Armando L. Brinca

Physics of Mass Loaded Plasmas

In space plasmas the phenomenon of mass loading is common. Comets are one of the most evident objects where mass loading controls to a large extent the structure and dynamics of its plasma environment. New charged material is implanted to the fast streaming solar wind by planets, moons, other solar system objects, and even by the interstellar neutral gas flowing through our solar system. In this review we summarize both the current observations and the relevant theoretical approaches. First we survey the MHD methods, starting with a discussion how mass loading affects subsonic and supersonic gasdynamics flows, continuing this with single and multi-fluid MHD approaches to describe the flow when mass, momentum and energy is added, and we finish this section by the description of mass loaded shocks. Next we consider the kinetic approach to the same problem, discussing wave excitations, pitch angle and energy scattering in linear and quasi-linear approximations. The different descriptions differ in assumptions and conclusions; we point out the differences, but it is beyond the scope of the paper to resolve all the conflicts. Applications of these techniques to comets, planets, artificial ion releases, and to the interplanetary neutrals are reviewed in the last section, where observations are also compared with models, including hybrid simulations as well. We conclude the paper with a summary of the most important open, yet unsolved questions.

Unusual characteristics of electromagnetic waves excited by cometary newborn ions with large perpendicular energies

Solution of the linear kinetic dispersion equation shows that cometary new-born ions with large perpendicular energies (large initial angles between the solar wind velocity and the IMF) can excite a wave mode with rest frame frequencies of the order of the heavy ion cyclotron frequency which exhibits maximum growth slightly away (6°–15°) from parallel propagation, together with high mass density compression ratios and almost linear polarization. The association of these properties is relevant to space observations of MHD-like waves at comets and in planetary foreshocks and has not been reported in the vast literature on possible instability mechanisms. Dilute drifting oxygen ion and photoelectron rings with finite thermal spreads model the free energy source in the solar wind frame. For parallel propagation, the mode of interest is left-hand circularly polarized, moves in the direction of the newborn particles, and is predominantly fed by the perpendicular energy of the heavy ions; as obliquity increases, its polarization and compression change rapidly. Modification of the free energy source parameters (ring density, drift velocity, perpendicular speed and thermal spread) determines their relative influence, defines the domain of existence of the mode, and shows that it might evolve into a nonoscillatory, purely growing structure distinct from the mirror wave. The results are also applicable to non-cometary environments.

On the stability of nongyrotropic ion populations: A first (analytic and simulation) assessment

The wave and dispersion equations for perturbations propagating parallel to an ambient magnetic field in magnetoplasmas with nongyrotropic ion populations show, in general, the occurrence of coupling between the parallel (left- and right-hand circularly polarized electromagnetic and longitudinal electrostatic) eigenmodes of the associated gyrotropic medium. These interactions provide a means to driving linearly one mode with free-energy sources of other modes in homogeneous media. Different types of nongyrotropy bring about distinct classes of coupling. The stability of a hydrogen magnetoplasma with anisotropic, nongyrotropic protons that only couple the electromagnetic modes to each other is investigated analytically (via solution of the derived dispersion equation) and numerically (via simulation with a hybrid code). Nongyrotropy enhances growth and enlarges the unstable spectral range relative to the corresponding gyrotropic situation. The relevance of the properties of nongyrotropic populations to space plasma environments is also discussed.

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.

Nongyrotropy as a source of instability and mode coupling

Nongyrotropic particle populations can bring about linear mode coupling in homogeneous media among the three eigen-modes of parallel propagation in gyrotropic magnetoplasmas. These interactions stimulate, in general, wave activity that does not occur in corresponding (random gyrophase) gyrotropic ambients. Solutions of the dispersion equation illustrate that simple introduction of gyrophase organization can (i) excite electrostatic (and electromagnetic) perturbations in media whose free energy sources are solely electromagnetic, and (ii) drive hybrid (both electrostatic and electromagnetic) wave growth in thoroughly stable Maxwellian plasmas.

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.

Properties of whistler mode wave packets at the leading edge of steepened magnetosonic waves: Comet Giacobini-Zinner

Abstract We examine the physical characteristics of high frequency wave packets which are detected at the steepened edge of magnetosonic waves near comet Giacobini-Zinner. Peak-to-peak wave amplitudes (ΔB) can be large, with Δ B |B|≈ 1.5. The waves are left-hand polarized in the spacecraft frame and typically develop as a smooth progression in amplitude and phase from the leading edge of the low frequency magnetosonic waves. In over half of the cases examined, the wave oscillations decrease approximately linearly with time, ruling out electron Laundau damping as the dominant mechanism for the amplitude fall off. We surmise that the wave packets play an integral role in the reorientation and reduction in field magnitude from the steepened magnetosonic waves to the upstream ambient field. Observed wave packets have durations between 6 and 45 s with an average value near 18s. The number of oscillations within a packet can vary from one to over 20. In general, packets with more oscillations have the lowest wave periods. The wave period within any given packet is approximately constant, but it can vary from 1.3 to 11 s from event to event. Small frequency shifts are also observed within each packet. Generally, higher frequencies occur at the small amplitude (upstream) end. Although one might expect a relationship between the above wave properties and the field gradient across the steepened edge of the magnetosonic waves, no obvious correlations were found. Since the high frequency wave packets are observed to be attached to the long period magnetosonic waves, they too propagate upstream and are blown back by the solar wind across the ICE spacecraft. The observed properties of the wave packets are therefore consistent with anomalously Doppler shifted righthand polarized waves. The frequency in the solar wind frame is computed to be between 1 and 10 times the proton gyrofrequency. An empirical model for the structure of the wave packets is presented and potential theories for the generation mechanisms are discussed.

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.

On the stability of stationary nongyrotropic distribution functions: Coupling and purely growing waves

Unperturbed particle distributions exhibiting gyrophase bunching can linearly couple the eigenmodes of parallel propagation and destabilize otherwise stable magnetoplasmas. Previous research on the parallel stability of nongyrotropic populations has either considered homogeneous but time-varying distributions (TNG) associated with closed phase spaces (solutions of the homogeneous Vlasov equation) or homogeneous, stationary distributions (SNG) arising from open phase spaces (solutions of the Vlasov, or transport equation with source and sink terms). Destabilization of otherwise stable media in the TNG case was only reported when the unperturbed gyrophase bunching generated a finite perpendicular current that coupled the electrostatic and electromagnetic parallel eigenmodes at frequencies shifted by multiples of the cyclotron frequency of the TNG species. Here we demonstrate that introduction of SNG in a particle population of an otherwise stable magnetoplasma can (1) bring about unstable linear mode coupling among the parallel eigenmodes for finite perpendicular currents (as was the case for TNG distributions) albeit now without the frequency shifts, and (2) stimulate nonoscillatory purely growing waves (zero real frequency within a finite wavenumber band) in nongyrotropic environments that only couple parallel electromagnetic waves (zero unperturbed perpendicular current), behavior not encountered in TNG media.

Linear coupling effects originated in electron nongyrotropy

In analogy with ion nongyrotropy the gyrophase organization of electron populations also linearly couples the eigenmodes of gyrotropic parallel propagation in magnetoplasmas. The interaction, besides intensifying preexisting instabilities, excites new types of wave activity. Solutions of the nongyrotropic parallel dispersion equation and particle simulations illustrate instability enhancements and the stimulation of electrostatic and electromagetic wave growth in media that, apart from the electron gyrophase organization, are devoid of free energy sources.