Tenuous Plasma Research Articles

Interactions of solar wind plasma with dust grains: Effects of strong plasma anisotropy

In this paper we re-examined the fundamental physics of charging of a dust particle in the moon environment by tenuous anisotropic solar wind plasma. The majority of work on dusty (complex) plasmas is largely concerns with laboratory plasmas, in which charging process of dust grains is very fast, thus making practical the working concept of dynamically equilibrium floating potential and grain charge. However, solar wind plasma parameters are considerably different at the moon orbit, and we found the characteristic charging time of lunar dust grains to be considerably longer, ranging from 3 to 4.6 min for micron size particles, and up to 7.6 h for 10-nm grains, depending on the value of plasma streaming velocity. These findings make it clear that the transient stage of charging process is important in the moon environment, and equilibrium floating potential and grain charge could be considered as long time asymptotic values. For this reason we re-formulated the moon dust charging process as an inherently time-dependent problem and derived the time-dependent charging equation for the grain potential for general case of anisotropic solar wind plasma. Using the results of our kinetics analysis we found that the distribution of charge density over grain surface submerged into solar wind plasma is highly anisotropic, thus making the OML model, which is based on the assumption of isotropic distribution of surface charge density, not applicable to the grain charging problem by the solar wind plasma.

Correction to “Electron density estimations derived from spacecraft potential measurements on Cluster in tenuous plasma regions”

Correction to “Electron density estimations derived from spacecraft potential measurements on Cluster in tenuous plasma regions”

Statistical analysis of injection/dispersion events in Saturn's inner magnetosphere

In the inner magnetosphere of a rapidly rotating planet such as Jupiter and Saturn, radial transport of plasma mainly comprises hot, tenuous plasma moving inward and cold, denser plasma moving outward. A distinctive phenomenon resulting from the drift dispersion of injecting hot plasma provides direct evidence for this convective motion. Particle instruments aboard the Cassini spacecraft, including the Magnetospheric Imaging Instrument (MIMI) and the Cassini Plasma Spectrometer (CAPS), have made numerous observations of such signatures. The statistics of the properties of such events are studied in this paper by analyzing CAPS data from 26 Cassini orbits. A statistical picture of their major characteristics is developed, including the distributions of ages, longitudinal widths, radial distances, and longitudes and local times of injection. An unexpected longitude modulation of these events appears in the old SLS longitude system, which is based on the Saturn kilometric radiation (SKR) observations by Voyager around 1980, while no such modulation seems to exist in the new SKR longitude system of the Cassini era. A Lomb periodogram analysis, however, reveals no significant periodic modulation of these events. The injection structures are found to occupy a small fraction (∼5–10) of the available longitude space.

Electron density estimations derived from spacecraft potential measurements on Cluster in tenuous plasma regions

Spacecraft potential measurements by the EFW electric field experiment on the Cluster satellites can be used to obtain plasma density estimates in regions barely accessible to other type of plasma experiments. Direct calibrations of the plasma density as a function of the measured potential difference between the spacecraft and the probes can be carried out in the solar wind, the magnetosheath, and the plasmashere by the use of CIS ion density and WHISPER electron density measurements. The spacecraft photoelectron characteristic (photoelectrons escaping to the plasma in current balance with collected ambient electrons) can be calculated from knowledge of the electron current to the spacecraft based on plasma density and electron temperature data from the above mentioned experiments and can be extended to more positive spacecraft potentials by CIS ion and the PEACE electron experiments in the plasma sheet. This characteristic enables determination of the electron density as a function of spacecraft potential over the polar caps and in the lobes of the magnetosphere, regions where other experiments on Cluster have intrinsic limitations. Data from 2001 to 2006 reveal that the photoelectron characteristics of the Cluster spacecraft as well as the electric field probes vary with the solar cycle and solar activity. The consequences for plasma density measurements are addressed. Typical examples are presented to demonstrate the use of this technique in a polar cap/lobe plasma.

Analysis of plasma waves observed within local plasma injections seen in Saturn's magnetosphere

Plasma injections or density depletion regions have been reported to be a prolific feature of Saturn's inner magnetosphere. They are characterized by flux tubes of warm, tenuous plasma in a cooler, locally produced plasma background. The injected plasma undergoes dispersion in energy due to gradient and curvature drifts as the flux tube transports. The plasma waves within these injections are of at least two types. Above the electron cyclotron frequency, fce, very intense and narrow‐banded emissions resembling electrostatic cyclotron harmonics (ECH) are often observed. Below fce, whistler mode chorus is sometimes observed. Inside the plasma injections there exists a low‐energy (<100 eV) electron component which tends to have a field‐aligned pitch angle distribution, and a warmer (E > 1000 eV) component with “pancake” pitch angle distributions (peaked at 90°). We model the electron plasma distributions observed inside one injection to conduct a linear dispersion analysis of the wave modes. The results suggest that the ECH emissions can be generated by phase space density gradients associated with a narrow loss cone that is likely to be present but not observed because the electron detectors field of view did not include the magnetic field line at the time of the observations. The whistler mode chorus emission can be generated by the pancake‐like distribution and temperature anisotropy (T⊥/T∥ > 1) of the warmest plasma population. Some interesting anomalies between the results and the observations may be resolved by analyses of additional injection events.

Effects of the stellar wind on X-ray spectra of Cygnus X-3

We study X-ray spectra of Cyg X-3 from BeppoSAX, taking into account absorption and emission in the strong stellar wind of its companion. We find the intrinsic X-ray spectra are well modelled by disc blackbody emission, its upscattering by hot electrons with a hybrid distribution, and by Compton reflection. These spectra are strongly modified by absorption and reprocessing in the stellar wind, which we model using the photoionization code CLOUDY. The form of the observed spectra implies the wind is composed of two phases. A hot tenuous plasma containing most of the wind mass is required to account for the observed features of very strongly ionized Fe. Small dense cool clumps filling ≤0.01 of the volume are required to absorb the soft X-ray excess, which is emitted by the hot phase but not present in the data. The total mass-loss rate is found to be (0.6-1.6) x 10 -5 M ⊙ yr -1 . We also discuss the feasibility of the continuum model dominated by Compton reflection, which we find to best describe our data. The intrinsic luminosities of our models suggest that the compact object is a black hole.

Photon acceleration in the laser wakefield

On the SILEX-Ⅰlaser facility, we measured the transmission spectra after ultrashort and ultraintense laser pulse interaction with tenuous plasma. Due to the existence of the wakefield, the time-dependent refractive index of the plasma can accelerate or decelerate photons of laser pulse when the laser pulse propagates in the plasma. Its main features are the spliting of the fundamental frequency and its evident broadening in the laser transmission spectra.The side_band wave which is often associated with Raman and Self modulation were not observed in our experiments. Utilizing the LPIC++ code, we also simulated the transmission spectrum after the ultrashort_ultrahigh intensity laser pulse interacts with the tenuous plasma. The simulation results are in good agreement with the experiment results.

Electron flat‐top distributions around the magnetic reconnection region

Cluster multisatellite observations provide snapshots of electron distributions around the magnetic neutral line. An isotropic flat‐top‐type electron distribution in phase space is frequently observed around the X line, together with large ion velocities and a Hall quadrupole‐like magnetic field inside the hot and tenuous plasma sheet in the magnetotail. The flat‐top distributions are also associated with a finite magnetic field in the direction normal to the neutral sheet, and the cross‐tail current density is sometimes very small. These results indicate that the flat‐top‐type distribution is mainly located near the outer boundary of the ion diffusion region in the plasma sheet outflow region, before reaching the pileup region with large normal component of the magnetic field. Simultaneously with the flat‐top distributions, strong field‐aligned electron beams mainly toward the X line are occasionally observed. This type of beam is mainly observed in the off‐equatorial plasma sheet and also appears well inside the plasma sheet. Typical energies of the beam are 4–10 keV, which is comparable to the upper edge of flat‐top energy. These highly accelerated electron distributions have a steep decrease in phase space density at the high‐energy end, and it is found that they are not correlated with the increase of the higher‐energy electrons related to suprathermal acceleration (>30 keV). This result indicates that the electron acceleration processes for the flat‐top‐type distributions are different from the suprathermal components, both of which are beyond the conventional MHD outflow acceleration and considered to be associated with some kinetic processes.

Electrostatic structure around spacecraft in tenuous plasmas

Most satellite‐based in situ plasma experiments are affected in some manner by the electrostatic structure surrounding the spacecraft. In order to better understand this structure, we have developed a fully three‐dimensional self‐consistent model that can accept realistic spacecraft geometry, including both thin (∼10−4 m) wires and long (∼102 m) booms, with open boundary conditions. The model uses an integral formulation incorporating boundary element, multigrid and fast multipole methods to overcome problems associated with the large range in scale sizes and inherently three‐dimensional structure. By applying the model to the Cluster spacecraft, we show that the electric potential structure is dominated by the charge on the wire booms, with the spacecraft body contributing at small distances. Consequently, the potential near the EFW (Electric Fields and Waves experiment) probes at the end of the wire booms is typically significantly above the true plasma potential. For the Cluster spacecraft, we show that this effect causes a 19% underestimation of the spacecraft potential and 13% underestimation of the ambient electric field. We further assess the electric field due to the sunward‐oriented photoelectron cloud, showing that the cloud contributes little to the observed spurious sunward field in the EFW data.

Filamentation of ultrashort laser pulses propagating in tenuous plasmas

The filamentation of ultrashort laser pulses (shorter than a plasma period) propagating in tenuous plasmas is studied. In this regime relativistic and ponderomotive nonlinearities tend to cancel each other. Time-dependent residual nonlinear plasma response brings about the dynamical filamentation with the maximum unstable transverse wave number decreasing in the course of laser pulse propagation. Dynamics of a hot spot that seeds the filamentation instability is studied numerically and reveals a good agreement with the analytical results.

Radio Emissions from Solar Active Regions

Solar active region coronae are known for strong magnetic fields permeating tenuous plasma, which makes them an ideal astronomical laboratory for magnetohydrodynamics research. It is, however, relatively less known that this physical condition also permits a very efficient radiation mechanism, gyro-resonant emission, produced by hot electrons gyrating in the coronal magnetic field. As a resonant mechanism, gyro-emission produces high enough opacity to fully reveal the coronal temperature, and is concentrated at a few harmonics of the local gyrofrequency to serve as an excellent indicator of the magnetic field. In addition, the polarization of the ubiquitous free–free emission and a phenomenon of depolarization due to mode coupling extend the magnetic field diagnostic to a wide range of coronal heights. The ability to measure the coronal temperature and magnetic field without the complications that arise in other radiative inversion problems is a particular advantage for the active region radio emissions available only at these wavelengths. This article reviews the efforts to understand these radiative processes, and use them as diagnostic tools to address a number of critical issues involved with active regions.

Johannes Geiss: Explorer of the Elements

The extraordinary life and scientific achievements of Johannes Geiss span an almost impossible breadth of scientific topics, from the study of rocks to tenuous plasmas, from volcanoes to meteorites. But, his impact also extends way beyond the field of science. Professor Geiss is a well-known teacher and a highly successful science leader whose impact has been felt at the University of Bern, in Switzerland, and around the globe. We present here a brief summary of this highly successful career via a pictorial overview and a movie compiled by a former student who had the good luck to work with Professor Geiss during his years at the University of Bern.

Tethys and Dione as sources of outward-flowing plasma in Saturn’s magnetosphere

Rotating at over twice the angular speed of Earth, Saturn imposes a rapid spin on its magnetosphere. As a result, cold, dense plasma is believed to be flung outward from the inner magnetosphere by centrifugal force and replaced by hotter, more tenuous plasma from the outer magnetosphere. The centrifugal interchange of plasmas in rotating magnetospheres was predicted many years ago and was conclusively demonstrated by observations in Jupiter's magnetosphere, which--like that of Saturn (but unlike that of Earth)--is rotationally dominated. Recent observations in Saturn's magnetosphere have revealed narrow injections of hot, tenuous plasma believed to be the inward-moving portion of the centrifugal interchange cycle. Here we report observations of the distribution of the angle between the electron velocity vector and the magnetic field vector ('pitch angle') obtained in the cold, dense plasma adjacent to these inward injection regions. The observed pitch-angle distributions are indicative of outward plasma flow and consistent with centrifugal interchange in Saturn's magnetosphere. Further, we conclude that the observed double-peaked ('butterfly') pitch-angle distributions result from the transport of plasma from regions near the orbits of Dione and Tethys, supporting the idea of distinct plasma tori associated with these moons.

Simulations of multiphase turbulence in jet cocoons

The interaction of optically emitting clouds with warm X-ray gas and hot, tenuous radio plasma in radio jet cocoons is modelled by 2D compressible hydrodynamic simulations. The initial setup is the Kelvin–Helmholtz instability at a contact surface of density contrast 104. The denser medium contains clouds of higher density. Optically thin radiation is realized via a cooling source term. The cool phase effectively extracts energy from the other gas which is both, radiated away and used for acceleration of the cold phase. This increases the system's cooling rate substantially and leads to a massively amplified cold mass dropout. We show that it is feasible, given small seed clouds of the order of 100 M⊙, that all of the optically emitting gas in a radio jet cocoon may be produced by this mechanism on the propagation time-scale of the jet. The mass is generally distributed as T−1/2 with temperature, with a prominent peak at 14 000 K. This peak is likely to be related to the counteracting effects of shock heating and a strong rise in the cooling function. The volume filling factor of cold gas in this peak is of the order of 10−5–10−3 and generally increases during the simulation time. The simulations tend towards an isotropic scale-free Kolmogorov-type energy spectrum over the simulation time-scale. We find the same Mach-number density relation as Kritsuk & Norman and show that this relation may explain the velocity widths of emission lines associated with high-redshift radio galaxies, if the environmental temperature is lower, or the jet-ambient density ratio is less extreme than in their low-redshift counterparts.

Ponderomotive acceleration of injected electrons in tenuous plasmas by intense laser pulses

The trapping and acceleration of an electron by forward ponderomotive force associated with intense short laser pulses, propagating in homogeneous rarefied plasmas is analyzed. This is done not by solving the motion equations but by energy conservation law and Lorentz transformation. This method is able to the treat the ponderomotive acceleration regardless of laser polarization. It is shown that the gain of acceleration increases linearly with the field strength of the laser and the relativistic factor of the group velocity of the laser in the plasma, while the minimum injection energy necessary for trapping the electron decreases with the laser field strength and increases slowly with the group velocity of the laser.

Evolution of the fastest-growing relativistic mixed mode instability driven by a tenuous plasma beam in one and two dimensions

Particle-in-cell simulations confirm here that a mixed plasma mode is the fastest growing when a highly relativistic tenuous electron-proton beam interacts with an unmagnetized plasma. The mixed modes grow faster than the filamentation and two-stream modes in simulations with beam Lorentz factors Γ of 4, 16, and 256, and are responsible for thermalizing the electrons. The mixed modes are followed to their saturation for the case of Γ=4 and electron phase space holes are shown to form in the bulk plasma, while the electron beam becomes filamentary. The initial saturation is electrostatic in nature in the considered one- and two-dimensional geometries. Simulations performed with two different particle-in-cell simulation codes evidence that a finite grid instability couples energy into high-frequency electromagnetic waves, imposing simulation constraints.

Double-Probe Measurements in Cold Tenuous Space Plasma Flows

Cold flowing tenuous plasmas are common in the terrestrial magnetosphere, particularly in the polar cap and tail lobe regions, which are filled by the supersonic plasma flow known as the polar wind. Electric field measurements with double-probe instruments in these regions suffer mainly from two error sources: 1) an apparent sunward electric field due to photoemission asymmetries in the probe-boom system and 2) an enhanced negatively charged wake forming behind the spacecraft, which will affect the probe measurements. The authors investigate these effects experimentally by Fourier analysis of the spin signature from the double-probe instrument Electric Fields and Waves (EFW) on the Cluster spacecraft. They show that while the signature due to photoemission asymmetry is very close to sinusoidal, the wake effect is characterized by a spectrum of spin harmonics. The Fourier decomposition can therefore be used for identifying wake effects in the data. As a spin-off, the analysis has also given information on the cold flowing ion population

Wake formation behind positively charged spacecraft in flowing tenuous plasmas

Spacecraft in tenuous plasmas become positively charged because of photoelectron emission. If the plasma is supersonically drifting with respect to the spacecraft, a wake forms behind it. When the kinetic energy of the positive ions in the plasma is not sufficient to overcome the electrostatic barrier of the spacecraft potential, they scatter on the potential structure from the spacecraft rather than get absorbed or scattered by the spacecraft body. For tenuous plasmas with Debye lengths much exceeding the spacecraft size, the potential structure extends far from the spacecraft, and consequently in this case the wake is of transverse dimensions much larger than the spacecraft. This enhanced wake formation process is demonstrated by theoretical analysis and computer simulations. Comparison to observations from the Cluster satellites shows good agreement.

Filamentation of a relativistic short pulse laser in a plasma

An intense short pulse laser propagating through a plasma undergoes filamentation instability under the combined effects of relativistic mass variation and ponderomotive force-induced electron density depression. These two nonlinearities superimpose each other. In a tenuous plasma, the filament size scales as where ω p is the plasma frequency, a0 is the normalized laser amplitude and γ 0 is the relativistic gamma factor.

Ponderomotive acceleration of electrons in the interaction of arbitrarily polarized laser pulse with a tenuous plasma

The forward ponderomotive force associated with an intense short laser pulse, propagating in a tenuous plasma, accelerates the electrons to velocities higher than the group velocity of the laser. In this work, a simple general solution for ponderomotive acceleration is presented for arbitrary polarization. The circular polarization is more efficient than linear polarization, since the threshold laser intensity needed for electron acceleration is lower for a circularly polarized laser pulse.