Plasma Torus Research Articles

Io's nonlinear MHD‐wave field in the heterogeneous Jovian magnetosphere

Io's relative motion in the plasma torus strongly perturbs the incident magnetoplasma. The waves generated by Io then propagate through the dense plasma torus, the low‐density magnetospheric plasma and finally reach the Jovian ionosphere producing the well‐known Io footprint. Direct spacecraft observations by the Voyager and Galileo spacecraft demonstrated that Io's interaction is nearly fully saturated, i.e. the plasma flow close to Io is nearly brought to a halt in conjunction with a strong magnetic field perturbation. Here we use a nonlinear, three‐dimensional, time‐dependent MHD model to examine how the Io‐generated waves propagate, are partly reflected at plasma density gradients, and nonlinearly interact. In this work, we concentrate on the basic properties of the wave propagation based on a simplified magnetic field geometry. Despite the idealization, structural features such as the shape and morphology of the Io footprint and its wake can be qualitatively compared to measured data. We show that a strong and saturated interaction fundamentally modifies Io's wave field from the linear wave morphology picture traditionally studied. In particular, we find that due to the strong and thus nonlinear interaction the standard law of reflection completely breaks down. Io's Alfvén waves are reflected in Jupiter's ionosphere nearly anti‐parallel to the incident wave. We also notice overlapping and blending together of the multiply reflected Alfvén wings with increasing strength of Io's interaction. This could be a possible explanation for the disappearance of multiple footprints when Io moves to the torus center.

Two-Dimensional Double Probe Study of the Temporal Evolution of the Charge Carrier Density in a Pulsed Magnetron

An asymmetric-bipolar pulsed magnetron discharge was spatially studied in two dimensions with a time-resolved Langmuir double probe. The cylindrical magnetron was equipped with an Mg target. The development of the charge carrier density during one pulse reveals the same characteristic as the two-peak structure at the beginning of the ‘on’ phase which we have observed earlier. This temporal peak structure changes with position within the volume of the discharge between target and substrate. While the first maximum is dominant in the discharge centre with the plasma torus of the magnetron only weakly pronounced, the second peak is very intense in the torus region and comparably weak in the whole rest of the discharge. This peak distribution is explained by a simple model based on the acceleration of electrons left over from the previous pulse.

The Variable Rotation Period of the Inner Region of Saturn's Plasma Disk

We show that the plasma and magnetic fields in the inner region of Saturn's plasma disk rotate in synchronism with the time-variable modulation period of Saturn's kilometric radio emission. This relation suggests that the radio modulation has its origins in the inner region of the plasma disk, most likely from a centrifugally driven convective instability and an associated plasma outflow that slowly slips in phase relative to Saturn's internal rotation. The slippage rate is determined by the electrodynamic coupling of the plasma disk to Saturn and by the drag force exerted by its interaction with the Enceladus neutral gas torus.

Amalthea's modulation of Jovian decametric radio emission

Most modulation lanes in dynamic spectra of Jovian decametric emission (DAM) are formed by radiation scattering on field-aligned inhomogeneities in the Io plasma torus. The positions and frequency drift of hundreds of lanes have been measured on the DAM spectra from UFRO archives. A special 3D algorithm is used for localization of field-aligned magnetospheric inhomogeneities by the frequency drift of modulation lanes. It is found that some lanes are formed near the magnetic shell of the satellite Amalthea mainly at longitudes of (north) and (south). These disturbances coincide with regions of plasma compression by the rotating magnetic field of Jupiter. Such modulations are found at other longitudes too (189° to 236°) with higher sensitivity. Amalthea's plasma torus could be another argument for the ice nature of the satellite, which has a density less than that of water.

Positional coincidence between an H2O maser and a plasma torus in NGC 1052

AbstractWe present VLBA observation towards the nucleus of a nearby radio galaxy NGC 1052. In NGC 1052, two-sided jet structure and a dense plasma circumnuclear torus with a radius of 0.7 pc have been found around the central mass. It emits a H2O megamaser, which is redshifted with respect to the systemic velocity of the galaxy (1491 kms−1) with a large velocity width of 100 kms−1 (FWHM). The maser gas is found at the inner jet components of both the approaching and receding jets. The maser gas is positionally coincident with a plasma torus. The maser gas in NGC 1052 could be explained as a circumnuclear torus or disk, as found for the nucleus of NGC 4258.

Ultraviolet Io footprint short timescale dynamics

The electromagnetic interaction between Io and Jupiter's magnetic field leads to single or multiple ultraviolet spots near the feet of the Io flux tube. Variations of spot numbers and brightness and of inter‐spot distances have been observed to be linked to Io's position in its plasma torus. We have studied the evolution of the Io UV footprints with a time resolution of a few tens of seconds using the Space Telescope Imaging Spectrograph (STIS) in time‐tag mode. We present evidence of systematic strong brightness variations of the main spots (up to 50%) with a typical growth time of 1 minute. Additionally, unanticipated simultaneous fluctuations of both primary and secondary spots have also been found in the southern hemisphere. Our findings suggest that the footprint brightness is not only actively controlled by the plasma directly interacting with Io but also by the poorly constrained electron acceleration region between Io and Jupiter.

Simulations of field‐aligned currents: Application of theory of thin filament motion to Io's plasma wake

Io's plasma wake was treated as a tail of magnetic flux tubes perturbed by Io successively. The evolution of an Io‐perturbed flux tube was studied numerically via magnetohydrodynamics (MHD) approach of a thin filament. Our simulations suggest that the mechanism for producing wake aurora could not be explained by either Alfvén wave or electric circuit alone, rather, the underlying physics possesses the characteristics typical for both Alfvén wave and corotational lag models. An upstream‐coming flux tube must be in contact with Io for approximately 500 s, until a tilt angle of about 4° has been developed, before it is released downstream. A magnetic field depression forms downstream as a result of the continual departure of the flux tubes from Io, which in turn has significant influence on the motion of a flux tube. A perturbed flux tube would undergo a subcorotational motion in Io's plasma wake. This motion is inevitably modulated by Alfvén wave bouncing back and forth between the equatorial plane and the boundary of Io plasma torus. The scale of the subcorotation region is in the order of 1 Jovian radius (RJ). The distribution of the simulated field‐aligned currents downstream is consistent with the observed wake aurora brightness profile; in particular, the periodic structure in the current distribution is in agreement with recent infrared and FUV observations showing the presence of secondary spots in the auroral emissions.

X-rays from solar system objects

During the last few years our knowledge about the X-ray emission from bodies within the solar system has significantly improved. Several new solar system objects are now known to shine in X-rays at energies below 2 keV. Apart from the Sun, the known X-ray emitters now include planets (Venus, Earth, Mars, Jupiter, and Saturn), planetary satellites (Moon, Io, Europa, and Ganymede), all active comets, the Io plasma torus (IPT), the rings of Saturn, the coronae (exospheres) of Earth and Mars, and the heliosphere. The advent of higher-resolution X-ray spectroscopy with the Chandra and XMM-Newton X-ray observatories has been of great benefit in advancing the field of planetary X-ray astronomy. Progress in modeling X-ray emission, laboratory studies of X-ray production, and theoretical calculations of cross-sections, have all contributed to our understanding of processes that produce X-rays from the solar system bodies. At Jupiter and Earth, both auroral and non-auroral disk X-ray emissions have been observed. X-rays have been detected from Saturn's disk, but no convincing evidence of an X-ray aurora has been observed. The first soft (0.1–2 keV) X-ray observation of Earth's aurora by Chandra shows that it is highly variable. The non-auroral X-ray emissions from Jupiter, Saturn, and Earth, those from the disk of Mars, Venus, and Moon, and from the rings of Saturn, are mainly produced by scattering of solar X-rays. The spectral characteristics of X-ray emission from comets, the heliosphere, the geocorona, and the Martian halo are quite similar, but they appear to be quite different from those of Jovian auroral X-rays. X-rays from the Galilean satellites and the IPT are mostly driven by impact of Jovian magnetospheric particles. This paper reviews studies of the soft X-ray emission from the solar system bodies, excluding the Sun. Processes of production of solar system X-rays are discussed and an overview is provided of the main source mechanisms of X-ray production at each object. A brief account on recent development in the area of laboratory studies of X-ray production is also provided.

Numerical simulation of fine structure in the Io plasma torus produced by the centrifugal interchange instability

The Io plasma torus as a whole has a radial width scale ∼1 RJ, much larger than the width of the localized Io plasma source (∼1 RIo ≈ RJ/39). One of the most prominent features of the Io torus observed by the Voyager spacecraft and Earth‐based instruments is the “ribbon” structure near Io's orbit. The stability properties of this narrower ribbon structure embedded within the larger torus have been investigated by the Rice Convection Model for Jupiter. Four initial plasma distributions having different radial widths are each represented by 82 longitudinally symmetric edges establishing 41 levels of the flux tube mass content η with the peak η value at Io's orbit. The same initial perturbation is put on each of these edges and is subjected to centrifugal interchange. Our simulations produce regularly spaced long, thin fingers moving outward from the outer edges. It is shown that the azimuthal width of the interchange convection cells (the distance between outflowing fingers in the nonlinear stage of development) is proportional to the radial width scale of the initial distribution that produced them. The constant of proportionality is ≈0.5. Since the exponential growth rate is essentially proportional to the azimuthal wave number of the disturbance and hence is inversely proportional to its azimuthal width, the ribbon‐scale interchange structures grow faster than torus‐scale interchange structures.

Possible eigenmode trapping in density enhancements in Saturn's inner magnetosphere

We report a re‐examination of observations of radio waves and banded fine structure observed by Cassini in the Saturn inner magnetosphere. These emissions have been identified in the past as Z‐mode emission from possible density cavities along the inner edge of the Saturn plasma torus. A careful examination of the emission fine structure in a region where the ratio of plasma to cyclotron frequency fp/fce < 1 suggests that the emission bands may be due to eigenmode trapping from density enhancement source regions. The observations are similar to those made at both the Earth and Jupiter and suggest that radio emission fine structure is a remote diagnostic of density fluctuations in planetary magnetospheres.

Pile-up Type Magnetic Reconnection Experiment by Compression of Current Sheet

Transient effect of fast magnetic reconnection was investivated in TS-4 torus plasma merging device. Reconnection speed of two merging tokamaks under low inflow and strong guiding(toroidal) field B⊥ agrees with the theoretical value for the Sweet-Parker model. However, when current-sheet was compressed by two pairs of acceleration coils, the reconnection speed became three times as much as the value for the Sweet-Parker model under constant resistivity of the sheet. The inflow flux was found to exceed the outflow flux, indicating density piled-up in current sheet. The measured inflow speed is consistent with a new model where the flux pile-up effect is inclued in convensional Sweet-Parker model.

Development of Neutral Molecular Beam Injector for Two-Dimensional Edge Density Measurement

A sheet-shaped thermal lithium beam probe has been developed for two-dimensional density measurements in the edge region of the torus plasma. For the production of an intense and uniform beam, the optimization of the nozzle shape and slit width of the beam injector has been performed. Beam density profiles at the observing region in the plasma were estimated with numerical calculations using a Monte Carlo technique for several kinds of nozzles and combinations with slits. Experiments on the test stand were also carried out, and pretty good agreement was found between numerical and experimental results. Based on these examination and optimization for the nozzle and slits, we have constructed a sheet-shaped beam injector for the large helical device. In spite of the long distance (about 3 m) from the injector, it is estimated that the thickness of the sheet-shaped beam is ∼90 mm (full width at half maximum).

Stochastic Acceleration of Energetic Particles in the Magnetosphere of Saturn

Voyager's plasma probe observations suggest that there are at least three fundamentally different plasma regimes in Saturn: the hot outer magnetosphere, the extended plasma sheet, and the inner plasma torus. At the outer regions of the inner torus some ions have been accelerated to reach energies of the order of 43 keV. We develop a model that calculates the acceleration of charged particles in the Saturn's magnetosphere. We propose that the stochastic electric field associated to the observed magnetic field fluctuations is responsible of such acceleration. A random electric field is derived from the fluctuating magnetic field – via a Monte Carlo simulation – which then is applied to the momentum equation of charged particles seeded in the magnetosphere. Taking different initial conditions, like the source of charged particles and the distribution function of their velocities, we find that particles injected with very low energies ranging from 0.129 eV to 5.659 keV can be strongly accelerated to reach much higher energies ranging from 22.220 eV to 9.711 keV as a result of 125,000 hitting events (the latter are used in the numerical code to produce the particle acceleration over a predetermined distance).

Motion of the ablation cloud in torus plasmas

The notion of an ablation cloud is investigated in a tokamak by using ideal masnetohydrodynamic equations with ablation processes. The doud quickly expands along the magnetic field and simultaneously drifts to the lower field side due to a tire tube force and 1/R force. It is found that the acceleration of the cloud increases and subsequently decreases due to the energy deposit from the hulk plasma and, finally, the cloud is stopped due to the poloidal field.

Simulation of sheet-shaped lithium beam probe performance for two-dimensional edge plasma measurement

A sheet-shaped thermal lithium beam probe has been developed for two-dimensional density measurements in the edge region of the torus plasma. A numerical simulation was carried out to confirm the validity of the diagnostics for fast and transient phenomena such as edge localized modes or blobs, etc., where the velocity of blobs is faster than that of the probe beam. It was found in the simulation that the density of the blob itself is reconstructed to be low and unexpected ghosts appear in the reconstructed density profile near the blob, if the conventional reconstruction method is employed. These results invite our attention to the numerical errors in the density reconstruction process. On the other hand, the errors can be corrected by using the simulation results.

Magnetic probe response function calibrations for plasma equilibrium reconstructions of CDX-U

A novel response function calibration technique has been developed to account for time-dependent nonaxisymmetric eddy currents near magnetic sensors in toroidal magnetic confinement devices. The response function technique provides a means to cross calibrate against all available external field coil systems to calculate the absolute sensitivity of each magnetic field sensor, even when induced eddy currents are present in the vacuum vessel wall. The response function information derived in the calibration process can be used in equilibrium reconstructions to separate plasma signals from signals due to externally produced eddy currents at magnetic field sensor locations, without invoking localized wall current distribution details. The response function technique was used for the first ever equilibrium reconstructions of spherical torus plasmas, when applied to the Current Drive Experiment-Upgrade (CDX-U) device. In conjunction with the equilibrium and stability code (ESC), equilibria were obtained for recent CDX-U experiments with lithium plasma-facing components. A description of the CDX-U magnetic sensor configuration and the response function calibration technique will be presented along with examples of resulting plasma equilibrium for CDX-U lithium wall operations.

Measurement of global instability of compact torus by three-dimensional tomography

The new three-dimensional tomography reconstruction system based on the novel scheme suggested by Balandin et al. [Eur. Phys. J. D 14, 97 (2001)] was developed for a powerful monitor of compact torus (CT) plasma using 16 two-dimensional (2D) projections of visible light. This system can obtain many (16) 2D projections with one detector (a framing camera) by use of light collecting optical probes and optical fiber bundles. Sixteen optical probes, which measure emission of plasmas, were installed on a sphere outside the separatrix of CT inside the vacuum vessel. Optical radiation was collected by the wide-angle lenses attached to the optical probes and transported by the fiber bundles to a high-speed framing camera. The line-integrated 2D images measured with this system were inverted to local emissivity using the software. The n=1 (n: toroidal mode number) tilt motion of field reversed configuration plasma was successfully measured.

Implication for the solar wind effect on the Io plasma torus

Sporadic enhancements of [SII] 673.1 nm emissions from the Io plasma torus were found in ground‐based observations in 1998 and 1999. Just after the onset of the enhancement on September 21, 1999, narrow‐band kilometric (nKOM) radiation began to be observed by the Galileo/PWS in the Jovian magnetosphere. During this period, quasi‐periodic burst groups (Morioka et al., 2006) were also observed by the PWS, and Jovian “auroral flare” (Waite et al., 2001) events were found on the same day by the Hubble Space Telescope. The sudden appearances of both nKOM radiation and auroral flare suggest the arrival of a remarkable solar‐wind disturbance at the Jovian magnetosphere. In this paper, we present evidence that these enhancements are stochastic, related to solar wind disturbances.

Two-dimensional MHD simulations of accretion disk evaporation

AbstractWe simulate the accretion disk evaporation to study the changes of accretion disk structure during the state transition from the soft state to the hard state. We performed 2 dimensional MHD simulations by including the heat conduction process. We assume the axisymmetric accretion and put a cold rotating gas torus in a hot halo in hydrostatic equilibrium initially. Weak magnetic fields are threaded vertically. Heat conduction equation and MHD equations are solved separately according to the time splitting method. We obtained the result that accretion disk is heated by the hot corona and the hot gas evaporates from the accretion disk surface. We found that magnetic fields lines bended by disk rotation restrict the energy transport vertically and make disk evaporation ineffective.

Influence of vacuum toroidal field on two-fluid flowing equilibria of helicity-driven spherical torus plasmas

Two-fluid flowing equilibrium configurations of a helicity-driven spherical torus (HD-ST) in the realistic confinement region, including a flux conserver and a coaxial helicity source, are numerically determined by means of the combination of the finite difference and the boundary element methods. It is found from the numerical results that electron fluid near the central conductor is tied to a vacuum toroidal field and ion fluid is not. The magnetic configurations change from the high-q HD-ST (safety factor, q>1) with a paramagnetic toroidal field and low-β (volume average β value, ⟨β⟩≈2%) through the helicity-driven spheromak and reversed-field pinch to the ultra-low-q HD-ST (0<q<1) with a diamagnetic toroidal field and high-β (⟨β⟩≈18%) as the vacuum toroidal field at the inner edge regions decreases and reverses the sign. The two-fluid effects are more significant in this equilibrium transition when the ion diamagnetic drift has the same direction as the E×B one.