Engineering topics

Akimoto, K.

Publications and source records attributed to Akimoto, K..

14 records

NASA NTRS1993

Nonlinear evolution of electromagnetic ion beam instabilities

A comparative study of the ion/ion right-hand resonant instability and the ion/ion nonresonant instability is carried out to investigate and contrast their properties. Linear analysis demonstrates that the nonresonant instability becomes resonant if the density of the ion beam is sufficiently high. Hybrid simulations show that both the resonant and nonresonant instabilities result in the formation of nonlinear pulses, called pulsations, but with distinct features. For example, the pulsations generated by the resonant instability have a positive correlation with the ion density, while those generated by the nonresonant instability are likely to have a relatively weak negative correlation. The waves generated by the nonresonant instability are subject to a parametric decay instability and tend to form a state of condensate where the turbulence becomes nearly monochromatic.

Akimoto, K.↗

NASA NTRS1991

Steepening of parallel propagating hydromagnetic waves into magnetic pulsations - A simulation study

The steepening mechanism of parallel propagating low-frequency MHD-like waves observed upstream of the earth's quasi-parallel bow shock has been investigated by means of electromagnetic hybrid simulations. It is shown that an ion beam through the resonant electromagnetic ion/ion instability excites large-amplitude waves, which consequently pitch angle scatter, decelerate, and eventually magnetically trap beam ions in regions where the wave amplitudes are largest. As a result, the beam ions become bunched in both space and gyrophase. As these higher-density, nongyrotropic beam segments are formed, the hydromagnetic waves rapidly steepen, resulting in magnetic pulsations, with properties generally in agreement with observations. This steepening process operates on the scale of the linear growth time of the resonant ion/ion instability. Many of the pulsations generated by this mechanism are left-hand polarized in the spacecraft frame.

Akimoto, K.↗

NASA NTRS1990

Particle acceleration by a wave in a strong magnetic field - Regular and stochastic motion

A general theory for the acceleration of a charged particle by a coherent wave of arbitrary polarization, propagation angle, and phase velocity in the presence of a uniform and strong magnetic field is presented. It is shown that the Hamiltonian surfaces are topologically open for waves with parallel phase velocity, equal to or larger than the speed of light. The trapping width is found to be a strong function of the index of refraction (N), and for N = 1 the trapping width increases as a function of the harmonic number. A novel mechanism for coherently accelerating particles to unlimited energies is presented.

Karimabadi, H.↗

NASA NTRS1990

Generation of strong MHD Alfvenic turbulence

Strong Alfvenic turbulence containing a number of solitonlike structures propagating at super-Alfvenic speeds is generated self-consistently and studied by means of computer simulation. A one-dimensional hybrid (kinetic ions, fluid electrons) code is used to investigate the nonlinear evolution of an electromagnetic ion-beam instability that generates low-frequency Alfven-like waves. As the instability develops, the field-aligned hydromagnetic waves steepen, forming a soliton that bifurcates several times, leading to a fully turbulent state.

Akimoto, K.↗

NASA NTRS1989

Particle-in-cell simulations of stochastic electron acceleration

The results of a series of particle-in-cell simulations of stochastic wave-particle interaction are presented. The threshold for stochasticity was confirmed. The simulations demonstrate that in a strong magnetic field plasma waves with quiver velocities much less than the speed of light but above a certain threshold can stochastically accelerate electrons to energies far greater than 1 MeV. Moreover, self-consistency effects drive return currents and produce energetic runaway electrons that violate an invariant of motion.

Akimoto, K.↗

NASA NTRS1989

Nonlinear generation of whistler waves by an ion beam

An electromagnetic hybrid code is used to simulate a new mechanism for whistler wave generation by an ion beam. First, a field-aligned ion beam becomes unstable to the electromagnetic ion/ion right-hand resonant instability which generates large amplitude MHD-like waves. These waves then trap the ion beam and increase its effective temperature anisotropy. As a result, the growth rates of the electron/whistler instability are significantly enhanced, and whistlers start to grow above the noise level. At the same time, because of the reduced parallel drift speed of the ion beam, the frequencies of the whistlers are also downshifted. Full simulations were performed to isolate and separately investigate the electron/ion whistler instability. The results are in agreement with the assumption of fluid electrons in the hybrid simulations and with the linear theory of the instability.

Akimoto, K.↗

NASA NTRS1988

Comment on 'Generation of broadband noise in the magnetotail by the beam acoustic instability' by P.B. Dusenbery

It is shown here that of the three plasma models for the generation of broadband noise in the magnetotail proposed by Dusenbery and Lyons (1985), two result in the same types of instabilities that are also excited in the Grabbe and Eastman (GE, 1984) model, while the third model introduces other modes not present in the GE model. For the plasma parameters given in the GE model, the ion/ion acoustic instability is a nonresonant one, even though the phase velocity of the excited waves falls within the distribution function of the core ions. The nonlinear saturation mechanism of the ion/ion acoustic instability is the trapping of both the core and the beam ions. In a reply, Dusenbery addresses several ongoing controversies which have resulted from studies of wave particle interactions in the plasma sheet boundary layer.

Omidi, N.↗

NASA NTRS1988

Nature and the nonlinear evolution of electrostatic waves associated with the AMPTE solar wind releases

The nonlinear evolution of the electrostatic waves observed during the AMPTE (Active Magnetosphere Particle Tracer Explorers) solar wind releases is investigated. Previous linear studies indicate two distinct sets of instabilities may be responsible for the generation. One set consists of ion-acoustic type instabilities which are insensitive to the presence of a background magnetic field, while the other group corresponds to the modified two stream instabilities and requires the solar wind flow to be across the ambient magnetic field. To establish which set of instabilities is more viable a detailed linear Vlasov theory has been conducted by numerically solving the full electromagnetic dispersion relation. In addition both the plasma wave as well as the magnetic field measurements by the IRM (Ion Release Module) spacecraft were used to correlate the frequency and the power of the observed waves with the magnitude and the direction of the solar wind magnetic field. The results of these analyses indicate that the ion-acoustic type instabilities have growth rates that are an order of magnitude or more larger than those of the modified two stream instabilities. Results show that both the solar wind protons and the released ions may be heated and accelerated in the directions oblique to the solar wind flow velocity.

Omidi, N.↗

NASA NTRS1987

Electron/ion whistler instabilities and magnetic noise bursts

Two whistler instabilities are investigated by means of the linear Vlasov dispersion equation. They are called the electron/ion parallel and oblique whistler instabilities, and are driven by electron/ion relative drifts along the magnetic field. It is demonstrated that the enhanced fluctuations from these instabilities can explain several properties of magnetic noise bursts in and near the plasma sheet in the presence of ion beams and/or field-aligned currents. At sufficiently high plasma beta, these instabilities may affect the current system in the magnetotail.

Akimoto, K.↗

NASA NTRS1986

The generation of broadband electrostatic noise by an ion beam in the magnetotail

The results of a theoretical investigation of the generation of broadband electrostatic noise (BEN) by an ion beam in the earth's magnetotail are presented. It is shown that at low beam temperatures an ion beam can generate BEN at wave normal angles (theta) between zero and 80 deg with the maximum growth occurring at theta - 0 deg when the beam velocity is small and at large theta (about 70 deg) for higher beam velocities. It is also shown that two types of instability are responsible for the wave amplification. One is the ion acoustic instability associated with the electron-ion beam relative drift, and the other is the ion-ion instability. The broad frequency and angular spectra of BEN can be explained by the presence of the two instabilities. These instabilities are shown to be insensitive to the presence of a background magnetic field.

Akimoto, K.↗

NASA NTRS1985

Ion-acoustic-like waves excited by the reflected ions at the earth's bow shock

Some model distributions based on recent observations and simulations of the plasma at the earth's bow shock are found to be unstable to obliquely propagating electrostatic instabilities. The model distributions consist of either an ion beam or an ion velocity ring accompanied by a bi-Maxwellian background ion distribution and a flattop electron distribution. Ion anisotropies and nonthermal electrons are capable of significantly lowering the threshold of the ion beam instability. The generated waves share many properties in common with the ion acoustic waves that have been observed at the earth's bow shock. The results also indicate the importance of an anisotropy in the background ion velocity distribution in identifying sources of the ion-acoustic-like waves observed in the ion foreshock and the solar wind.

Akimoto, K.↗

NASA NTRS1985

Lower-hybrid instabilities driven by an ion velocity ring

The lower-hybrid instabilities in high-beta (ratio of plasma to magnetic pressure) plasmas driven by ring-ion distributions in velocity space are investigated. A dispersion equation including electromagnetic effects is derived. In the low-beta limit, analytic expressions are obtained which illuminate the physical nature of the instabilities. The complete dispersion equation is solved numerically as a function of ring speed and plasma beta for several types of ring distribution. Electromagnetic effects are important for relatively energetic rings even in the low-beta regime, suppressing growth rates and shifting the angle of propagation to more oblique angles. Stabilization by thermal effects is also discussed. Application of these results to the earth's bow shock, AMPTE, comets and solar flares is suggested.

Akimoto, K.↗

NASA NTRS1985

Ion-acoustic instabilities driven by an ion velocity ring

A ring distribution of ions in velocity space can generate electrostatic waves which propagate predominantly along an ambient magnetic field at frequencies comparable with the ion plasma frequency. A dispersion equation which accounts for these waves is presented, and solved analytically and numerically. It was found that ion-acoustic-like waves are excited in a plasma even if the electron temperature is comparable with the in temperature under the assumption of an anisotropic ion distribution.

Akimoto, K.↗

NASA NTRS1984

Microinstabilities associated with a high Mach number, perpendicular bow shock

Instability analyses incorporating insights gained through ISEE observations and hybrid simulations are used in an examination of the instabilities associated with a high Mach number perpendicular shock akin to the earth's bow shock. In the regions in front of, and at, the shock transition the cross-field instabilities are subdivided into the ion-ion streaming, kinetic cross-field streaming, and drift lower hybrid instability low frequency modes, as well as the electron cyclotron drift, ion sound, and electron whisker instability high frequency modes. Further downstream, ion ring-like and anisotropy-driven instabilities are considered. The implications of these results for wave signatures, plasma heating and acceleration are noted.

Wu, C. S.↗