Large Plasma Flow Research Articles

Modeling study of divertor particle and heat flux asymmetries for EAST H-mode discharges

The BOUT++ transport code is run to study the effects of plasma drifts on the divertor out-in asymmetries (DOIAs) of particle and heat fluxes and their decay widths for EAST lower single null H-mode discharges. The diamagnetic drift seems to have no effects on the DOIAs of total particle and heat fluxes due to its divergence-free nature. However, it could significantly increase the DOIAs of peak particle and heat fluxes and the flux decay widths. The E × B drift is found to induce a large plasma flow to the divertor region, enhancing the DOIAs of both total and peak particle and heat fluxes and the flux decay widths. Both the radial and poloidal components of the E × B drift are necessary in increasing the DOIAs, however, the radial E × B drift seems to play a more important role. The effects on the DOIAs caused by both diamagnetic and E × B drifts are reversed with the reverse of toroidal magnetic field. The heat flux decay width λq and spreading width Sq are important physical and engineering parameters for the divertors and could be obtained by fitting the heat flux profiles at divertor targets. The λq at the outer target from the simulation case with all drifts could well match with the multi-machine scaling proposed by Eich and the DOIA of λq is in reasonable agreement with the scaling proposed by Goldston.

MAST and the impact of low aspect ratio on tokamak physics

Low aspect ratio plasmas in devices such as the mega ampere spherical tokamak (MAST) are characterized by strong toroidicity, strong shaping and self fields, low magnetic field, high beta, large plasma flow and high intrinsic E × B flow shear. These characteristics have important effects on plasma behaviour, provide a stringent test of theories and scaling laws and offer new insight into underlying physical processes, often through the amplification of effects present in conventional tokamaks (e.g. impact of fuelling source and magnetic geometry on H-mode access). The enhancement of neoclassical effects makes MAST ideal for the study of particle pinch processes and neoclassical resistivity corrections, which can be assessed with unique accuracy. MAST data have an important influence on scaling laws for confinement and H-mode threshold power, exerting strong leverage on the form of these scaling laws (e.g. scaling with aspect ratio, beta, magnetic field, etc). The high intrinsic flow shear is conducive to transport barrier formation by turbulence suppression. Internal transport barriers are readily formed in MAST with both co- and counter-NBI, and electron and ion thermal diffusivities have been reduced to the ion neoclassical level. The strong variation in toroidal field (∼ × 5 in MAST) between the inboard and outboard plasma edges, provides a useful test of edge models prompting, for example, a comparison of inboard and outboard scrape-off-layer transport to highlight magnetic field effects. Low aspect ratio plasmas are also an ideal testing ground for plasma instabilities, such as neoclassical tearing modes, edge localized modes (ELMs) and Alfvén eigenmodes, which are readily generated due to the supra-Alfvénic ion population. Examples of how MAST is providing new insights into such instabilities (e.g. ELM structure) are described.

Multi-instrument study of the dynamic cusp during dominant IMF <i>B<sub>y</sub></i> conditions

Abstract. We present multi-instrument observations using the meridian scanning photometer (MSP) at NyAlesund, the EISCAT Svalbard radar (ESR) and the CUTLASS Finland HF radar, to investigate the dynamics of the cusp region during pulsed reconnection events. The optical data obtained from the MSP indicate the presence of several poleward-moving auroral forms (PMAFs) which have been previously identified as the auroral signature of pulsed reconnection. Furthermore, the optical green line (557.7 nm) luminosity indicates a loss of emission equatorward of the location of the onset of the PMAFs, characteristic of magnetospheric plasma escaping to the magnetosheath along newly opened field lines. This reduction in green line luminosity creates a "dark region", the equatorward edge of which is found to lie close to the boundary between high and low spectral widths observed by the CUTLASS Finland radar. High spectral widths on the dayside have previously been identified as a good indicator of cusp backscatter. Both of these boundaries have been suggested to provide an accurate representation of the location of the open/closed field line boundary. The ESR observations show enhancements in electron density and electron temperature occurring in conjunction with the optical PMAFs. The observations demonstrate some correspondence with the theoretical predictions of Davis and Lockwood (1996), who used an auroral precipitation model to predict ESR observations in the vicinity of the cusp. However, the limitations of this model are apparent under conditions of large plasma flows in the ionosphere. Finally, convection velocities obtained from the HF radar data illustrate a flow regime similar to that predicted to be driven by strong IMF By, as described by Cowley and Lockwood (1992), demonstrating an initial azimuthal flow followed by a rotation to more poleward directions.Key words. Ionosphere (ionosphere-magnetosphere interactions; particle precipitation) – Magnetospheric physics (magnetopause, cusp and boundary layers)

Conjunction of tail satellites for substorm study: ISTP event of 1997 January 2

The interval of 1997 January 1–2 was identified as a favorable conjunction of Geotail and IMP‐8 to examine substorm activity in the mid‐tail region prior to data acquisition. On January 2, 1997, global auroral observations from Polar indicated a substorm onset at ∼0120 UT followed by a substorm intensification at ∼0154 UT at a local time spatially separated from the initial substorm activity region. During this event, both Geotail and IMP‐8 were in the mid‐tail near the midnight meridian (Geotail (X, Y) ≈ (−30, −3)RE and IMP‐8 (X, Y) ≈ (−37, 2)RE). Observations indicated that the substorm onset activity was localized in the postmidnight region. After the onset, Geotail detected a transient dipolarization which was not accompanied by large plasma flows (i.e., | Vx |≤ 200 km/s). The subsequent substorm intensification produced enduring dipolarization at Geotail and highly fluctuating magnetic field (mostly northward Bz) at IMP‐8. Observations for this substorm showed no indication of mid‐tail activities occurring prior to auroral brightening for both onset and intensification even though the satellites observed activities subsequently. Close examination of data indicates that the delays were not due to a dawn‐dusk expansion of mid‐tail activity. These results are consistent with substorm activity beginning in the near‐Earth region first, followed by activity in the mid‐tail region later.

Active region EUV transient brightenings – First Results by EIT of SOHO JOP 80

On 13 May 1998, the Extreme-Ultraviolet Imaging Telescope (EIT, on board SOHO) has produced a unique image sequence operating in 'shutterless mode' (SOHO JOP 80). In JOP 80, EIT is the lead instrument, followed by several space-born instruments (SXT, TRACE, MDI, CDS, SUMER), as well as two observatories on the ground (in La Palma and Sac Peak). The target of the campaign was a relatively small but rapidly evolving active region (AR 8218). For the EIT contribution, a 15 s cadence was achieved in the Fe xii bandpass at 195 A by leaving EIT's shutter open for 1 h and operating the CCD in frame-transfer mode. In this paper, we start the analysis of the huge data set, by making an inventory of the transients observed in the EIT image sequence. Besides scatter plots of duration, size and radiative output of the detected EUV brightenings, we discuss in full detail the morphology and evolution of several typical events. These transients range from a B3.5 flare producing a large plasma flow along pre-existing loops, to EUV versions of active region transient brightenings as previously observed by SXT on board Yohkoh. In addition, a new class of weaker footpoint brightenings is discussed that produce wave-like disturbances propagating along quasi-open field lines. This new class of propagating disturbances extends the wide variety of transient phenomena that we discovered in the EIT data, and makes the potential for inter-instrumental studies of the JOP 80 data all the more exciting. We stress the necessity of such forthcoming studies to reach an instrument-independent classification of small-scale solar transients.

Modeling the formation of polar cap patches using large plasma flows

Recent measurements made with the Sondrestrom incoherent scatter radar have indicated that the formation of polar cap patches can be closely associated with the flow of a large plasma jet. In this paper, we report the results of a numerical study to investigate the role of plasma jets on patch formation, to determine the temporal evolution of the density structure, and to assess the importance of O+ loss rate and transport mechanisms. We have used a time‐dependent model of the high‐latitude F region ionosphere and model inputs guided by data collected by radar and ground‐based magnetometers. We have studied several different scenarios of patch formation. Rather than mix the effects of a complex of variations that could occur during a transient event, we limit ourselves here to simulations of three types to focus on a few key elements. The first attempt employed a Heelis‐type pattern to represent the global convection and two stationary vortices to characterize the localized velocity structure. No discrete isolated patches were evident in this simulation. The second modeling study allowed the vortices to travel according to the background convection. Discrete density patches were seen in the polar cap for this case. The third case involved the use of a Heppner and Maynard pattern of polar cap potential. Like the second case, patches were seen only when traveling vortices were used in the simulation. The shapes of the patches in the two cases of moving vortices were defined by the geometrical aspect of the vortices, i.e. elliptical vortices generated elongated patches. When we “artificially” removed the Joule frictional heating, and hence any enhanced O+ loss rate, it was found that transport of low density plasma from earlier local times can contribute to ∼60% of the depletion. We also found that patches can be created only when the vortices are located in a narrow local time sector, between 1000 and 1200 LT and at latitudes close to the tongue of ionization.

Formation of the magnetopause boundary layer by magnetic reconnection

Magnetic reconnection at the earth's magnetopause can lead to the formation of the magnetopause boundary layer. In this paper, our recent hybrid simulations of the structure of this reconnection layer are summarized. The results are compared with those from the ideal MHD formulation and resistive MHD simulations. In the ideal MHD, rotational discontinuities, slow shocks, slow expansion waves, and contact discontinuity are present in the reconnection layer. In the resistive MHD simulation, the rotational discontinuity is replaced by an intermediate shock or time-dependent intermediate shock (TDIS). In the hybrid simulation, the TDIS quickly evolves to a steady rotational discontinuity. The contact discontinuity cannot be identified due to the mixing of ions from the magnetosheath and magnetosphere, and slow shocks and slow expansion waves are modified. At the dayside magnetopause, a thin rotational discontinuity bounds the reconnection layer from the magnetosheath side, and a high-speed accelerated plasma flow is present on the magnetospheric side of the rotational discontinuity. At the flank of the magnetopause, where a large plasma flow is present in the magnetosheath, the magnetic field transition region is thick, and the accelerated flow exists in the entire field transition region.

Reconnection layer at the flank magnetopause in the presence of shear flow

We present hybrid simulations of reconnection layer at the flank magnetopause, where a large plasma flow speed is present in the magnetosheath. It is found that there exists a threshold flow speed υ* such that for the magnetosheath flow speed υs < υ* (υs > υ* ), the rotational discontinuity with a larger field rotation angle exists on the magnetosheath (magnetospheric) side of the reconnection layer. The threshold speed is found to be υ* = υAm ‐ υAs, where υAm (υAs) is the Alfvén speed on the magnetospheric (magnetosheath) side of the reconnection layer. Furthermore, for υs << υ*, the rotational discontinuity on the magnetosheath side is very thin, and an accelerated high‐speed flow is located earthward of the rotational discontinuity, as observed at the dayside magnetopause. For υs ∼ υ*, the magnetic field transition region is thick, and the accelerated flow is present in the entire field transition region, as observed at the flank magnetopause.

Structure of the tail plasma/current sheet at ∼11 RE and its changes in the course of a substorm

At the end of April 2, 1978, the ISEE 1 and 2 spacecraft moved inbound at ∼11 RE on the nightside (0130 MLT). Due to a flapping motion of the plasma sheet the spacecraft crossed the neutral sheet region (central region of the plasma sheet) more than 10 times in the hour between 2115 and 2215 UT. This provided a unique opportunity to study the structure of the plasma/current region and its evolution during substorm growth and early expansion before the final disruption of the current sheet. Using minimum variance analysis of the magnetic field variations during the crossings as well as finite ion gyroradius diagnostics, we determine the orientation of the current sheet (CS) and then estimate the CS thickness as well as the value of its normal component, Bn. Typically, the current distribution was inferred to be very inhomogeneous with a current concentrated in a very thin CS (only 0.2 to 0.8 RE as thick) embedded inside the thicker plasma sheet. Current sheet crossings could be classified as regular or turbulent. The first type prevailed during the growth phase and at the initial stage of expansion when the spacecraft were well outside (in longitude) of the active region of the substorm and no large plasma flow was detected. The normal field component Bn was typically very small (∼1 nT) in the CS center in comparison to the larger shear magnetic By component. In the course of the growth phase we inferred an increase of the lobe field Bx and a decrease of the CS half thickness h (from h∼3000 km to ∼800 km just before the expansion onset), i.e., a very large increase (up to an order of magnitude) of the current density. At the same time, in disagreement with the usual cartoon picture of magnetic reconfiguration, the magnetic field magnitude in the CS center increased (instead of decreased) at the expense of the shear component. Three turbulent crossings were found during substorm expansion within the longitude range of the substorm current wedge (SCW). The second of them was detected ∼1 min before the main dipolarization and was characterized by a rather small CS thickness (h < 600 km), by strong earthward plasma flow and by a positive normal magnetic field component. That period showed signatures of concentration of both cross‐B and field‐aligned current at the outer edge of CS and may indicate a nearby reconnection region. The main result of this study is that the region of very thin current sheet (thickness of the order of the gyroradius of thermal protons in the field just outside the current sheet), which contained a very small normal component, clearly appeared in the near tail prior to the sudden onset of current disruption as predicted by some quantitative models of quasi‐static evolution of earthward convecting plasma sheet flux tubes. Comparing these observations to theoretical results, we find that the threshold conditions for the growth of the tearing mode instability in sheared magnetic fields were apparently satisfied in this case, but the growth rate was too slow for sudden initiation of substorm expansion.

Large ionospheric plasma flows along magnetic field lines: EISCAT observations and their interpretation

EISCAT measurements of plasma velocities in the auroral ionosphere have indicated very large plasma flows along magnetic field lines. Velocities exceeding 500 ms −1 have been observed on several occasions, always in association with large electric fields. Observations with the EISCAT radar pointing along its local field line have provided height profiles of the various plasma parameters during periods when large field-aligned flows occur. The most important factor in driving the plasma is found to be the gradient in plasma pressure. Electric fields up to 50 mVm −1 cause frictional heating of the ion population, but little change in recombination coefficient so that increased plasma pressure in the F-region drives plasma upwards into the topside. However, when the electric field is even larger then the rate of recombination increases sharply, the electron concentration is depleted, F-region plasma pressure decreases and plasma is driven downwards.

Modelling and experimental studies of impurity control in JET X-point configurations

The scrape-off layer of a JET X-point discharge has been modelled with a multi-fluid plasma model linked to a Monte Carlo model for the neutral deuterium and impurity species. A relatively complete set of boundary plasma data has been obtained, is shown to be consistent within the experimental uncertainties and suggests significant shielding of the carbon impurities released at the graphite target plate. The agreement with the main experimental parameters, n e, T e, is good, as also is the match with the observed radiation and impurity distribution provided that a rather large plasma flow is included in the model.

Magnetic tearing of plasma discharges due to nonuniform resistivity

The rearrangement of current in a plasma discharge in response to resistivity nonuniformities within a magnetic surface is studied. It is shown that macroscopic magnetic islands develop about those surfaces where the nonuniformity is aligned with the magnetic field. If the nonuniformity and the field are not aligned anywhere, there is no current rearrangement; instead, relatively large plasma flows are set up. Such resistivity inhomogeneities can obtain in solar coronal loops and, in some circumstances, in tokamak discharges.

On the generation of field‐aligned plasma flow at the boundary of the plasma sheet

This paper deals with a possible cause of the large plasma flow velocities parallel to the magnetic field observed near the boundary of the plasma sheet in the earth's magnetotail. For a large class of steady state configurations with typical cases involving magnetic reconnection we first show qualitatively that high parallel flow velocities can be expected to exist on field lines connecting to a region of weak magnetic field. For reconnection configurations this weak field region contains the diffusion region. The maximum value of the parallel flow velocity is sensitive to the lowest magnetic field magnitude just outside the diffusion region. The physical mechanism causing large values of the parallel velocity component υ∥ can be visualized as a strong imbalance of perpendicular mass flux into and out of magnetic flux tubes passing through regions where the magnetic field is weak and inhomogeneous. In a steady state the unbalanced perpendicular flow requires large parallel flow to establish mass conservation. The presence of a parallel velocity is not unusual in MHD systems. The new (generic) aspect is that near the separatrix, parallel flow is the dominant form of plasma transport. For a quantitative evaluation we consider an MHD model specialized for the domains where the inertia force can be neglected. By a self‐consistent treatment we evaluate υ∥ and find that |υ∥| can substantially exceed the perpendicular velocity υ⊥ = E/B; in a typical example with stretched magnetic field lines we obtain |υ∥| ≈ 40υ⊥. We apply our results to the earth's magnetotail and conclude that this mechanism is able to explain the parallel flow velocities near the boundary of the plasma sheet in the range of several hundreds of kilometers per second.