Dust grains in plasma acquire a large negative charge, and can constitute a strongly coupled system. If the plasma is stationary, the plasma-mediated electrostatic potential around a single grain can be calculated by orbital-motion-limited (OML) theory, including ion absorption at the grain surface. This potential is repulsive at all ranges, and falls off as r−2 at long range. Nonlinear modifications occur when there are several grains, but the interaction is still repulsive. If the plasma is streaming by the grains, each grain generates a wake field potential which can be calculated via linear response theory, and which attracts other grains to stationary points behind the grain. There is in addition an attractive force between grains, due to ion-impact momentum deposition. In certain parameter regimes, this “shadowing” force can yield a weak net attraction at long range. Trapped-ion effects are significant at high plasma density, but have not yet been calculated.

[1] We present a numerical study of the propagation of VLF whistler waves in the magnetospheric plasma. In this study the plasma is considered to be homogeneous in the direction along the ambient magnetic field and strongly inhomogeneous across it. The goal of this investigation is to understand whistler propagation in magnetic-field-aligned channels (also called ducts) with either enhanced or depleted plasma density. In particular, the paper is focused on situations where the transverse scale size of the duct is comparable to or smaller than the perpendicular wavelength of the whistler. In this case, classical analysis of the whistler dynamics based on the geometrical optic approximation becomes invalid, and numerical solutions of the full wave equations should be performed. Our simulations extend the earlier analysis based on the ray-tracing technique and analytical studies of the very low frequency wave equations. We show that high-density ducts are inherently leaky and this leakage depends on the perpendicular wavelength of the wave inside the duct. We also show that whistler trapping occurs not only at density maxima and minima but also at critical points along a density gradient. This effect can explain whistler guiding along strong transverse plasma density gradients at the plasmapause.

The problem of electrostatic shielding around a small spherical collector immersed in nonflowing plasma, and the related problem of electron and ion flow to the collector, date to the origins of plasma physics. Calculations have typically neglected collisions, on the grounds that the mean free path is long compared to the Debye length. However, it has long been suspected that negative-energy trapped ions, created by occasional collisions, could be important. This paper presents self-consistent analytic calculations of the density and distribution function of trapped and untrapped ions, the potential profile, the ion and electron current to the collector, and the floating potential and charge of the collector. Under typical conditions for dust grains immersed in a discharge plasma, trapped ions are found to dominate the shielding near the grain, substantially increase the ion current to the grain, and suppress the floating potential and grain charge, even when the mean free path is much greater than the Debye length.

Recent observations indicate that low‐altitude (below 1500 km) ion energization and thermal ion upwelling are colocated in the convective flow reversal region. In this region the convective velocity V⊥ is generally small but spatial gradients in V⊥ can be large. As a result, Joule heating is small. The observed high level of ion heating (few electron volts or more) cannot be explained by classical Joule heating alone but requires additional heating sources such as plasma waves. At these lower altitudes, sources of free energy are not obvious and hence the nature of ion energization remains ill understood. The high degree of correlation of ion heating with shear in the convective velocity (Tsunoda et al., 1989) is suggestive of an important role of velocity shear in this phenomenon. We provide more recent evidence for this correlation and show that even a small amount of velocity shear in the transverse flow is sufficient to excite a large‐scale Kelvin‐Helmholtz mode, which can nonlinearly steepen and give rise to highly stressed regions of strongly sheared flows. Furthermore, these stressed regions of strongly sheared flows may seed plasma waves in the range of ion cyclotron to lower hybrid frequencies, which are potential sources for ion heating. This novel two‐step mechanism for ion energization is applied to typical observations of low‐altitude thermal ion upwelling events.

A kinetic theory in the form of an integral equation is provided to study the electrostatic oscillations in a collisionless plasma immersed in a uniform magnetic field and a nonuniform transverse electric field. In the low temperature limit (kyρi ≪1, where ky is the wave vector in the y direction and ρi is the ion gyroradius) the dispersion differential equation is recovered for the transverse Kelvin–Helmholtz modes for arbitrary values of k∥, where k∥ is the component of the wave vector in the direction of the external magnetic field assumed in the z direction. For higher temperatures (kyρi>1) the ion-cyclotron-like modes described earlier in the literature by Ganguli, Lee, and Palmadesso [Phys. Fluids 28, 761 (1985)] are recovered. In this article the integral equation is reduced to a second-order differential equation and a study is made of the kinetic Kelvin–Helmholtz and the ion-cyclotron-like modes that constitute the two branches of oscillation in a magnetized plasma including a transverse inhomogeneous dc electric field.

The threshold current for the ion-acoustic branch is shown to be significantly lower than the ioncyclotron branch and insensitive to the ion͞electron temperature ratio if there is a transverse gradient in the relative magnetic field aligned drift, V d , and j͑k y ͞k z ͒ ͑1͞V i ͒ ͑dV d ͞dx͒j is sufficiently large. The effect persists even when jdV d ͞dxj ! 0 provided ͑k z ͞k y ͒ ! 0, where k z and k y are wave vectors along and across the magnetic field and V i is the ion gyrofrequency. Therefore, the ionacoustic branch is more central to the plasma processes in the ionosphere than is currently believed.[S0031-9007 (97)05043-6] PACS numbers: 94.20. -y, 52.35.FpA fundamental reality throughout the space plasmas is the existence of magnetic field-aligned flows and currents. It is well known that a field-aligned current can support a host of plasma fluctuations and that these fluctuations can, in turn, affect the plasma steady state. The work of Kindel and Kennel [1], which considered the effects of a field-aligned current on ionospheric plasmas, has influenced and guided the interpretation and analysis of in situ observations for over two and a half decades. Kindel and Kennel show that, in an infinite homogeneous plasma, the threshold current necessary for the currentdriven electrostatic ion-cyclotron (CDEIC) instability [2] is the lowest for ionospheric conditions and, therefore, it is the most likely source for the observed plasma waves which correlate with a field-aligned current.Although there are some in situ ionospheric observations that support the classical CDEIC instability [3,4], a large number of them are at odds with it [5,6]. In particular, the observed signatures are often in the subcyclotron frequency range and resemble more closely the ion-acoustic branch [7][8][9]. A problem of identifying these as the ion-acoustic mode is that they occur for ion/electron temperature ratios of order unity or larger where the classical ion-acoustic modes are severely ion Landau damped [1,10], and, in addition, they are frequently observed for subthreshold currents. We have earlier shown that the inclusion of a localized transverse dc electric field can introduce substantial modifications to the ion-cyclotron wave properties and these modifications can better account for the observed signatures provided that the spatial gradient in the dc electric field is sufficiently strong [11]. In this Letter, we report that, even in the absence of a transverse dc electric field, an infinitesimal transverse gradient in the field-aligned flow can alter the plasma dispersion characteristics sufficiently and make the ion-acoustic branch dominant even when the ion temperature is greater than the electron temperature. This is in sharp contradiction to the behavior in a homogeneous plasma [1].The effect of a shear in the parallel ion drift was first addressed by D'Angelo [12], who showed the existence of a nonresonant instability whose real frequency is zero in the ion frame but whose growth rate depends on the spatial gradient in the p...

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Highly charged dust grains in plasma discharges reside at the sheath edge, where the ions stream toward the electrode at speed approximately c(s). Above a critical pressure P(crit), the grains lose their kinetic energy and reach a strongly coupled crystalline state, but for P P(crit) by the combined effect of ion-molecule and grain-molecule collisions.