Mieko Toida

Damping of perpendicular magnetosonic pulses in a two-ion-species plasma

The damping of finite-amplitude magnetosonic pulses propagating perpendicular to a magnetic field in a collisionless plasma containing two ion species (majority light ions and minority heavy ions) is studied through theory and simulation. A magnetosonic pulse accelerates heavy ions in the direction parallel to the wave front; this cross-filed motion of heavy ions then generates a long-wavelength perturbation behind the original pulse. Because of these processes the original pulse is damped. The damping rate of a solitary pulse is theoretically obtained. It decreases with increasing amplitude. The damping is further investigated by means of a one-dimensional, fully electromagnetic code based on a three-fluid model. The theoretically obtained damping rate is in good agreement with the simulation result. Also, it is confirmed that small-amplitude periodic waves are not damped.

Oblique Ion Acoustic Wave Instabilities in a Multi-Ion Plasma and 3He-Rich Events

Oblique ion acoustic waves in a current-carrying, magnetized plasma are investigated. For a multi-ion plasma whose dominant components are hydrogen and helium, it is found that for some plasma parameters, oblique ion acoustic waves can have positive growth rates at frequencies ω≃Ω 3 He ( 3 He cyclotron frequency) and, at the same time, negative growth rates at ω≃Ω 4 He . It is then suggested that these waves can play an essential role in 3 He-rich solar flares.

Collective behavior of ion Bernstein waves in a multi-ion-species plasma

Collective behavior of ion Bernstein waves propagating perpendicular to an external magnetic field is studied with attention to the effect of multiple-ion species. In a thermal-equilibrium, multi-ion-species plasma, a great number of Bernstein waves are excited near the harmonics of many different ion cyclotron frequencies. The autocorrelation function of the quasimode consisting of these waves is initially damped and is not recovered to its initial value. This is predicted by the theory and is confirmed by numerical calculations and by particle simulations. It is also demonstrated by particle simulations that a perpendicular macroscopic disturbance is damped in a multi-ion-species plasma. The electric-field energy associated with this disturbance is significantly reduced and is transferred to the ions, indicating that the presence of multiple-ion species affects the energy transport.

Effect of ion composition on ion acceleration by magnetosonic shock waves

The study of heavy-ion acceleration by magnetosonic shock waves in multi-ion-species plasmas [M. Toida and Y. Ohsawa, Solar Physics 171, 161 (1997)] is extended to the case in which the ion masses are of the same order of magnitude; specifically, the effect of mass and density ratios is examined for H-T and D-T plasmas with three-dimensional, electromagnetic, particle simulations. The frequency difference Δω, where Δω=(ω+0−ω−r)∕ω+0 with ω+0 the cut-off frequency of the high-frequency magnetosonic mode and ω−r the resonance frequency of the low-frequency mode, is a key parameter in the generation of shock waves from a disturbance. In H-T plasmas with nH⪡nT and with nH⪢nT and in D-T plasmas with any density ratio, Δω is small, and the high-frequency-mode shock wave is mainly generated and plays a central role in ion energization processes. In H-T plasmas with nH=nT, for which Δω is much greater, both the high- and low-frequency-mode shock waves are generated and contribute to the acceleration.

Effect of Ion Composition on Magnetosonic Waves

The propagation of the two types of fast magnetosonic waves, i.e., low- and high-frequency modes, in a two-ion-species plasma is studied theoretically and numerically. It is analytically found that the KdV equation for the low-frequency mode is valid for amplitudes e 2Δ ω . With electromagnetic particle simulations, the evolution of the low- and high-frequency-mode pulses is investigated for various density and cyclotron frequency ratios and is compared with theoretical predictions. In particular, it is shown that high-frequency-mode pulses are generated from a long-wavelength low-frequency-mode pulse if its amplitude e exceeds 2Δ ω .

Simulation studies of heavy ion heating by current-driven instabilities

Nonlinear evolution of current-driven instabilities and associated energy transport among different particle species are studied by means of a two-dimensional, electrostatic, particle simulation code with full ion and electron dynamics. The plasma is assumed to consist of hydrogen (H) and helium (He) ions and electrons with the electron temperature larger than the ion temperatures; the electrons drift along a uniform magnetic field with an initial speed equal to the thermal speed. Then, simulations show that after the development of ion acoustic waves and fundamental H cyclotron waves, second harmonic waves are destabilized due to the change in the electron velocity distribution function parallel to the magnetic field, fe(v∥). Even though the linear theory based on the initial conditions predicts that the second harmonics are only marginally unstable, they eventually grow to the largest amplitudes and heat He ions more significantly than H ions. The instabilities of these three kinds of modes with differe...

Elemental compositions of high-energy ions produced by magnetosonic waves in quiescent plasmas

The structure of a nonlinear magnetosonic wave in a multiple-ion-species plasma is analytically investigated, and the elemental composition of ions reflected by the magnetosonic wave is studied. Firstly, stationary solutions of magnetosonic waves propagating perpendicularly to a magnetic field in a plasma consisting of electrons and two-species ions are obtained from a fluid model. Secondly, on the basis of the solitary wave solution, conditions for ion reflection are discussed. Then, the fraction of high-energy ions produced by a magnetosonic wave is expressed in terms of the ion mass, charge, thermal speed, Alfven speed, and wave amplitude. It rapidly decreases with increasing mass. It is found that most of the light ions can be reflected in a multiple-ion-species plasma even for small-amplitude waves if the masses of the main component ions are sufficiently heavy.

Low-frequency electromagnetic fluctuations in thermal-equilibrium, multi-ion-species plasmas

Low-frequency electromagnetic thermal fluctuations propagating perpendicular to a magnetic field are theoretically studied with attention to the effect of multiple ion species. In the frequency regime lower than the lower hybrid frequency, there exist three types of modes; magnetosonic mode with ω≃kvA, ion cyclotron modes with ω≃nΩi, and heavy-ion cutoff modes with frequencies slightly higher than the ion-ion hybrid resonance frequencies. The power spectra of magnetic fluctuations due to these modes are obtained analytically and numerically. In a single-ion-species plasma, the magnetosonic mode is an overwhelmingly dominant mode. The autocorrelation function Ck(τ) is thus given by a cosine function with a constant amplitude. In a multi-ion-species plasma, however, the amplitudes of the heavy-ion cutoff modes can be comparable to that of the magnetosonic mode. Therefore, Ck(τ) is initially damped, and its recurrence time is extremely long.

Effects of trapped electrons on ion reflection in an oblique shock wave

A magnetosonic shock wave propagating obliquely to an external magnetic field can trap electrons and accelerate them to ultrarelativistic energies. The trapped electrons excite two-dimensional (2D) electromagnetic fluctuations with finite wavenumbers along the shock front. We study effects of the trapped electrons on ion motions through the 2D fluctuations. It is analytically shown that the fraction of ions reflected from the shock front is enhanced by the 2D fluctuations. This is confirmed by 2D (two space coordinates and three velocities) relativistic, electromagnetic particle simulations with full ion and electron dynamics and calculation of test ions in the electromagnetic fields averaged along the shock front. A comparison between 2D and one-dimensional electromagnetic particle simulations is also shown.

Damping of magnetohydrodynamic disturbances in multi-ion-species plasmas

The evolution of macroscopic magnetohydrodynamic disturbances across a magnetic field is studied, with particular attention to the effect of multiple ion species. Analyses are carried out on disturbances where the initial magnetic profiles are sinusoidal. Both the theory and electromagnetic simulations show that, in a single-ion-species plasma, the disturbance is undamped, with its energy oscillating between the magnetic field and ion velocity. In a multi-ion-species plasma, however, it is initially damped, owing to the phase mixing of the magnetosonic mode and the modes having ion-ion hybrid cutoff frequencies. Furthermore, it is found from long-time simulations that the amplitude of the disturbance continues to decrease in a multi-ion-species plasma. This is due to nonlinear mode couplings. The magnetic energy is irreversibly transferred to the ions.