H. K. Wong

Observational constraints on the dynamics of the interplanetary magnetic field dissipation range

The dissipation range for interplanetary magnetic field fluctuations is formed by those fluctuations with spatial scales comparable to the gyroradius or ion inertial length of a thermal ion. It is reasonable to assume that the dissipation range represents the final fate of magnetic energy that is transferred from the largest spatial scales via nonlinear processes until kinetic coupling with the background plasma removes the energy from the spectrum and heats the background distribution. Typically, the dissipation range at 1 AU sets in at spacecraft frame frequencies of a few tenths of a hertz. It is characterized by a steepening of the power spectrum and often demonstrates a bias of the polarization or magnetic helicity spectrum. We examine Wind observations of inertial and dissipation range spectra in an attempt to better understand the processes that form the dissipation range and how these processes depend on the ambient solar wind parameters (interplanetary magnetic field intensity, ambient proton density and temperature, etc.). We focus on stationary intervals with well-defined inertial and dissipation range spectra. Our analysis shows that parallel-propagating waves, such as Alfven waves, are inconsistent with the data. MHD turbulence consisting of a partly slab and partly two-dimensional (2-D) composite geometry is consistent with the observations, while thermal paxticle interactions with the 2-D component may be responsible for the formation of the dissipation range. Kinetic Alfven waves propagating at large angles to the background magnetic field are also consistent with the observations and may form some portion of the 2-D turbulence component.

A cross‐field current instability for substorm expansions

We investigate a cross-field current instability (CFCI) as a candidate for current disruption during substorm expansions. The numerical solution of the linear dispersion equation indicates that (1) the proposed instability can occur at the inner edge or the midsection of the neutral sheet just prior to the substorm expansion onset although the former environment is found more favorable at the same drift speed scaled to the ion thermal speed, (2) the computed growth time is comparable to the substorm onset time, and (3) the excited waves have a mixed polarization with frequencies near the ion gyrofrequency at the inner edge and near the lower hybrid frequency in the midtail region. On the basis of this analysis we propose a substorm development scenario in which plasma sheet thinning during the substorm growth phase leads to an enhancement in the relative drift between ions and electrons. This results in the neutral sheet being susceptible to the CFCI and initiates the diversion of the cross-tail current through the ionosphere. Whether or not a substorm current wedge is ultimately formed is regulated by the ionospheric condition. A large number of substorm features can be readily understood with the proposed scheme. These include (1) precursory activities (pseudobreakups) prior to substorm onset, (2) substorm initiation region to be spatially localized, (3) three different solar wind conditions for substorm occurrence, (4) skew towards evening local times for substorm onset locations, (5) different acceleration characteristics between ions and electrons, (6) tailward spreading of current disruption region after substorm onset, and (7) local time expansion of substorm current wedge with possible discrete westward jump for the evening expansion.

Dissipation range dynamics : Kinetic Alfvén waves and the importance of βe

In a previous paper we argued that the damping of obliquely propagating kinetic Alfven waves, chiefly by resonant mechanisms, was a likely explanation for the formation of the dissipation range for interplanetary magnetic field fluctuations. This suggestion was based largely on observations of the dissipation range at 1 AU as recorded by the Wind spacecraft. We pursue this suggestion here with both a general examination of the damping of obliquely propagating kinetic Alfven waves and an additional examination of the observations. We explore the damping rates of kinetic Alfven waves under a wide range of interplanetary conditions using numerical solutions of the linearized Maxwell-Vlasov equations and demonstrate that these waves display the nearly isotropic dissipation properties inferred from the previous paper. Using these solutions, we present a simple model to predict the onset of the dissipation range and compare these predictions to the observations. In the process we demonstrate that electron Landau damping plays a significant role in the damping of interplanetary magnetic field fluctuations which leads to significant heating of the thermal electrons.

Contribution of Cyclotron-resonant Damping to Kinetic Dissipation of Interplanetary Turbulence

We examine some implications of inertial range and dissipation range correlation and spectral analyses extracted from 33 intervals of Wind magnetic field data. When field polarity and signatures of cross helicity and magnetic helicity are examined, most of the data sets suggest some role of cyclotron-resonant dissipative processes involving thermal protons. We postulate that an active spectral cascade into the dissipation range is balanced by a combination of cyclotron-resonant and noncyclotron-resonant kinetic dissipation mechanisms, of which only the former induces a magnetic helicity signature. A rate balance theory, constrained by the data, suggests that the ratio of the two mechanisms is of order unity. While highly simplified, this approach appears to account for several observed features and explains why complete cyclotron absorption, and the corresponding pure magnetic helicity signature, is usually not observed.

Electromagnetic cyclotron-loss-cone instability associated with weakly relativistic electrons

The amplification of fast extraordinary mode waves with frequencies very close to the electron cyclotron frequency is investigated for a plasma which consists of a weakly relativistic electron component with a loss-cone type distribution and a cold background electron component. The basic mechanism of the amplification is attributed to a relativistic cyclotron resonance between the wave and the energetic electrons. The method employed in the present analysis enables us to solve the dispersion relation in a self-consistent manner for arbitrary ratio of the densities of the energetic and background electrons. It is found that the maximum growth rates occur at certain values of ω 2 pe /Ω 2 e and the angular dependence of the growth rate is sensitive to the ratios ω 2 pe /Ω 2 e and n e / n b . Here ω pe and Ω e are the electron plasma frequency and the electron cyclotron frequency, respectively, and n e and n b denote the number densities of the energetic and background electrons, respectively.

Two-fluid theory of drift-kink instability in a one-dimensional neutral sheet

A thin current sheet with thickness comparable to the thermal ion gyroradius possesses free energy capable of driving a number of plasma instabilities for its destabilization. Among these instabilities is the drift-kink instability (DKI) unveiled through previous numerical simulations. To advance our theoretical understanding of DKI, we formulate a two-fluid theory with finite ion and electron temperatures in a one-dimensional (Harris) current sheet to examine the linear properties of DKI. We reduce the stability analysis to solving a nonlocal eigenvalue equation. We find that the eigenvalue equation gives a number of growing modes with both symmetric and antisymmetric magnetic perturbations with respect to the neutral sheet. These solutions have high growth rates, generally of the same order as their real frequencies, which are sizable fractions of the ion gyrofrequencies evaluated outside the current sheet. Near the center of the current sheet, the eigenfunction exhibits fine spatial structures with dimensions much smaller than the ion inertial length. These structures become progressively broadened as the ion to electron mass ratio is arbitrarily reduced. This theoretical result is potentially useful in assessing the impact of adopting unrealistic ion to electron mass ratio in numerical simulations of DKI or thin current sheet stability.

Electron cyclotron wave generation by relativistic electrons

We show that an energetic electron distribution which has a temperature anisotropy (T⊥b > T∥b), or which is gyrating about a DC magnetic field, can generate electron cyclotron waves with frequencies below the electron cyclotron frequency. Relativistic effects are included in solving the dispersion equation and are shown to be quantitatively important. The basic idea of the mechanism is the coupling of the beam mode to slow waves. The unstable electron cyclotron waves are predominantly electromagnetic and right-hand polarized. For a low-density plasma in which the electron plasma frequency is less than the electron cyclotron frequency, the excited waves can have frequencies above or below the electron plasma frequency, depending upon the parameters of the energetic electron distribution. This instability may account for observed Z mode waves in the polar magnetosphere of the Earth and other planets.

A purely growing electromagnetic mode operative in the geomagnetic tail

This paper discusses a purely growing mode driven by a cross-field current. The study was motivated by a recent article by Chang et al. (1990). The present discussion pays special attention to two aspects. One is to generalize the analysis by Chang et al. (1990) so that the unmagnetized-ion approximation used by these authors is removed, and the other is to apply the theory to several regions in the magnetotail where the value of the plasma beta is in general very high. The present analysis is restricted to waves propagating along the ambient magnetic field. The high ion beta limit is discussed by considering two different situations. The first is to fix the strength of the ambient magnetic field but to increase the plasma temperature, and the second is to maintain the plasma temperature but to decrease the ambient magnetic field strength. It is found that in the former case the mode stabilizes when βi → ∞, but in the latter case the instability persists even if βi → ∞ (although the growth rate is significantly reduced). The present theory is applied to three regions in the magnetotail. These are: (1) the inner edge region, (2) the midtail region, and (3) the neutral sheet of a distant magnetotail. It is found that, among these three regions, for a given value of υ0 / αi, where υ0 is the net cross-field drift speed between the electrons and the ions and αi is the ion thermal speed, the growth rate in the neutral sheet is found to be the largest.

Electron beam excitation of upstream waves in the whistler mode frequency range

We examine whistler mode instabilities arising from electron beams in interplanetary space at 1 AU. Both parallel and obliquely propagating solutions are considered. We demonstrate that the generation of two simultaneous whistler mode waves is possible, and even reasonably likely, for beam parameters frequently encountered upstream of the Earths bow shock and at interplanetary shocks. We also explore the generation of left-hand polarized waves at whistler mode frequencies under these same conditions. We offer both parametric variations derived from numerical solutions of the various instabilities as well as an analytical treatment of the problem which succeeds in unifying the various numerical results.

Association of electron conical distributions with upper hybrid waves

The particle and plasma wave data of the DE 1 and Swedish Viking satellites shows that incense (> 1 mV/m) upper hybrid emissions are sometimes present in the mid-altitude polar magnetosphere on both the dayside cusp/cleft and the nightside auroral regions and that waves near the upper hybrid frequency are often associated with electron conical distributions. These observations are consistent with the production of at least some electron conical distributions by oblique heating of the electrons by upper hybrid waves. Examination of the wave data to establish the role of parallel heating remains to be performed.