Magnetohydrodynamic Structure Research Articles

Impact of COMPASS-U vacuum vessel and the first wall structures on signals of in-vessel magnetic diagnostic coils

The paper evaluates the impact of metallic structures of the COMPASS-U tokamak design on signals measured by in-vessel magnetic diagnostic coils. The eddy current shielding caused by the tokamak conductive structures of the vacuum vessel and plasma facing components is evaluated using finite element analysis performed by ANSYS Maxwell software. The simulation of the sudden shifts in the radial or vertical position of the plasma column returns 33–45% amplification of the tangential field component that from the most part would be analogous to a simple lumped circuit model approximation, despite the complex 3D shape of the nearby plasma facing components. The simulation of a rotating helical filament close to plasma edge (approximating a rotating magnetic island magnetohydrodynamic (MHD) structure) reveals an unexpected spatial modulation of the magnitude attenuation coefficient of up to 45% for the tangential field component across different positions relative to the PFC holders. Both results have led to the described conclusions for the sensor positioning design, specifically to the decision to install plasma position feedback sensors in between the robust holders of the PFC tiles, and the plasma MHD activity sensors strictly symmetrically with respect to these PFC structures.

Analytic solution of a magnetized tori with magnetic polarization around Kerr black holes

We present the first family of magnetically polarized equilibrium tori around a Kerr black hole. The models were obtained in the test fluid approximation by assuming that the tori is a linear media, making it is possible to characterize the magnetic polarization of the fluid through the magnetic susceptibility χm. The magnetohydrodynamic (MHD) structure of the models was solved by following the Komissarov approach, but with the aim of including the magnetic polarization of the fluid, the integrability condition for the magnetic counterpart was modified. We build two kinds of magnetized tori depending on whether the magnetic susceptibility is constant in space or not. In the models with constant χm, we find that the paramagnetic tori ( χm > 0) are more dense and less magnetized than the diamagnetic ones ( χm < 0) in the region between the inner edge, rin, and the center of the disk, rc; however, we find the opposite behavior for r > rc. Now, in the models with non-constant χm, the tori become more magnetized than the Komissarov solution in the region where ∂χm/∂r < 0, and less magnetized when ∂χm/∂r > 0. Nevertheless, it is worth mentioning that in all solutions presented in this paper the magnetic pressure is greater than the hydrodynamic pressure. These new equilibrium tori can be useful for studying the accretion of a magnetic media onto a rotating black hole.

Juno observations of large‐scale compressions of Jupiter's dawnside magnetopause

AbstractWe investigate the structure of Jupiter's dawnside magnetopause using observations obtained by particle and fields instrumentation on the Juno spacecraft. Characterization of Jupiter's magnetopause is critical for the understanding of mass and energy transport between the solar wind and the magnetosphere. We find an extended magnetopause boundary layer (MPBL) during a magnetopause crossing on 14 July 2016. This thick MPBL, in combination with a large magnetic field component normal to the magnetopause boundary, suggests that strong magnetospheric compression enhances mass transport across the magnetopause via magnetic reconnection. We further identify ~2 h increases in the total magnetospheric pressure adjacent to the magnetopause on 14 July 2016 and 1 August 2016. These large‐scale structures provide evidence of focused energy transport into the magnetosphere via magnetohydrodynamic structures.

Coronal mass ejections are not coherent magnetohydrodynamic structures

Coronal mass ejections (CMEs) are episodic eruptions of solar plasma and magnetic flux that travel out through the solar system, driving extreme space weather. Interpretation of CME observations and their interaction with the solar wind typically assumes CMEs are coherent, almost solid-like objects. We show that supersonic radial propagation of CMEs away from the Sun results in geometric expansion of CME plasma parcels at a speed faster than the local wave speed. Thus information cannot propagate across the CME. Comparing our results with observed properties of over 400 CMEs, we show that CMEs cease to be coherent magnetohydrodynamic structures within 0.3 AU of the Sun. This suggests Earth-directed CMEs are less like billiard balls and more like dust clouds, with apparent coherence only due to similar initial conditions and quasi homogeneity of the medium through which they travel. The incoherence of CMEs suggests interpretation of CME observations requires accurate reconstruction of the ambient solar wind with which they interact, and that simple assumptions about the shape of the CMEs are likely to be invalid when significant spatial/temporal gradients in ambient solar wind conditions are present.

Reconstruction of the electron diffusion region

AbstractWe discuss mathematical tools for the reconstruction of two‐dimensional, time‐independent magnetic field and flow in the electron diffusion region, at a site of antiparallel magnetic reconnection. The basic assumptions are that the ions are stationary and have constant density. The width of the reconnection layer is of the order of the electron gyroradius or the electron inertial length. Our model includes the axial electron pressure term in Ohm's law developed by M. Hesse and coworkers. We demonstrate the feasibility of doing reconstruction of electron magnetohydrodynamic (EMHD) structures for a simplified system with zero electron inertia. The code is benchmarked using an exact solution that has antiparallel unidirectional magnetic fields, plus out‐of‐plane quadrupolar Hall fields, as well as the expected slow electron inflow and rapid exit jets. The inertialess reconstruction is then applied to synthetic data from a 2‐D, particle‐in‐cell, simulation of antiparallel reconnection. We find that the inertialess reconstruction of its electron diffusion region works reasonably well only when the spacecraft path passes close to the center of the reconnection site where the magnetic field is zero and the electron flow has a stagnation point. When the path is located farther away, the effects of electron inertia, and probably also deviations from the Hesse formula, cause the quality of the reconstruction to deteriorate. Electron inertia is included in the theoretical development presented here but requires a more complicated numerical reconstruction code. The development and testing of such a code is underway and will be presented separately.

Features of the behavior of some magnetohydrodynamic structures in the interplanetary space

Constant pressure structures moving together with a solar wind flux are described based on magnetohydrodynamic representations. The action of magnetic hole type structures on the bow shock wave before the Earth’s magnetosphere is thus considered. In the presence of a rotational discontinuity in the magnetic cloud, it is shown that the decomposition of an arbitrary discontinuity generates a plateau with an increase in the density of charged particles and a decrease in the intensity of the interplanetary magnetic field, which is repeatedly observed on spacecrafts. The plateau coincides with the properties of the magnetic hole to a large extent but is of another origin (in fact, local).

Magnetohydrodynamic structure of a plasmoid in fast reconnection in low-beta plasmas: Shock-shock interactions

The shock structure of a plasmoid in magnetic reconnection in low-beta plasmas is investigated by two-dimensional magnetohydrodynamic simulations. Using a high-accuracy code with unprecedented resolution, shocks, discontinuities, and their intersections are resolved and clarified. Contact discontinuities emanate from triple-shock intersection points, separating fluids of different origins. Shock-diamonds inside the plasmoid appear to decelerate a supersonic flow. New shock-diamonds and a slow expansion fan are found inside the Petschek outflow. A sufficient condition for the new shock-diamonds and the relevance to astrophysical jets are discussed.

3D Gray Radiative Properties of Accretion Shocks in Young Stellar Objects

We address the problem of the contribution of radiation to the structure and dynamics of accretion shocks on Young Stellar Objects. Solving the 3D RTE (radiative transfer equation) under our “gray LTE approach”, i.e., using appropriate mean opacities computed in local thermodynamic equilibrium, we post-process the 3D MHD (magnetohydrodynamic) structure of an accretion stream impacting the stellar chromosphere. We find a radiation flux of ten orders of magnitude larger than the accreting energy rate, which is due to a large overestimation of the radiative cooling. A gray LTE radiative transfer approximation is therefore not consistent with the given MHD structure of the shock. Further investigations are required to clarify the role of radiation, by relaxing both the gray and LTE approximations in RHD (radiation hydrodynamics) simulations. Post-processing the obtained structures through the resolution of the non-LTE monochromatic RTE will provide reference radiation quantities against which RHD approximate solutions will be compared.

Kinetic effects on robustness of electron magnetohydrodynamic structures

Following recent remarkable progress in the development of high-power short-pulse lasers, exploration is ongoing into hitherto unknown phenomena at fast time scales of electrons, the understanding of which is becoming crucial. For a simplified description of such phenomena, the Electron Magnetohydrodynamics (EMHDs) fluid description is often adopted. For the possibility of electron transport in high-density plasma, exact solutions of the EMHD model in the form of electron vortex currents, together with their associated magnetic fields, have been considered. However, the fluid EMHD model does not incorporate kinetic effects. Here, the finite Larmor radius effects owing to a finite electron temperature on the robustness of the exact EMHD structures are investigated using two-dimensional particle-in-cell simulations. It is found that larger EMHD vortex structures can sustain themselves for long periods, even in high temperature plasma; however, sustaining structures at higher temperatures tends to be difficult. With increasing temperature, electrons with finite Larmor radii become disengaged from the localized region. It is also shown that structures localized in smaller regions are more difficult to sustain. A quantitative criterion in terms of the structure size and Larmor radius has been established by simulations over a wide range of parameters. Finally, we conclude that a structure, larger than about eight times the typical Larmor radius at r=R, could form and exist even under the effects of finite electron temperature.

The space–time CE/SE method for solving one-dimensional special relativistic magnetohydrodynamics equations

The development of powerful computational tools for simulating phenomena that inhibits the relativistic magnetohydrodynamic (MHD) structure has become one of the core research issues in astrophysics and is one of the compelling fields. The relativistic MHD equations are more complex than the non-relativistic ones due to non-linear relations between conserved and state (primitive) variables. The non-linearity of the systems and the flow near speed of light pose major challenges to the theoretical investigation of the models and to the numerical solution techniques. The current numerical study is related to the implementation of the space–time conservation element and solution element (CE/SE) method for solving one-dimensional special relativistic magnetohydrodynamic (SRMHD) equations. In contrast to the existing upwind finite volume schemes, the Riemann solver and reconstruction procedure are not the building blocks of the suggested method. The method differs from previous techniques because of global and local flux conservation in a space–time domain without resorting to interpolation or extrapolation. For validation, the numerical results of the method are compared with the second order central and kinetic flux-vector splitting schemes. The one-dimensional computations of this paper verify the method’s efficiency, robustness and accuracy which are the key parameters in the context of astrophysical scenario.

Magnetohydrodynamic structure of a plasmoid in fast reconnection in low-beta plasmas

Plasmoid structures in fast reconnection in low-beta plasmas are investigated by two-dimensional magnetohydrodynamic simulations. A high-resolution shock-capturing code enables us to explore a variety of shock structures: vertical slow shocks behind the plasmoid, another slow shock in the outer-region, and the shock-reflection in the front side. The Kelvin–Helmholtz-like turbulence is also found inside the plasmoid. It is concluded that these shocks are rigorous features in reconnection in low-beta plasmas, where the reconnection jet speed or the upstream Alfvén speed exceeds the sound speed.

Variational derivation of the dispersion relation of kinetic coherent modes in the acoustic frequency range in tokamaks

In the present paper, we compare two modes with frequencies belonging to the acoustic frequency range: the geodesic acoustic mode (GAM) and the Beta Alfvén eigenmode (BAE). For this, a variational gyrokinetic energy principle coupled to a Fourier sidebands expansion is developed. High order finite Larmor radius and finite orbit width effects are kept. Their impact on the mode structures and on the Alfvén spectrum is calculated and discussed. We show that in a local analysis, the degeneracy of the electrostatic GAM and the BAE dispersion relations is verified to a high order and based in particular on a local poloidal symmetry of the two modes. When a more global point of view is taken, and the full radial structures of the modes are computed, differences appear. The BAE structure is shown to have an enforced localization, and to possibly connect to a large magnetohydrodynamic structure. On the contrary, the GAM is seen to have a wavelike, nonlocalized structure, as long as standard slowly varying monotonic profiles are considered.

Intermittent MHD structures and classical discontinuities

We examine statistics of rapid spatial variations of the magnetic field in simulations of magnetohydrodynamic (MHD) turbulence, by analyzing intermittency properties, and by using classical methods for identifying discontinuities. The methods identify similar structures, and give very similar event distribution functions. When the results are scaled to the correlation length, the average waiting times agree with typically reported waiting times between solar wind discontinuities. Thus discontinuities may be related to flux tube boundaries and intermittent structures that appear spontaneously in MHD turbulence.

Propagation of electron magnetohydrodynamic structures in a two-dimensional inhomogeneous plasma

The fully three-dimensional governing equations in the electron magnetohydrodynamic (EMHD) regime for a plasma with inhomogeneous density are obtained. These equations in the two-dimensional limit can be cast in terms of the evolution of two coupled scalar fields. The nonlinear simulations for the two-dimensional case are carried out to understand the propagation of EMHD magnetic structures in the presence of inhomogeneity. A novel effect related to the trapping of dipolar magnetic structures in the high density plasma region in the EMHD regime is observed. The interpretation of this phenomena as well as its relevance to the problem of hot spot generation in the context of fast ignition is presented.

Fossil AGN jets as ultrahigh-energy particle accelerators

Remnants of AGN jets and their surrounding cocoons leave colossal magnetohydrodynamic (MHD) fossil structures storing total energies �10 60 erg. The original active galacic nucleus (AGN) may be dead but the fossil will retain its stable configuration resembling the reversed-field pinch (RFP) encountered in laboratory MHD experiments. In an RFP the longitudinal magnetic field changes direction at a critical distance from the axis, leading to magnetic re-connection there, and to slow decay of the largescale RFP field. We show that this field decay induces large-scale electric fields which can accelerate cosmic rays with an E 2 power-law up to ultra-high energies with a cutoff depending on the fossil parameters. The cut-off is expected to be rigidity dependent, implying the observed composition would change from light to heavy close to the cut-off if one or two nearby AGN fossils dominate. Given that several percent of the universe’s volume may house such slowly decaying structures, these fossils may even reenergize ultra-high energy cosmic rays from distant/old sources, offsetting the “GZKlosses” due to interactions with photons of the cosmic microwave background radiation and giving evidence of otherwise undetectable fossils. In this case the composition would remain light to the highest energies if distant sources or fossils dominated, but otherwise would be mixed. It is hoped the new generation of cosmic ray experiments such as the Pierre Auger Observatory and ultra-high energy neutrino telescopes such as ANITA and lunar Cherenkov experiments will clarify this.

Tomographic analysis of the central magnetohydrodynamic oscillations on the HT-7 tokamak

Multi-channel soft x-ray (SX) detectors are applied to generate images of magnetohydrodynamic (MHD) oscillation on the HT-7 tokamak and the data from SX cameras are analysed by using the Fourier–Bessel harmonic reconstruction method and the singular value decomposition. The image reconstruction of SX emissivity is obtained on the assumption of plasma rigid rotation. One of the important phenomena in the HT-7 discharge is the transition from the sawtooth oscillations to the MHD oscillations when the plasma density grows higher. The MHD structure observed in the SX tomography is featured as follows: the magnetic surface of MHD structure is made up of the crescent-shaped ``hot core'' and the circular ``cold bubble''. The structure of the magnetic surface is relatively stable. It rotates in the direction of the electron diamagnetic drift at a frequency being the oscillation frequency of the MHD oscillations.

Acceleration of charged particles by gyroresonant surfing at quasi-parallel shocks

We discuss an acceleration mechanism of charged particles by magnetohydrodynamic (MHD) structures, such as quasi-parallel shocks and short-large amplitude magnetic structures (SLAMS). In the presence of electromagnetic waves and an electrostatic electric field, particles are accelerated efficiently in the perpendicular direction to the background magnetic field by the combination of two effects, the trapping of particles by the wave and the dragging by the electrostatic field to keep the resonance condition. This allows particles to propagate downstream even when they initially have smaller kinetic energy than the potential. We show the fundamental properties of this mechanism, referred to as gyroresonant surfing.

Role of electron inertia in nonlinear electron magnetohydrodynamics

This paper reports experimental investigations of nonlinear electron magnetohydrodynamic (EMHD) structures of the size of skin depth. It is shown that the difficulty of exciting EMHD structures with k⊥de∼1 is not related to any specifics of antenna–plasma coupling mechanisms but to a general attenuation scaling with antenna size. Formation of null-point or mirror geometry with contribution from antenna current leads to nonlinear effects in case of EMHD structures with k⊥de≪1. However, there is weakening of nonlinearity in case of k⊥de∼1. Observations indicate that reconnection rates in EMHD may not be independent of the plasma skin depth, in contrast to theoretical predictions. It is pointed out that electron inertia takes the role of magnetic field in the region where adiabatic constants of motion do not remain constant.

Relativistic magnetized star with poloidal and toroidal fields

We study the inner magnetohydrodynamic structure of a general relativistic magnetized star, with poloidal and toroidal fields. The star is taken to be differentially rotating, stationary, axisymmetric, and made from perfect, infinitely conducting fluid. Strong toroidal fields of up to ${10}^{17} \mathrm{G}$ can be created from the initial poloidal field by a variation of the mechanism proposed by Meier et al. and Klu\ifmmode \acute{z}\else \'{z}\fi{}niak and Ruderman. It is also found that the redshifted toroidal field and the redshifted chemical potential are constants along a magnetic surface. We prove that a spacetime containing an ideal magnetohydrodynamic fluid which flows only azimuthally is circular in the sense of Carter if, and only if, the magnetic field has only poloidal or only toroidal components. Further, we show through post Newtonian analysis that, even when this criterion is breached, spacetime inside astrophysical compact objects where the magnetic field is less than ${10}^{19} \mathrm{G}$ can be considered circular. In both cases the metric inside the star assumes a simple form, with only one nonvanishing off diagonal term. It is shown that imposing chemical equilibrium forces the magnetic field to assume a force-free configuration. We derive the form of the electric 4-current in force-free relativistic magnetohydrodynamics. The connection between both field components is then given through the vector potential and is used to rule out some field configurations. We derive a new separability condition on the metric which shows that not every pure fluid metric can be dressed with a frozen in, force-free field.

Spacecraft studies of the interplanetary magnetic field

In situ studies of the magnetic fields in space by spacecraft (S/C) began 4 decades ago. The experiments were initially difficult to perform and interpret accurately because of contamination by S/C‐generated magnetic fields, partial measurements of the vector field, limited mass and power for the instrumentation, low‐data‐rate sampling, discontinuous or spotty data coverage, and sometimes unknown orientation or location of the S/C. As each of these challenges was overcome, an initial view of the macrostructure of the interplanetary magnetic field (IMF) emerged, substantiating the Archimedean spiral structure and solar origin of the IMF as proposed theoretically by E. N. Parker. An important mesoscale feature noted early on was the sectoring of the IMF into an ordered sequence of alternating polarities associated with solar sources. Subsequent studies catalogued the microscale magnetohydrodynamic structures: shock waves, and tangential and rotational discontinuities and fluctuations. Correlations with solar photospheric magnetic fields, coronal holes and in situ plasma, and energetic particles elaborated more completely on the structure and dynamics of the IMF. This paper will provide a brief historical summary of the study of the IMF or, as it is now more commonly referred to, the heliospheric magnetic field (HMF).