Global Magnetic Configuration Research Articles

Optimisation of the poloidal field system for advanced divertor configurations in STEP

The power exhaust proves to be one of the most challenging and concept–defining aspects in the design of a commercial fusion power plant, while the magnetic coil system, capable of supporting advanced exhaust solutions, emerges as one of the main design and cost drivers. Consequently, much effort should be dedicated to the optimisation of a robust global magnetic configuration, which integrates both the plasma and edge scenarios, while ensuring engineering feasibility and compatibility with the available technology. Here we present a multidisciplinary framework employed to analyse, evaluate, and optimise the Spherical Tokamak for Energy Production (STEP) equilibrium configuration, coupled with a viable divertor solution, and a compatible poloidal field coil system. The complexity of this task leads to a multitude of potentially conflicting requirements and competing constraints. We identify interfaces and conflicts between the aspects of the design that were previously considered independently, and highlight the key benefits, trends, and trade–offs between alternative configurations. We demonstrate that advanced exhaust solutions, simultaneously applied to both inboard and outboard divertors, are accessible with feasible coil sets under conditions relevant for STEP. We show that the most promising inner–X geometry, paired with the outer super–X configuration, can significantly enhance divertor’s power handling capability, allowing access to stable detached regimes. The coil set feasibility is further assessed considering its compatibility with the assumed plasma initiation scenario, and with the most demanding plasma current density profiles utilising alternative heating and current drive schemes.

Three-dimensional Multispecies Simulation of the Solar Wind Interaction with Mars Under Different Interplanetary Magnetic Field Orientations

Abstract Without the intrinsic magnetic field, the solar wind interaction with Mars can be significantly different from the interaction with Earth and other magnetized planets. In this paper, we investigate how a global configuration of the magnetic structures, consisting of the bow shock, the induced magnetosphere, and the magnetotail, is modulated by the interplanetary magnetic field (IMF) orientation. A 3D multispecies numerical model is established to simulate the interaction of solar wind with Mars under different IMF directions. The results show that the shock size including the subsolar distance and the terminator radius increases with Parker spiral angle, as is the same case with the magnetotail radius. The location and shape of the polarity reversal layer and inverse polarity reversal layer in the induced magnetotail are displaced to the y < 0 sector for a nonzero flow-aligned IMF component, consistent with previous analytical solutions and observations. The responses of the Martian global magnetic configuration to the different IMF directions suggest that the external magnetic field plays an important role in the solar wind interaction with unmagnetized planets.

Global Simulation of the Jovian Magnetosphere: Transitional Structure From the Io Plasma Disk to the Plasma Sheet

AbstractJupiter has a strong magnetic field, and a huge magnetosphere is formed through the solar wind‐Jupiter interaction. The generated magnetosphere–ionosphere system is reproduced based on the 9‐component Magnetohydrodynamics (MHD) and the current conservation in the ionosphere. Assuming Io plasma emission rate 1.4 t/sec, this paper reproduces self‐consistently global magnetic configuration, generations of the field‐aligned current (FAC) and aurora, formation of the Io plasma disk at 8–20 RJ, plasma corotation, instability in the plasma disk, transition from the Io plasma disk to the plasma sheet at 20–150 RJ, and the plasmoid ejection. The rotating Io plasma in the disk forms instabilities that promotes radial diffusion. H+ is supplied from the ionosphere along high‐latitude magnetic field lines and mixed with heavy ions around 15–20 RJ. Beyond 20 RJ, mixed plasma diffuses further outward by the centrifugal force that can exceed magnetic tension. In the ionosphere, the main oval occurs at 13.7°–15.5° colatitude. The Io disk is inner side of magnetic field lines traced from the low‐latitude edge of the main oval. Along magnetic field lines, the main oval is mapped from the outer edge of the Io disk to the entire plasma sheet accompanying rotation delay. Due to the corotation limit, convection is accompanied by plasmoid ejection. Back reaction of plasmoid ejection affects even transport process in the Io disk. The downward FAC occurs in the polar cap showing variability. The region of externally driven Dungey convection seems quite narrow.

Dynamo generated magnetic configurations in accretion discs and the nature of quasi-periodic oscillations in accreting binary systems

Magnetic fields are important for accretion disc structure. Magnetic fields in a disc system may be transported with the accreted matter. They can be associated with either the central body and/or jet, and be fossil or dynamo excited in situ. We consider dynamo excitation of magnetic fields in accretion discs of accreting binary systems in an attempt to clarify possible configurations of dynamo generated magnetic fields. We first model the entire disc with realistic radial extent and thickness using an alpha-quenching non-linearity. We then study the simultaneous effect of feedback from the Lorentz force from the dynamo-generated field. We perform numerical simulations in the framework of a relatively simple mean-field model which allows the generation of global magnetic configurations. We explore a range of possibilities for the dynamo number, and find quadrupolar-type solutions with irregular temporal oscillations that might be compared to observed rapid luminosity fluctuations. The dipolar symmetry models with $R_\alpha<0$ have lobes of strong toroidal field adjacent to the rotation axis that could be relevant to jet launching phenomena. We have explored and extended the solutions known for thin accretion discs.

Recurrent filament eruptions and associated CMEs

AbstractWe investigate the violent events in the cluster of two active regions (ARs), NOAA numbers 11121 and 11123, observed on 11 November 2010 by the Solar Dynamics Observatory (SDO). Within one day the magnetic field intensity increased by 70% with the emergence of new groups of bipoles in AR 11123, where three filaments are seen along the complex inversion line. The destabilization of the filaments led to flares and CMEs. The CMEs around 08:24 UT and 17:00 UT are directly related to the partial eruption of one filament in the new AR, as shown by a topology computation and analysis. The other CMEs on this day are due to either other ARs or to the destabilization of the global magnetic configuration of the two ARs. This conclusion can be only reached by using the three eyes of SOHO, STEREO and SDO.

MAGNETIC INTERACTIONS IN PRE-MAIN-SEQUENCE BINARIES

Young stars typically have strong magnetic fields, so that the magnetospheres of newly formed close binaries can interact, dissipate energy, and produce synchrotron radiation. The V773 Tau A binary system, a pair of T Tauri stars with a 51 day orbit, displays such a signature, with peak emission taking place near periastron. This paper proposes that the observed emission arises from the change in energy stored in the composite magnetic field of the system. We model the fields using the leading order (dipole) components and show that this picture is consistent with current observations. In this model, the observed radiation accounts for a fraction of the available energy of interaction between the magnetic fields from the two stars. Assuming antisymmetry, we compute the interaction energy $E_{\rm int}$ as a function of the stellar radii, the stellar magnetic field strengths, the binary semi-major axis, and orbital eccentricity, all of which can be measured independently of the synchrotron radiation. The variability in time and energetics of the synchrotron radiation depend on the details of the annihilation of magnetic fields through reconnection events, which generate electric fields that accelerate charged particles, and how those charged particles, especially fast electrons, are removed from the interaction region. However, the major qualitative features are well described by the background changes in the global magnetic configuration driven by the orbital motion. The theory can be tested by observing a collection of pre-main-sequence binary systems.

Rolling motion in erupting prominences observed by STEREO

We analyze the large-scale dynamical forms of three erupting prominences (filaments) observed by at least one of the two STEREO spacecraft and which reveal evidence of sideways rolling motion beginning at the crest of the erupting filament. We find that all three events were also highly non-radial and occurred adjacent to large coronal holes. For each event, the rolling motion and the average non-radial outward motion of the erupting filament and associated CME were away from a neighboring coronal hole. The location of each coronal hole was adjacent to the outer boundary of the arcade of loops overlying the filaments. The erupting filaments were all more non-radial than the CMEs but in the same general direction. From these associations, we make the hypothesis that the degree of the roll effect depends on the level of force imbalances inside the filament arcade related to the coronal hole and the relative amount of magnetic flux on each side of the filament, while the non-radial motion of the CME is related to global magnetic configuration force imbalances. Our analyses of the prominence eruption best observed from both STEREO-A and STEREO-B shows that its spine retained the thin ribbon-like topology that it had prior to the eruption. This topology allows bending, rolling, and twisting during the early phase of the eruption.

The YOHKOH survey of partially occulted flares in hard X-rays

Flares being partially occulted by the solar limb, are the best reservoir of our knowledge about hard X-ray loop-top sources. Recently, the survey of partially occulted flares observed by the RHESSI has been published (Krucker & Lin 2008). The extensive YOHKOH database still awaits such activities. This work is an attempt to fill this gap. Among from 1286 flares in the YOHKOH Hard X-ray Telescope Flare Catalogue, for which the hard X-ray images had been enclosed, we identified 98 events that occurred behind the solar limb. We investigated their hard X-ray spectra and spatial structure. We found that in most cases the hard X-ray spectrum of partially occulted flares consists of two components, non-thermal and thermal, which are co-spatial. The photon energy spectra of the partially occulted flares are systematically steeper than spectra of the non-occulted flares. Such a difference we explain as a consequence of intrinsically dissimilar conditions ruling in coronal parts of flares, in comparison with the footpoints which usually dominate the hard X-ray emission of disk flares. At least two reasons of the difference should be taken into consideration: (1) stronger contamination of hard X-rays with emission of thermal plasma, (2) different mechanism in which non-thermal electrons radiate their energy. A schematic picture, in which thin-target mechanism is responsible for hard X-ray emission of loop-top sources and thick-target mechanism -- for emission of footpoint sources, can be modified by the presence of some coronal thick-target sources. At least a part of them suggests a magnetic trapping. Investigated flares do not respond the global magnetic configuration of the solar corona. For their characteristics conclusive is rather the local magnetic configuration in which they were developed.

A Possible Structure of the Magnetic Field in Solar Filaments Obtained by Flux Emergence

Abstract We report a result, that the emergence of a subsurface magnetic field naturally reproduces the feature of the global magnetic configuration observed in solar filaments. This was obtained by performing a three-dimensional simulation of a twisted flux tube that emerges in the shape of a multi-$\Omega$-loop in a highly stratified solar atmosphere, extending from the subsurface layer to the corona. One of the key findings is that a kinking of the twisted flux tube occurs at the site of a linkage between adjacent $\Omega$-loops. This develops a magnetic structure that has remarkable similarities to the structure of a filament, in that the inner part of the twisted flux tube can be applied to the main body of a filament, while the outer part is reminiscent of observations of filament feet and a coronal arcade overlying the main body of a filament. Based on the result, we concluded that the orientation of filaments observed on the Sun may have its origin in the handedness of a twisted magnetic field below the solar surface.

A new model of Mercury’s magnetospheric magnetic field

Two new missions will arrive at Mercury within the next decade. Interest in this enigmatic planet has therefore been revived in the magnetospheric community, and new models of Mercury’s magnetic field are demanded. In the past, there have been several attempts to model the magnetosphere of Mercury based on simply scaling models of the Earth’s magnetosphere. Although these scaled models have enjoyed many successes, their global magnetic configurations are determined by measurements made at the Earth, not at Mercury. In this paper, we develop a new empirical model of the Hermean field which is constrained wherever possible by the Mariner 10 dataset. We also explore the response of the model to upstream solar wind conditions. Data-based empirical models are an invaluable tool in magnetospheric physics at the Earth, and will doubtless prove to be just as useful at Mercury. The model developed here is generally applicable, and can be used to fit a much larger spacecraft dataset when it becomes available.

Catastrophe of Coronal Magnetic Flux Ropes Caused by Photospheric Motions

Using a 2.5-D, time-dependent ideal MHD model in Cartesian coordinates, we carried out numerical simulations to investigate the equilibrium and evolution properties of a magnetic configuration that consists of a coronal magnetic flux rope and a partly open photospheric background field, which is equivalent to that produced by a two-patch magnetic source on the photospheric surface. The axial and annular magnetic fluxes of the flux rope are given and fixed. The global magnetic configuration evolves in response to three types of changes of the background field: decreasing of the distance between the two sources, shrinking of the size of each source, and increasing of the shear in the closed component of the background field. As a result, the geometrical parameters of the flux rope, i.e. the height of the rope axis, the half-width of the rope and the length of the vertical current sheet below the rope, change due to the variation of the background field. It is shown that for a given coronal magnetic flux rope in a partly open background field, the variation of the geometrical parameters of the flux rope displays a catastrophic behavior, namely, there exists a critical point for each case, at which an infinitesimal change of the background field leads to a loss of equilibrium, and thus a jump of the flux rope. The implication of such a catastrophe in solar active phenomena is briefly discussed.

General architecture, implementation and performance of the digital feedback control in RFX

RFX is a large toroidal machine in the European Programme for nuclear fusion research. It uses digital active feedback for both control of global magnetic configuration and correction of local magnetic field errors. These controls have different time requirements. Global controls must produce output reference waveforms with updating frequencies of some kSample/s and must allow a maximum delay between input and corresponding output of few ms. Conversely, the local controls need faster response with updating frequencies of the outputs up to 20 kSample/s and maximum delay of 200 /spl mu/s. The I/O requirements are also different, ranging from few to many tens of input signals, depending on the specific controls. A general scalable distributed architecture has been designed and realized for the RFX feedback control systems. It is based on processor networks implemented by means of Digital Signal Processors equipped with built-in facilities for interprocessor communication and connected to each other either directly or via a fiber-optic based network. The paper summarizes the design concepts of the system architecture and, after the description of the hardware and software structure, reports the performances obtained.

The model magnetic configuration of the extended corona in the solar wind formation region

The coupling between small and large scale structures and processes on the Sun and in the heliosphere is important in the relation to the global magnetic configuration. Thin heliospheric current sheets play the leading role in this respect.The simple analytical model of the magnetic field configuration is constructed as a superposition of the three sources: 1) a point magnetic dipole in the center of the Sun, 2) a thin ring current sheet with the azimuthal current density jϕ ∼ r−3 near the equatorial plane and 3) a magnetic quadrupole in the center of the Sun.The model reproduces, in an asymptotically correct manner, the known geometry of the field lines during the declining phase and solar minimum years near the Sun (the dipole term) as well as at large distances in the domain of the superalfvenic solar wind in the heliosphere, where the thin current sheet dominates and ∥Br (θ)∥ = const according to Ulysses observations (Balogh et al., 1995; Smith et al., 1995). The model with the axial quadrupole term is appropriate to describe the North-South asymmetry of the field lines.The model may be used as a reasonable analytical interpolation between the both extreme asymptotic domains (inside the region of the intermediate distances ∼ (1−10Rbd) when considering the problems of the solar wind dynamics and cosmic ray propagation theories.

Magnetic coupling between polar plumes and the heliospheric current sheet

A simple analytical model for the global magnetic configuration in the heliosphere is described as a superposition of the solar magnetic multipoles and a thin equatorial current sheet. Polar plumes are tracing these global field lines and inhomogeneous field-aligned currents. Both internal and external boundary conditions are important for the polar plume formation and dynamics.

Galactic Dynamo Theory Confronted with Observations

Kinematic models of galactic dynamo in axisymmetric disk show remarkable agreement with observations. We argue that nonlinear effects are relatively weak in galactic dynamos and consider the properties of linear dynamo models inherited by global magnetic configurations in spiral galaxies as well as nonlinear distortions of the linear solutions.