L. Gomberoff

Acceleration and heating of heavy ions by circularly polarized Alfvén waves

We study the dispersion relation of left-hand-polarized Alfven waves in multicomponent plasmas. If initially the plasma components are not drifting relative to each other, the Alfven waves propagate until they meet the gyrofrequency of the species with the largest Ml = ml/zlmp value (ml is the ion mass, zl is the degree of ionization, and mp is the proton mass). As a result of resonance absorption, these ions are heated and accelerated by quasi-linear resonant interaction. When these ions reach a given velocity, the dispersion relation of the Alfven waves changes drastically. The Alfven branch of the dispersion relations no longer goes to the gyrofrequency of the ion species with the largest Ml value, but it goes to the gyrofrequency of the species with the second largest Ml value. In this way, more and more channels open up. We apply these results to high-speed solar wind streams, and we argue that Alfven waves generated in coronal holes or in nearby regions can heat and accelerate heavy ions.

Excitation and parametric decay of electromagnetic ion cyclotron waves in high‐speed solar wind streams

In a recent paper, Hollweg et al. (1993) studied the parametric decay of Alfven waves in high-speed solar wind streams. Following this analysis, we consider the nonlinear decay of left-hand-polarized ion cyclotron waves. It is shown that in a solar wind type plasma composed of electrons, protons, and alpha particles drifting relative to the protons, both branches of the dispersion relation of the circularly polarized waves can be excited by observed thermal anisotropies (Gomberoff and Elgueta, 1991). Guided by this analysis, the parametric decay of each branch of the dispersion relation is discussed. It is shown that the presence of drifting alpha particles introduces new wave couplings in the system that lead to new instabilities. Some of these instabilities involve sound waves supported essentially by the alpha particles, which, due to Landau damping, can be very efficient in the energization of alpha particles. Other instabilities involve ordinary sound waves that can lead to proton heating. A modulational instability that involves two electromagnetic daughters is also found. We have also found that a strong pump can force decays of modes that do not satisfy the resonance conditions when the pump intensity is vanishingly small. Finally, it is shown that both branches of the dispersion relation, particularly the branch close to the Doppler-shifted alpha particle resonance, are highly unstable even for small intensities of the pump wave.

Minor heavy ion electromagnetic beam–plasma interactions in the solar wind

It is shown that the velocity threshold of the right-hand-polarized ion-ion resonant instability decreases with decreasing gyrofrequency values of the beam ions, and also, the system can become unstable for very small beam concentrations. In order to illustrate the results, a plasma consisting of a proton background and a very tenuous O + ion beam is considered. It is shown that for an ion beam of n O +/n p = 10 -4 , the threshold velocity is V O +∼1.36V A (V A is the Alfven velocity), and the maximum growth rate is of the order of 1O -3 Ω p (Ω p is the proton gyrofrequency). We apply these results to high-speed solar-wind-type plasmas consisting of a proton background, an alpha particle beam, and a beam of O + ions. It is also shown that the presence of alpha particles reduces the velocity instability threshold of the O + -p resonant instability.

Ion acoustic damping effects on parametric decays of Alfvén waves: Right-hand polarization

We study ion acoustic damping effects on parametric decays of right-hand-polarized electromagnetic waves. We do this because ion beams have been observed in a variety of space environments, and consequently, these waves can exist in those places. Damping effects are incorporated into the model by adding to the longitudinal component of the equation of motion a collision-like term. Like for left-hand-polarized waves, the effect of damping is twofold. On the one hand, damping decreases the maximum growth rate of the existing instabilities while increasing the instability range, and on the other hand, it destabilizes regions that are stable in the absence of damping. Thus, for low-frequency pump waves, ω 0 << ω ci , and for low β = υ t /υ A (ω ci is the ion gyrofrequency, and υ t and υ A are the thermal and the Alfven velocities, respectively), where the only parametric instability is a decay instability, damping destabilizes the frequency range between ω = 0 and the threshold of the decay instability. As β increases, two new parametric instabilities develop: One of them is a modulational instability, and above some threshold value of the pump wave amplitude, there is also a beat wave instability. The decay instability is also possible for pump wave amplitude above some threshold. For even larger β the decay instability is no longer possible. In all cases, damping effects reduce the growth rate of the existing instabilities and destabilize regions which are stable in the absence of damping. These results are in agreement with those obtained by Vasquez [1995]. It is also shown that for large-frequency pump waves, electron/ion whistler waves, the decay instability reappears even for large β and has very large growth rates.

Nonlinear decay of electromagnetic ion cyclotron waves in the magnetosphere

The authors study the parametric decays of left-hand polarized electromagnetic ion cyclotron waves, propagating parallel to the external magnetic field, in the magnetosphere. They show that the presence of He{sup +} ions and a mixed population of thermal and hot protons give rise to new wave couplings. These couplings lead to a number of new instabilities. Some of the instabilities involve sound waves carried mainly by the He{sup +} ions, which can be very efficient in heating up the bulk of the He{sup +} ions via Landau damping. Other instabilities involve the branch of the left-hand polarized electromagnetic ion cyclotron waves which has a resonance at the He{sup +} ion gyrofrequency. These instabilities can also play a role in the energy transfer from the pump wave to the He{sup +} ions through resonance absorption, preferably in the direction perpendicular to the external magnetic field. The new couplings give rise to several types of parametric instabilities such as ordinary decay instabilities, beat wave instabilities, and modulational instabilities. There are also couplings where the pump wave decays into the two electromagnetic sideband waves. 42 refs., 10 figs.

Electrostatic instabilities induced by large‐amplitude left‐hand polarized waves

[1] We study the effect of a large-amplitude left-hand polarized wave on ion acoustic instabilities. We show that the presence of a finite amplitude left-hand polarized wave produces electrostatic instabilities above a threshold amplitude. These instabilities occur when the phase velocities of the counter-streaming ion acoustic waves become equal, due to the action of the nonlinear wave. They do not exist in the absence of large-amplitude waves. We examine their growth rates and threshold amplitude behavior as a function of the heam speed. temperature, and large-amplitude wave frequency.

Behavior of linear beam-plasma instabilities in the presence of finite amplitude circularly polarized waves

We review the effect of finite amplitude circularly polarized waves on the behavior of linear ion-beam plasma instabilities. It has been shown that left-hand polarized waves can stabilize linear right-handed instabilities [1]. It has also been shown that for beam velocities capable of destabilizing left-handed waves, left-hand polarized large amplitude waves can also stabilize these waves. On the other hand, when the large amplitude wave is right-hand polarized, they can either stabilize or destabilize right-handed instabilities depending on the wave frequency and beam speed [2]. Finally, we show that the presence of large amplitude left-hand polarized waves can also trigger electrostatic ion-acoustic instabilities by forcing the phase velocities of two ion acoutic waves to become equal, above a threshold amplitude value.

Ion acoustic‐like waves triggered by nonlinear circularly polarized waves in the presence of an alpha particle beam

[1] We consider a fast solar wind-type plasma consisting of electrons, a protons background, and a much more tenuous alpha particle beam. It is shown that such system is unstable against ion acoustic-like waves triggered by finite-amplitude left-hand polarized waves propagating in the direction of the interplanetary magnetic field. The ion acoustic-like waves have similar properties to those found in a plasma with a much denser proton beam (Gomberoff, 2006b). Although the alpha particle beam is much more tenuous than the proton beam, the system is equally able to trigger ion acoustic-like waves. Therefore we believe that this effect should be taken into account in the evolution of the fast solar wind.

Parametric decays of electromagnetic ion cyclotron waves in a H+ ‐He+ ‐O+ magnetosphericlike plasma

Parametric decays of large-amplitude electromagnetic ion cyclotron waves (EICW) due to a minor O+ ion component in the magnetosphere are studied. It is shown that the presence of O+ ions leads to a number of new wave couplings which in turn lead to new instabilities. Some couplings involve sound waves carried mainly by the O+ ions, and a sideband EICW which has a resonance at the O+ ion gyrofrequency. These are decay instabilities which can lead to O+ heating through Landau damping and/or resonance absorption. There is also a modulational instability involving two sideband EICW, one propagating forward and the other propagating backward relative to the external magnetic field. These waves can also transfer energy to the O+ ions through resonance absorption. The other branches of the dispersion relation, namely, the He+ and proton branch, have additional decay instabilities (Gomberoff et al., 1995) due to the presence of a minor O+ ion component. It is also shown that in the fluid description, the decays to sound waves associated with the minority heavy ion species have growth rates comparable to, or even larger than, the decays to the acoustic branch corresponding to the majority proton species.

Nonlinear ion‐acoustic waves supported by an ion beam

[1] It is shown that in a plasma composed by electrons, a proton core, and a proton beam propagating along an external magnetic field, finite amplitude left-hand polarized waves propagating backward relative to the beam direction can trigger nonlinear ion-acoustic waves supported by the beam. A detailed study of such instabilities is carried out. The instability bounds are such that if there are large amplitude left-hand polarized waves propagating toward the Sun, nonlinear ion-acoustic waves supported by the beam should exist in high-speed solar wind streams.