S. P. Gary

Identification of low-frequency fluctuations in the terrestrial magnetosheath

On the basis of MHD theory we develop a scheme for distinguishing among the four low-frequency modes which may propagate in a high-β anisotropic plasma such as the magnetosheath: the fast and slow magnetosonic, the Alfven, and mirror modes. We use four parameters: the ratio of transverse to compressional powers in the magnetic field, the ratio of the wave powers in the thermal pressure and in the magnetic field, the ratio of the perturbations in the thermal and magnetic pressures, and the ratio of the wave powers in the velocity and in the magnetic field. In the test case of an Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) magnetosheath pass near the Sun-Earth line downstream of a quasi-perpendicular shock, the four modes can be clearly distinguished both spatially and spectrally. Near the bow shock, the waves are Alfvenic in a large frequency range, 1 to 100 mHz. In the middle and inner magnetosheath, the waves below 10 mHz are Alfvenic. The fast mode waves occur in the higher-frequency end of the enhanced spectrum, 80 mHz for the middle magnetosheath and 55 mHz for the inner sheath. The wave enhancement in the intermediate frequencies is slow modes in the inner sheath and mirror modes in the middle sheath. This confirms the earlier report of the existence of the slow mode waves near the magnetopause. These slow waves provide evidence that the magnetopause is an active source of the waves in the sheath. We also show that the measured frequency of a wave is close to an invariant if the magnetosheath flow is in a steady state. Therefore changes in the frequencies of enhanced waves indicate emergence, or damping, or mode conversion of the waves.

Electron temperature in the ambient solar wind: Typical properties and a lower bound at 1 AU

Our understanding of what controls the solar wind electron temperature is far from complete. Previous studies from the Vela and IMP spacecraft have suggested that twice the proton temperature or an assumed average of ∼ 150,000 K are reasonable estimations of total electron temperature at 1 AU. Eighteen months of continuous ISEE 3 solar wind data are analyzed in this paper and are found to have a mean electron temperature of 141,000 ± 38,000 K, in good agreement with past measurements. No correlation is found between electron temperature and other solar wind parameters, including proton temperature. However, a very distinct lower bound on the electron temperature is found; this bound increases with proton temperature and is observed by both ISEE 3 and Ulysses spacecraft. The bound is also found to vary with bulk solar speed of the solar wind and with distance from the Sun. Solar wind plasma observed following stream interactions are often associated with temperatures near this bound, and enhanced electromagnetic wave activity in the 18–100 Hz range is coincident with intervals where this apparent temperature coupling is observed, suggesting the possible presence of wave-particle interactions. Possible explanations for the existence of this electron temperature bound are explored, but no definitive answer has been found at this time.

Two-dimensional simulations of ion anisotropy instabilities in the magnetosheath

Ion populations with large perpendicular temperature anisotropies in the magnetosheath can excite both the mirror and proton cyclotron anisotropy instabilities. We compare kinetic aspects of these two instabilities using two-dimensional hybrid simulations, expanding upon an earlier work using one-dimensional simulations. Three simulation runs are examined: one in which the proton cyclotron instability has a higher linear growth rate, the second in which the mirror instability grows more rapidly, and the third in which the linear growth rates are identical. The last two runs include a large density of isotropic He++ ions to suppress the cyclotron instability. We find that initial growth occurs in all three runs at approximately the frequencies, wavenumbers, and obliquities expected by linear theory. As the system evolves, the power in both instabilities shifts to longer wavelength modes, and the characteristic frequency of the proton cyclotron instability decreases. In the first two runs, the instability with the larger linear growth rate dominates the wave energy at saturation, and in the third, the two instabilities contribute about equally. The proton cyclotron instability is relatively more important to wave-particle energy exchange than the mirror instability; its importance to proton isotropization is generally greater than that suggested by its contribution to the total wave energy, and it is almost solely responsible for heating the helium ions.

Dispersion relation analysis of solar wind turbulence

Frequency versus wave number diagram of turbulent magnetic fluctuations in the solar wind was determined for the first time in the wide range over three decades using four Cluster spacecraft. Almost all of the identified waves propagate quasi‐perpendicular to the mean magnetic field at various phase speeds, accompanied by a transition from the dominance of outward propagation from the Sun at longer wavelengths into mixture of counter‐propagation at shorter wavelengths. Frequency‐wave number diagram exhibits largely scattered populations with only weak agreement with magnetosonic and whistler waves. Clear identification of a specific normal mode is difficult, suggesting that nonlinear energy cascade is operating even on small‐scale fluctuations.

Time History of Events and Macroscale Interactions during Substorms observations of a series of hot flow anomaly events

[1] A series of seven hot flow anomaly (HFA) events has been observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) C spacecraft just upstream from the subsolar bow shock from 0100 to 1300 UT on 19 August 2008. Both young (no shocks at edges, two distinct ion populations) and mature (strong shocks at edges, a single hot ion population) HFAs have been observed. Further upstream, THEMIS B observed four proto‐HFAs (density and magnetic field strength depletions, plasma heating but no flow deflections) which later developed into HFAs observed by THEMIS C. We present evidence indicating that electromagnetic right‐hand resonant ion beam instabilities heat ions inside HFAs. Observations of small‐amplitude perturbations (DB/B < 50%) consistent with the resonant ion beam instability in a proto‐HFA, 30 s electromagnetic waves (DB/B ∼ 1) in a young HFA, and magnetic pulsations in a mature HFA (DB/B ∼ 4) indicate that they are at early, middle, and late (nonlinear) stages of the electromagnetic right‐hand resonant ion beam instabilities. Both young and mature HFAs are associated with strong electromagnetic waves near the lower hybrid frequency (0.1–1 Hz). The lower hybrid waves are the likely source of the electron heating inside HFAs. THEMIS B observations of four proto‐HFAs which later developed into HFAs observed by THEMIS C indicate that these four HFAs might extend beyond 14 RE upstream from the bow shock, while the other three HFAs may extend between 5 and 14 RE upstream from the bow shock. We present an example of an HFA that lies displaced toward the side of the tangential discontinuity with a quasi‐parallel bow shock configuration rather than lying centered on the driving interplanetary magnetic field discontinuity.

Kinetic physics of the mirror instability

Kinetic mechanisms for the growth and saturation of the mirror instability are described using one-dimensional hybrid simulations. Two parameter regimes are considered. In the first regime, a relatively small ion anisotropy excites a slowly growing instability that produces small-amplitude waves; most ions respond to the waves as an adiabatic fluid. In the second regime a large anisotropy excites a rapidly growing instability that generates large-amplitude waves; the response of many ions in this case is nonadiabatic. The difference in ion response is due to the relative importance of two ion populations, resonant and nonresonant. Resonant ions, those ions with low velocities parallel to the background magnetic field, contribute to the growth of the instability as a result of their gyrointeractions with the noncoplanar component of the wave electric field and respond to the mirror waves nonadiabatically. Nonresonant ions, those with large parallel velocities, respond as an adiabatic fluid. In both regimes, ion anisotropy is reduced by means of the magnetic mirror force; in the second regime, the anisotropy is further reduced as a consequence of the field rotation at magnetic minima. The anisotropy reduction reduces the free energy available for wave growth and leads to the saturation of the mirror instability.

Steepening of parallel propagating hydromagnetic waves into magnetic pulsations: A simulation study

The steepening mechanism of parallel propagating low-frequency MHD-like waves observed upstream of the earths quasi-parallel bow shock has been investigated by means of electromagnetic hybrid simulations. It is shown that an ion beam through the resonant electromagnetic ion/ion instability excites large-amplitude waves, which consequently pitch angle scatter, decelerate, and eventually magnetically trap beam ions in regions where the wave amplitudes are largest. As a result, the beam ions become bunched in both space and gyrophase. As these higher-density, nongyrotropic beam segments are formed, the hydromagnetic waves rapidly steepen, resulting in magnetic pulsations, with properties generally in agreement with observations. This steepening process operates on the scale of the linear growth time of the resonant ion/ion instability. Many of the pulsations generated by this mechanism are left-hand polarized in the spacecraft frame.

Hybrid simulations of plasma transport by Kelvin‐Helmholtz instability at the magnetopause: Density variations and magnetic shear

[1] Two-dimensional hybrid (kinetic ions and massless fluid electrons) simulations of the Kelvin-Helmholtz instability (KHI) for a magnetopause configuration with a varying density jump and magnetic shear across the boundary are carried out to examine how the transport of magnetosheath plasma into the magnetosphere is affected by these conditions. Low magnetic shear conditions where the magnetosheath magnetic field is within 30° of northward is included in the simulations because KHI is thought to be important for plasma transport only for northward or near-northward interplanetary magnetic field orientations. The simulations show that coherent vortices can grow for these near-northward angles and that they are sometimes more coherent than for pure northward conditions because the turbulence which breaks down these vortices is reduced when there are magnetic tension forces. With increasing magnetic shear angle and increasing density jump, the growth rate is reduced, and the vortices do not grow to as large of a size, which reduces the plasma transport. By tracking the individual particle motions, diffusion coefficients can be obtained for the system, where the diffusion is not classical in nature but instead has a time dependence resulting from both the increasingly large-scale vortex motion and the small-scale turbulence generated in the breakdown of the instabilities. Results indicate that diffusion on the order of 10 9 m 2 /s could possibly be generated by KHI on the flanks of the magnetosphere.

The great solar energetic particle events of 1989 observed from geosynchronous orbit

Los Alamos energetic proton instruments at geosynchronous orbit observed more major solar energetic particle events during 1989 than any other year since this series of detectors began observations in 1976. The temporal flux profiles of four intervals, which contain six distinct events, are compared illustrating the uniqueness of each event. Characteristic risetime and decay time are computed for each event. During two of these events, brief order-of-magnitude increases of the proton flux are observed. They are associated with sudden commencement events and dramatic changes in the solar wind. We conclude that these two brief events are likely the result of shock acceleration in the solar wind. We have fit the measured count rates to a spectral form which is exponential in rigidity, and we have examined the changes in spectral slope with time for each of the four intervals. In general, harder spectra are measured near the onset of an event followed by a softening of the spectrum as the fluxes decay. We have also investigated the effects of these events on geomagnetic activity by comparing the fluxes of >30-keV electrons at geosynchronous orbit and Kp geomagnetic index during the early part of two of the solar energetic particle events.

Kinetic properties of mirror waves in magnetosheath plasmas

Linear and nonlinear properties of waves excited by the mirror instability in high beta, low anisotropy (β⟂ > β‖ ≥ 1) plasmas characteristic of the magnetosheath are investigated using linear theory and one-dimensional hybrid simulations. The mechanisms for wave growth and saturation at low amplitudes are discussed. A new method is considered for generating the large amplitude mirror waves observed in the magnetosheath based on external compression of magnetic flux tubes. Simulations in which the anisotropy is maintained by recycling the ions shows this process can inhibit the growth of ion cyclotron waves and enhances the growth of mirror waves.