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Results for “beam instabilities”

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NASA NTRS1985

Electromagnetic ion beam instabilities - Hot beams at interplanetary shocks

This paper considers Vlasov instabilities driven by a very hot ion beam streaming parallel to a magnetic field. The linear theory of electromagnetic instabilities driven by such a beam in a homogeneous, collisionless, nonrelativistic plasma is used. Numerical solutions of the full linear dispersion equation for bi-Maxwellian distribution functions are presented. If the thermal speed of the beam is greater than its drift speed, the two dominant modes are the right-hand and left-hand resonant ion beam instabilities. The parametric dependencies of the threshold drift speeds and growth rates for both modes are presented. In particular, the thresholds are shown to be sensitive functions of the beam temperature anisotropies. Arguments are presented that these two modes are driven by the suprathermal ion component at interplanetary shocks, and that the growth of these modes is sufficient to account for the amplitudes of MHD-like waves observed at such shocks.

Gary, S. P.↗

NASA NTRS1985

Electromagnetic electron beam instabilities - Hot, isotropic beams

This paper considers the linear theory of electromagnetic instabilities driven by an electron beam in a homogeneous, nonrelativistic, Vlasov plasma. The beam is relatively hot, isotropic in its own frame, and streams parallel or antiparallel to a magnetic field B. Numerical solutions of the full dispersion equation for propagation parallel or antiparallel to B are presented, and the linear properties of the whistler heat flux and electron beam firehose instabilities are exhibited and compared. Under a broad range of parmameters the former mode has the lower beam speed threshold, and the larger maximum growth rate. In addition, it is demonstrated that, for a sufficiently large relative beam density, relative beam temperature, and plasma beta the whistler heat flux instability has a much lower beam speed threshold than the electrostatic electron beam instability. The application of these instabilities to first-order Fermi acceleration of electrons at space plasma shocks is discussed.

Gary, S. P.↗

NASA NTRS1986

Computer simulations of electromagnetic cool ion beam instabilities

Electromagnetic ion beam instabilities driven by cool ion beams at propagation parallel or antiparallel to a uniform magnetic field are studied using computer simulations. The elements of linear theory applicable to electromagnetic ion beam instabilities and the simulations derived from a one-dimensional hybrid computer code are described. The quasi-linear regime of the right-hand resonant ion beam instability, and the gyrophase bunching of the nonlinear regime of the right-hand resonant and nonresonant instabilities are examined. It is detected that in the quasi-linear regime the instability saturation is due to a reduction in the beam core relative drift speed and an increase in the perpendicular-to-parallel beam temperature; in the nonlinear regime the instabilities saturate when half the initial beam drift kinetic energy density is converted to fluctuating magnetic field energy density.

Gary, S. P.↗

NASA NTRS1993

Generation of waves in the Venus mantle by the ion acoustic beam instability

The ion acoustic beam instability is suggested as a mechanism to produce wave turbulence observed in the Venus mantle at frequencies 100 Hz and 730 Hz. The plasma is assumed to consist of a stationary cold O(+) ion plasma and a flowing, shocked solar wind plasma. The O(+) ions appear as a beam relative to the flowing ionosheath plasma which provides the free energy to drive the instability. The plasma is driven unstable by inverse electron Landau damping of an ion acoustic wave associated with the cold ionospheric O(+) ions. The instability can directly generate the observed 100 Hz waves in the Venus mantle as well as the observed 730 Hz waves through the Doppler shift of the frequency caused by the satellite motion.

Huba, J. D.↗

NASA NTRS1975

Experimental observation of the ion-ion beam instability in a cylindrical geometry

The ion-ion beam instability is investigated in a cylindrical geometry. In the presence of a cylindrically symmetric ion beam a coherent ion-ion beam instability is observed for a well-defined range of beam velocity. This instability gives rise to cylindrical standing waves whose frequency depends upon beam velocity. The beam distribution function is measured to be a ring in velocity space. The dispersion relation appropriate for the cylindrical standing waves is solved numerically and is shown to be in good agreement with the experimentally observed instability.

Romesser, T.↗

NASA NTRS1985

Electromagnetic ion beam instabilities. II

The results of Gary et al. (1984) on the properties of the right-hand resonant and nonresonant electromagnetic ion beam instabilities for relatively cool beam temperatures are extended. In particular, the parametric dependence of the real frequency at maximum growth of these modes is examined. It is demonstrated that the right-hand resonant ion beam instability can have maximum growth at frequencies near the ion-cyclotron frequency if the beam main component relative drift speed is about twice the Alfven speed and at least one of two conditions holds: that the ion beta or the beam main component relative temperature are sufficiently small, or the perpendicular-to-parallel beam temperature ratio is sufficiently large. These results support the identification of the right-hand resonant instability as the source of the large amplitude magnetic fluctuations observed upstream of slow shocks in the earth's magnetotail.

Gary, S. P.↗

NASA NTRS1984

Diffuse ions produced by electromagnetic ion beam instabilities

The evolution of the electromagnetic ion beam instability driven by the reflected ion component backstreaming away from the earth's bow shock into the foreshock region is studied by means of computer simulation. The linear and quasi-linear stages of the instability are found to be in good agreement with known results for the resonant mode propagating parallel to the beam along the magnetic field and with theory developed in this paper for the nonresonant mode, which propagates antiparallel to the beam direction. The quasi-linear stage, which produces large amplitude delta B approximately B, sinusoidal transverse waves and 'intermediate' ion distributions, is terminated by a nonlinear phase in which strongly nonlinear, compressive waves and 'diffuse' ion distributions are produced. Additional processes by which the diffuse ions are accelerated to observed high energies are not addressed. The results are discussed in terms of the ion distributions and hydromagnetic waves observed in the foreshock of the earth's bow shock and of interplanetary shocks.

Winske, D.↗

NASA NTRS1985

Observations of the right-hand resonant ion beam instability in the distant plasma sheet boundary layer

Examples of the onset of a resonant ion beam instability are illustrated on the basis of ISEE 3 observations of 66 large amplitude wave events at the plasma sheet boundary layer. Analysis of the observational data showed that the observed wave and ion beam parameters were, in general, in agreement with linear theoretical predictions of resonant ion beam instability. The distribution of ion energies at the onset of the instability is given in a table.

Tsurutani, B. T.↗

NASA NTRS1988

Comet-solar wind interaction through ion-proton beam instability

The acceleration of cometary ions in the distant cometary tail by an electromagnetic beam instability, which is caused by the relative drift velocity between the cometary ions and solar protons is studied. The linear phase of the instability is analyzed by numerical evaluation of the Vlasov dispersion relation. A particle simulation is used to study the nonlinear phase of the instability. Right-hand polarized cyclotron waves are made unstable by the presence of ion beams. The cometary tail ions are accelerated mainly during the reduced-growth phase of the instability. The cyclotron waves cause momentum transfer from solar wind protons to cometary tail ions. The acceleration of cometary ions through the electromagnetic beam instability is found to be 500-5000 cm/s-squared.

Price, Channon P.↗

NASA NTRS1987

Electromagnetic ion beam instabilities - Growth at cyclotron harmonic wave numbers

The linear theory of electromagnetic ion beam instabilities for arbitrary angles of propagation is studied, with an emphasis on the conditions necessary to generate unstable modes at low harmonics of the ion cyclotron resonance condition. The present results extend the analysis of Smith et al. (1985). That paper considered only the plasma parameters at a time during which harmonic wave modes were observed in the earth's foreshock. The parameters of that paper are used as the basis of parametric variations here to establish the range of beam properties which may give rise to observable harmonic spectra. It is shown that the growth rates of both left-hand and right-hand cyclotron harmonic instabilities are enhanced by an increase in the beam temperature anisotropy and/or the beam speed. Decreases in the beam density and/or the core-ion beta reduce the overall growth of the cyclotron harmonic instabilities but favor the growth of these modes over the growth of the nonresonant instability and thereby enhance the observability of the harmonics.

Smith, Charles W.↗

NASA NTRS1981

The electromagnetic ion beam instability upstream of the earth's bow shock

The linear theory of the electromagnetic ion beam instability for arbitrary angles of propagation has been studied. The parameters considered in the theory are typical of the solar wind upstream of the earth's bow shock when a 'reflected' proton beam is present. Maximum growth occurs for propagation parallel to the ambient magnetic field B, but this instability also displays significant growth at wave-vectors oblique to B. Oblique, unstable modes seem to be the likely source of the compressive magnetic fluctuations recently observed in conjunction with the 'diffuse' ion population. An energetic ion beam does not directly give rise to linear growth of either ion acoustic or whistler mode instabilities.

Gary, S. P.↗

NASA NTRS1986

Electrostatic beam instabilities in a positive/negative ion plasma

This paper examines the linear theory of electrostatic waves and instabilities in an unmagnetized, homogeneous Vlasov plasma. The first configuration considered is that of a stable plasma with three Maxwellian components: electrons, positive ions, and negative ions. The dispersion properties of the lightly damped ion modes are studied as a function of relative electron density and relative component temperatures. The second configuration considered is that of a tenuous electron-ion beam with drift speed streaming through a negative/positive ion plasma. If the beam is very tenuous, an electron beam instability is excited; if the beam is more dense, the instability becomes Buneman-like with parabolic dispersion. The thresholds and maximum growth rates of these instabilities are described as functions of the beam density and drift speed.

Galvez, Miguel↗

NASA NTRS1989

Simulations relevant to the beam instability in the foreshock

The results presently obtained from two-dimensional simulations of the reactive instability for Maxwellian beams and cutoff distributions are noted to be consistent with recent suggestions that electrons backstreaming into earth's foreshock have steep-sided cutoff distributions, which are initially unstable to the reactive instability, and that the back-reaction to the wave growth causes the instability to pass into its kinetic phase. It is demonstrated that the reactive instability is a bunching instability, and that the reactive instability saturates and passes over into the kinetic phase by particle trapping.

Cairns, I. H.↗

NASA NTRS1989

The electron beam instability revisited: Growth above and below f sub p

The growth of electrostatic waves near the plasma frequency (fp) due to an unstable electron beam is investigated by solving the unmagnetized electrostatic dispersion equation numerically. The numerical solutions are compared with analytic theories for reactive (or fluid-type) and kinetic versions of the beam instability, and for the O'Neil/Malmberg connection of the beam and Langmuir modes. Conditions for growth significantly above or below fp are given. Three general results are found: (1) The unstable waves do not grow on a mode with Langmuir dispersion except in the limit of a very dilute beam with growth on O'Neil/Malmberg's connected mode; (2) The properties of the unstable mode depend strongly on beam parameters such as beam density, speed and temperature; and (3) The frequency of maximum growth frequently lies significantly above or below fp, and differs significantly from that predicted by the Langmuir dispersion relation. Results imply important consequences for theories of strong turbulence and nonlinear wave-wave processes, and observational identification of fp from observed wave frequencies.

Cairns, I. H.↗

NASA NTRS1984

Electromagnetic ion beam instabilities

The linear theory of electromagnetic instabilities driven by an energetic ion beam streaming parallel to a magnetic field in a homogeneous Vlasov plasma is considered. Numerical solutions of the full dispersion equation are presented. At propagation parallel to the magnetic field, there are four distinct instabilities. A sufficiently energetic beam gives rise to two unstable modes with right-hand polarization, one resonant with the beam, the other nonresonant. A beam with sufficiently large T (perpendicular to B)/T (parallel to B) gives rise to the left-hand ion cyclotron anisotropy instability at relatively small beam velocities, and a sufficiently hot beam drives unstable a left-hand beam resonant mode. The parametric dependences of the growth rates for the three high beam velocity instabilities are presented here. In addition, some properties at oblique propagation are examined. It is demonstrated that, as the beam drift velocity is increased, relative maxima in growth rates can arise at harmonics of the ion cyclotron resonance for both right and left elliptically polarized modes.

Gary, S. P.↗

NASA NTRS2022

Linear Theory of Electromagnetic Ion Beam Instabilities in the Earth’s Forshock: Peter Gary’s Contributions (1981-1991)

We discuss Peter Gary’s contributions to the understanding of the origin and properties of ultra-low frequency (ULF) waves in the Earth’s foreshock during the period when the International Sun Earth Explorer spacecraft (ISEE-1 and -2) provided unique data about the plasma and wave environment in this region. Peter’s work concerning the linear theory of electromagnetic ion beam instabilities is contained in five journal articles and then summarized in a review article, all of which are discussed here. Brief summaries of observations and theory prior to ISEE as well as to later work are also included.

Dan Winske↗

NASA NTRS1984

Generation of broadband electrostatic noise by ion beam instabilities in the magnetotail

Particle data from ISEE 1 sampled in the earth's magnetotail show the presence of energetic ion beams in the boundary layer of the plasma sheet. A theory of instabilities driven by the beams is developed and compared with wave data sampled simultaneously to the particle data. It is concluded that the ion beams generate broadband electrostatic bursts of noise. The electrostatic noise correlates well with the occurrence of the beams, and the spectrum is consistent with that predicted from a negative energy beam instability under magnetotail conditions. The theory predicts that a spectrum of growing waves can be driven for frequencies from 0.001 omega(pe) up to omega(pe), the electron plasma frequency, with a spectral peak typically near 0.01 omega(pe) or lower, in agreement with the wave data. Furthermore, as one moves away from the source region perpendicular to the magnetic field, the high frequency components of the observed wave spectra are predicted to disappear gradually, leaving the low frequency part of the spectrum, also as is observed. Evidence is given for significant pitch angle scattering of the beams by the broadband electrostatic noise, leading to more isotropic ion distributions.

Grabbe, C. L.↗

NASA NTRS1982

Nonlinear evolution of the ion-ion beam instability

The criterion for the existence of vortex-like ion phase-space configurations, as obtained by a standard pseudopotential method, is found to coincide with the criterion for the linear instability for two (cold) counterstreaming ion beams. A nonlinear equation is derived, which demonstrates that this instability actually evolves into such phase-space configurations. A small, but nonzero, ion temperature turns out to be essential for the saturation into stationary structures

Pecseli, H. L.↗