations of Pc 1 waves (∼0.6 Hz) that occurred shortly after a strong (>20 nPa) compression of Earth's magnetosphere at 1321 UT, 18 March 2002. Intense Pc 1 waves were observed at several high‐latitude ground stations in Antarctica and Greenland from 1321 UT to beyond 1445 UT. Two wave bursts were recorded at the Polar satellite at 1338 and 1343–1344 UT as it passed outbound in the Southern Hemisphere at 1154 local time (SM magnetic latitude of −22° and near L = 7.5) in good magnetic conjunction with the Antarctic. The pressure increase created a significant population of protons between a few hundred eV and several keV, whose fluxes were mostly perpendicular to B. These protons seem to have replaced the quiescent stream of protons (presumably convected from the plasma sheet) that existed before this increase. There was also a nearly two‐order‐of‐magnitude increase in the population of thermal/suprathermal (0.32–410 eV) protons. The generation of ion cyclotron waves is expected to limit the proton temperature anisotropy A, defined as T⊥/T∥ − 1. The ion cyclotron instability driven by the observed hot ion temperature anisotropy is studied using two models, with and without the presence of cold background plasma. Peaks in the calculated instability as a function of time show excellent agreement with the times of the Polar wave bursts, which were measured a few tens of seconds after maxima in the instability calculation. The time delay is consistent with the propagation time to the spacecraft from a source nearer to the equatorial plane. The hot proton population at Polar appears to be driven back to stability by a sudden increase in very field‐aligned protons having energies less than the hot perpendicular population, suggesting a different source for the two populations. These observations confirm the importance of both the energization and/or increase in population of protons transverse to B in the several keV range (possibly betatron acceleration as a result of the pressure pulse), and the presence of greatly increased fluxes of lower energy protons (100s of eV to a few keV), predominantly aligned along B, in determining whether the particle population is unstable at a given time.

On Collisionless Electron‐Ion Temperature Equilibration in the Fast Solar Wind

We explore a mechanism, entirely new to the fast solar wind, of electron heating by lower hybrid waves to explain the shift to higher charge states observed in various elements in the fast wind at 1 AU relative to the original coronal hole plasma. This process is a variation on that previously discussed for two-temperature accretion flows by Begelman & Chiueh. Lower hybrid waves are generated by gyrating minor ions (mainly α-particles) and become significant once strong ion cyclotron heating sets in beyond 1.5 R☉. In this way the model avoids conflict with SUMER electron temperature diagnostic measurements between 1 and 1.5 R☉. The principal requirement for such a process to work is the existence of density gradients in the fast solar wind, with scale length of similar order to the proton inertial length. Similar size structures have previously been inferred by other authors from radio scintillation observations and considerations of ion cyclotron wave generation by global resonant MHD waves.

The kinetic shell model of coronal heating and acceleration by ion cyclotron waves: 3. The proton halo and dispersive waves

The kinetic shell model seeks to test the collisionless kinetic response of coronal hole protons to the quasi‐linear resonant cyclotron dissipation of outwardly directed, parallel propagating ion cyclotron waves, which is often invoked as the mechanism responsible for generating the fast solar wind. In this model, we make the approximation that the resonant cyclotron interaction proceeds much faster than any other process affecting the protons, so that the resonant portions of the distribution are maintained in a state of marginal stability with respect to this interaction. Under this approximation, the resonant protons are organized on nested shells of constant density in velocity space, defined by the condition that the proton energy is conserved in the rest frame moving with the phase speed of the resonant wave. These shells then evolve on the slower nonresonant timescale, responding to the shell‐averaged forces of gravity, charge‐separation electric field, mirror force, and an inertial force due to the fact that the waves are propagating in the accelerating plasma frame. Since these kinetic shell distributions are held in a marginally stable state, they cannot exchange any more energy with the waves than that taken to maintain that state. Thus their evolution corresponds to the effect of the maximum possible dissipation of outward propagating waves. Our earlier work was simplified in two major respects: we considered only dispersionless waves and we assumed that all the antisunward protons were resonantly interacting with self‐generated inward propagating waves. In this paper, we first present an improved treatment of the antisunward protons which yields more plausible speeds and temperatures than obtained previously. We then incorporate the effects of ion cyclotron dispersion into the model. Dispersion creates resonant shells which are broader in υ∥ than our previous dispersionless distributions. We find that the nonresonant forces acting on these dispersive shell distributions invariably produce only weak acceleration and result in perpendicular cooling rather than the heating that is generally required for fast solar wind speeds. These effects are attributed to the weaker inertial force on the sunward dispersive shells and are essentially independent of the behavior of the antisunward particles. Since these results correspond to the maximum possible dissipation by this mechanism, we conclude that the heating of coronal hole protons and the generation of the fast solar wind are not caused by the collisionless dissipation of parallel propagating ion cyclotron waves.

Ion cyclotron waves at Io: implications for the temporal variation of Io's atmosphere

When the flowing torus plasma encounters the upper atmosphere of Jupiter's moon, Io, newly created ions are rapidly accelerated by the motional electric field. Many of these ions are reneutralized and form a spray of fast neutrals that travel far away from Io before being reionized by photoionization and impact. These ions, now far from Io, are unstable to the generation of ion cyclotron waves. These waves in turn act as a mass spectrometer allowing Galileo magnetic measurements to be used to probe the composition of the atmosphere of Io and how it varies in time and in space. We now have six Galileo passes by Io on which we have measurements with sufficient cadence to examine the ion cyclotron waves. One of these passes, on Galileo's 32nd orbit has not been discussed previously. These passes provide sufficient observations to begin to distinguish the sources of variability. We find that while the atmosphere of Io varies temporally throughout the mission, it also has a spatial variation in composition at any instant of time.

Observations of two types of Pc 1–2 pulsations in the outer dayside magnetosphere

Analysis of high time resolution magnetometer data from the Polar satellite and from an array of high‐latitude ground stations in Antarctica has identified 20 simultaneously observed Pc 1–2 wave events in the outer dayside magnetosphere during the first 17 months of Polar operations. In contrast to most earlier satellite studies of Pc 1–2 waves, based on data from equatorial satellites near apogee which moved only slowly across L shells if at all, the initial orbit of Polar allowed it to rapidly cross outer magnetospheric L shells, but significantly away from the magnetic equator. Consistent with several previous studies of outer magnetospheric Pc 1–2 waves, the majority of these events (75%) were associated with significant compressions of the magnetosphere. Seven of the events occurred simultaneously in satellite and ground data, with wave bursts temporally associated with compressions. These events, most at L values > 9, confirm the suggestion of Anderson and Hamilton [1993] that the outer dayside magnetosphere is often near marginal stability for electromagnetic ion cyclotron (EMIC) wave generation, so waves can be stimulated by even modest magnetospheric compressions. However, 10 of the wave events (five associated with compressions, and five not) were highly localized in L shell. In these “spatially localized” cases, continuous wave emissions were seen on the ground for extended periods of time, while in space waves were observed for only a few minutes and occurred only in radially narrow regions. The noncompressional events, all spatially localized, appear to be the first examples identified in space of the class of wave events known as Pc 1–2, sustained narrowband emissions which have durations at high‐latitude ground stations of the order of 10 hours in the day and dusk local time sectors. All 10 of the spatially localized events, whether compressional or not, followed intervals of at least 12 hours of sustained very quiet magnetospheric conditions. Energetic ion observations from Polar confirm earlier suggestions that drifting plasma sheet ions are the source of dayside Pc 1–2 waves in the outer dayside magnetosphere, but they also show different particle configurations for the spatially localized and temporally ordered event categories. Events in both categories occurred within radially extended regions with ring‐like, moderately anisotropic distributions of ≥5 keV protons and with deep minima in the flux distributions at energies <5 keV. However, spatially localized wave events occurred only in association with radially localized regions that also contained highly anisotropic fluxes of ∼0.5–3 keV protons, at a considerable distance from the magnetospheric boundary. In contrast, no such radial structure was evident in any of the temporally ordered events, or in three “uncertain” events. The association of the spatially localized events with highly structured populations of plasma sheet protons of keV and higher energies indicates an important but unanticipated role for these protons in destabilizing plasma in the outer dayside magnetosphere, possibly through increasing the local plasma beta near the magnetic equator.

Generation of electromagnetic ion cyclotron waves in the ionosphere by localized transverse dc electric fields

Localized dc electric fields directed perpendicular to the geomagnetic field are studied as a possible ionospheric source of electromagnetic ion cyclotron waves. The eigenvalue formulation of the problem is solved numerically and a class of unstable electromagnetic ion cyclotron eigenmodes are found that have properties consistent with some observations of broadband waves near auroral arcs. The frequency range of instability is comparable to the spectral width measured from photometer observations of broadband auroral flickering. Eigenmodes are associated with electric field polarization reversals and E/B ratios similar to that encountered by satellites and sounding rockets traversing auroral arcs. The dc electric field affects ion resonance conditions and allows for resonant interaction within the extent of the eigenfunction.

The kinetic shell model of coronal heating and acceleration by ion cyclotron waves: 2. Inward and outward propagating waves

We extend the kinetic shell model of the cyclotron resonant interaction between coronal hole protons and outward propagating ion cyclotron waves presented in the first paper of this series [Isenberg et al., 2001]. That work showed that the resonant dissipation of outward propagating waves produced proton distributions that were unstable to the generation of inward propagating waves. Here we include the kinetic shell interaction with the inward waves, assuming that both the wave generation and wave dissipation proceed much faster than all other processes in the collisionless coronal hole. In this case, the entire proton distribution will be resonant with one set of waves or the other and is thus composed of constant‐energy shells in both the sunward and antisunward regions of velocity space. The evolution of the distribution as the plasma flows away from the Sun is then described by following the motion of these shells under the action of the nonresonant forces in the coronal hole. We find that the distribution consists of a core population which circulates through velocity space and a halo which continually expands to higher energy. The halo population is shown to be essential to obtain acceleration of the bulk proton plasma through its response to the mirror force. We present an illustrative calculation of this system which assumes the waves to be dispersionless. We find for this case that the halo particles soon reach extremely high energies, leading to a continuous, rather than declining, acceleration of the plasma. We suggest that these properties are due to the dispersionless assumption and that an improved model incorporating wave dispersion may give more reasonable quantitative results.

The Io mass‐loading disk: Constraints provided by ion cyclotron wave observations

Galileo measurements on the first two encounters with Io have revealed that ion cyclotron waves associated with SO2+ and SO+ can extend to at least 20 Io radii (RIo) on the anti‐Jupiter side of Io but to only ∼7 RIo on the Jupiter facing side. In the flow direction the waves extend downstream at least 10 RIo, but not upstream of Io. Earlier Voyager observations, 11 RIo below Io, also show no evidence for the generation of ion cyclotron waves. Quite unlike expectations from growth rate calculations in a uniform infinite plasma, the waves are frequently found to be propagating at an oblique angle to the magnetic field. The absence of waves upstream of Io and the wide dispersal of the ions creating the waves inward and outward from Io are consistent with a mass‐loading disk created by a short period of acceleration of the ions by the Jovian corotation electric field followed by reneutralization, producing a spray of SO, SO2, S, and possibly H2S across the magnetic field similar to the spray proposed for the neutral sodium cloud by Wilson and Schneider [1999]. When these fast neutrals are reionized farther from Io, they lead to wave growth.

Generation of Ion Cyclotron Waves in Coronal Holes by a Global Resonant Magnetohydrodynamic Mode

The damping of ion cyclotron waves is a process that can provide effective coronal heating and solar wind acceleration. However, the mechanism of generation of these waves in the solar corona remains unclear. We suggest that the ion cyclotron waves in coronal holes are excited by plasma instability that is driven by current fluctuations of a global resonant magnetohydrodynamic (MHD) mode. The frequency of the MHD mode is much lower than the proton gyrofrequency, so that its direct cyclotron damping is absent. At the same time, this frequency must be high enough to effectively generate the instability. We discuss the sources of such MHD waves in the coronal holes. The generated ion cyclotron waves are highly oblique with respect to the background magnetic field. We present qualitative arguments that these waves contribute to coronal heating and solar wind acceleration in a manner similar to that of the parallel-propagating waves usually used to explain the heating and acceleration. The scales of spatial inhomogeneity suggested by the observations and the amplitudes of the magnetic field fluctuations put limitations on the currents associated with MHD waves. We show that even with these limitations the current can still be large enough to drive the instability.

Is the plasmapause a preferred source region of electromagnetic ion cyclotron waves in the magnetosphere?

It has generally been assumed in the past that the plasmapause is a preferred region for the generation and propagation of electromagnetic ion cyclotron (EMIC) waves in the Earth's magnetosphere. One assumption invoked the overlap of the expanding cold plasmapause with the inner edge of the hot ring current during storm recovery to provide favourable conditions for EMIC instability. The plasmapause was also expected to provide a convenient gradient for guiding the waves from equatorial sources to higher latitudes. The paper commences from a historic perspective and reviews the development of the ideas from the 1960s that relate the source of EMIC waves to the plasmapause. CRRES spacecraft observations of EMIC wave events over L = 3.5 –8 and associated plasmapause locations indicate that the plasmapause is a region of wave generation and propagation, with significant wave power seen in the plasmatrough, but is not necessarily the preferred region. Other results show that wave occurrence predominates in the afternoon and increases with radial distance and a He + slot is seen in the data. These agree with earlier AMPTE-CCE results from Anderson et al. (J. Geophys. Res. 97 (1992) 3075, 3089). New results from CRRES show all wave polarisations (left-hand, linear, and right-hand) are seen within 8° of the equator whereas linear predominates over 20–30° latitude. Waves are observed in background plasma densities of 4– 300 cm −3 . Wave frequencies above the He + cyclotron frequency are concentrated outside the plasmapause in lower density regions of 2– 30 cm −3 .

Ion-cyclotron waves at Io

The massloading of the near Io torus by sulfur dioxide leads to the generation of intense ion cyclotron waves, near the singly ionized sulfur dioxide ion gyro frequency, that have been detected by the Galileo magnetometer. While other ions are produced near Io, the Io torus plasma contains nearly isotropic Maxwellian distributions of these ions that damp any waves near their ion cyclotron frequencies. Since SO2+ dissociates rapidly in the Io torus environment, there is no background of damping ions at the SO2+ ion gyro frequency. To zeroth order these waves grow at the ion cyclotron frequency, are left-hand circularly polarized and propagate along the magnetic field. In actuality the waves grow over a finite bandwidth, propagate at an angle to the field and are elliptically polarized. At the upper edge of the band of wave growth the waves propagate at their greatest angles to the magnetic field, and are more elliptically polarized. In fact in some regions the waves become slightly right-handed. These polarizations may arise due to the propagation of the waves in an increasing field strength in a multi ion environment. Finally, we examine the data obtained by the Voyager spacecraft along the “same” field line as Galileo but 16.5 years earlier and 12 RIo southward of Io. No ion cyclotron waves are seen here or anywhere else along the Voyager Io passage. If we assume that ion cyclotron waves were generated at the same strength in 1979, then the waves must have been significantly attenuated by propagation or absorption.

Fast magnetosonic and fast Alfvén waves in an elliptic plasma cylinder

The propagation of ion-cyclotron waves in an extended cylindrical plasma is analyzed analytically. The results obtained show progress in constructing a two-dimensionally-nonuniform analytical theory.

Influence of particle trapping on the propagation of ion cyclotron waves

The influence of particle trapping on the nonlinear dynamics of ion cyclotron waves is investigated. It is shown that in the case when the frequency of externally applied electromagnetic waves is close to that of the ion cyclotron waves, the nonlinear coupling between the ion cyclotron waves and the resulting longitudinal perturbations is mainly caused by the pondermotive potential of ions. Due to the trapping of particles, the characteristics of the propagation of the stationary ion cyclotron waves, as well as the condition for their existence, change. Under some circumstances there is no possibility for their propagation. Depending on the amplitude, oscillatory and localized shapes are possible. Trapping of particles makes the characteristic length of modulation larger than the case when it is neglected.

Three-dimensional observations of electromagnetic ion cyclotron wave propagation in a laboratory plasma column

The propagation of electromagnetic ion cyclotron waves as launched by an antenna of small transverse scale has been studied in a laboratory plasma. Effects of the finite perpendicular size of the plasma column on the wave magnetic field pattern are observed, and their analysis is aided by visualizing the wave fields in three dimensions.

On the generation and propagation of auroral electromagnetic ion cyclotron waves

On a recent sounding rocket flight, electromagnetic ELF waves below the proton gyrofrequency were detected in an auroral arc. These waves would be resonant with ƒcH+ or ƒcO+ in the auroral acceleration region; the latter waves are associated with modulated parallel electron fluxes. Following previous papers, we use homogeneous linear theory to calculate temporal and convective growth rates for electron beam‐driven electromagnetic ion cyclotron (EMIC) waves in various plasmas with H+, He+, and O+ ions, although the inhomogeneity in the source region makes this approximation questionable. We find that all three EMIC modes are unstable with growth rates inversely proportional to the mass of the ion associated with the mode; maximum convective growth occurs at frequencies of 0.8–1.0Ωi. However, the growth rate of the O+ EMIC mode is too low to account for the observations unless relatively large beam densities are used, implying that nonlinear or inhomogeneous plasma effects must play an important role in the instability. We also assess the propagation of these EMIC waves in a cold plasma. Our ray tracing calculations show that propagation effects can explain both the localization of the O+ waves and the absence of He+ waves at lower altitudes. We also show that the wider latitudinal spread and low power spectral density of the H+ waves can also be explained by propagation effects, but the H+ rays which can reach the ionosphere are not the rays for which the beam‐driven instability produces the highest growth rates in a homogeneous plasma.