Magnetic Pressure Gradient Research Articles

Excitation Magnetohydrodynamic Wave by Gravitational Wave Produced by Binary of Neutron Stars

The gravitational wave, through the strongly magnetized plasma surrounding the neutron stars, in the [Formula: see text]-direction, deforms plasma particle rings in ellipses, alternating axes periodically along the direction of the magnetic field ([Formula: see text]-axis) and of the [Formula: see text]-axis. The uniform field leads to a modulation of the magnetic field, which results in magnetic pressure gradients (magneto-acoustic mode) or in the shear of the magnetic field lines (Alfvén mode). The gravitational wave drives MHD modes and transfers energy to the plasma, can become an important alternative process for the acceleration of baryons to high Lorentz factors observed in short GRBs. The total amount of energy that is transferred from the gravitational wave to the plasma is estimated ([Formula: see text]J - [Formula: see text] J), with [Formula: see text]. We compare our results with previously obtained results by other works.

Similarity Solution of Unsteady MHD Boundary Layer Flow and Heat Transfer past a Moving Wedge in a Nanofluid using the Buongiorno Model

The present work is focused on the unsteady MHD boundary layer flow and heat transfer over a wedge stretching surface moving in a nanofluid with the effects of various dimensionless parameters by using the Boungiorno model. The solution for the velocity, temperature and nanoparticle concentration depends on parameters like Prandtl number Pr, Brownian motion Nb, thermophoresis Nt, unsteadiness parameter A, velocity ratio parameter λ, pressure gradient parameter β and magnetic parameter M. The local similarity transformation is used to convert the governing partial differential equations into coupled higher order non-linear ordinary differential equations. These equations are numerically solved by using fourth order RungeKutta method along with shooting technique. Numerical results are obtained for distributions of velocity, temperature and nanoparticle concentration, as well as, for the skin friction, local Nusselt number and local Sherwood number for several values of governing parameters. The results are shown in graphically and as well as in a tabular form. From the graph the results indicate that the velocity increases for increasing values of magnetic parameter, unsteadiness parameter and pressure gradient parameter but decreases for velocity ratio parameter. The temperature profile increases for thermophoresis and Brownian motion parameter but reverse results arises for Prandtl number and velocity ratio parameter. On the other hand, nanoparticle concentration decreases for thermophoresis parameter, Lewis number and velocity ratio parameter. But in case of Brownian motion parameter the concentration decreases up to η < 1 and then increases. Besides, the present results are compared with previously published work and found to be in good agreement.

Solution of Falkner- Skan Unsteady MHD Boundary Layer Flow and Heat Transfer Past a Moving Porous Wedge in a Nanofluid

The present problem deals with the effect of various dimensionless parameters on the forced convection unsteady MHD boundary layer flow and heat transfer over a permeable wedge stretching surface in a nanofluid. The effects of Brownian motion and thermophoresis are considered. The solution for the velocity, temperature and nanoparticle concentration depends on various dimensionless parameters like Prandtl number Pr, velocity ratio parameter λ, permeability parameter K*, unsteady parameter A, pressure gradient parameter β, magnetic parameter M. The local similarity transformation is used to convert the governing partial differential equations into coupled third order non-linear ordinary differential equations. These equations are numerically solved using suitable method along with shooting technique. Numerical results are obtained for distributions of velocity, temperature and nanoparticle concentration, as well as, for the skin friction, local Nusselt number and local Sherwood number for several values of governing parameters. The numerical results are shown graphically and also in a tabular form. The velocity decreases for increasing values of velocity ratio parameter but increases for magnetic parameter, unsteady parameter, permeability parameter and pressure gradient parameter. The temperature profiles decreases for thermophoresis parameter Prandtl number and velocity ratio parameter whereas the temperature remains unchanged for Brownian motion parameter. Again, nanoparticle concentration profiles increases for Brownian motion parameter but decreases for thermophoresis parameter, Lewis number and velocity ratio parameter. Besides, the results of rate of velocity and rate of heat transfer are compared with previously published work for the validity and accuracy and found to be in good agreement.

Generalized magnetotail equilibria: Effects of the dipole field, thin current sheets, and magnetic flux accumulation

AbstractGeneralizations of the class of quasi‐1‐D solutions of the 2‐D Grad‐Shafranov equation, first considered by Schindler in 1972, are investigated. It is shown that the effect of the dipole field, treated as a perturbation, can be included into the original 1972 class solution by modification of the boundary conditions. Some of the solutions imply the formation of singularly thin current sheets. Equilibrium solutions for such sheets resolving their singular current structure on the scales comparable to the thermal ion gyroradius can be obtained assuming anisotropic and nongyrotropic plasma distributions. It is shown that one class of such equilibria with the dipole‐like boundary perturbation describes bifurcation of the near‐Earth current sheet. Another class of weakly anisotropic equilibria with thin current sheets embedded into a thicker plasma sheet helps explain the formation of thin current sheets in a relatively distant tail, where such sheets can provide ion Landau dissipation for spontaneous magnetic reconnection. The free energy for spontaneous reconnection can be provided due to accumulation of the magnetic flux at the tailward end of the closed field line region. The corresponding hump in the normal magnetic field profile Bz(x,z = 0) creates a nonzero gradient along the tail. The resulting gradient of the equatorial magnetic field pressure is shown to be balanced by the pressure gradient and the magnetic tension force due to the higher‐order correction of the latter in the asymptotic expansion of the tail equilibrium in the ratio of the characteristic tail current sheet variations across and along the tail.

Particle-in-cell simulation study of a lower-hybrid shock

The expansion of a magnetized high-pressure plasma into a low-pressure ambient medium is examined with particle-in-cell simulations. The magnetic field points perpendicular to the plasma's expansion direction and binary collisions between particles are absent. The expanding plasma steepens into a quasi-electrostatic shock that is sustained by the lower-hybrid (LH) wave. The ambipolar electric field points in the expansion direction and it induces together with the background magnetic field a fast E cross B drift of electrons. The drifting electrons modify the background magnetic field, resulting in its pile-up by the LH shock. The magnetic pressure gradient force accelerates the ambient ions ahead of the LH shock, reducing the relative velocity between the ambient plasma and the LH shock to about the phase speed of the shocked LH wave, transforming the LH shock into a nonlinear LH wave. The oscillations of the electrostatic potential have a larger amplitude and wavelength in the magnetized plasma than in an unmagnetized one with otherwise identical conditions. The energy loss to the drifting electrons leads to a noticeable slowdown of the LH shock compared to that in an unmagnetized plasma.

INSTABILITY OF NON-UNIFORM TOROIDAL MAGNETIC FIELDS IN ACCRETION DISKS

ABSTRACT We present a new type of instability that is expected to drive magnetohydrodynamic (MHD) turbulence from a purely toroidal magnetic field in an accretion disk. It is already known that in a differentially rotating system, the uniform toroidal magnetic field is unstable due to magnetorotational instability (MRI) under a non-axisymmetric and vertical perturbation, while it is stable under a purely vertical perturbation. Contrary to the previous study, this paper proposes an unstable mode completely confined to the equatorial plane, driven by the expansive nature of the magnetic pressure gradient force under a non-uniform toroidal field. The basic nature of this growing eigenmode, which we name “magneto-gradient driven instability,” is studied using linear analysis, and the corresponding nonlinear evolution is then investigated using two-dimensional ideal MHD simulations. Although a single localized magnetic field channel alone cannot provide sufficient Maxwell stress to contribute significantly to the angular momentum transport, we find that the mode coupling between neighboring toroidal fields under multiple localized magnetic field channels drastically generates a highly turbulent state and leads to the enhanced transport of angular momentum, which is comparable to the efficiency seen in previous studies on MRIs. This horizontally confined mode may play an important role in the saturation of an MRI through complementray growth with the toroidal MRIs and coupling with magnetic reconnection.

Two-dimensional inflow-wind solution of black hole accretion with an evenly symmetric magnetic field

We solve the two-dimensional magnetohydrodynamic (MHD) equations of black hole accretion with the presence of magnetic field. The field includes a turbulent component, whose role is represented by the viscosity, and a large-scale ordered component. The latter is further assumed to be evenly symmetric with the equatorial plane. The equations are solved in the $r-\theta$ plane of a spherical coordinate by assuming time-steady and radially self-similar. An inflow-wind solution is found. Around the equatorial plane, the gas is inflowing; while above and below the equatorial plane at a certain critical $\theta$ angle, $\theta\sim 47^{\circ}$, the inflow changes its direction of radial motion and becomes wind. The driving forces are analyzed and found to be the centrifugal force and the gradient of gas and magnetic pressure. The properties of wind are also calculated. The specific angular momentum of wind is found to be significantly larger than that of inflow, thus wind can transfer angular momentum outward. These analytical results are compared to those obtained by the trajectory analysis based on MHD numerical simulation data and good agreements are found.

Magnetic relaxation and the Taylor conjecture

A one-dimensional model of magnetic relaxation in a pressureless low-resistivity plasma is considered. The initial two-component magnetic field $\boldsymbol{b}(\boldsymbol{x},t)$ is strongly helical, with non-uniform helicity density. The magnetic pressure gradient drives a velocity field that is dissipated by viscosity. Relaxation occurs in two phases. The first is a rapid initial phase in which the magnetic energy drops sharply and the magnetic pressure becomes approximately uniform; the helicity density is redistributed during this phase but remains non-uniform, and although the total helicity remains relatively constant, a Taylor state is not established. The second phase is one of slow diffusion, in which the velocity is weak, though still driven by persistent weak non-uniformity of magnetic pressure; during this phase, magnetic energy and helicity decay slowly and at constant ratio through the combined effects of pressure equalisation and finite resistivity. The density field, initially uniform, develops rapidly (in association with the magnetic field) during the initial phase, and continues to evolve, developing sharp maxima, throughout the diffusive stage. Finally it is proved that, if the resistivity is zero, the spatial mean $\langle (\boldsymbol{b}\boldsymbol{\cdot }\boldsymbol{{\rm\nabla}}\times \boldsymbol{b})/b^{2}\rangle$ is an invariant of the governing one-dimensional induction equation.

Magnetic forces associated with bursty bulk flows in Earth's magnetotail

AbstractWe present the first direct measurements of magnetic forces acting on bursty bulk flow plasma in the magnetotail. The magnetic forces are determined using Cluster multispacecraft measurements. We analyze 67 bursty bulk flow (BBF) events and show that the curvature part of the magnetic force is consistently positive, acting to accelerate the plasma toward Earth between approximately 10 and 20 RE geocentrical distances, while the magnetic field pressure gradient increasingly brakes the plasma as it moves toward Earth. The net result is that the magnetic force accelerates the plasma at distances greater than approximately 14 RE, while it acts to decelerate it within that distance. The magnetic force, together with the thermal pressure gradient force, will determine the dynamics of the BBFs as they propagate toward the near‐Earth tail region. The determination of the former provides an important clue to the ultimate fate of BBFs in the inner magnetosphere.

NUMERICAL SIMULATION OF HOT ACCRETION FLOWS. III. REVISITING WIND PROPERTIES USING THE TRAJECTORY APPROACH

Previous MHD simulations have shown that wind must exist in black hole hot accretion flows. In this paper, we continue our study by investigating the detailed properties of wind, such as mass flux and poloidal speed, and the mechanism of wind production. For this aim, we make use of a three dimensional GRMHD simulation of hot accretion flows around a Schwarzschild black hole. The simulation is designed so that the magnetic flux is not accumulated significantly around the black hole. To distinguish real wind from turbulent outflows, we track the trajectories of the virtual Largrangian particles from simulation data. We find two types of real outflows, i.e., a quasi-relativistic jet close to the axis and a sub-relativistic wind subtending a much larger solid angle. Most of the wind originates from the surface layer of the accretion flow. The poloidal wind speed almost remains constant once they are produced, but the flux-weighted wind speed roughly follows $v_{\rm p, wind}(r)\approx 0.25 v_k(r)$. The mass flux of jet is much lower but the speed is much higher, $v_{\rm p,jet}\sim (0.3-0.4) c$. Consequently, both the energy and momentum fluxes of the wind are much larger than those of the jet. We find that the wind is produced and accelerated primarily by the combination of centrifugal force and magnetic pressure gradient, while the jet is mainly accelerated by magnetic pressure gradient. Finally, we find that the wind production efficiency $\epsilon_{\rm wind}\equiv\dot{E}_{\rm wind}/\dot{M}_{\rm BH}c^2\sim 1/1000$, in good agreement with the value required from large-scale galaxy simulations with AGN feedback.

Theoretical analyses of immiscible MHD pipe flow

In this study, effect of normally applied magnetic field on immiscible fluid flow hydrodynamic flow characteristic has been analytically investigated. Flow case modelled for two different flow patterns. The first one is low electrically conductive fluid (gasoline) inner core and higher one (salty water) outer flow regional pattern and the second one is high electrically conductive fluid inner core and lower one is outer regional flow case in a vertical circular pipe. Flow models have been modelled in differential form theoretical equations and solved with Laplace Transform method. Solved equation systems presented the dependent statement of the local flow velocity and flow rates to the magnetic field induction, pipe radius and pressure gradient. It is estimated that high electrically conductive fluid was directly affected by the magnetic forces which is created by the magnetic field, and also low electrically conductive fluid was indirectly affected from the magnetic field because of the interfacial tension between the fluids. As a result, overall velocity of the flow domain was reduced more by the magnetic forces in the event of low conductive fluid inner core and high conductive fluid outer regional flow case.

2-D MHD Simulation of Two Axially Colliding FRCs Accelerated by Magnetic Pressure Gradient

2-D MHD Simulation of Two Axially Colliding FRCs Accelerated by Magnetic Pressure Gradient

The role of pressure gradients in driving sunward magnetosheath flows and magnetopause motion

AbstractWhile pressure balance can predict how far the magnetopause will move in response to an upstream pressure change, it cannot determine how fast the transient response will be. Using Time History of Events and Macroscale Interactions during Substorms (THEMIS), we present multipoint observations revealing, for the first time, strong (thermal + magnetic) pressure gradients in the magnetosheath due to a foreshock transient, most likely a hot flow anomaly, which decreased the total pressure upstream of the bow shock. By converting the spacecraft time series into a spatial picture, we quantitatively show that these pressure gradients caused the observed acceleration of the plasma, resulting in fast sunward magnetosheath flows ahead of a localized outward distortion of the magnetopause. The acceleration of the magnetosheath plasma was fast enough to keep the peak of the magnetopause bulge at approximately the equilibrium position, i.e., in pressure balance. Therefore, we show that pressure gradients in the magnetosheath due to transient changes in the total pressure upstream can directly drive anomalous flows and in turn are important in transmitting information from the bow shock to the magnetopause.

Magnetic turbulence and pressure gradient feedback effect of the 1/2 mode soft-hard magnetohydrodynamic limit in large helical device

The aim of this study was to analyze the feedback process between the magnetic turbulence and the pressure gradients in Large Helical Device (LHD) inward-shifted configurations as well as its role in the transition between the soft-hard magnetohydrodynamic (MHD) regimes for instabilities driven by the mode 1/2 in the middle plasma. In the present paper, we summarize the results of two simulations with different Lundquist numbers, S=2.5×105 and 106, assuming a plasma in the slow reconnection regime. The results for the high Lundquist number simulation show that the magnetic turbulence and the pressure gradient in the middle plasma region of LHD are below the critical value to drive the transition to the hard MHD regime, therefore only relaxations in the soft MHD limit are triggered (1/2 sawtooth-like events) [Phys. Plasmas 19, 082512 (2012)]. In the case of the simulation with low Lundquist number, the system reaches the hard MHD limit and a plasma collapse is observed.

Maximum mass of stable magnetized highly super-Chandrasekhar white dwarfs: stable solutions withvarying magnetic fields

We address the issue of stability of recently proposed significantly super-Chandrasekhar white dwarfs.We present stable solutions of magnetostatic equilibrium models for super-Chandrasekhar white dwarfs pertaining to various magnetic field profiles. This has been obtained by self-consistently including the effects of the magnetic pressure gradient and total magnetic densityin a general relativistic framework. We estimate that the maximum stable mass of magnetized white dwarfscould be more than 3 solar mass. This is very useful to explain peculiar, overluminous type Iasupernovae which do not conform to the traditional Chandrasekhar mass-limit.

Effect of an Inclined Magnetic Field on Steady Poiseuilleflow between Two Parallel Porous Plates

The present work is to investigate the magnetohydrodynamic behaviour of two dimensional Poiseuille flow under the influence of an inclined magnetic field and constant pressure gradient for a viscous, incompressible and electrically conducting fluid between two infinite parallel plates of which the lower plate is taken to be porous. The solution of the fluid flow governing equation has been obtained by analytical method and expressed in terms of Hartmann number. The fluid flow profiles are represented through graphs for different values of Hartmann number with different angles of inclination for injection/suction. The effects of the other parameters such as pressure gradient, inclined magnetic field and suction/injection on the flow are also studied graphically. The resulting equation of motion has preferably been solved by the method of variation of parameters.

Non-symmetric magnetohydrostatic equilibria: a multigrid approach

Aims. Linear magnetohydrostatic (MHS) models of solar magnetic fields balance plasma pressure gradients, gravity and Lorentz forces where the current density is composed of a linear force-free component and a cross-field component that depends on gravitational stratification. In this paper, we investigate an efficient numerical procedure for calculating such equilibria. Methods. The MHS equations are reduced to two scalar elliptic equations - one on the lower boundary and the other within the interior of the computational domain. The normal component of the magnetic field is prescribed on the lower boundary and a multigrid method is applied on both this boundary and within the domain to find the poloidal scalar potential. Once solved to a desired accuracy, the magnetic field, plasma pressure and density are found using a finite difference method. Results. We investigate the effects of the cross-field currents on the linear MHS equilibria. Force-free and non-force-free examples are given to demonstrate the numerical scheme and an analysis of speed-up due to parallelization on a graphics processing unit (GPU) is presented. It is shown that speed-ups of x30 are readily achievable.

Nonlinear Alfvén wave dynamics at a 2D magnetic null point: ponderomotive force

Context : In the linear, {\beta}=0 MHD regime, the transient properties of MHD waves in the vicinity of 2D null points are well known. The waves are decoupled and accumulate at predictable parts of the magnetic topology: fast waves accumulate at the null point; whereas Alfv\'en waves cannot cross the separatricies. However, in nonlinear MHD mode conversion can occur at regions of inhomogeneous Alfv\'en speed, suggesting that the decoupled nature of waves may not extend to the nonlinear regime. Aims: We investigate the behaviour of low-amplitude Alfv\'en waves about a 2D magnetic null point in nonlinear, {\beta}= 0 MHD. Methods: We numerically simulate the introduction of low-amplitude Alfv\'en waves into the vicinity of a magnetic null point using the nonlinear LARE2D code. Results: Unlike in the linear regime, we find that the Alfv\'en wave sustains cospatial daughter disturbances, manifest in the transverse and longitudinal fluid velocity, owing to the action of nonlinear magnetic pressure gradients (viz. the ponderomotive force). These disturbances are dependent on the Alfv\'en wave and do not interact with the medium to excite magnetoacoustic waves, although the transverse daughter becomes focused at the null point. Additionally, an independently propagating fast magnetoacoustic wave is generated during the early stages, which transports some of the initial Alfv\'en wave energy towards the null point. Subsequently, despite undergoing dispersion and phase-mixing due to gradients in the Alfv\'en-speed profile (Grad c_A \neq 0) there is no further nonlinear generation of fast waves. Conclusions: We find that Alfv\'en waves at 2D cold null points behave largely as in the linear regime, however they sustain transverse and longitudinal disturbances - effects absent in the linear regime - due to nonlinear magnetic pressure gradients.

Numerical investigation of buoyancy effects on hydromagnetic unsteady flow through a porous channel with suction/injection

The paper focus on first and second laws analysis for flow and heat transfer inside a vertical channel made of two uniformly porous parallel plates with suction/injection under the combined action of buoyancy force, transverse magnetic field and constant pressure gradient. Both vertical walls are kept isothermal at the same temperatures and the flow of the conducting fluid is assumed to be unsteady with variable viscosity. The nonlinear governing equations in Cartesian coordinate are obtained and solved numerically using semi-implicit finite difference techniques to develop expressions for velocity and temperature profiles. The entropy generation number, irreversibility distribution ratio and Bejan number are presented graphically and discussed quantitatively for various values of the embedded parameters.

Comparing Poynting flux dominated magnetic tower jets with kinetic-energy dominated jets

Magnetic towers represent one of two fundamental forms of MHD outflows. Driven by magnetic pressure gradients, these flows have been less well studied than magneto-centrifugally launched jets even though magnetic towers may well be as common. Here we present new results exploring the behavior and evolution of magnetic tower outflows and demonstrate their connection with pulsed power experimental studies and purely hydrodynamic jets which might represent the asymptotic propagation regimes of magneto-centrifugally launched jets. High-resolution AMR MHD simulations (using the AstroBEAR code) provide insights into the underlying physics of magnetic towers and help us constrain models of their propagation. Our simulations have been designed to explore the effects of thermal energy losses and rotation on both tower flows and their hydro counterparts. We find these parameters have significant effects on the stability of magnetic towers, but mild effects on the stability of hydro jets. Current-driven perturbations in the Poynting Flux Dominated (PDF) towers are shown to be amplified in both the cooling and rotating cases. Our studies of the long term evolution of the towers show that the formation of weakly magnetized central jets within the tower are broken up by these instabilities becoming a series of collimated clumps which magnetization properties vary over time. In addition to discussing these results in light of laboratory experiments, we address their relevance to astrophysical observations of young star jets and outflow from highly evolved solar type stars.