Volume 13, Issue 5, May 2006
 LETTERS


Larmor radius size density holes discovered in the solar wind upstream of Earth’s bow shock
View Description Hide DescriptionThe Cluster and Double Star satellites recently observed plasma density holes upstream of Earth’s collisionless bow shock to apogee distances of and 13 earth radii, respectively. A survey of 147 isolated density holes using time resolution data shows they have a mean duration of , but holes as short as are observed. The average fractional density depletion inside the holes is . The upstream edge of density holes can have enhanced densities that are five or more times the solar wind density. Particle distributions show the steepened edge can behave like a shock. Multispacecraft analyses show the density holes move with the solar wind, can have an ion gyroradius scale, and could be expanding. A small normal electric field points outward. Similarly shaped magnetic holes accompany the density holes indicating strong coupling between fields and particles. The density holes are only observed with upstream particles, suggesting that backstreaming particles interacting with the solar wind are important.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Bounded dustacoustic waves in a cylindrically bounded collisional dusty plasma with dust charge variation
View Description Hide DescriptionTaking into account the boundary, particle collisions, and dust charging effects, dustacoustic waves in a uniform cylindrically bounded dusty plasma is investigated analytically, and the dispersion relation for the dustacoustic wave is obtained. The effects of boundary, dust charge variation, particle collision, and dust size on the dustacoustic wave are discussed in detail. Due to the bounded cylindrical boundary effects, the radial wave number is discrete, i.e., the spectrum is discrete. It is shown that the discrete spectrum, the adiabatic dust charge variation, dust grain size, and the particle collision have significant effects on the dustacoustic wave.

Generation of filamentary structures by beamplasma interaction
View Description Hide DescriptionThe previous simulations by Wang and Lin [Phys. Plasmas.10, 3528, (2003)] showed that filaments, frequently observed in space plasmas, can form via the interaction between an ion beam and a background plasma. In this study, the physical mechanism for the generation of the filaments is investigated by a twodimensional hybrid simulation, in which a fieldaligned ion beam with relative beam density and beam velocity is initiated in a uniform plasma. Righthand nonresonant ion beam modes, consistent with the linear theory, are found to be dominant in the linear stage of the beamplasma interaction. In the later nonlinear stage, the nonresonant modes decay and the resonant modes grow through a nonlinear wave coupling. The interaction among the resonant modes leads to the formation of filamentary structures, which are the fieldaligned structures of magnetic field, density, and temperature in the final stage. The filaments are nonlinearly generated in a preypredator fashion by the parallel and oblique resonantion beam modes, which meanwhile evolve into two types of shear Alfvén modes, with one mainly propagating along the background field and the other obliquely propagating. The filamentary structures are found to be phase standing in the plasma frame, but their amplitude oscillates with time. In the dominant filament mode, fluctuations in the background ion density, background ion temperature, and beam density are in phase with the fluctuations in , whereas the significantly enhanced beamtemperature is antiphase with . It is found that the filaments are produced by the interaction of at least two ion beam modes with comparable amplitudes, not by only one single mode, thus their generation mechanism is different from other mechanisms such as the stimulated excitation by the decay of an Alfvén wave.

Properties of the acoustic mode in partially ionized and dusty plasmas
View Description Hide DescriptionThe properties of currentdriven ionacoustic (IA) and dustacoustic (DA) modes in partially ionized plasmas are investigated. The current is oriented along the magnetic field lines and the mode is assumed to propagate at an angle with respect to the current. For highly collisional light plasma components, the fluid equations in the parallel direction are used. In the case of unmagnetized heavy species, which provide the mode inertia (ions for the IA mode and grains for the DA mode), the oblique perturbations will have an acoustic nature. For an arbitrary collision frequency of heavy species with neutrals, a kinetic description is used for the heavy species. For the DA mode, the dispersionequation is solved first in the limits of an electrondepleted plasma, showing that the mode has a minimum instability threshold at a large angle of propagation. This feature is primarily due to the collisions. For higher values of the charge on the grains, this minimum vanishes but the threshold becomes considerably lower. The full dispersionequation, with electrons having a current with an opposite sign compared to ions, is solved numerically yielding both a lower frequency and a smaller increment. A similar angledependent threshold and increment are found for the IA mode as well.

Lower hybrid turbulence driven by parallel currents and associated electron energization
View Description Hide DescriptionParallel currents are usually in the form of fieldaligned electron drifts in collisionless plasmas. The fieldaligned drifts often drive instabilities. For instance, Drake et al. [Science, 299, 873 (2003)] found growth of parallel propagating turbulence initially and strong levels of oblique lower hybrid (LH) waves at later times; substantial parallel electron acceleration was also found. We use collisionless linear theory and quasilinear simulations to study wave growth and parallel electron dynamics in similar systems. In low plasmas with intense parallel currents and both with or without parallel fields, LH waves are shown to grow even for electron distributions stable to the parallel Buneman instability and to accelerate electrons parallel to very rapidly. This instability may be seen as the oblique limit of the ion acoustic and Buneman instabilities. The quasilinear diffusion via LH waves may release almost all of the available drift energy, and produce stronger electron acceleration and heating than the parallel Buneman instability alone.

Transport processes of a nonneutral plasma coupled to an external rotating wave
View Description Hide DescriptionExperimental investigations are carried out on radial transport phenomena of a pure electron plasma under the application of an external rotating wave that belongs to TrivelpieceGould (TG) modes. Substantial radial compression of the density distribution is achieved by application of a properly controlled rotating electric field to one side of the plasma. Analyses of the observed plasma wave indicate that the efficient increase of the onaxis density entails the substantial damping of the TG wave propagating in the plasma. The radial particle flux observed during the density compression is consistent with that of the theoretical model based on the driftkinetic Vlasov equation [Y. Kiwamoto et al., Phys. Plasmas12, 094501 (2005)]. This result implies that the radial compression of the plasma density distribution consists of the transverse drift of particles subject to resonant waveparticle interaction in the axial dynamics.

Effect of selfgravitation on the energy loss of pair of projectiles in dusty plasma
View Description Hide DescriptionThe effect of selfgravitation of massive dust grains is investigated on the shielded potential and the energy loss of pair of charged projectiles passing through a dustcontaminated plasma. Analytical general expressions are derived for the shielded potential and for the energy loss by incorporating twobody correlation effects. An interference contribution of these projectiles to the shielded potential and energy loss is observed that depends upon their orientation and separation distance. It is found that for two collinear projectiles the potential is enhanced by increasing dust Jeans frequency for separation less than Debye length and the energy loss versus projectile velocity decreases with the increase of Jeans frequency for arbitrary separation. The effect of inclination of two noncollinear projectiles on energy loss is also investigated for a fixed value of Jeans frequency . The contribution to the energy loss due to the interference term has been separately calculated for a typical Jeans frequency. The present investigation would be useful to explain the coagulation of dust particles in the molecular clouds and in the ionbeamdriven inertial confinement fusion approach.

Magnetohydrodynamics of fractal media
View Description Hide DescriptionThe fractal distribution of charged particles is considered. An example of this distribution is the charged particles that are distributed over the fractal. The fractional integrals are used to describe fractal distribution. These integrals are considered as approximations of integrals on fractals. Typical turbulent media could be of a fractal structure and the corresponding equations should be changed to include the fractal features of the media. The magnetohydrodynamics equations for fractal media are derived from the fractional generalization of integralMaxwell equations and integralhydrodynamics (balance) equations. Possible equilibrium states for these equations are considered.

Green’s function of compressible Petschektype magnetic reconnection
View Description Hide DescriptionWe present a method to analyze the wave and shock structures arising from Petschektype magnetic reconnection. Based on a timedependent analytical approach developed by Heyn and Semenov [Phys. Plasmas3, 2725 (1996)] and Semenov et al. [Phys. Plasmas11, 62 (2004)], we calculate the perturbations caused by a delta functionshaped reconnection electric field, which allows us to achieve a representation of the plasma variables in the form of Green’s functions. Different configurations for the initial conditions are considered. In the case of symmetric, antiparallel magnetic fields and symmetric plasma density, the wellknown structure of an Alfvén discontinuity, a fast body wave, a slow shock, a slow wave, and a tube wave occurs. In the case of asymmetric, antiparallel magnetic fields, additionally surface waves are found. We also discuss the case of symmetric, antiparallel magnetic fields and asymmetric densities, which leads to a faster propagation in the lower half plane, causing side waves forming a Mach cone in the upper half plane. Complex effects like anisotropic propagation characteristics, intrinsic wave coupling, and the generation of different nonlinear and linear wave modes in a finite plasma are retained. The temporal evolution of these wave and shock structures is shown.

Differential equation model of an Alfvén wave maser
View Description Hide DescriptionA mathematical model describing the operation of an Alfvén wavemaser in a laboratory environment is constructed from a continuous differential equation. The model incorporates the essential features of maser operation, namely, a resonator with a semitransparent boundary, a unidirectional amplification region, and a nonuniform magnetic field that provides frequency filtering. The spectral features and the parameter scaling predicted by the model are in agreement with laboratory measurements of an Alfvén wavemaser [J. E. Maggs and G. J. Morales, Phys. Rev Lett.91, 035004 (2003)]. The model provides a useful tool to explore a variety of operational scenarios for users of this novel wave source, including studies of Alfvénic interactions of relevance to space and fusion plasmas.

Simulation of disk and bandlike voids in dusty plasma systems
View Description Hide DescriptionThe minimumenergy configurations of systems of multispecies charged grains of different mass and charge with an interaction potential including longrange repulsive as well as shortrange attractive components are studied by molecular dynamics simulation. The grains are also subject to a radial drag force and a quadratic confining potential. It is found that central as well as bandlike void regions separating grains of different species can exist as well as coexist, depending on the species parameters. The results are consistent with the horizontal crosssections of the structures found in a recent experiment on selforganization of chemically synthesizing grains [Huang et al.Chin. Phys. Lett.21, 121 (2004)].

Dynamics of potential surface modes in a nonuniform plasma waveguide with finite electron temperature
View Description Hide DescriptionIn this paper we study the mechanisms of waveenergy transformation in nonuniform waveguide plasmas maintained by surface modes in the presence of local upperhybrid resonance. The dispersion equation is derived for such waves in the potential case with the following effects taken into account: the finite magnetic field, the finite electron temperature, and electronneutral collisions. Numerical and analytical solutions for the dispersion and for eigenfield distributions of the waves are presented. The thermal conversion of the surface wave into the upperhybrid volume modes at a finite electron temperature is shown to completely change the structure of the wave field distribution and to make the surface wave wavelength and the spatial damping rate very sensitive to slight shifts of the surface wave frequency.

Unstable longitudinal plasma oscillations in a magnetic field: Nonrelativistic and relativistic considerations
View Description Hide DescriptionThe nonrelativistic and relativistic stability properties are investigated of longitudinal waves propagating in a plasma embedded in an ambient magnetic field, when the wave propagation direction is not necessarily either parallel or perpendicular to the ambient magnetic field. The analysis is based on the concept introduced by Harris [Phys. Rev. Lett.2, 34 (1959)] of neutral points in wavenumber space to determine plasma instability to one side or the other of such neutral points. The critical need is to determine whether a particular plasmadistribution function permits the existence of a neutral point. Relativistic considerations, although necessary to include for many astrophysical plasmas, complicate significantly the determination of instability conditions. In this paper it is shown how one can provide a general argument for such neutral point determination and for determining instability rates in the neighborhood of such neutral points. Only waves independent of resonant waveparticle effects are considered.

Dispersion properties of compressional electromagnetic waves in quantum dusty magnetoplasmas
View Description Hide DescriptionA new dispersion relation for lowfrequency compressional electromagneticwaves is derived by employing quantum magnetohydrodynamic model and Maxwell equations in cold quantum dusty magnetoplasmas. The latter is composed of inertialess electrons, mobile ions, and immobile charged dust particulates. The dispersion relation for the lowfrequency compressional electromagnetic modes is further analyzed for the waves propagating parallel, perpendicular, and oblique to the external magnetic field direction. It is found theoretically and numerically that the quantum parameter affects the real angular frequencies and the phase speeds of the compressional electromagnetic modes. Here, is the equilibrium number density of the ions (electrons), is the electron (ion) mass, and is the Plank constant divided by .

Charging properties of a dust grain in collisional plasmas
View Description Hide DescriptionCharging related properties of a small spherical grain immersed in a collisional plasma are investigated. Asymptotic expressions for charging fluxes, grainsurface potential, long range electrostatic potential, and the properties of grain charge fluctuations due to the discrete nature of the charging process are obtained. These analytical results are in reasonable agreement with the available results of numerical modeling.

Electron parallelflow shear driven lowfrequency electromagnetic modes in collisionless magnetoplasma
View Description Hide DescriptionThe free energy associated with shear in the equilibrium parallel electron velocity is shown to be responsible for the excitation of lowfrequency electromagnetic waves in collisionless magnetoplasma. New dispersion relations are derived by using the hydrodynamic equations for the electron fluid, the magneticfieldaligned (parallel) drift of which varies in one of the perpendicular directions, and by using a kinetic ion model, together with Ampère’s law and Poisson’s equation. The dispersion relations are analyzed both analytically and numerically for a set of parameters representative of a laboratory experiment. New filamentary instabilities are predicted.

The excitation of extraordinary and ordinary waves in a magnetized plasma medium by a rotating electron beam
View Description Hide DescriptionThe possibility of the excitation of extraordinary and ordinary cyclotron waves by interaction of a rotating electron beam propagating parallel to the external magnetic field with a cold plasma medium is investigated. By obtaining the growth rates, the influence of dissipation on the development of extraordinary and ordinary cyclotron waves are studied. By comparing the development of the cyclotron instability with the dissipative instability in extraordinary and ordinary cyclotron waves excitation in the beamplasma system, the dependency of the growth rates of these instabilities on the physical parameters is discussed.

Modulated electrostatic modes in pair plasmas: Modulational stability profile and envelope excitations
View Description Hide DescriptionA pair plasma consisting of two types of ions, possessing equal masses and opposite charges, is considered. The nonlinear propagation of modulated electrostatic wave packets is studied by employing a twofluid plasma model. Considering propagation parallel to the external magnetic field, two distinct electrostatic modes are obtained, namely a quasiacoustic lower moddfe and a Langmuirlike, as optictype upper one, in agreement with experimental observations and theoretical predictions. Considering small yet weakly nonlinear deviations from equilibrium, and adopting a multiplescale technique, the basic set of model equations is reduced to a nonlinear Schrödinger equation for the slowly varying electric field perturbation amplitude. The analysis reveals that the lower (acoustic) mode is stable and may propagate in the form of a darktype envelope soliton (a void) modulating a carrier wave packet, while the upper linear mode is intrinsically unstable, and may favor the formation of brighttype envelope soliton (pulse) modulated wave packets. These results are relevant to recent observations of electrostatic waves in pairion (fullerene) plasmas, and also with respect to electronpositron plasma emission in pulsar magnetospheres.

Charging of dust grains in a plasma with negative ions
View Description Hide DescriptionThe effect of negative ions on the charging of dust particles in a plasma is investigated experimentally. A plasma containing a very low percentage of electrons is formed in a singleended machine when is admitted into the vacuum system. The relatively cold machine electrons readily attach to molecules to form negative ions. Calculations of the dust charge indicate that for electrons, negative ions, and positive ions of comparable temperatures, the charge (or surface potential) of the dust can be positive if the positive ion mass is smaller than the negative ion mass and if , the ratio of the electron to positive ion density, is sufficiently small. The machine plasma is operated with positive ions (mass ) and negative ions (mass ), and also utilizes a rotating cylinder to dispense dust into the plasma column. Analysis of the currentvoltage characteristics of a Langmuir probe in the dusty plasma shows evidence for the reduction in the (magnitude) of the negative dust charge and the transition to positively charged dust as the relative concentration of the residual electrons is reduced. Some remarks are offered concerning experiments that could become possible in a dusty plasma with positive grains.

Experimental study of twofluid effects on magnetic reconnection in a laboratory plasma with variable collisionality
View Description Hide DescriptionThis article describes the recent findings on twofluid effects on magnetic reconnection in plasmas with variable collisionality in the magnetic reconnection experiment (MRX) [M. Yamada et al., Phys. Plasmas4, 1936 (1997)]. The MRX device has been upgraded to accommodate a variety of reconnection operation modes and high energy density experiments by increasing its capacitor bank energy and extending the discharge duration. As our experimental operation regime has moved from the collisional to the collisionfree, twofluid effects have become more evident. It is observed that the twodimensional profile of the neutral sheet is changed significantly from the rectangular shape of the familiar SweetParker type to a double wedge shape as the collisionality is reduced and the reconnection rate increases. The recent evolution of our experimental research from the magnetohydrodynamics(MHD) to the twofluid analysis is presented to illuminate the physics of Hall MHD in a collisionfree reconnection layer. In particular, a clear experimental verification of an outofplane quadrupole field, a characteristic signature of the Hall MHD, has been made in the MRX neutral sheet, where the sheet width is comparable to the ion skin depth. It is important to note that the Hall effect, which occurs due to twodimensional laminar flows of electrons in the reconnection plane, is observed together with the presence of low and high frequency magnetic turbulence, which often has threedimensional structures. These observations in MRX have striking similarities to the recent magnetospheric measurements of reconnection region, in which the quadrupole component has been detected together with magnetic fluctuations.