Tenuous Plasma Research Articles

Magnetic Stress in Solar System Plasmas

Magnetic stresses play an important role in the dynamics of geophysical systems, from deep inside the Earth to the tenuous plasmas of deep space. In the magnetic dynamos inside the sun and the planets, the magnetic stresses of necessity rival the interior mechanical stresses. In solar system plasmas, magnetic stresses play critical roles in the transfer of mass, momentum and energy from one region to another. Coronal mass ejections are rapidly expelled from the sun and their interplanetary manifestations plough through the pre-existing solar wind. Some of these structures resemble flux ropes, bundles of magnetic field wrapped around a central core, and some of these appear to be almost force-free. These structures and similar ones in planetary magnetospheres appear to be created by the mechanism of magnetic reconnection. Solar system plasmas generally organise themselves in giant cells in which the properties are rather uniform, separated by thin current layers across which the properties change rapidly. When the magnetic field on the two sides of one of these current layers changes direction significantly (by over 90°), the magnetic field on opposite sides of the boundary may become linked across the current sheet. If the resulting magnetic stress can accelerate the plasma out of the reconnection region, the process will continue uninterrupted. If not, the process will shut itself off. Such continuous reconnection can occur at the Earth’s magnetopause and those of the magnetised planets. Reconnection in the terrestrial magnetotail current sheet and the jovian current sheet occurs in a setting in which the flow can be blocked on one side, causing reconnection to be inherently time-varying. At Jupiter, this mechanism also separates heavy ions from magnetospheric flux tubes so that the ions can escape but Jupiter can retain its magnetic field. Despite the very wide range of parameters and scales encountered in heliospheric plasmas, there is surprising coherence in the mechanisms in these varying environments.

Magnetic field generated in a plasma by a short, circularly polarized laser pulse

We study the generation of a quasistatic magnetic field by a short, circularly polarized laser pulse in a tenuous cold uniform plasma. It is shown that two physical mechanisms are responsible for the generation of the various components of the magnetic field. One mechanism is due to the ponderomotive forces and governs the generation of the azimuthal component of the magnetic field. The other is similar to the inverse Faraday effect (IFE) in a nonuniform plasma and gives rise to axial and radial components of the magnetic field. At moderate radiative intensities, all magnetic field components are proportional to the squared intensity. The spatial structure of the magnetic field depends strongly on the pulse shape and the plasma density.

Flux transfer events produced by bursty merging at a single X line

We present predictions for the signatures of flux transfer events (FTEs) at the dayside magnetopause produced by bursty merging at a single extended X line. In this MHD model, the time‐dependent enhanced local resistivity turns on at a low northern latitude for a brief time and then turns off. Realistic ratios of magnetosheath to magnetosphere parameters are chosen to conform with observations and our previous simulations [Ku and Sibeck, 1997, 1998]: ρmsh/ρsph = 10, Bmsh/Bsph = 0.5, and Tmsh/Tsph = 0.175. The burst of merging produces FTEs marked by strongly asymmetric bipolar magnetic field and plasma velocities normal to the magnetopause in the magnetosheath and no significant signatures in the magnetosphere, just as in the previously studied case for the step‐function onset of merging [Ku and Sibeck, 1997]. However, the simulation produces other FTE signatures that differ strikingly from those generated by the step‐function onset of merging. After the resistivity is switched off, one or more secondary reconnection sites appear on the trailing edge of the events. Merging at these sites inflates and propels the main FTE as it moves out of the simulation domain. Merging at these sites also fills a magnetic island with hot tenuous plasma near the subsolar (symmetry) point. Transient magnetic islands form on the trailing edge of the main FTE, but these remain small and may disappear. Scatterplots of various parameters versus the plasma density reveal that steep changes for the temperature kT, magnetic field BZ, and the Alfvén velocity occur on the magnetosheath side immediately adjacent to the magnetopause and also reflect the fact that the events scarcely deform the magnetopause.

The Fast Auroral SnapshoT (FAST) Mission

The FAST satellite mission investigates plasma processes occurring in the low altitude auroral acceleration region, where magnetic field‐aligned currents couple global magnetospheric current systems to the high latitude ionosphere. In the transition region between the hot tenuous magnetospheric plasma and the cold, dense ionosphere, these currents give rise to parallel electric fields, particle beams, plasma heating, and a host of wave‐particle interactions. FAST instruments provide observations of plasma particles and fields in this region, with excellent temporal and spatial resolution combined with high quantitative accuracy. The spacecraft data system performs on‐board evaluation of the measurements to select data “snapshots” that are stored for later transmission to the ground. New measurements from FAST show that upward and downward current regions in the auroral zone have complementary field and particle features defined by upward and downward directed parallel electric field structures and corresponding electron and ion beams. Direct measurements of wave particle interactions have led to several discoveries, including Debye‐scale electric solitary waves associated with the acceleration of upgoing electron beams and ion heating, and the identification of electrons modulated by ion cyclotron waves as the source of flickering aurora. Detailed quantitative measurements of plasma density, plasma waves, and electron distributions associated with auroral kilometric radiation source regions yield a consistent explanation for AKR wave generation.

Electrostatic expansion of a tenuous plasma produced by excimer laser-irradiation of MgO

We describe the expansion dynamics of the tenuous plasmas formed by irradiating single crystal MgO with a KrF excimer laser (248 nm) in vacuum. At low fluences (0.03–0.5 J/cm 2), the positive ion kinetic energy distribution peaks at about 11 eV and is relatively narrow (±2 eV). As the fluence is increased, the energy distribution broadens dramatically. Also, we find that a portion of the positive ions have been turned around – i.e., significant ion intensities can be detected behind the sample. We argue that these effects result from the electrostatic expansion of a net positive, tenuous plasma, where the ions outnumber the electrons by approximately 0.02% of the total charged particle density. A one dimensional model which simulates this expansion mechanism is presented.

Coronal Heating Mechanism in the Presence of a Flow: A Numerical Approach

The solar corona is a hot, tenuous plasma permeated with the structured magnetic fields. A variety of waves is generated in the corona due to the convective upwelling motion in the photosphere. The excitation of MHD fluctuations is generated by the footpoint motion of the field lines in the photosphere. Resonant absorption of the Alfvén waves in an inhomogeneous plasma has been suggested as a means of driving current and plasma heating in the corona (Sakurai et al., 1991). We study this problem in the presence of flow.

Enhanced Dielectronic Recombination in Crossed Electric and Magnetic Fields

Photorecombination is the inverse of photoionization and is the process by which a free electron simultaneously emits a photon and makes a transition to a bound state. It is the purpose of this paper to show that certain types of photorecombination processes can be greatly enhanced in static, crossed electric and magnetic fields. Besides being intrinsically interesting, this enhancement may affect the interpretation of recent experiments on photorecombination in static electric fields and affect the recombination rate in some tenuous astrophysical plasmas. Burgess and Summers [1] suggested that under some circumstances the dielectronic recombination (DR) rate could be enhanced due to angular momentum redistribution during three-body collisions. DR is photorecombination that proceeds through the capture of an electron by an ion in an autoionizing state; the incident electron induces a transition in the ion but loses so much energy it is captured into a resonance state. Before the electron can regain the energy from the core electrons and leave the ion, a photon is emitted and the electron is captured into a bound state. Jacobs et al. [2,3] showed that , redistribution of the autoionizing state due to plasma (electric) microfields can strongly enhance the DR rate. The prototypical example for studying field effects is the DR of a Li-like ion. The initial state is a free electron, and the ion is in the 1s 2 2s configuration. The autoionizing state is 1s 2 2pn,. The system is stabilized by the 2p electron emitting a photon, leaving the system in the 1s 2 2sn, bound state. The physical reason for enhanced DR in a static field can be understood in a time dependent picture. The electron is initially captured in an n, state while simultaneously exciting the core. If an electric field is on, the Rydberg state will precess into states of higher angular momentum. Since the autoionization rate rapidly decreases with angular momentum, this has the effect of making photon emission more probable. In the more traditional time independent picture, the

Effects of High Temperature Radiation on Electrostatic Plasma Waves

The effects of black-body radiation emitted from a high temperature radiation source on electrostatic plasma wave characteristics in a relatively cold tenuous plasma are theoretically investigated. The ponderomotive force of the photons perturbed by the electron density fluctuation is taken into account. It is shown that the effect is in general very small, but has a logarithmic singularity which corresponds to a radiation induced zero-sound. A significant effect can be expected in a narrow frequency range in which the zero-sound couples with the electron plasma wave.

A statistical survey of auroral solitary waves and weak double layers: 2. Measurement accuracy and ambient plasma density

Solitary electrostatic waves (SWs) were frequently observed by the Viking satellite in the auroral plasma. Previously, Mälkki et al. [1993] reported occurrence statistics and voltage data for these structures. In this paper, we make a detailed study of the measurement process in order to establish what further knowledge can be extracted from the data. The possibilities that SWs are spacecraft‐generated pulses or signatures of rectification of hypothetical wave packets are investigated and rejected. We analyze Langmuir probe data from several SW events and show that densities are at least as low as previously reported (a few cm−3). In order to assess the quality of the SW observations in this very tenuous plasma, we use a semiempirical model of the complete probe‐spacecraft‐plasma system, including nonlinearities in the probe sheaths. The usefulness of the model is shown by its ability to model details in the measured fluctuations. It is found that the errors in the voltage (electric field) measurements of solitary waves are very small, while the density fluctuation estimates may be severely affected by spurious signals. Therefore details in the shape of the observed probe current signature cannot be used to study the detailed shape of the density depletion. This limits the possibility of identifying weak double layer (WDL) characteristics in SWs, as effects of time evolution result in signatures similar to those of net potential drops.

Kinetic modeling of intense, short laser pulses propagating in tenuous plasmas

Fast time averaged equations are derived for the motion of particles and the generation of electromagnetic wake fields under the action of the ponderomotive potential of an ultraintense laser pulse propagating through a tenuous plasma. Based on these averaged equations, a new particle code is designed which calculates the particle trajectories on the plasma period time scale. The regime of total cavitation of the plasma is investigated. It is found that stable propagation over a long distance is possible in this regime, and that energetic electrons are produced with a simple characteristic dependence of their angle of deflection on energy. This new code allows for computationally efficient modeling of pulse propagation over great distances.

Alternating Conductivity in a Weakly Relativistic Gas Plasma

Characteristics of alternating conductivity in a weakly relativistic gas plasma are investigated based on the relativistic equation for the electron drift velocity, and the existence of negative conductivity, which leads to the amplification of an electromagnetic wave, in a slightly ionized, tenuous gas plasma is reported.

Generation of the Auroral Kilometric Radiation in plasma cavities—I. Experimental study

According to the Viking observations, the sources of the Auroral Kilometric Radiation (AKR) are plasma cavities, filled by a hot and tenuous plasma, separated from the denser and colder external plasma by sharp density gradients. These plasma inhomogeneities constitute interfaces that affect the wave propagation and then could play an important role in the generation of the radiation. These effects on the propagation are analyzed using Viking measurements obtained during crossings of AKR sources. It is shown that the electromagnetic energy, generated on the X mode, in the close vicinity of the local electron gyrofrequency, is first confined inside the cavity structure. Indeed, in a small frequency domain that extends a few kHz above the local electron gyrofrequency the “internal” X mode waves can only be connected to “external” Z or O mode waves. The corresponding transmission coefficients are measured as being an attenuation larger than 25 dB. At higher frequencies, the internal waves are connected to the external X mode with a transmission coefficient close to 1. However, this connection is only possible after an upward propagation of the electromagnetic energy inside the source. The progressive rotation of the wave vector associated to the wave refraction is estimated. An analysis of the polarization also shows that the radiating diagram likely does not have a cylindrical symmetry with respect to the geomagnetic field: the radiation is mainly emitted tangentially to the laminar structure constituted by the source. The existence of a preferential direction in the radiating diagram is a direct observable consequence of the finite extension of the sources. This could have important consequences for remote sensing studies.

Electron cavitation and acceleration in the wake of an ultraintense, self-focused laser pulse.

A particle code is used to calculate the plasma response to the relativistic ponderomotive force of a short intense laser pulse propagating in a tenuous plasma. The regime of total cavitation of the plasma is investigated. It is found that stable propagation over a long distance is possible in this regime, and the energetic electrons are produced with a simple characteristic dependence of their angle of deflection on energy.

Is the magnetopause Rayleigh‐Taylor unstable sometimes?

At the terrestrial magnetopause the dense magnetosheath flowing around the magnetosphere interfaces with the normally much more tenuous magnetospheric plasma. Temporal variations in the solar wind dynamic pressure cause the magnetopause to be in continuous motion. When it decelerates and rebounds sunward after a strong and rapid drop in the solar wind dynamic pressure, we have a situation of a heavy‐over‐light plasma in an effective gravitational field. In this paper we investigate theoretically the conditions under which this configuration may become Rayleigh‐Taylor (RT) unstable. The motion of the magnetopause is studied using a gas‐dynamic model which takes into account forces arising from hypersonic flow past an object of prescribed shape. Thus the effect of the magnetic field near the magnetopause is not included. For sufficiently large and rapid dynamic pressure variations we obtain accelerations ≥ 1kms−2 lasting for periods of the order of 1–2 min. We discuss the possible instability of both global (λ ≫ Δ, thickness of the magnetopause) and internal (λ < Δ) modes, taking into account the stabilizing effects of (1) magnetic shear across the magnetopause and (2) viscosity of the magnetosheath plasma. We find that both modes are stable when the magnetic shear across the magnetopause is large. When the magnetosheath field points strongly northward, however, we find that both modes may be RT unstable. Among the effects of RT instability on the magnetopause and its environs are (1) large distortions of the magnetic field lines, (2) localized compressions of the magnetospheric magnetic field, (3) increased magnetopause thickness, and (4) reduced average density gradient across the magnetopause. (Since we only concentrate here on the linear stage of the development of the instability, no mass transfer into the magnetosphere is included.) These are all, in principle, observable effects, though the rapid motion of the magnetopause would require a fortuitous spacecraft‐magnetopause configuration for them to be actually seen in single‐spacecraft data. This work may be considered äs a contribution to the current research on the structure and physics of the low‐shear magnetopause.

Flows in clumpy planetary nebulae

The dynamics of clumps observed in planetary nebulae are considered. The possibility that SiO maser spots in evolved stars and the planetary nebula clumps are formed by the Parker instability behind shocks in pulsating stars' atmospheres is raised. Molecular observations of the clumps are suggested. The effects of the ablation of clumps on the global flow structure of a more tenuous plasma in which they are embedded are reviewed.

Wide intermediate scale structures in mass-loaded flows

We showed in earlier work that a clump embedded in tenuous plasma, which is flowing subsonically relative to the clump, will develop a long thin tail whether the gas in the tail behaves adiabatically or isothermally. This paper presents a criterion for determining if viscous dissipation in a tail with result in heating that will cause such a tail to be wide.

Stellar prominences

Stellar coronae, like the interstellar medium, are multi-phase media. Physical conditions in different parts of the corona allow material to exist in thermal equilibrium either as a tenuous X-ray plasma at temperatures of several million degrees, or in a denser form at temperatures low enough for hydrogen to recombine.

X-ray astronomy: past achievements and future prospects

X-ray astronomy can only be carried out from above the Earth atmosphere which absorbs this radiation. Its development could only occur with the advent of instruments carried in space on board of rockets and satellites. The first x-ray star was discovered in June 1962 in a rocket flight and found to emit an extraordinary and unexpectedly large flux of x-rays, equal to 10−7 erg cm−2s−1 at Earth and exceeding by a thousand times the visible light emission. In the subsequent ten years with the advent of "Uhuru", the first orbiting observatory entirely devoted to x-ray astronomy, the physical nature of such celestial sources was revealed. They consist of binary stellar systems containing a normal star and a collapsed companion, a neutron star or a black hole. Gas from the atmosphere of the normal star acquires energy when falling in the deep potential well of the compact companion. The energy which can be acquired and liberated per nucleon is much larger then can be extracted per nucleon during nuclear burning. This source of energy, gravitational infall onto massive compact objects, forms today the generally accepted explanation for the release of the stupendous quantity of energy emitted by the quasars and other active galactic nuclei. Also in the 70's and early 80's, a new type of matter was discovered; a tenuous high temperature plasma filling the spaces between galaxies in galaxy clusters. This gas, which could not be detected except for its x-ray emission, contains as much mass in baryons as that known to exist in the galaxies themselves, thus doubling the luminous mass of clusters. Given the high X luminosity of the clusters, x-ray observations have permitted us to study the dynamic state of evolution of clusters through their morphological appearance. We find that clusters formation has occurred relatively recently and is in fact ongoing at the present epoch. With the advent in the 70's and 80's of focusing x-ray telescopes capable of angular resolutions comparable to thoe obtained in visible light, the scope of x-ray astronomy has expanded to include all known classes of celestial objects. We have observed fluorescent x-rays from the moon; the coronas of main sequence stars; novas and supernovas; binary x-ray systems in our own and external galaxies; normal galaxies of all morphological types; active galactic nuclei in Seyferts, BL Lac, QSO's and radio galaxies; clusters of galaxies and the sources of the x-ray background, possibly early QSO's or Seyferts. In many of these objects the energy loss in x-rays equals that in the visible. In the 30 years since the discovery of the first x-ray stars, the sensitivity of x-ray astronomical observations has improved by more then one hundred million times, comparable to the improvement in the optical from the naked eye to Mt. Palomar which occurred in the last three centuries. In fact in some observations the sensitivity of x-ray detectors has outstripped the capacity of optical ground based telescopes to observe the optical counterparts and the new generation of 8 meter class telescopes will be required to carry out correlated measurements. X-ray astronomy will permit us therefore to study in the next decades the problem of formation of structures in the early universe, their evolution and dynamical development.

Relativistic harmonic generation by intense laser in plasmas

Starting from the nonlinear equations describing intense laser plasma interaction, without the supposition of tenuous plasma, weak relativity and a priori assumption φf≪1+φs, slowly varying part and rapidly varying part of scalar potential φ, and ratio of third harmonic and pumping laser a3/a1 in plasmas pumped by intense laser (≥1018 W/cm2) are obtained. Plasma and laser parameters suitable to generation of harmonic are discussed.

Auroral radiation ray distribution in the light of Viking observations of AKR

The observations of the Viking satellite have shown that the AKR sources are narrow structures with sharp density edges, filled with hot tenuous plasma, into which the wave fields are trapped over some distance before escaping into free space. We examine the consequences of these features for the free propagation of the waves and for the distribution of their energy through space. The length of the trapping is estimated in a realistic situation corresponding to orbit 237 of Viking. The AKR energy is found to be confined inside an emission cone. It is shown that 60% of the wave energy concentrates at the thin edges of the cone, corresponding to an angular range of wave vectors oriented slightly downward. It is shown that in realistic situations met by Viking, the effect of a broad auroral cavity is to stretch significantly the pattern in longitude.