Magnetic Pressure Gradient Research Articles

Curved Jet Motion. I. Orbiting and Precessing Jets

Astrophysical jets are often observed as bent or curved structures. We also know that the different jet sources may be binary in nature, which may lead to a regular, periodic motion of the jet nozzle, an orbital motion, or precession. Here we present the results of 2D (M)HD simulations in order to investigate how a precessing or orbiting jet nozzle affects the propagation of a high-speed jet. We have performed a parameter study of systems with different precession angles, different orbital periods or separations, and different magnetic field strengths. We find that these kinds of nozzles lead to curved jet propagation, which is determined by the main parameters that define the jet nozzle. We find C-shaped jets from orbiting nozzles and S-shaped jets from precessing nozzles. Over a long time and long distances, the initially curved jet motion bores a broad channel into the ambient gas that is filled with high-speed jet material whose lateral motion is damped, however. A strong (longitudinal) magnetic field can damp the jet curvature that is enforced by either precession or orbital motion of the jet sources. We have investigated the force balance across the jet and ambient medium and found that the lateral magnetic pressure and gas pressure gradients are almost balanced, but that a lack of gas pressure on the concave side of the curvature is leading to the lateral motion. Magnetic tension does not play a significant role. Our results are obtained in code units, but we provide scaling relations such that our results may be applied to young stars, microquasars, symbiotic stars, or active galactic nuclei.

Interaction of coronal mass ejections and the solar wind

Aims.Our goal is to thoroughly analyse the dynamics of single and multiple solar eruptions, as well as a stealth ejecta. The data were obtained through self-consistent numerical simulations performed in a previous study. We also assess the effect of a different background solar wind on the propagation of these ejecta to Earth.Methods.We calculated all the components of the forces contributing to the evolution of the numerically modelled consecutive coronal mass ejections (CMEs) obtained with the 2.5D magnetohydrodynamics (MHD) module of the code MPI-AMRVAC. We analysed the thermal and magnetic pressure gradients and the magnetic tension dictating the formation of several flux ropes in different locations in the aftermath of the eruptions. These three components were tracked in the equatorial plane during the propagation of the CMEs to Earth. Their interaction with other CMEs and with the background solar wind was also studied.Results.We explain the formation of the stealth ejecta and the plasma blobs (or plasmoids) occurring in the aftermath of solar eruptions. We also address the faster eruption of a CME in one case with a different background wind, even when the same triggering boundary motions were applied, and attribute this to the slightly different magnetic configuration and the large neighbouring arcade. The thermal pressure gradient revealed a shock in front of these slow eruptions, formed during their propagation to 1 AU. The double-peaked magnetic pressure gradient indicates that the triggering method affects the structure of the CMEs and that a part of the adjacent streamer is ejected along with the CME.

Density filamentation nonlinearly driven by the Weibel instability in relativistic beam plasmas

Density filamentation has been observed in many beam-plasma simulations and experiments. Because current filamentation is a pure transverse mode, charge density filamentation cannot be produced directly by the current filamentation process. To explain this phenomenon, several mechanisms are proposed such as the coupling of the Weibel instability to the two-stream instability, coupling to the Langmuir wave, differences in thermal velocities between the beam and return currents, the magnetic pressure gradient force, etc. In this paper, it is shown that the gradient of the Lorentz factor can, in fact, represent the nonlinear behavior of a plasma fluid and further that the nonuniform Lorentz factor distribution can give rise to electrostatic fields and density filaments. Simulation results together with theoretical analyses are presented.

High-density disc reflection spectroscopy of low-mass active galactic nuclei

ABSTRACTThe standard alpha-disc model predicts an anticorrelation between the density of the inner accretion disc and the black hole mass times square of the accretion rate, as seen in higher mass (MBH > 106 M⊙) active galactic nuclei (AGNs). In this work, we test the predictions of the alpha-disc model and study the properties of the inner accretion flow for the low-mass end (MBH ≈ 105–6 M⊙) of AGNs. We utilize a new high-density disc reflection model where the density parameter varies from ne = 1015 to 1020 cm−3 and apply it to the broad-band X-ray (0.3–10 keV) spectra of the low-mass AGN sample. The sources span a wide range of Eddington fractions and are consistent with being sub-Eddington or near-Eddington. The X-ray spectra reveal a soft X-ray excess below ∼1.5 keV which is well modelled by high-density reflection from an ionized accretion disc of density ne ∼ 1018 cm−3 on average. The results suggest a radiation pressure-dominated disc with an average of 70 per cent fraction of the disc power transferred to the corona, consistent with that observed in higher mass AGNs. We show that the disc density higher than 1015 cm−3 can result from the radiation pressure compression when the disc surface does not hold a strong magnetic pressure gradient. We find tentative evidence for a drop in black hole spin at low-mass regimes.

Double‐diffusive convection on peristaltic flow of hyperbolic tangent nanofluid in non‐uniform channel with induced magnetic field

Consequence of thermal and concentration convection on peristaltic pumping of hyperbolic tangent nanofluid in a non‐uniform channel and induced magnetic field is discussed in this article. The brief mathematical modeling, along with induced magnetic field, of hyperbolic tangent nanofluid is given. The governing equations are reduced to dimensionless form by using appropriate transformations. Exact solutions are calculated for temperature, nanoparticle volume fraction, and concentration. Numerical technique is manipulated to solve the highly non‐linear differential equations. The roll of different variables is graphically analyzed in terms of concentration, temperature, volume fraction of nanoparticles, axial induced magnetic field, magnetic force function, stream functions, pressure rise, and pressure gradient.

Gravity versus Magnetic Fields in Forming Molecular Clouds

Abstract Magnetic fields are dynamically important in the diffuse interstellar medium. Understanding how gravitationally bound, star-forming clouds form requires modeling of the fields in a self-consistent, supernova-driven, turbulent, magnetized, stratified disk. We employ the FLASH magnetohydrodynamics code to follow the formation and early evolution of clouds with final masses of 3–8 × 103 M ⊙ within such a simulation. We use the code’s adaptive mesh refinement capabilities to concentrate numerical resolution in zoom-in regions covering single clouds, allowing us to investigate the detailed dynamics and field structure of individual self-gravitating clouds in a consistent background medium. Our goal is to test the hypothesis that dense clouds are dynamically evolving objects far from magnetohydrostatic equilibrium. We find that the cloud envelopes are magnetically supported with field lines parallel to density gradients and flow velocity, as indicated by the histogram of relative orientations and other statistical measures. In contrast, the dense cores of the clouds are gravitationally dominated, with gravitational energy exceeding internal, kinetic, or magnetic energy and accelerations due to gravity exceeding those due to magnetic or thermal pressure gradients. In these regions, field directions vary strongly, with a slight preference toward being perpendicular to density gradients, as shown by three-dimensional histograms of relative orientation.

Magnetic Energy Conversion in Magnetohydrodynamics: Curvature Relaxation and Perpendicular Expansion of Magnetic Fields

Abstract The mechanisms and pathways of magnetic energy conversion are an important subject for many laboratory, space, and astrophysical systems. Here, we present a perspective on magnetic energy conversion in magnetohydrodynamics through magnetic field curvature relaxation (CR) and perpendicular expansion (PE) due to magnetic pressure gradients, and quantify their relative importance in two representative cases, namely 3D magnetic reconnection and 3D kink-driven instability in an astrophysical jet. We find that the CR and PE processes have different temporal and spatial evolutions in these systems. The relative importance of the two processes tends to reverse as the system enters the nonlinear stage from the instability growth stage. Overall, the two processes make comparable contributions to magnetic energy conversion, with the PE process somewhat stronger than the CR process. We further explore how these energy conversion terms can be related to particle energization in these systems.

Physical and Chemical Vertical Structure of Magnetostatic Accretion Disks of Young Stars

The vertical structure of the accretion disks of young stars with fossil large-scale magnetic field is studied. The equations of magnetostatic equilibrium of the disk are solved taking into account the stellar gravity, gas and magnetic pressure, turbulent heating, and heating by stellar radiation. The modelled physical structure of the disk is used to simulate its chemical structure, in particular, to study the spatial distribution of CN molecules. The disk of the typical T Tauri star is considered. Simulations show that the temperature within the disk in the region $r<50$ au decreases with height and density profiles are steeper than in the isothermal case. Outside the `dead' zone, vertical profiles of the azimuthal component of the magnetic field are nonmonotonic, and the magnetic field strength maximum is reached within the disk. The magnetic pressure gradient can cause an increase in the disk thickness in comparison with the hydrostatic one. The CN molecule concentration is maximum near the photosphere and in the disk atmosphere where the magnetic field strength at chosen parameters is $\sim 0.01$ G. Measurements of the Zeeman splitting of CN lines in the submm range can be used to determine the magnetic field strength in these regions of accretion disks.

Analysis of Deformation and Erosion during CME Evolution

Magnetised coronal mass ejections (CMEs) are quite substantially deformed during their journey form the Sun to the Earth. Moreover, the interaction of their internal magnetic field with the magnetic field of the ambient solar wind can cause deflection and erosion of their mass and magnetic flux. We here analyse axisymmetric (2.5D) MHD simulations of normal and inverse CME, i.e., with the opposite or same polarity as the background solar wind, and attempt to quantify the erosion and the different forces that operate on the CMEs during their evolution. By analysing the forces, it was found that an increase of the background wind density results in a stronger plasma pressure gradient in the sheath that decelerates the magnetic cloud more. This in turn leads to an increase of the magnetic pressure gradient between the centre of the magnetic cloud and the separatrix, causing a further deceleration. Regardless of polarity, the current sheet that forms in our model between the rear of the CME and the closed field lines of the helmet streamer, results in magnetic field lines being stripped from the magnetic cloud. It is also found that slow normal CMEs experience the same amount of erosion, regardless of the background wind density. Moreover, as the initial velocity increases, so does the influence of the wind density on the erosion. We found that increasing the CME speed leads to a higher overall erosion due to stronger magnetic reconnection. For inverse CMEs, field lines are not stripped away but added to the magnetic cloud, leading to about twice as much magnetic flux at 1 AU than normal CMEs with the same initial flux.

MHD simulation of supersonic FRC merging corrected by non-invasive magnetic measurements

In this study, a newly developed correction method with external magnetic measurements for the magnetohydrodynamics (MHD) simulation of the collisional merging formation of a field-reversed configuration (FRC) realized the estimation of the internal structure of the FRCs without invasive internal measurements. In the collisional merging formation of FRCs, an FRC is formed via merging of two initial FRC-like plasmoids at supersonic/Alfvénic velocity. An invasive diagnostic may also interfere with the collisional merging formation process. A two-dimensional resistive MHD simulation was conducted to evaluate the global behavior and internal structure of FRCs in the collisional merging formation process without invasive measurements. This code simulated the initial formation and collisional merging processes of FRCs including discharge circuits. However, the translation velocity and the pressure of initial FRCs did not simultaneously agree with the experimental values because the magnetic pressure gradient in each formation region could not be reproduced without the artificial adjustment of the initial condition. The experimentally measured current distribution was given as the initial condition of the circuit calculation in the developed correction method. The initial FRCs were successfully translated at the translation velocity and plasma pressure in the corrected simulation, both of which were equivalent to the experiments. The properties of the merged FRCs in the experiments such as volume, total temperature, and average electron density were reproduced in the corrected simulation. The detailed radial profile of the internal magnetic field of the FRC was also measured and found to agree very well with the simulation results.

Analysis of influences of pressure anisotropies on the 3D MHD equilibrium in LHD

3D equilibria with an anisotropic pressure component in the large helical device are analyzed with respect to their magnetic axis locations. The anisotropic extension of the 3D equilibrium solver variational moments equilibrium code, anisotropic neumann inverse moments equilibrium code, is used to compute fixed-boundary plasma equilibria based on a bi-Maxwellian distribution function describing the anisotropic particles. Different heating scenarios are assessed by means of parallel and perpendicular pressure anisotropies with different radial anisotropic pressure profiles imposed. A theoretical predicted scaling of the magnetic axis location with the auxiliary parameter βeq as predicted for classical stellarators and heliotrons by Hitchon [Nucl. Fusion 23, 383 (1983)] is found to be applicable to the large helical device in the case of a flat hot-particle profile for parallel or weak perpendicular dominated anisotropies with β⊥/β∥≤2. For strong perpendicular or non-flat hot-particle profiles, a deviation from the predicted scaling of the magnetic axis location is found. Whereas center-peaked profiles show a stronger shift of the magnetic axis, edge-peaked profiles show no significant change of its radial location. High critical magnetic fields are identified as a necessary condition for strong perpendicular anisotropies. The observed deviations are ascribed to the magnetic field structure and negative pressure gradients. The invalidity of the theoretical predictions in the case of certain configurations is found to be caused by higher-order terms in the pressure components, which are not accounted for by the ordering on which the theory is based.

The Causes of Peripheral Coronal Loop Contraction and Disappearance Revealed in a Magnetohydrodynamic Simulation of Solar Eruption

Abstract The phenomenon of peripheral coronal loop contraction during solar flares and eruptions, recently discovered in observations, has gradually drawn the attention of solar physicists. However, its underlying physical mechanism is still uncertain. One possible mechanism is Hudson’s implosion conjecture, which attributes the contraction of peripheral coronal loops to magnetic pressure reduction in the magnetic energy liberation core, while other researchers proposed alternative explanations. In previous observational studies we also note the disappearance of peripheral shrinking loops in the late phase, of which there is a lack of investigation and interpretation. In this paper, we exploit a full MHD simulation of solar eruption to study the causes of the two phenomena. It is found that the loop motion in the periphery is well correlated with magnetic energy accumulation and dissipation in the core, and the loop shrinkage is caused by a more significant reduction in magnetic pressure gradient force than in magnetic tension force, consistent with the implosion conjecture. The peripheral contracting loops in the late phase act as inflow to reconnect with central erupting structures, which destroys their identities and naturally explains their disappearance. We also propose a positive feedback between the peripheral magnetic reconnection and the central eruption.

Heat transfer from a ferrofluid during generalized Couette flow through parallel plates in the presence of an orthogonal magnetic field

In the present work, we report heat transfer during the unidirectional laminar flow of ferrofluids in a parallel plate channel in the presence of a homogeneously applied magnetic field perpendicular to the flow direction. We have considered generalized Couette flow which results from the combination of shear stress, due to the relative motion between the parallel plates, and applied pressure gradient. For a comprehensive investigation, four different thermal boundary conditions have been considered. The application of magnetic field modifies the viscosity of the colloidal ferrofluid and thereby causes variations in the local flow field. Further, it also modifies the thermal conductivity of the fluid by changing particle orientation. As a consequence, the temperature distribution between the parallel plate channel is uniquely modified. The complex interplay between the hydrodynamics and the magnetics results in implicitly coupled governing differential equations, which have been solved numerically. We report that the local temperature in the domain could be reduced by increasing the applied field strength irrespective of the imposed thermal boundary conditions. However, the influence of the applied field finally gets saturated. We also discuss the effect of Brinkman number, loading of magnetic particles and pressure gradient on temperature distribution. Finally, analytical solutions, which presents closed-form expressions of the temperature profiles for four different types of boundary conditions, have been proposed.

Experimental evidence for super-Alfvénic acceleration of the field-reversed configuration due to a magnetic pressure gradient

The super-Alfvénic acceleration of an extremely high beta plasmoid due to a magnetic pressure gradient is evidenced in translation experiments involving the field-reversed configuration (FRC) in the FRC amplification via the translation-collisional merging (FAT-CM) device. Inside the FRC, there is a population of unmagnetized particles that forms a volume with extremely high beta. Formed using the field-reversed theta pinch method, the FRC is accelerated and injected into a quasi-static confinement magnetic field that is weaker than the formation and acceleration fields. Because the FRC is injected with nonzero axial translation speed and cannot slow down, its speed exceeds the Alfvénic and sonic speed upon entering the confinement field. This phenomenon is predicted from two-dimensional resistive magnetohydrodynamic simulations, and experimental results that are consistent with the magnetohydrodynamic approximation show that the FRC acceleration process is due mainly to magnetic pressure on the thin magnetized layer.

The Rise and Emergence of Untwisted Toroidal Flux Ropes on the Sun

Abstract Magnetic flux ropes (MFRs) rising buoyantly through the Sun’s convection zone are thought to be subject to viscous forces preventing them from rising coherently. Numerous studies have suggested that MFRs require a minimum twist in order to remain coherent during their rise. Furthermore, even MFRs that get to the photosphere may be unable to successfully emerge into the corona unless they are at least moderately twisted, since the magnetic pressure gradient needs to overcome the weight of the photospheric plasma. To date, however, no lower limit has been placed on the critical minimum twist required for an MFR to rise coherently through the convection zone or emerge through the photosphere. In this paper, we simulate an untwisted toroidal MFR that is able to rise from the convection zone and emerge through the photosphere as an active region that resembles those observed on the Sun. We show that untwisted MFRs can remain coherent during their rise and then pile up near the photosphere, triggering undular instability, allowing the MFR to emerge through the photosphere. We propose that the toroidal geometry of our MFR is critical for its coherent rise. Upon emergence, a pair of lobes rises into the corona. The two lobes then interact and reconnect, resulting in a localized high speed jet. The resulting photospheric magnetogram displays the characteristic salt-and-pepper structure often seen in observations. Our major result is that MFRs need not be twisted to rise coherently through the convection zone and emerge through the photosphere.

Mathematical model on magneto-hydrodynamic dispersion in a porous medium under the influence of bulk chemical reaction

The mathematical model of hydrodynamic dispersion through a porous medium is developed in the presence of transversely applied magnetic fields and axial harmonic pressure gradient. The solute introduce into the flow is experienced a first-order chemical reaction with flowing liquid. The dispersion coefficient is numerically determined using Aris’s moment equation of solute concentration. The numerical technique employed here is a finite difference implicit scheme. Dispersion coefficient behavior with Darcy number, Hartmann number and bulk flow reaction parameter is investigated. This study highlighted that the dependency of Hartmann number and Darcy number on dispersion shows different natures in different ranges of these parameters.

MESSENGER Observations of Flow Braking and Flux Pileup of Dipolarizations in Mercury's Magnetotail: Evidence for Current Wedge Formation

AbstractSimilar to Earth, Mercury's magnetotail experiences frequent dipolarization of the magnetic field. These rapid (~2 s) increases in the northward component of the tail field (ΔBz ~ 30 nT) at Mercury are associated with fast sunward flows (~200 km/s) that enhance local magnetic field convection. Differences between the two magnetospheres, namely Mercury's smaller spatiotemporal scales and lack of an ionosphere, influence the dynamics of dipolarizations in these magnetotails. At Earth, the braking of fast dipolarization flows near the inner magnetosphere accumulates magnetic flux and develops the substorm current wedge. At Mercury, flow braking and flux pileup remain open topics. In this work, we develop an automated algorithm to identify dipolarizations, which allows for statistical examination of flow braking and flux pileup in Mercury's magnetotail. We find that near the inner edge of the plasma sheet, steep magnetic pressure gradients cause substantial braking of fast dipolarization flows. The dipolarization frequency and sunward flow speed decrease significantly within a region ~500 km thick located at ~900 km altitude above Mercury's local midnight surface. Due to the close proximity of the braking region to the planet, we estimate that ~10–20% of dipolarizations may reach the nightside surface of the planet. The remaining dipolarizations exhibit prolonged statistical flux pileup within the braking region similar to large‐scale dipolarization of Earth's inner magnetosphere. The existence of flow braking and flux pileup at Mercury indicates that a current wedge may form, although the limitations imposed by Mercury's magnetosphere require the braking of multiple, continuous dipolarizations for current wedge formation.

Particle-in-cell Simulations of Secondary Magnetic Islands: Ion-scale Flux Ropes and Plasmoids

Abstract Ion-scale flux ropes and plasmoids are secondary magnetic islands produced during magnetic reconnection in various heliospheric plasma environments. Here we study the structure of secondary islands and the particle dynamics within them using particle-in-cell simulations. Ion-scale flux ropes (secondary islands with a strong core field) are formed in a strong guide field regime, whereas ion-scale plasmoids (secondary islands with a weak core field) are formed in a weak guide field regime. Currents in both types of secondary islands are carried primarily by electrons. Both types of secondary islands have a magnetic tension force pointing radially inward toward their center. In the flux rope type, this inward tension force is balanced by an outward magnetic pressure gradient of the strong core field; in the plasmoid type, it is countered by an outward thermal pressure gradient caused by electron and ion energizations. The transition between these two types occurs when B g /B 0 = 0.1–0.2 (B g is the guide field, and B 0 is the asymptotic magnetic field).

The Effect of Varying Magnetic Number, Reynolds Number and Pressure Gradient on Velocity Profiles in an MHD Flow

Magnetohydrodynamic ow of a hot viscous electrically conducting incompressible uid through parallel plates is studied. In the study, the e ect of Hartmann number (M), pressure gradient and Reynolds number (Re) on the velocity eld is investigated. The Navier-stokes equations were coupled with Ohms law and then solved using nite di erence method (FDM). The velocity eld was computed for various values of the physical parameters and shown graphically. It was found that as the Hartmann number M increases, the velocity pro les decreased due to increased Lorents force while an increase in Reynolds number causes an increase in the velocity of the uid. All these analysis was done using MATLAB program and the results were presented in tables and graphs.

Ion‐Scale Flux Rope Observed inside a Hot Flow Anomaly

AbstractWe report an earthward moving ion‐scale flux rope embedded within the trailing edge of a hot flow anomaly (HFA) observed by the Magnetospheric Multiscale satellite constellation on 17 December 2016 upstream of Earth's quasi‐parallel bow shock. The driver of the HFA, a tangential discontinuity, was observed by the Wind spacecraft without flux rope signatures around it in the solar wind. This suggests that the earthward moving flux rope was generated inside the HFA. This ion‐scale flux rope is not a force free structure and expands due to a strong magnetic pressure gradient force. Solar wind ions are decelerated inside the flux rope by the static electric field likely caused by the charge separation of solar wind particles. Our observations imply that magnetic reconnection may have occurred inside the HFA. Reconnection and flux ropes may play a role in particle acceleration/heating inside foreshock transients.