S. Peter Gary

Ion-Driven Instabilities in the Solar Wind: Wind Observations of 19 March 2005

Abstract Intervals of enhanced magnetic fluctuations have been frequently observed in the solar wind. But it remains an open question as to whether these waves are generated at the Sun and then transported outward by the solar wind or generated locally in the interplanetary medium. Magnetic field and plasma measurements from the Wind spacecraft under slow solar wind conditions on 19 March 2005 demonstrate seven events of enhanced magnetic fluctuations at spacecraft‐frame frequencies somewhat above the proton cyclotron frequency and propagation approximately parallel or antiparallel to the background magnetic field B o. The proton velocity distributions during these events are characterized by two components: a more dense, slower core and a less dense, faster beam. Observed plasma parameters are used in a kinetic linear dispersion equation analysis for electromagnetic fluctuations at k x B o = 0; for two events the most unstable mode is the Alfvén‐cyclotron instability driven by a proton component temperature anisotropy T⊥/T|| > 1 (where the subscripts denote directions relative to B o), and for three events the most unstable mode is the right‐hand polarized magnetosonic instability driven primarily by ion component relative flows. Thus, both types of ion anisotropies and both types of instabilities are likely to be local sources of these enhanced fluctuation events in the solar wind.

Electromagnetic ion beam instabilities

The linear theory of electromagnetic instabilities driven by an energetic ion beam streaming parallel to a magnetic field in a homogeneous Vlasov plasma is considered. Numerical solutions of the full dispersion equation are presented. At propagation parallel to the magnetic field, there are four distinct instabilities. A sufficiently energetic beam gives rise to two unstable modes with right‐hand polarization, one resonant with the beam, the other nonresonant. A beam with sufficiently large T⊥/T∥ gives rise to the left‐hand ion cyclotron anisotropy instability at relatively small beam velocities, and a sufficiently hot beam drives unstable a left‐hand beam resonant mode. The parametric dependences of the growth rates for the three high beam velocity instabilities are presented here. In addition, some properties at oblique propagation are examined. It is demonstrated that, as the beam drift velocity is increased, relative maxima in growth rates can arise at harmonics of the ion cyclotron resonance for both...

Magnetic spectral signatures in the Earth's magnetosheath and plasma depletion layer

Correlations between plasma properties and magnetic fluctuations in the subsolar magnetosheath downstream of a quasi-perpendicular shock have been found and indicate that mirror and ion cyclotronlike fluctuations correlate with the magnetosheath proper and plasma depletion layer, respectively (Anderson and Fuselier, 1993). We explore the entire range of magnetic spectral signatures observed from the AMPTE/CCE spacecraft in the magnetosheath downstream of a quasi-perpendicular shock. The magnetic spectral signatures typically progress from predominantly compressional fluctuations, δB∥/δB⊥ ≈ 3, with F/Fp <0.2 ( F and Fp are the wave frequency and proton gyrofrequency, respectively) to predominantly transverse fluctuations, δB∥/δB⊥ ≈ 0.3, extending up to Fp. The compressional fluctuations are characterized by anticorrelation between the field magnitude and electron density, ne, and by a small compressibility, Ce ≡ (δne/ne)2(B/δB∥)2 ≈ 0.13, indicative of mirror waves. The spectral characteristics of the transverse fluctuations are in agreement with predictions of linear Vlasov theory for the H+ and He2+ cyclotron modes. The power spectra and local plasma parameters are found to vary in concert: mirror waves occur for β∥p (β∥p ≡ 2µ0npkT∥p/B2) ≈ 2, Ap ≡ T⊥p/T⊥ - 1 ≈ 0.4, whereas cyclotron waves occur for β∥p ≈ 0.2 and Ap ≈ 2. The transition from mirror to cyclotron modes is predicted by linear theory. The spectral characteristics overlap for intermediate plasma parameters. The plasma observations are described by Ap = 0.85β∥p−0.48 with a log regression coefficient of −0.74. This inverse Ap - β∥p correlation corresponds closely to the isocontours of maximum ion anisotropy instability growth, γm/Ω = 0.01, for the mirror and cyclotron modes. The agreement of observed properties and predictions of local theory suggests that the spectral signatures reflect the local plasma environment and that the anisotropy instabilities regulate Ap. We suggest that the spectral characteristics may provide a useful basis for ordering observations in the magnetosheath and that the Ap - β ∥p inverse correlation may be used as a beta-dependent upper limit on the proton anisotropy to represent kinetic effects.

Electromagnetic ion/ion instabilities and their consequences in space plasmas - A review

This paper reviews recent research on the theory and computer simulations of electromagnetic ion/ion instabilities and their consequences in space plasmas. ‘Ion/ion’ instabilities are growing modes in a collisionless plasma driven unstable by the relative streaming velocity v0of two distinct ion components such that v0is parallel or antiparallel to the uniform background magnetic field B00. The space physics regimes which display enhanced fluctuations due to these instabilities and which are reviewed in this paper include the solar wind, the terrestrial foreshock, the plasma sheet boundary layer, and distant cometary environments.

The mirror and ion cyclotron anisotropy instabilities

The linear dispersion equation for fully electromagnetic waves and instabilities at arbitrary directions of propagation relative to a background magnetic field Bo in a homogeneous Vlasov plasma is solved numerically for bi-Maxwellian particle distributions. For isotropic plasmas the dispersion and damping of the three modes below the proton cyclotron frequency are studied as functions of βi and Te/Ti. The transport ratios of helicity, cross-helicity, Alfven ratio, compressibility, and parallel compressibility are defined. Under the condition that the proton temperature perpendicular to Bo is greater than the parallel temperature, the growth rates and transport ratios of the mirror instability and the ion cyclotron anisotropy instability are examined and compared. Both the proton parallel compressibility and the proton Alfven ratio are significantly different for the two growing modes.

The electron‐acoustic mode

This paper examines electrostatic modes in an unmagnetized, homogeneous, Vlasov plasma with three Maxwellian components: ions, hot electrons, and cool electrons. In such a plasma, the electron‐acoustic mode with frequencies between the ion and electron plasma frequencies may propagate with light damping. The conditions that allow propagation of this mode, which is distinct from the well‐known ion‐acoustic and Langmuir waves, are given in detail; approximate necessary conditions are 10≲Th/Tc and 0<nc<0.8ne, where the subscripts c, h, and e refer to the cool and hot electron components and the total electron population, respectively.

Solar wind magnetic fluctuation spectra: Dispersion versus damping

Solar wind magnetic fluctuation power spectra at frequencies ƒ < 1 Hz are commonly observed to have the approximate power law dependence ƒ−5/3. These observations may be described by Kolmogorov diffusion in wavenumber space which defines what is called the inertial range. At wavelengths shorter than the inertial range, spectra often are observed to have steeper power laws. This intermediate wavelength regime is sometimes called the dissipation range because it has been suggested that this steeper slope is caused by collisionless damping of Alfven and magnetosonic waves. Here it is argued that, at intermediate wavenumbers, Alfven fluctuations are suppressed by proton cyclotron damping, so the observed power spectra are likely to consist of weakly damped magnetosonic/whistler waves which have an increased wavenumber diffusion rate due to their dispersion. Numerical calculations at βp ≲ 2.5 with a model representing this picture yield fluctuation spectra with steep power laws at intermediate wavenumbers and with sharp cutoffs due to electron cyclotron damping at still shorter wavelengths. The term “dispersion range” is more appropriate to describe steep power law spectra in the intermediate wavenumber regime.

Ion anisotropy instabilities in the magnetosheath

Recent observations in Earths magnetosheath have delineated several different kinds of magnetic fluctuation spectra below the proton cyclotron frequency. This paper provides a theoretical interpretation for some of these observations, describing solutions of the linear Vlasov dispersion equation for fully electromagnetic instabilities for particle distributions which model those observed in the magnetosheath. The model assumes a high-[beta], homogeneous plasma with bi-Maxwellian, anisotropic (T[sub [perpendicular]p]/T[sub [parallel]p]) protons; a similarly anisotropic, tenuous, doubly ionized helium component; and isotropic cool electrons. This model also assumes a uniform magnetic field and no relative average drifts among the components. This model yields three growing modes: the proton cyclotron anisotropy, the helium cyclotron anisotropy, and the mirror instabilities. The regimes in [beta][sub p] versus T[sub [perpendicular]p]/T[sub [parallel]p] space which correspond to maximum growth rates of the mirror and proton cyclotron anisotropy instabilities are computed, including helium densities appropriate for the magnetosheath; the results show very good agreement with the observations of mirror-like and proton-cyclotron-like events. This agreement with observations implies that the transition between cyclotron and mirror fluctuation dominance is consistent with linear theory. 33 refs., 7 figs., 1 tab.

Wind/SWE observations of firehose constraint on solar wind proton temperature anisotropy

�2 , and the fitting parameters Sp � 1a ndap ’ 0.7. Observations from the Wind spacecraft are reported here. These measurements show for the first time with a comprehensive plasma and magnetic field data set that this constraint is statistically satisfied in the solar wind near 1 AU, with best-fit values of Sp = 1.21 ± 0.26 and ap = 0.76 ± 0.14. INDEX TERMS: 7871 Space Plasma Physics: Waves and instabilities; 7867 Space Plasma Physics: Wave/particle interactions; 2164 Interplanetary Physics: Solar wind plasma. Citation: Kasper, J. C., A. J. Lazarus, and S. P. Gary, Wind/SWE observations of firehose constraint on solar wind proton temperature anisotropy, Geophys. Res. Lett., 29(17), 1839, doi:10.1029/ 2002GL015128, 2002.

The proton cyclotron instability and the anisotropy/β inverse correlation

Spacecraft observations in the strongly compressed subsolar magnetosheath show an inverse correlation between the proton temperature anisotropy (T{sub {perpendicular}p}/T{sub {parallel}p} > 1 where {perpendicular} and {parallel} denote directions perpendicular and parallel to the background magnetic field) and the parallel proton {beta}({beta}{sub {parallel}p}). This manuscript uses one-dimensional hybrid simulations of the proton cyclotron anisotropy instability in homogeneous electron-proton plasmas to study this correlation which may represent a limited closure relation for fluid theories of anisotropic space plasmas. The emphasis is on driven simulations which increase the temperature anisotropy by periodically reducing the magnetic-field-aligned velocities of the protons. The late-time states from ensembles of both initial value and driven simulations yield very similar expressions for the proton anisotropy/{beta}{sub {parallel}p} inverse correlation, and provide a basis for explaining differences between sheath observations from different spacecraft. The driven simulations also yield expressions for the maximum instability growth rate and the fluctuating field energy as functions of {beta}{sub {parallel}p} and a parameter characterizing the anisotropy driver. 50 refs., 5 figs.