An explanation for experimental observations of harmonic cyclotron emission induced by fast ions (original) (raw)
Related papers
Ion cyclotron harmonic wave generation by ring protons in space plasmas
Journal of Geophysical Research, 1993
Spacecraft such as the Active Magnetospheric Particle Tracer Explorers, Giotto and the Combined Release and Radiation Effects Spacecraft have shown that suprathermal protons with ringlike or shell-like distributions in velocity space exist in many space plasmas. Examples include the radiation belts, the auroral zones, the bow shock, and the solar wind. Ring proton distributions may excite obliquely propagating fast Alfv4n waves at harmonics of the ion cyclotron frequency. In this paper we carry out an analytical study of this instability, restricting our attention to strictly perpendicular propagation. In the case of a monoenergetic proton ring in a cold plasma, we show that cyclotron harmonics can have a higher growth rate than parallel-propagating ion cyclotron waves, depending on the ratio of the proton ring speed to the Alfv•n speed. Analytical stability boundaries in parameter space are determined, indicating that the threshold for the growth of cyclotron harmonics depends critically on the ion plasma beta, •3i. If •3i • 1, for example in the radiation belts or the auroral zones, the concentration of ring protons required for instability is very low. If, on the other hand, •3i ~ 1, for example in the bow shock or the solar wind, instability will only occur if the ring protons constitute a large fraction of the total ion density. In the bow shock case, the required concentrations may indeed occur. Growth rates are also calculated for the case of a ring proton distribution with a Gaussian spread of velocities, and it is shown that the instability persists even if the velocity dispersion is comparable to the ring speed itself. Our analysis is consistent with the observed excitation of perpendicular-propagating ion cyclotron harmonics in the vicinity of the Earth's plasmapause. cometary nuclei [Johnstone, 1990] or from the interstellar medium [ Wu and Davidson, 1972]. Many authors have studied wave excitation by anisotropic ions, and applied their results to space plasmas. To a large extent this work has been concerned with wave propagation along the magnetic field: it is commonly assumed that parallel-propagating waves have the highest growth rates. Kennel and Petschek [1966] pointed out that protons trapped in the magnetosphere could excite left circularly polarized electromagnetic waves with frequencies below the ion cyclotron frequency. Cornwall and Schulz [1971] Published in 1993 by the American Geophysical Union. Paper number 93JA00158. and Cuperman et al. [1975] studied this instability for the case of a bi-Maxwellian ion distribution interacting with a cold background plasma. Wu and Davidson [1972] considered the case of a cold ion ring drifting with respect to the background ions. Freund and Wu [198S] investigated the stability of shell-like ion distributions, finding that parallelpropagating ion cyclotron waves could be excited by an incomplete shell: it was found that complete (i.e., spherically symmetric) shells did not excite such waves. This study was motivated by observations of cometary ions picked up by the solar wind, indicating the presence of spherical shell distributions [Balsiger et al., 1986].
Stability of Electrostatic Ion Cyclotron Harmonic Waves in a Multi-ion Plasma
V and dH V + , respectively, along the ambient magnetic field and positively and negatively charged oxygen ions constitute the plasma under consideration. This composition very well approximates the plasma environment around a comet. Analytical expressions for the frequency and growth / damping rate of the EIC waves around the higher harmonics of hydrogen ion gyrofrequency have been derived. The EIC waves propagate at frequencies around the harmonics of the hydrogen ion gyrofrequency and the wave growth decreases rapidly for higher harmonics. We find that, the wave can be driven unstable by the hydrogen ion drift velocity dH V + alone, at small k⊥ρLH+ as well as electron drift velocity de V at large LH k ρ ⊥ + . Also, the growth rate is dependent on the densities and temperature anisotropies of the various constituent ions.
Physica Scripta, 1994
A common mechanism for the observed excitation of waves at sequential multiple ion cyclotron harmonics in the JET tokamak and in various space plasmas is proposed. It is suggested that, in each case, waves on the fast Alfven-ion Bernstein branch are excited by the magnetoacoustic cyclotron instability, which is driven by an energetic ion distribution with a local maximum at ul # 0, where vI is the velocity component perpendicular to the magnetic field.
Harmonic generation and parametric decay in the ion cyclotron frequency range
1984
Harmonic generation and parametric decay are examined in a toroidal ACT-I plasma using electrostatic plate antennas. The harmonic generation, which is consistent with sheath rectification, is sufficiently strong that the nonlinearly generated harmonic modes themselves decay parametrically. Resonant and nonresonant parametric decay of the second harmonic are observed and compared with uniform pump theory. Resonant decay of lower hybrid waves
Double-peaked electrostatic ion cyclotron harmonic waves
Journal of Geophysical Research, 1990
Electrostatic H + cyclotron harmonic waves are often observed along the auroral field lines at altitudes of 1-3.5 Roe by the Dynamics Explorer 1 satellite. A small fraction of these waves are found to have two peaks associated with each harmonic instead of one peak. The waves occur below the lower hybrid frequency and are usually relatively weak, about a factor of 4 smaller than typical electric field amplitudes of other H + cyclotron harmonic wave events. For two events the separation between the spectral peaks is found to be proportional to the harmonic number. The/double-peaked spectral signature is believed to be produced by Doppler shifts arising from the satellit/e velocity relative to the plasma rest frame. The wavelength and phase velocity of the harmonics can be determined by measuring the separation between the peaks. The waves were found to have wavelengths of the order of 300 m and phase velocities of the order of 150 km/s. The proportiOnality between the peak separation and the harmonic number indicates that the phase velocity i• approximately constant, independent of harmonic number. Assuming that the phase velocity is constant, it is shown that as Ak/k increases, the double peaks merge to form a single spectral peak. For the wave events presented in this paper, Ak/k is estimated to be less than 0.1.
Measurements of fast-ion acceleration at cyclotron harmonics using Balmer-alpha spectroscopy
Plasma Physics and Controlled Fusion, 2007
Combined neutral beam injection and fast wave heating at the fourth and fifth cyclotron harmonics accelerate fast ions in the DIII-D tokamak. Measurements with a nine-channel fast-ion D-alpha (FIDA) diagnostic indicate the formation of a fast-ion tail above the injection energy. Tail formation correlates with enhancement of the d-d neutron rate above the value that is expected in the absence of fast-wave acceleration. FIDA spatial profiles and fast-ion pressure profiles inferred from the equilibrium both indicate that the acceleration is near the magnetic axis for a centrally located resonance layer. The enhancement is largest 8-10 cm beyond the radius where the wave frequency equals the cyclotron harmonic, probably due to a combination of Doppler-shift and orbital effects. The fast-ion distribution function calculated by the CQL3D Fokker-Planck code is fairly consistent with the data.
Generation of cyclotron harmonic waves in the ionospheric modification experiments
1994
In the present paper, the parametric decay instability of the pump X-mode into electron Bernstein wave (EBW) near second harmonics of electron cyclotron frequency and IBVV at different harmonics (LJ < nui cl ;n = 2,3,4) is examined. Expressions are derived for homogenous threshold, growth rate and convective threshold for this instability. Applications and relevances of the present investigation to ionospheric modification experiment in the F-layer of the ionosphere as well as during intense electron cyclotron resonance heating in the upcoming MTX tokamak have been given.
Excitation of helium cyclotron harmonic waves during quiet magnetic conditions
Journal of Geophysical Research, 1998
A general approach to the generation of ion cyclotron harmonic waves observed on board the Akebono satellite in the deep plasmasphere is presented. It is shown that during quiet magnetic conditions the development of the hydrodynamic cyclotron instability with growth rate ,-/oc n•/2 where ni is the number density of the hot heavy ions, is suppressed by the field-aligned inhomogeneity of the dipole magnetic field. The instability is, in this case, controlled by the weak resonant interaction of the waves and the trapped particles with growth rate-/oc ni. The waves are generated by a kinetic instability involving hot helium ions with a ring-like distribution. Such ions are present in the magnetosphere during quiet magnetic conditions. A simple analytical model of this instability accounting for the inhomogeneity of the ambient magnetic field is used. It is shown that the ULF wave observations during quiet times on board the Akebono satellite are in a reasonable agreement with the present theoretical approach.
Nonlinear interaction of ion-cyclotron waves with fast protons in the magnetosphere
sta of1 lnv the magnetoýphe]e Se1 wit еп€ tiol ter ým of1 tha nat, mai the Tht ord finc whC flec rот .ра B(z and vaI 1. INTB,oDUCTIoN 1Ъе conseqrrenceý of t}e feaonant int€ ractlona Ьеhrееп waveв and parH,cleB in а plaвma confhed Ьу а magnetic fteld аrе регtiпепt to tЪе physicB of laboratory plaBmаs and plaBmaB in sрасе, as can Ье shоvrп Ьу Bimply cltiпg thе rQgеатсh оп instabilities in adiabatic confinement вуst€ mя,i the тевеаrсh оп cyclotTon heating (вее Ref. 2 Bnd the Цtвrаtчrе cited tbeTe), tlв elfoЁ to int€ rpret the паfirrаl and maJr-made vlf апd цlf гаdiаtiоп iп the magпеtоярhеrэ,8rа Bnd the рrоьlеm of int€ Ipr€ ting the fine ýtпrсtчrе of the вроrаФс воlдт radio еmlsвiоп.5 Tlmofeevz hаs studied in detail the попliпеаr tnteraction of сirсчlаrlу polarized waveB with сhаrgеd раrtlсlеs 1п cormectlon with the theoly of the сусlоtrоп heating of рlаsmав 1п magBetic conflnement sуst€ mý. Under cyclotron-heating condftions, the саsе of цовt intereBt 1в kпча << О (k rr iB the longttudinal wave пчmЬоr, апd v1, and Qс аr€ tb thеrmаl and cyclotron velocities of Фе раrticles of thе соrrевропФпg BlBcieB), and this is tЪе саsе dealt wtth 1п Ref. 2. In the рrФlеm of the interaction of cyclotron wачеs with fast раrfi,сlеs in Фе mцпеtоврhеrе, lt Ь песеssаry to trest the саве of large DoppleT BhlftB Вцч9 -Qg, пrhете vq is фе characteriýtic velocity of the faBt particles). Тhis сirсчmstапсе rафсаllу сhапgеg the пдfurе of the рrоЬlеm. Until rэсепtlу, this latter саве hаs been analyzed аввчmirц that the characterlstic tlmeB т of the рrосеsýеs чпdеr сопsldеrаff,оп (8ее the фsсчýslоп belщI for mоrе detatls) ýatbfy the сопфtiоп т << ,Q6{, whеr€ Qь is t}e сhаrасtеrlstiс bounce frequency of the partlcles between the magnetlc mirrоrs. Мчсh rроrk has Ьееп carried out чпdеr thls aBsumption (вее Ref. б and the litеrаtчrе cited there). For ýpical magnetospheric condltions, фе inequality , .. O5t соrrеsропds to rаthег large wave amplltudes (оr growth rаtеs).
Stability of electrostatic ion cyclotron waves in a multi-ion plasma
Pramana-journal of Physics, 2009
We have studied the stability of the electrostatic ion cyclotron wave in a plasma consisting of isotropic hydrogen ions (H+) and temperature-anisotropic positively (O+) and negatively (O−) charged oxygen ions, with the electrons drifting parallel to the magnetic field. Analytical expressions have been derived for the frequency and growth/damping rate of ion cyclotron waves around the first harmonic of both hydrogen and oxygen ion gyrofrequencies. We find that the frequencies and growth/damping rates are dependent on the densities and temperatures of all species of ions. A detailed numerical study, for parameters relevant to comet Halley, shows that the growth rate is dependent on the magnitude of the frequency. The ion cyclotron waves are driven by the electron drift parallel to the magnetic field; the temperature anisotropy of the oxygen ions only slightly enhance the growth rates for small values of temperature anisotropies. A simple explanation, in terms of wave exponentiation times, is offered for the absence of electrostatic ion cyclotron waves in the multi-ion plasma of comet Halley.