Background Plasma Flow Research Articles

Effect of gravitational stratification, longitudinal temperature inhomogeneity, radiative cooling and background plasma flow on torsional Alfvén oscillations of a coronal loop

Effect of gravitational stratification, longitudinal temperature inhomogeneity, radiative cooling and background plasma flow on torsional Alfvén oscillations of a coronal loop

Model for plasma transport due to drift waves based on mappings including finite Larmor radius

Abstract Turbulence in plasmas is modeled by fluctuating electric fields that determine particle motion through the E × B drift velocity which can lead to chaotic behavior. When applied to an ensemble of plasma particles in the chaotic regime their transport is studied: the anomalous plasma transport. The fluctuations are modeled by a spectrum of traveling waves with a wide frequency span which converts the equations of motion into an iterative mapping. The statistical properties of transport are derived. When the waves amplitude is small the particle orbits are regular, but as it is increased the behavior becomes increasingly chaotic. The effect of finite Larmor radius and the presence of a background plasma flow is also studied. We show that when a thermal population of particles is considered, the transport becomes non-local, as evidenced by a non-Gaussian particle distribution function (PDF). We have analyzed two different kinds of sheared flows: (1) a monotonic velocity shear and (2) a non-monotonic shear. The presence of transport barriers associated with the sheared velocity is also studied. We also present the application of the same techniques to study the transport of energetic particles that are born with a monoenergetic distribution.

Interpretation of the North‐South Asymmetric Oxygen Aurora Morphology on Europa Using Test Particle Simulation

AbstractSeveral observations using the Hubble Space Telescope reported that the brightness morphology of the oxygen OI] 135.6 nm emissions on Europa's atmosphere has a north‐south asymmetry which changes with the position of Europa with respect to the Jovian magnetospheric plasma sheet. Similar north‐south asymmetry of Io's auroral limb glow has been explained by higher electron flux into the atmosphere on the hemisphere that faces the plasma sheet center. This explanation, however, has not yet been evaluated for the case of Europa quantitatively. In this study, we used a test particle simulation for the Jovian magnetospheric electrons to estimate the brightness of the 135.6 nm aurora in Europa's atmosphere and evaluate the cause of the north‐south asymmetry with the previously suggested idea, in which the strong deceleration of the magnetospheric flux tube results in the inhomogeneous electron flux into Europa's atmosphere (the “slow‐down effect”). Our simulation successfully recreates the systematically changing north‐south asymmetry of Europa's oxygen aurora brightness using the “slow‐down effect.” With deceleration into 10% of the background plasma flow, the maximum north‐to‐south brightness ratio is estimated at 2.17 and 2.56 on the trailing (plasma‐upstream) and leading (downstream) side, respectively. However, the previously observed brightness ratio is larger on the trailing side (up to ∼5). The results indicate that additional model scenarios are required to fully explain the north‐south asymmetry of Europa's oxygen aurora morphology.

Study of turbulent transport in magnetized plasmas with flow using symplectic maps

Turbulent transport in a magnetized plasma of the kind found in tokamaks is modeled by a 2D wave spectrum that allows reduction to a symplectic map. The properties of particle transport when chaos sets in are analyzed in various circumstances including finite Larmor radius (FLR) effects and a background plasma flow. For large wave amplitudes, regular particle orbits become chaotic, which represents a type of Lagrangian turbulence. When chaos becomes global, it leads to the loss of particle confinement. Poloidal flows tend to decrease the chaos in some regions, and they can give rise to the formation of transport barriers. FLR effects not only reduce chaos but also give rise to non-local behavior. Thus, when the particles have a thermal distribution of Larmor radii, a non-Gaussian particle distribution function in space is obtained. However, the transport preserves its diffusive scaling when there is no flow. Previous results about the dependence of the diffusion coefficient with amplitude are re-derived analytically and numerically taking into account FLR effects. In the presence of general poloidal flows, the transport has to be described by a two-step map. They modify the nature of transport in the direction of the flow from diffusive to ballistic to super-ballistic, depending on the type of flow. The transverse transport, in turn, shows suppression of the oscillations with wave amplitude that are present in the absence of flow. When the plasma flow varies linearly with radius, the transport can be studied with a similar single-step map, and the transverse diffusion coefficient is reduced while parallel transport can become super-ballistic. For non-monotonic flows, there are accelerating modes that can produce ballistic-like particles while the bulk of the particles behaves diffusively.

I. The effect of symmetric and spatially varying equilibria and flow on MHD wave modes: slab geometry

ABSTRACT Realistic theoretical models of magnetohydrodynamic wave propagation in the different solar magnetic configurations are required to explain observational results, allowing magnetoseismology to be conducted and provide more accurate information about local plasma properties. The numerical approach described in this paper allows a dispersion diagram to be obtained for any arbitrary symmetric magnetic slab model of solar atmospheric features. This proposed technique implements the shooting method to match necessary boundary conditions on continuity of displacement and total pressure of the waveguide. The algorithm also implements fundamental physical knowledge of the sausage and kink modes such that both can be investigated. The dispersion diagrams for a number of analytic cases that model magnetohydrodynamic waves in a magnetic slab were successfully reproduced. This work is then extended by considering density structuring modelled as a series of Gaussian profiles and a sinc(x) function. A further case study investigates properties of MHD wave modes in a coronal slab with a non-uniform background plasma flow, for which the governing equations are derived. It is found that the dispersive properties of slow body modes are more greatly altered than those of fast modes when any equilibrium inhomogeneity is increased, including background flow. The spatial structure of the eigenfunctions is also modified, introducing extra nodes and points of inflexion that may be of interest to observers.

ULF waves in the outer magnetosphere: Geotail observation 1 transverse waves

A survey of ULF waves in the Pc 5 frequency range during the six year period (1995–2000) has been made for the field line resonance waves in the outer magnetosphere, using plasma flow and magnetic field measurements with the Geotail spacecraft. On the morning side a series of wave trains often appears during several hours. In contrast such a series of wave trains with a long duration is rarely observed in the afternoon to evening sector. Most of waves in the afternoon sector are isolated, of which duration is approximately one hour. The existence of a set of preferential frequencies and the pronounced dawn-dusk asymmetry of wave occurrence and wave features, which were found in ground-based and ionospheric measurements of geomagnetic ULF pulsations, were statistically confirmed. It is also noted that the background plasma flow is sunward in the evening sector without exception. Transverse waves are generally observed in the condition of plasma β below 1.5. High β cases are mostly associated with events on the dusk side. As for the relation to solar wind conditions, Pc 5 waves tend to occur under the condition of more radial than the average IMF spiral angle, or of low cone angle. Key words: Pc 5 waves, dawn-dusk asymmetry of occurrence, radial IMF.

Transport Characteristics in the Stochastic Magnetic Boundary of LHD: Magnetic Field Topology and Its Impact on Divertor Physics and Impurity Transport

Transport characteristics of the stochastic magnetic boundary of the Large Helical Device (LHD) are investigated, based on three-dimensional Monte-Carlo Braginskii-type fluid model code, EMC3, coupled with the kinetic neutral transport code EIRENE, in direct comparison with experimental observations for aspects of the relation between the magnetic topology and the resulting transport in terms of counter acting flux tube flows and impurity screening/transport. Divertor probe measurements show a rather weak divertor parameter dependence on upstream density in contrast to those of tokamaks at high-recycling regime. This is found to be due to the loss of parallel momentum via cross-field interaction between the stochastic flux tubes, where strong flow shear exists. The three-dimensional modeling predicts an impurity screening potential of the stochastic scrape-off layer (SOL) at high densities. The remnant island geometry affects the energy transport, which leads to suppression of the thermal forces by increasing cross-field energy flux across islands at high collisionality. The screening effect is most pronounced at the edge surface layers with a strong friction force exerted by the background plasma flow, where the flow toward divertor is enhanced due to the rich ionization source. Modeling results are compared to the edge carbon emission obtained in experiments, where a reasonable agreement on the density dependence is found, indicating the existence of the impurity screening mechanism in the stochastic SOL of LHD.

Heavy coronal ions in the heliosphere

<i>Aims. <i/>Our aim is to investigate and study the de-charging of the elements C, N, O, Mg, Si and S-ions, and assess the fluxes of the resulting ENA in the heliosphere.<i>Methods. <i/>The model treats the heavy ions as test particles convected by (and in a particular case also diffusing through) a hydrodynamically calculated background plasma flow from 1 AU to the termination shock (TS), the heliosheath (HS) and finally the heliospheric tail (HT). The ions undergo radiative and dielectronic recombinations, charge exchanges, photo- and electron impact ionizations with plasma particles, interstellar neutral atoms (calculated in a Monte-Carlo model) and solar photons.<i>Results. <i/>Highly-charged heavy coronal ions flowing with the solar wind undergo successive de-ionizations, mainly in the heliosheath, which leads to charge-states much lower than in the supersonic solar wind. If Coulomb scattering is the main ion energy-loss mechanism, the end product of these deionizations are fluxes of ENA of <i>∼<i/>1 keV/nucleon originating in the upwind heliosheath that for C, Mg, Si and S may constitute sources of pickup ions (PUI), significantly exceeding the interstellar supply.<i>Conclusions. <i/>Discussed processes result in (i) distinct difference of the ion charge <i>q<i/> in the supersonic solar wind (approximately <i>q<i/><i>≥<i/>+Z/2, <i>Z<i/> = atomic number) compared to that in the HS (approximately 0<i>≤<i/>q<i>≤<i/>+Z/2)); (ii) probable concentration of singly ionized atoms (<i>q<i/> = +1) in the heliosheath towards the heliopause (HP) and in the HT; (iii) possible significant production of ENA in the HS offering natural explanation for production of PUI, and – after acceleration at the TS – anomalous cosmic rays (ACR) of species (like C, Mg, Si, S) unable to enter the heliospheric cavity from outside because of their total ionization in the local interstellar medium.

Measurements and simulations of scrape-off layer flows in the DIII-D Tokamak

Flow velocities of the order 10–20 km/s in the direction of the high-field side divertor have been measured for deuterons and low charge-state carbon ions in the scrape-off layer at the crown of low-density L-mode plasmas, suggesting that these carbon ions at the crown move with the background plasma flow. Simulations with the multi-fluid edge code UEDGE including cross-field drifts due to E × B and B × ∇B yield calculated divertor conditions which are more consistent with the measurements, but flows at the crown that are stagnant or in the opposite direction than observed. The simulations indicate that both the ion temperature gradient force and deuteron frictional drag play a role in determining the flow direction and magnitude of low charge-state carbon ions. The effect of the assumed radial transport model, toroidal core rotation, and neutral pumping at the divertor plates on the flow at the crown is investigated.

Plasma transport along discrete auroral arcs and its contribution to the ionospheric plasma convection

Abstract. The role of intense high-altitude electric field (E-field) peaks for large-scale plasma convection is investigated with the help of Cluster E-field, B-field and density data. The study covers 32 E-field events between 4 and 7 RE geocentric distance, with E-field magnitudes in the range 500–1000 mV/m when mapped to ionospheric altitude. We focus on E-field structures above the ionosphere that are typically coupled to discrete auroral arcs and their return current region. Connected to such E-field peaks are rapid plasma flows directed along the discrete arcs in opposite directions on each side of the arc. Nearly all the E-field events occur during active times. A strong dependence on different substorm phases is found: a majority of intense E-field events appearing during substorm expansion or maximum phase are located on the nightside oval, while most recovery events occur on the dusk-to-dayside part of the oval. For most expansion and maximum phase cases, the average background plasma flow is in the sunward direction. For a majority of recovery events, the flow is in the anti-sunward direction. The net plasma flux connected to a strong E-field peak is in two thirds of the cases in the same direction as the background plasma flow. However, in only one third of the cases the strong flux caused by an E-field peak makes an important contribution to the plasma transport within the boundary plasma sheet. For a majority of events, the area covered by rapid plasma flows above discrete arcs is too small to have an effect on the global convection. This questions the role of discrete auroral arcs as major driver of plasma convection.

Plasma flow and carbon production and circulation with the ergodic divertor of Tore Supra

This paper presents a detailed study of carbon production and transport from the ergodic divertor (ED) target plates to the plasma core in the Tore Supra tokamak. Adapted experimental and numerical modelling techniques have been used to describe each of the main phenomena in play. Edge electron density and temperature are measured with Langmuir probes. The C II, C III and Hα emission is measured with optical fibres and cameras. The background plasma flow is calculated consistently with the observed recycling pattern by the neutral transport code EDCOLL for the two magnetic connection schemes of interest (short or long connection lengths). 3D Monte-Carlo modelling of carbon near the neutralizer plate (BBQ code) shows that the transport of carbon ions is governed by the friction force in addition to the electric field. Finally, a simplified 3D test particle model is used to estimate the core penetration fraction of carbon. A high value is found for the carbon screening efficiency (fraction of particles that does not penetrate in the plasma core), in the range 95–97% depending on the edge plasma conditions. This value, combined with the calculated carbon influxes, yields the first quantitative estimate of the carbon core contamination during ED operation. The paper shows that the screening of carbon and core contamination are mainly dependent on the carbon source (partially controlled with the ED) and the plasma flow distribution in the laminar region (magnetic topology and particle drifts).

Magnetohydrodynamic waves in non-uniform flows II: stress-energy tensors, conservation laws and Lie symmetries

The interaction of magnetohydrodynamic (MHD) waves in a non-uniform, time-dependent background plasma flow is investigated using Lasrangian field theory methods. The analysis uses Lagrangian maps, in which the exact position of the fluid element x* is expressed as a vector sum of the position vector x of the background plasma fluid element phis a Lagrangian displacement ξ(x,t) due to the waves. An exact theory for the wave and background stress energy tensors is developed based on the exact Lagrangian and the exact Lagrangian map. Noether's theorems are used in conjunction with tlie exact action out Lagrangian maps to determine the general form of conservation laws for the total system of waves and background plasma, corresponding to divergence symmetries of the action. The energy and momentum conservation laws of the system are derived from Noether's first theorem corresponding to the time and space translation symmetries of the action, respectively. As examples of the use of Noether's first theorem, we derive the conservation laws associated with tlie 10-parameter Galilean group admitted by the MHD equations. This includes the space and time translation symmetries, the space rotations, and the Galilean boosts. A class of solutions of the Lie determining equations for the infinife-dimensional MHD fluid relabeling symmetries are used to discuss the corresponding conservation laws. Ertel's theorem for the conservation of potential vorticity for the system of waves and background gas in ideal gas dynamics is derived from an infinite-dimensional fluid relabeling symmetry of the action.

Magnetohydrodynamic waves in non-uniform flows I: a variational approach

The interaction of magnetohydrodynamic (MHD) waves in a non- uniform, time-dependent background plasma flow is investigated using Lagrangian field theory methods. The analysis uses Lagrangian maps, in which the position of the fluid element x ∗ is expressed as a vector sum of the position vector x of the background plasma fluid element plus a Lagrangian displacement ξ(x ,t ) due to the waves. Linear, non-Wentzel-Kramer-Brillouin (WKB) wave interaction equations are obtained by expansion of the Lagrangian out to second order in ξ and ∆S, where ∆S is the Lagrangian entropy perturbation. The characteristic manifolds of the waves are determined by consideration of the Cauchy problem for the wave interaction equations. The manifolds correspond to the usual MHD waves modes, namely the Alfvwaves, the fast and slow magnetoacoustic waves and the entropy wave. The relationships between the characteristic manifolds, and the ray equations of geometrical MHD optics are developed using the theory of Cauchy characteristics for first-order partial differential equations. The first-order differential equations describing the singular manifolds are the dispersion equations for the MHD ei- genmodes, where the wave vector k = ∇φ and frequency ω = −φt correspond to the characteristic manifolds φ(x ,t )= constant. The form of the characteristic manifolds for both time-dependent and steady MHD flows are developed. The bi- characteristics for steady MHD waves in a steady background flow are related to the group velocity surface and Mach cone for the waves, and determine when the flow is elliptic, hyperbolic, or of mixed hyperbolic-elliptic type. The wave interaction equations are decomposed into coupled equations for the compressible and incompressible perturbations.

Impurity transport in a simulated gas target divertor

Future generation fusion reactors and tokamaks will require dissipative divertors to handle the high particle and heat loads leaving the core plasma (100–400 MW/m2 in ITER). A radiative divertor is proposed as a possible scenario, utilizing a hydrogen target gas to disperse the plasma momentum and trace impurity radiation to dissipate the plasma heat flux. Introducing an impurity into the target hydrogen gas enhances the radiative power loss but may lead to a significant impurity backflow to the main plasma. Thus, impurity flow control represents a crucial design concern. Such impurity flows are studied experimentally in this thesis. The PISCES-A linear plasma device (n ≤ 3 × 1019 m−3, kTe ≤ 20 eV) has been used to simulate a gas target divertor. To study the transport of impurities, a trace amount of impurity gas (i.e., neon and argon) is puffed near the target plate along with the hydrogen gas. Varying the hydrogen gas puffing rate permits us to study the effects of various background plasma conditions on the transport of impurities. A 1-1/2-D fluid code has been developed to solve the continuity and momentum equations for a neutral and singly ionized impurity in a hydrogen background plasma. The results indicate an axial reduction in the impurity concentration upstream from the impurity puffing source. Impurity entrainment is more effective for higher hydrogen target pressures (and for higher hydrogen plasma densities). However, if there is a reversal of the background plasma flow, impurity particles can propagate past the plasma flow reversal point and are then no longer entrained.

Convection and diffusion of cosmic rays in the heliosheath

Diffusion and convection of both anomalous (ACR) and galactic (GCR) cosmic rays in the heliosheath and adjacent local interstellar plasma flow is calculated by solving the appropriate CR transport equation in the test particle approximation (background plasma flow unaffected). Upon creation at the termination shock the ACR are convected towards the tail region and form there a huge particle reservoir. Ion transcharging on neutral galactic gases leads to apex/antiapex heliosphere asymmetries in the fluxes of energetic neutral atoms.