Superthermal ion cyclotron harmonic emission from fusion and space plasmas: A single physical mechanism (original) (raw)
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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].
Helium cyclotron harmonic waves in the magnetosheric plasma
Advances in Space Research, 1981
Observations on the satellite Geos 1 have shown the presence of banded emissions whose frequency spacing is of the order of the local He+ cyclotron frequency. Above the proton cyclotron frequency, the emissions are electrostatic in character whereas below they are electromagnetic. It is believed that these form the first observations of electrostatic ion cyclotron harmonic waves detected in the Earth's magnetosphere beyond~~lRE. An investigation of the cold ion population at the times of the observed events indicates that helium ions form a large percentage of the total concentration, and occasionally may entirely dominate the population. Energetic ion distribution functions appear to peak in the energy range 30 keV, and show anisotropic pitch-angle distributions. Preliminary instability calculations using these observations are beginning to yield an insight into a mechanism whereby the waves can be produced.
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.
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.
Velocity-shear-driven ion-cyclotron waves and associated transverse ion heating
Journal of Geophysical Research: Space Physics, 1998
Recent sounding rocket experiments, such as SCIFER, AMICIST, and ARCS-4, and satellite data from FAST, Freja, DE-2, and HILAT, provide compelling evidence of a correlation between small-scale spatial plasma inhomogeneities, broadband low-frequency waves, and transversely heated ions. These naturally arising, localized inhomogeneities can lead to sheared cross-magnetic-field plasma flows, a situation that has been shown to have potential for instability growth. Experiments performed in the Naval Research Laboratory's Space Physics Simulation Chamber demonstrate that broadband waves in the ion-cyclotron frequency range can be driven solely by a transverse, localized electric field, without the dissipation of a field-Migned current. Significant perpendicular ion energization resulting from these waves has been measured. Detailed comparisons with both theoretical predictions and space observations of electrostatic waves found in the presence of sheared cross-magnetic-field plasma flow are made. FAC is accompanied by sheared cross-field particle flows 1permanently at Sachs-Freeman Associates, Inc., Largo, Maryland.
An explanation for experimental observations of harmonic cyclotron emission induced by fast ions
Physics of Plasmas, 1994
An explanation, supported by numerical simulations and analytical theory, is given for the harmonic cyclotron emission induced by fast ions in tokamak plasmas—in particular, for the emission observed at low harmonics in deuterium–deuterium and deuterium–tritium experiments in the Joint European Torus [e.g., Phys. Rev. Lett. 60, 33 (1988)]. It is shown that the first proton harmonic, whose field energy amplitude scales as the 0.84 power of the proton density, is one of the highest spectral peaks, whereas the first alpha harmonic is weak. The relative spectral amplitudes of different harmonics are compared. The results are consistent with the experimental observations. The simulations verify that the instabilities are caused by a weak relativistic mass effect. Simulation also shows that a nonuniform magnetic field leads to no appreciable change in the growth rate and saturation amplitude of the waves.
Stability of electrostatic ion cyclotron waves in a multi-ion plasmay
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.
Plasma Physics and Controlled Fusion
Ion cyclotron emission (ICE) was the first collective radiative instability, driven by confined fusion-born ions, observed from deuterium-tritium plasmas in JET and TFTR. ICE comprises strongly suprathermal emission, which has spectral peaks at multiple ion cyclotron harmonic frequencies as evaluated at the outer mid-plane edge of tokamak plasmas. The measured intensity of ICE spectral peaks scaled linearly with measured fusion reactivity in JET. In other large tokamak plasmas, ICE is currently used as an indicator of fast ions physics. The excitation mechanism for ICE is the magnetoacoustic cyclotron instability (MCI); in the case of JET and TFTR, the MCI is driven by a set of centrally born fusion products, lying just inside the trapped-passing boundary in velocity space, whose drift orbits make large radial excursions to the outer mid-plane edge. Diagnostic exploitation of ICE in future experiments therefore rests in part on deep understanding of the MCI, and recent advances in c...
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.
Convection of ion cyclotron waves to ion-heating regions
Journal of Geophysical Research, 1991
Low-frequency waves associated with ion conics have been observed in the ;:entral plasma sheet, in a region where there are no obvious sources of free energy that could destabiliz•,these waves locally. We consider ion cyclotron waves generated in the equatorial plane b•. a i•rotot•' temperature anisotropy and use computed growth rates to create a model wave distri.bution. U•ing raj•t•'?• ing and conservation of the wave distribution function along phase space rays, w.e. th, eri map the; Wave intensities from the equatorial plane to the top of the ion-heating region. We find that •he spectral density at a geocentric distance of 2.8 RE will be about 10 times higher than that in the equatorial region. Thus, convection from the equatorial plane could explain the observed spectral density of 10-(; V 2 m-2 Hz-1 and the associated oxygen ion heating. iNTRODUCTION It is well known that ions in the ionosphere and magnetosphere can be heated perpendicularly to the geomagnetic field. These ions may then move adiabatically up the field lines of the inhomogeneous terrestrial magnetic field and form so-called conics in velocity space. Several alternative ion energization mechanisms have been suggested (see reviews by Klumpar [1986], Lysak [1986], and Chang et al., [1988], and references therein). However, the problem of ion conic generation is still the subject of vigorous debate. At least at altitudes above a few thousand kilometers, ion conics are often observed on the same field lines as relatively intense, broadband waves. Several studies indicate that the waves observed around the ion gyrofrequency may, via resonant cyclotron heating, generate the observed ion conics [Chang et al., 1986; Andrd et al., 1988, 1990; Crew et al., 1990]. Waves at roughly half the ion gyrofrequency may via double-cyclotron absorption contribute to the ion heating ITemerin and Roth, 1986; Ball, 1989; Ball and Andrd, 1991a]. Emissions at even lower frequencies may also cause some ion energization [Lundin and Hultqvist, 1989; Lundin et al., 1990; Ball and Andrd, 1991b]. Broadband waves and ion conics are often observed above the auroral zone and in the polar cusp/cleft region. Here local energy sources such as sharp gradients and drifting particles can possibly generate some of the waves. However, broadband waves associated with ion conics are also common in the central plasma sheet [Chang et ai., 1986]. In this region, equatorward of the auroral zone, there are no obvious local energy sources that can power the broadba• waves. This led Johnson et [1989] to suggest that the waves are generated by anisotropic ion distributions in the equatorial plane and that they then propagate down the field lines. Closely related ideas were considered recently •lso by Horne and Thorne [1990], who used ray tracing to study the propagation of ion cyclotron waves in the plasmapause region. They emphasized ion cyclotron damping at the second harmonic of the oxygen gyrofrequency. The path-integrated absorption they computed was used to estimate the ion heating qualitatively, but their method did not allow quantitative comparisons based on observed spectral densities.