Fausto T. Gratton

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.

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.

Concerning a problem on the Kelvin‐Helmholtz stability of the thin magnetopause

[1]xa0According to incompressible MHD theory, when the magnetopause is modeled as a tangential discontinuity with jumps in the field and flow parameters, it is Kelvin-Helmholtz (KH) stable when the following inequality is satisfied: (ρ0,1ρ0,2)(V,1 − V,2)2 < (4π)−1(ρ0,1 + ρ0,2)[(B,1)2 + (B,2)2] (a). Here the indices 1 and 2 refer to quantities on either side of the magnetopause, ρ0 is the plasma density, and V, Bκ are the projections of the plasma velocity and magnetic field on the direction of the wave vector , respectively. An example of a continuous velocity profile with finite thickness Δ that can be solved in closed form is presented for which condition (a) is satisfied. Yet the configuration can be shown to be KH unstable, and it approaches stability only in the limit Δ → 0. Using hyperbolic tangent profiles for ρ0, , and , and solving the stability problem numerically with parameters typical of the dayside magnetopause, we show cases of unstable configurations, all of which are stable according to (a). This possibility, as far as we know, has passed unnoticed in the literature. Being incompressible, the theory applies to subsonic regions of the dayside magnetopause. We conclude that condition (a) must be used with care in data analysis work.

On the possible excitation of electromagnetic ion cyclotron waves in solar ejecta

We study the possibility of exciting electromagnetic ion cyclotron waves (EICWs) in solar ejecta (CMEs) by a kinetic instability driven by ion temperature anisotropies. Our approach is to vary key parameters about assumed baseline values. Since Tp,‖ > Tp,⊥ in most solar ejecta, the polarization of the unstable waves is right-handed. If the average proton beta is low (βp ≤ 0.3), the activity is negligible for moderate temperature ratios, Tp,‖/Tp,⊥. Increasing βp increases both the frequency range and the instability growth rate. Increasing the temperature anisotropy brings about qualitatively similar effects as increasing βp, with comparable growth rates. Increasing the relative alpha-to-proton density ratio η has two effects: the active frequency range is shifted toward lower frequencies and the growth rate increases. Between η = 0 and η = 0.15, the maximum growth rate increases by a factor of ∼20, highlighting the importance of the alphas for generating this instability. A case that may represent some magnetic clouds with exceptional parameters, βp = 0.2, Tp,‖/Tp,⊥ = 10, and η = 0.08 − 0.15, is considered. The maximum growth rate is found to be twice the reference CME case, while the active frequency range is 3 times wider. We conclude that EICWs should be present in some ejecta and possibly also in those magnetic clouds with relatively weak magnetic field, high He++ content, and large Tp,‖/Tp,⊥ ratios, and whose βp is high, for example, through interaction with a succeeding fast stream. We also suggest that substantial changes with respect to normal conditions should occur in the power spectrum of EICWs in the terrestrial plasma depletion layer when a CME, or a magnetic cloud, with negative anisotropy passes Earth.

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.

Theoretical properties of electromagnetic ion cyclotron waves in the terrestrial, dayside, low‐latitude plasma depletion layer under uncompressed magnetosheath conditions

We present a numerical study of electromagnetic ion cyclotron wave (EICW) activity (growth and damping) in the terrestrial plasma depletion layer (PDL) under typical solar wind conditions. Aside from the α particles, all parameters in this study are as measured by AMPTE/IRM during 13 magnetosheath passes under low magnetic shear made in 1984-1985 and calculated with the superposed epoch analysis technique [Phan et al., 1994]. Three locations within the PDL are specified using minutes before the key time identifying the magnetopause crossing in the superposed epoch analysis (0, -5, -10 min). Our work thus characterizes average properties of EICWs along radial profiles in the PDL in the magnetic latitude and local time ranges of ±30° and 0800 to 1600 hours, respectively. In order to study the situation in the subsolar region more closely, we calculate also with PDL parameters acquired by AMPTE/IRM during the pass nearest to local noon (October 24, 1985). For a particle parameters we take reasonable estimates but also study the effect of varying the a particle temperature anisotropy. The influence of a small a particle-proton relative drift is also included. At the magnetopause we find one peak of activity in the frequency range from 0.2 to 0.3 Ω p (the proton cyclotron frequency), which is thus below the resonance frequency of the α particles (0.5 Ω p ) At locations 10 min and 5 min prior to the magnetopause crossing, the activity spectrum bifurcates, with both peaks below the α resonance and separated by an absorption band. Our explanation of this result is that as we move away from the magnetopause, the increasing plasma β enables the protons to overcome the a absorption. With a relative a particle-proton drift of 10% of the local Alfven speed, the absorption band is removed, but all excited frequencies are still less than 0.5 Ω p . The absorption band may also be removed by a high a particle thermal anisotropy coupled with a low a particle β along the magnetic field. On October 24, 1985, the PDL was characterized by a wider variation of β and a larger proton temperature anisotropy. Under these conditions a second EICW activity peak appears between 0.5 and 0.6 Ω p at the two locations away from the magnetopause, in addition to the α peak mentioned above, which we ascribe to the larger anisotropy of the protons. Again, a small α particle drift can remove the activity minimum. With the stagnation line flow pattern in the PDL found by AMPTE/IRM on these passes, we hypothesize that EICWs excited near noon subsequently travel along the magnetic field, bringing EICW activity to high latitudes. However, EICWs generated away from noon will be damped out while being carried to the magnetopause flanks.

Hydromagnetic oscillations of a tangential discontinuity in the Chew, Goldberger, and Low approximation

The differential equation for linear modes of oscillation of plane parallel flows of plasmas along an external magnetic field in the Chew, Goldberger, and Low approximation is obtained. Properties of modes for a tangential discontinuity are studied for the case when the surface is modulated along the magnetic field. Overstable modes found by other authors are shown to be spurious. Regions of existence of modes, proper frequencies, and spatial dependence of the perturbation are given. It is found that, broadly speaking, low β plasmas should be free of surface instabilities for all values of the flow velocity, whereas high β plasmas can be unstable if the flow velocity is nearly sonic. Changes in the anisotropy do not substantially affect the general picture of the problem.

Supersonic mixing layers: Stability of magnetospheric flanks models

Compressibility has a strong influence on the stability of velocity shear layers when the difference of velocity ?V across the flow becomes supersonic. The flanks of the Earths magnetopause are normally supersonic Ms > 1, and super-Alfv?nic MA > 1, depending on the distance from the dayside terminator (Ms and MA are the sonic and Alfv?n Mach numbers of the magnetosheath plasma, respectively). The stability of MHD supersonic flows depends, also on several other features, such as the finite thickness ? of the boundary layer, the relative orientation of velocity and magnetic fields, the density jump across the boundary and the magnetic shear angle. We analyze the MHD stability of some representative flank sites modeled after data from spacecraft crossings of the magnetopause under different interplanetary conditions, complementing these cases with extrapolations of likely conditions upstream, and downstream of the crossing site. Under northward interplanetary magnetic field conditions, there are solar wind regimes such that the near, but already supersonic, flank of the magnetopause may be locally stable. Stability is possible, e.g., when Ms becomes larger than ~1.2?1.4 while MA remains smaller than 1.2, and there is magnetic shear between the geomagnetic and the interplanetary magnetic field. Solar winds favouring local stability of the boundary layer are cold, not-too-dense plasmas, with strong magnetic fields, so that MA is smaller, while Ms is larger, than normal values of the magnetosheath flow. A gap between dayside and tail amplifying regions of Kelvin-Helmholtz disturbances over the magnetopause may exist when the above conditions are realized.