Volume 9, Issue 9, September 2002
 LETTERS


Unique capabilities of an intense heavy ion beam as a tool for equationofstate studies
View Description Hide DescriptionIntense heavy ion beams open new possibilities in highenergydensity matter research. Due to the unique feature of the energy deposition process of heavy ions in dense matter (volume character of heating) it is possible to generate high entropy states in matter without the necessity of shock compression. Previously, such high entropy states could only be achieved by using the most powerful shock wave generators, like nuclear explosions or powerful lasers. In this paper this novel technique of heavy ion heating and expansion is proposed to explore new fascinating regions of the phase diagram, including the liquid phase, the evaporation region with the critical point, and strongly coupled plasmas.

Production of ultracollimated bunches of multiMeV electrons by 35 fs laser pulses propagating in explodingfoil plasmas
View Description Hide DescriptionVery collimated bunches of high energy electrons have been produced by focusing superintense femtosecond laser pulses in submillimeter underdense plasmas. The density of the plasma, preformed with the laser explodingfoil technique, was mapped using Nomarski interferometry. The electron beam was fully characterized: up to electrons per shot were accelerated, most of which in a beam of aperture below with energies up to 40 MeV. These measurements, which are well modeled by threedimensional numerical simulations, validate a reliable method to generate ultrashort and ultracollimated electron bunches.

Kinetic simulation of a quasisteady state in collisionless ion temperature gradient driven turbulence
View Description Hide DescriptionExistence of a quasisteady state with a mean transport flux in the collisionless ion temperature gradient driven turbulence has been confirmed by means of a direct numerical simulation of a basic kinetic equation for the perturbed ion velocity distribution function The phase mixing generates finescale fluctuations of and leads to continuous growth of highorder moments which balances the transport flux. The phase relation between the temperature and the parallel heat flux is also examined and compared with a fluid closure model.
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 SPECIAL TOPICS SECTION: MODES OF TRANSPORT IN GEOSPACE


Modes of convection in the magnetotail
View Description Hide DescriptionThe flow of plasma in the Earth’s magnetotail cannot reach a steady state, since adiabatic convection would lead to exceedingly high pressure of the associated magnetic flux tubes closer to the Earth, the socalled pressure catastrophe. The natural way to avoid the pressure catastrophe is to significantly reduce the flux tube volume by reconnection, and observations show a nearEarth reconnection line typically around 20–25 Earth radii down tail. Earthward flows from this reconnection line are rather bursty and typically seen outside of 10 Earth radii. At this point they are strongly braked by the here dominant dipolar magnetic field. The pressure gradients piled up by the flow lead to the substorm current wedge, and possibly other substorm phenomena observed in the Earth’s ionosphere. When more and more flux tubes are piled up, the dipolarization front moves tailward and finally shuts off nearEarth reconnection.

Nonlinear dynamics of thin current sheets
View Description Hide DescriptionObservations indicate that the current sheet in the Earth’s geomagnetic tail may compress to a thickness comparable to an ion gyroradius prior to substorm onset. In recent years, there has been considerable controversy regarding the kinetic stability of these thin structures. In particular, the growth rate of the kink instability and its relevance to magnetotail dynamics is still being debated. In this work, a series of fully kinetic particleincell simulations are performed for a thin Harris sheet. The ion to electron mass ratio is varied between and careful comparisons are made with a formally exact approach to the linear Vlasov theory. At low mass ratio the simulations are in excellent agreement with the linear theory, but at high mass ratio the kink instability is observed to grow more rapidly in the kinetic simulations than predicted by theory. The resolution to this apparent discrepancy involves the lower hybrid instability which is active on the edge of the sheet and rapidly produces nonlinear modifications to the initial equilibrium. The nature of this nonlinear deformation is characterized and a simple model is proposed to explain the physics. After the growth and saturation of the lower hybrid fluctuations, the deformed current sheet is similar in structure to a Harris equilibrium with an additional background population. This may explain the large growth rate of the kink instability at later times, since this type of modification to the Harris sheet has been shown to greatly enhance the growth rate of the kink mode.

“Complexity” and anomalous transport in space plasmas
View Description Hide Description“Complexity” has become a hot topic in nearly every field of modern physics. Space plasma is of no exception. In this paper, it is demonstrated that the sporadic and localized interactions of magnetic coherent structures are the origin of “complexity” in space plasmas. The intermittent localized interactions, which generate the anomalous diffusion,transport, and evolution of the macroscopic state variables of the overall dynamical system, may be modeled by a triggered (fast) localized chaotic growth equation of a set of relevant order parameters. Such processes would generally pave the way for the global system to evolve into a “complex” state of longranged interactions of fluctuations, displaying the phenomenon of forced and/or selforganized criticality. An example of such type of anomalous transport and evolution in a sheared magnetic field is provided via twodimensional magnetohydrodynamic simulations. The coarsegrained dissipation due to the intermittent triggered interactions among the magnetic coherent structures induces a “fluctuationinduced nonlinear instability” that reconfigures the sheared magnetic field into an Xpoint magnetic geometry (in the mean field sense), leading to the anomalous acceleration of the magnetic coherent structures. A phenomenon akin to such type of anomalous transport and acceleration, the socalled bursty bulk flows, has been commonly observed in the plasma sheet of the Earth’s magnetotail.

Parallel electric fields in the upward current region of the aurora: Indirect and direct observations
View Description Hide DescriptionIn this article we present electric field,magnetic field, and charged particle observations from the upward current region of the aurora focusing on the structure of electric fields at the boundary between the auroral cavity and the ionosphere. Over 100 highresolution measurements of the auroral cavity that were taken by the Fast Auroral Snapshot (FAST) satellite are included in this study. The observations support earlier models of the auroral zone that held that quasistatic parallel electric fields are the primary acceleration mechanism. In addition to the statistical study, several examples of direct observations of the parallel electric fields at the lowaltitude boundary of the auroral cavity are put forth. These observations suggest that the parallel electric fields at the boundary between the auroral cavity and the ionosphere are selfconsistently supported as oblique double layers.

Parallel electric fields in the upward current region of the aurora: Numerical solutions
View Description Hide DescriptionDirect observations of the parallel electric field by the Fast Auroral Snapshot satellite and the Polar satellite suggest that the ionospheric boundary of the auroral cavity is consistent with an oblique double layer that carries a substantial fraction (roughly 5% to 50%) of the auroral potential. A numerical solution to the Vlasov–Poisson equations of a planar, oblique double layer reproduces many of the properties of the observed electric fields, electron distributions, and ion distributions. The solutions indicate that the electron and ion distributions that emerge from the ionospheric side dominate the structure of the double layer. The ionospheric electron distribution includes scattered and reflected (mirrored) primaries, auroral secondaries, photoelectrons, and a cold population. A large fraction of the ionospheric electrons is reflected by the parallel electric field whereas the ionospheric ions are strongly accelerated. The steep density gradient between the ionosphere and the auroral cavity results in a highly asymmetric double layer, with a strong, localized positive charge layer on the ionospheric side and a moderate, extended negative charge layer on the auroral cavity side. This structure results in an asymmetric electric field, a feature also seen in the observations. The electric field observations, however, do not always support a planar double layer since the parallel and perpendicular signals are not always well correlated. Fully twodimensional solutions are needed to better reproduce the observed features.

Counterstreaming ions as evidence of magnetic reconnection in the recovery phase of substorms at the kinetic level
View Description Hide DescriptionCounterstreaming ions embedded in hot isotropic ions are found at the front of fast earthward plasma flows in the recovery phase of substorms in the Earth’s magnetotail. The counterstreaming ions are present only when the northward component of the magnetic field increases in the equatorial plane. Hybrid simulations of magnetic reconnection have been carried out. It is found that counterstreaming ions form in the leading edge of jetting plasmas produced with magnetic reconnection, where the magnetic field lines pile up due to the preexisting stationary plasmas. These counterstreaming ions originate from cold ions on the northern and southern tail lobe field lines, and earthward transport of the reconnected field lines makes these cold ions flow into the equatorial plane. The present observations provide strong evidence of magnetic reconnection in the recovery phase of substorms at the kinetic level.

Geomagnetic transport in the solar wind driven nightside magnetosphere–ionosphere system
View Description Hide DescriptionA spatially resolved nonlinear dynamical model of the solar wind driven geomagnetic tail plasma is developed for the purpose of space weather predictions. The model represents the fluctuating electromagnetic fields and high pressure central plasma sheet by a large number of semiglobal coupled current loops ranging from the nearEarth geosynchronous orbit position for substorm dynamics to deep in the geotail. There is a spectrum of dynamical frequencies ranging from 1 h periods to 1 min and shorter periods. The lowfrequency modes are global and lead to the dynamics of the lowdimensional WINDMI model. The highfrequency dynamics are nonlinear compressionalrarefraction waves propagating up and down the geotail. The localized pulses start from sites of local reconnection set off either by the tearing mode unloading trigger or by localized solar wind disturbances acting on the nightside magnetopause. Larger unloading events lead to nonlinear steepening of the compressional pulsations which act to trigger secondary convection events under certain conditions.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Acceleration and heating of charged particles by a dispersive electrostatic pulse
View Description Hide DescriptionAcceleration and heating mechanisms of charged particles by a generalized electrostatic pulse, i.e., a dispersive and propagating pulse of various forms, are investigated analytically and numerically. Gaussianshaped pulses of arbitrary group as well as phasevelocities are considered, but the results should apply qualitatively to solitons and other pulses. Actions of ordinary wave packets, as well as extremely short pulses with less than one cycle (or wavelength) among others, are investigated. For these general pulses, it is found that there may exist four types of acceleration/heating mechanisms. If a particle’s initial velocity is sufficiently away from the central phasevelocity of the pulse and if the pulse is sufficiently short, there occurs a wellknown but generalized transittime acceleration. However, for relatively long pulses with more than a couple of wavelengths or cycles, the transittime acceleration becomes rapidly ineffective in accelerating particles. In the vicinity of the central phasevelocity of the pulse, a new process called “quasitrapping” occurs, usually giving rise to the greatest velocity shifts. It can at times also cause a novel particle reflection. Finally, in the vicinity of the group velocity of the pulse, there occurs another new reflection process driven by the ponderomotive force of the pulse. All of the analytic expressions obtained for these processes are generally in good to excellent agreement with numerical results.

Measurements of helicon wave propagation and Ar II emission
View Description Hide DescriptionWavemagnetic field, optical and Langmuir probemeasurements are carried out to examine fast and thermal electron contributions to plasma ionization in a helicon plasmasource. For the optical measurements, an Ar II line (443 nm) with a particularly short lifetime (7 ns) is chosen to resolve the excitation rate within a radio frequency period (73.7 ns). Information is then obtained regarding the acceleration processes and their action on the electron distribution that caused the excitation. Spatiotemporal measurements of 443 nm peak emission show that the emission is modulated at the source frequency. The peak count phase of the modulation propagates along the plasma at a comparable speed as the local helicon wave phase velocity. Computer modeling utilizing lab data is carried out to examine wave field effects on electron acceleration and ionization contributions arising from nonMaxwellian fast electrons.

Numerical simulation of ultracold plasmas
View Description Hide DescriptionIn recent experiments ultracold plasmas were produced by photoionizing small clouds of laser cooled atoms. This paper presents the results of molecular dynamic simulations for the early time evolution of such plasmas. Contrary to earlier speculation, no evidence of strong electron–electron correlations is observed in the simulations even if the initial value of the coupling parameter is much larger than unity. As electron–electron correlations begin to develop, the correlation energy is released to heat the electrons, raising the electron temperature to the point where and limiting further development of correlation. Further heating of the electrons occurs as a byproduct of threebody recombination. When a model of laser cooling is added to the simulation, the formation of strong ion–ion correlation is observed. Contrary to earlier suggestion, the rate of threebody recombination is observed to be in reasonable agreement with the traditional formula, but care must be taken to use the correct temporally evolving temperature, The simulations are challenging because it is necessary to follow threebody recombination into weakly bound (high quasiclassical) Rydberg states, and the time scale for such states is short compared to that for the plasma dynamics. This kind of problem was faced earlier in computational astrophysics when studying binary starformation in globular clusters and the simulation method used here is adapted from such studies.

Stimulated Brillouin scattering in a magnetized dusty plasma with charge fluctuation
View Description Hide DescriptionStimulated Brillouin scattering of an electromagnetic wave is investigated analytically in a magnetized dusty plasma. Variation of growth rates of the scatteredionacoustic wave and the electromagnetic wave have been analyzed with respect to the concentration of dust particles by taking account dust charge fluctuation.

Effective screened potentials of strongly coupled semiclassical plasma
View Description Hide DescriptionThe pseudopotentials of particle interaction, taking into account both quantummechanical effects of diffraction at short distances, and also screening field effects at large distances are obtained for a strongly coupled semiclassical plasma. The limiting cases of potentials are considered.

Local equilibrium of nonrotating plasmas
View Description Hide DescriptionWhat is the minimal information that must be supplied on a constant pressure surface of a toroidal plasma to completely specify a local magnetohydrodyamic equilibrium? Clearly the shape of the surface must be specified, but other information such as the separation between neighboring surfaces must also be given. This paper explores the conditions required for threedimensional equilibria and how local equilibrium constraints are related to global constraints. The resulting conditions provide useful checks on the accuracy of equilibrium solvers, particularly for solvers for threedimensional equilibria, and allow local refinements in the accuracy of equilibria.

Double mode condensates of a flowing plasma as possible relaxed states
View Description Hide DescriptionStationary energy (SE) states are found for a flowing twofluid plasma. These are of interest because quiescent relaxed plasma equilibria, if they exist, should be a subset of the SE states. The platform for the analysis is a twofluid flowing plasma, which is more realistic than the singlefluid model used in most treatments of relaxed plasmas. The twofluid model is characterized by two helicities, one for each species. Including flow allows for the possibility that flow may be an important ingredient in relaxed states. The analysis expands the flow and field vectors in a complete basis set of divergencefree vectors. This reduces the problem to algebra. It leads at once to the prediction of SE states that are a twopoint spectrum of the basis set, i.e., doublemode condensates. The properties of these SE states are shown to depend on their location in helicity space, a twodimensional parameter space of the ion and electron helicities. The migration of a SE state as a result of resistive and viscous dissipation is also shown.
 Nonlinear Phenomena, Turbulence, Transport

The Kelvin–Helmholtz instability, differential rotation, and threedimensional, localized, magnetic reconnection
View Description Hide DescriptionResults are presented from a study of threedimensional magnetic reconnection caused by a Kelvin–Helmholtz instability and differential rotation. Specifically, subsonic and subAlfvénic flow is considered, which is Kelvin–Helmholtz stable in the direction of the magnetic field, but unstable perpendicular to the magnetic field. The flow is modeled by the resistive magnetohydrodynamics equations in three dimensions with constant resistivity. As a result of differential rotation (a gradient in vorticity parallel to the initial field), localized transient reconnection is observed on the Kelvin–Helmholtz time scale. Current amplification is observed along with the generation of parallel current. Results indicate that the observed transient reconnection rate is insensitive to resistivity (even with a constant resistivity model), but is sensitive to the initial flow shear.

Sound waves in twoion plasmas
View Description Hide DescriptionThe dispersive and nonlinear characteristics of sound waves in plasmas that consist of warm electron and two different ion fluids are studied in detail. Analytical solutions are used to illustrate the characteristics of sound waves and to validate a numerical scheme that solves fluid and Poisson equations. This scheme can be used to model sound waves that participate in stimulated Brillouin scattering in twoion plasmas.

Fully nonlinear dust kinetic Alfvén waves
View Description Hide DescriptionExact localized nonlinear dust kinetic Alfvén waves are investigated by retaining the complete dust nonlinearity and by invoking small but finiteβ effects in a collisionless plasma, whose constituents are electrons, ions and negatively charged dust grains. SubAlfvénic, as well as superAlfvénic, solitary waves consisting of smooth density humps or dips are found to exist. Further, superAlfvénic kink type solitons are also found to occur. The soliton amplitude depends upon various plasma parameters along with the soliton propagation speed. The results of the present investigation may be useful in the diagnostics of dust in magnetized plasmas and in understanding the formation of coherent soliton structures in space and laboratory plasmas.