Magnetohydrodynamic Processes Research Articles

The Evershed effect as we understand it today

Following a description of the Evershed effect as observed by John Evershed in 1909 at the Kodaikanal Solar Observatory, we discuss the evolution of our scientific understanding that accompanied the development of the subject of magnetohydrodynamics (MHD). How the modern telescopic observations as well as the computational capabilities are serving to uncover the complex magnetohydrodynamic processes behind this highly dynamical phenomenon is discussed at length.

EXPLOSIVE INSTABILITY AND CORONAL HEATING

The observed energy-loss rate from the solar corona implies that the coronal magnetic field has a critical angle at which energy is released. It has been hypothesized that at this critical angle an explosive would occur, leading to an enhanced conversion of magnetic energy into heat. In earlier investigations, we have shown that a shear-dependent magnetohydrodynamic process called has many of the distinctive features of the hypothetical explosive instability. In this paper, we give the first demonstration that this occurs in a system with line-tied magnetic fields and boundary shearing—basically the situation described by Parker. We also show that, as the disturbance due to secondary instability attains finite amplitude, there is a transition to turbulence which leads to enhanced dissipation of magnetic and kinetic energy. These results are obtained from numerical simulations performed with a new parallelized, viscoresistive, three-dimensional code that solves the cold plasma equations. The code employs a Fourier collocation—finite difference spatial discretization, and uses a third-order Runge-Kutta temporal discretization.

MMTF-Hα AND HST -FUV IMAGING OF THE FILAMENTARY COMPLEX IN ABELL 1795

We have obtained deep, high spatial resolution images of the central region of Abell 1795 at Halpha and [NII] (6583A) with the Maryland Magellan Tunable Filter (MMTF), and in the far-ultraviolet (FUV) with the Advanced Camera for Surveys Solar Blind Channel on the Hubble Space Telescope (HST). The superb image quality of the MMTF data has made it possible to resolve the known SE filament into a pair of thin, intertwined filaments extending for ~50 kpc, with a width < 1 kpc. The presence of these thin, tangled strands is suggestive of a cooling wake where runaway cooling is taking place, perhaps aided by an enhanced magnetic field in this region. The HST data further resolve these strands into chains of FUV-bright stellar clusters, indicating that these filaments are indeed sites of on-going star formation, but at a rate ~2 orders of magnitude smaller than the mass-deposition rates predicted from the X-ray data. The elevated [NII]/Halpha ratio and large spatial variations of the FUV/Halpha flux ratio across the filaments indicate that O-star photoionization is not solely responsible for the ionization. The data favor collisional heating by cosmic rays either produced in-situ by magnetohydrodynamical processes or conducted in from the surrounding intracluster medium.

BLAZAR HALOS AS PROBE FOR EXTRAGALACTIC MAGNETIC FIELDS AND MAXIMAL ACCELERATION ENERGY

High-energy photons from blazars interact within tens of kpc with the extragalactic photon background, initiating electromagnetic pair cascades. The charged component of such cascades is deflected by extragalactic magnetic fields (EGMFs), leading to halos even around initially point-like sources. We calculate the intensity profile of the resulting secondary high-energy photons for different assumptions on the initial source spectrum and the strength of the EGMF, employing also fields found earlier in a constrained simulation of structure formation including magnetohydrodynamics processes. We find that the observation of halos around blazars like Mrk 180 probes an interesting range of EGMF strengths and acceleration models: in particular, blazar halos test if the photon energy spectrum at the source extends beyond ~100 TeV and how anisotropic this high-energy component is emitted.

Magnetohydrodynamic processes in the electromagnetic mixing of liquid phase in the ingot on continuous bar- and bloom-casting machines

The influence of electromagnetic mixing of liquid metal in the continuous-casting machine on the quality of continuous-cast bar and bloom depends on the mixing rate. In world practice, the mixing rate is characterized by the maximum linear velocity of the liquid metal in the flow. Physical and mathematical simulation reveals the relations between all the relevant parameters of the mixing stator that affect the mixing rate.

EFFICIENCY OF MAGNETIC TO KINETIC ENERGY CONVERSION IN A MONOPOLE MAGNETOSPHERE

Unconfined relativistic outflows from rotating, magnetized compact objects are often well modeled by assuming that the field geometry is approximately a split-monopole at large radii. Earlier work has indicated that such an unconfined flow has an inefficient conversion of magnetic energy to kinetic energy. This has led to the conclusion that ideal magnetohydrodynamical (MHD) processes fail to explain observations of, e.g., the Crab pulsar wind at large radii where energy conversion appears efficient. In addition, as a model for astrophysical jets, the monopole field geometry has been abandoned in favor of externally confined jets since the latter appeared to be generically more efficient jet accelerators. We perform time-dependent axisymmetric relativistic MHD simulations in order to find steady-state solutions for a wind from a compact object endowed with a monopole field geometry. Our simulations follow the outflow for 10 orders of magnitude in distance from the compact object, which is large enough to study both the initial 'acceleration zone' of the magnetized wind as well as the asymptotic 'coasting zone'. We obtain the surprising result that acceleration is actually efficient in the polar region, which develops a jet despite not being confined by an external medium. Our models contain jets thatmore » have sufficient energy to account for moderately energetic long and short gamma-ray burst (GRB) events ({approx}10{sup 51}-10{sup 52} erg), collimate into narrow opening angles (opening half-angle {theta} {sub j} {approx} 0.03 rad), become matter-dominated at large radii (electromagnetic energy flux per unit matter energy flux {sigma} < 1), and move at ultrarelativistic Lorentz factors ({gamma} {sub j} {approx} 200 for our fiducial model). The simulated jets have {gamma} {sub j}{theta} {sub j} {approx} 5-15, so they are in principle capable of generating 'achromatic jet breaks' in GRB afterglow light curves. By defining a 'causality surface' beyond which the jet cannot communicate with a generalized 'magnetic nozzle' near the axis of rotation, we obtain approximate analytical solutions for the Lorentz factor that fit the numerical solutions well. This allows us to extend our results to monopole wind models with arbitrary magnetization. Overall, our results demonstrate that the production of ultrarelativistic jets is a more robust process than previously thought.« less

Mathematical modeling of aluminum electrolysis over a long interval of time

A new approach is proposed to the modeling of magnetohydrodynamic and chemical processes involved in aluminum electrolysis. In this approach a medium under study is presented as a mixture of reagents in unknown concentrations, making it possible to describe mixing and the chemical interaction of the electrolyte and the metal melt. Therefore, all stages of the electrolysis process can be modeled over a long interval of time.

A mechanism for generating magnetic field by vortex processes in stellar plasma

The difference in viscous properties of proton and electron gases in fully ionized hydrogen plasma is shown to result in a possibility of generation in this plasma of the magnetic field, even if initially there was no field. As an example, a simplified planar stationary model of a magnetohydrodynamic process of the eddy sink type is considered. It is demonstrated that the intensity of the generated magnetic field strongly depends on plasma temperature, so that the range T = 100000–500000 K corresponds to the range of maximum magnetic induction B = 50–2700 G. Such values are frequently observed in ordinary stars, in particular, in solar flares.

Dynamics of a turbulent spin-down flow inside a torus

A turbulent screw flow (with Reynolds number exceeding 106) is generated by the sudden stop of a toroidal channel. The working fluid is a gallium alloy and velocity measurements are performed using a dual-axis potential probe. We describe the onset of motion, the development of strongly anisotropic fluctuations, and the final homogenization and decay of turbulence. Our observations are relevant for the relaxation of anisotropic turbulence; they are also in agreement with measurements of magnetohydrodynamic induction processes in this type of flow.

Stellar Convection and Magnetism across the H-R diagram: Theory and Models

Stars constitute undoublty one of the elementary blocks of the Universe and play as such a central role in determining for instance its chemical evolution. They can be seen as modern physics laboratory from which fundamental processes as diverse as atomic physics or turbulence can be studied and understood. Being able to model accurately their structure, dynamic and evolution is thus of fundamental importance and is the subject of intense research. In this short lecture we will first discuss the basic equations and processes, such as convection, turbulence, rotation, instabilities and dynamo action that are at the origin of the magnetic field observed in stars. We will then present some of the numerical simulations in three dimensions performed in recent years to model such complex objects and their nonlinear behavior, focussing mainly on results obtained with the anelastic spherical harmonic (ASH) code. Using the Sun as a reference star, we wish to gain insight the various magnetohydrodynamical processes that shape its large scale dynamics and magnetism, such as the Reynolds and Maxwell stresses, and the ω and α-effects. We will then extend our study to other stars, such as young Suns, massive stars or evolved RGB stars in order to identify which processes are at the origin of their significantly different dynamics.

Fluid Mechanics in Disks Around Young Stars

This article reviews hydrodynamic and magnetohydrodynamic processes in disks around young stars, encompassing the epochs of molecular-cloud turbulence, dense core collapse, disk formation, disk evolution, and planetesimal formation.

A Comparison of Spectral Element and Finite Difference Methods Using Statically Refined Nonconforming Grids for the MHD Island Coalescence Instability Problem

A recently developed spectral-element adaptive refinement incompressible magnetohydrodynamic (MHD) code [Rosenberg, Fournier, Fischer, Pouquet, J. Comp. Phys. 215, 59-80 (2006)] is applied to simulate the problem of MHD island coalescence instability (MICI) in two dimensions. MICI is a fundamental MHD process that can produce sharp current layers and subsequent reconnection and heating in a high-Lundquist number plasma such as the solar corona [Ng and Bhattacharjee, Phys. Plasmas, 5, 4028 (1998)]. Due to the formation of thin current layers, it is highly desirable to use adaptively or statically refined grids to resolve them, and to maintain accuracy at the same time. The output of the spectral-element static adaptive refinement simulations are compared with simulations using a finite difference method on the same refinement grids, and both methods are compared to pseudo-spectral simulations with uniform grids as baselines. It is shown that with the statically refined grids roughly scaling linearly with effective resolution, spectral element runs can maintain accuracy significantly higher than that of the finite difference runs, in some cases achieving close to full spectral accuracy.

Proposed national large solar telescope

Sun’s atmosphere is an ideal place to study and test many magnetohydrodynamic (MHD) processes controlling turbulent plasma. We wish to resolve some of the finest solar features (which remain unresolved presently) and study their dynamics. Indian Institute of Astrophysics has proposed to design, fabricate and install a 2-meter class solar telescope at a suitable site in India to resolve features on the Sun of the size of about 0.1 arcsec. The focal plane instruments will include a high resolution polarimeteric package to measure polarization with an accuracy of 0.01 per cent; a high spectral resolution spectrograph to obtain spectra in 5 widely separated absorption lines simultaneously and high spatial resolution narrow band imagers in various lines. The Himalayan region appears to be a good choice keeping in view the prevailing dry and clear weather conditions. We have started detailed analysis of the weather conditions in the area and at some other locations in India. The site characterization will be done using the Sun-photometer, S-DIMM and SHABAR techniques to determine the seeing conditions.

A NEW VIEW OF THE SUN FROM THE HINODE SPACE MISSION

The Japanese/US/UK space mission, Hinode, was launched successfully in September 2006. Now, more than a year after the commissioning of the spacecraft and instruments, Hinode is unveiling a new view of the Sun. Hinode's goal is to help us to understand solar activity, and to link activity on (and below) the surface to the outer corona. This review will describe how the initial results are overturning our understanding of the Sun and will look to the future to anticipate what further discoveries might be made. In particular I will describe the areas of basic magnetohydrodynamic (MHD) processes such as magnetic reconnection and Alfvén waves, the formation of both the fast and slow solar winds and the triggering of flares and coronal mass ejections.

Energy Extraction from a Rotating Black Hole by Magnetic Reconnection in the Ergosphere

We investigate mechanisms of energy extraction from a rotating black hole in terms of negative energy at infinity. In addition to the Penrose process through particle fission, the Blandford-Znajek mechanism by magnetic tension, and the magnetohydrodynamic Penrose process, we examine energy extraction from a black hole caused by magnetic reconnection in the ergosphere. The reconnection redistributes the angular momentum efficiently to yield the negative energy at infinity. We derive a condition for the process to operate in a simple situation, where the plasma is incompressible and the magnetic energy is converted completely to the plasma kinetic energy locally. Astrophysical situations of magnetic reconnection around the black holes are also discussed.

On the linear stability of weakly ionized, magnetized planar shear flows

We investigate the effects of ambipolar diffusion and the Hall effect on the stability of weakly-ionized, magnetized planar shear flows. Employing a local approach similar to the shearing-sheet approximation, we solve for the evolution of linear perturbations in both streamwise-symmetric and non-streamwise-symmetric geometries using WKB techniques and/or numerical methods. We find that instability arises from the combination of shear and non-ideal magnetohydrodynamic processes, and is a result of the ability of these processes to influence the free energy path between the perturbations and the shear. They turn what would be simple linear-in-time growth due to current and vortex stretching from shear into exponentially-growing instabilities. Our results aid in understanding previous work on the behaviour of weakly-ionized accretion discs. In particular, the recent finding that the Hall effect and ambipolar diffusion destabilize both positive and negative angular velocity gradients acquires a natural explanation in the more general context of this paper. We construct a simple toy model for these instabilities based upon transformation operators (shears, rotations, and projections) that captures both their qualitative and, in certain cases, exact quantitative behaviour.

On the Penetration of Meridional Circulation below the Solar Convection Zone

Meridional flows with velocities of a few meters per second are observed in the uppermost regions of the solar convection zone. The amplitude and pattern of the flows deeper in the solar interior, in particular near the top of the radiative region, are of crucial importance to a wide range of solar magnetohydrodynamical processes. In this paper, we provide a systematic study of the penetration of large-scale meridional flows from the convection zone into the radiative zone. In particular, we study the effects of the assumed boundary conditions applied at the radiative-convective interface on the deeper flows. Using simplified analytical models in conjunction with more complete numerical methods, we show that penetration of the convectively driven meridional flows into the deeper interior is not necessarily limited to a shallow Ekman depth but can penetrate much deeper.

Global models of the magnetic Sun

AbstractWe briefly present recent simulations of the internal magnetism of the Sun with the 3-D ASH code and with the 2-D STELEM code. The intense magnetism of the Sun is linked to local and global dynamo action within our star. We focus our study on how magnetohydrodynamical processes in stable (radiative) or unstable (convective) zones, nonlinearly interact to establish the solar differential rotation, meridional circulation, confine the tachocline, amplify and organise magnetic fields and how magnetic flux emerge to the surface. We also test the robustness of flux transport dynamo models to various profiles of circulation.

Magnetically Driven Explosions of Rapidly Rotating White Dwarfs Following Accretion‐Induced Collapse

We present 2D multi-group flux-limited diffusion magnetohydrodynamics (MHD) simulations of the Accretion-Induced Collapse (AIC) of a rapidly-rotating white dwarf. We focus on the dynamical role of MHD processes after the formation of a millisecond-period protoneutron star. We find that including magnetic fields and stresses can lead to a powerful explosion with an energy of a few Bethe, rather than a weak one of at most 0.1 Bethe, with an associated ejecta mass of ~0.1Msun, instead of a few 0.001Msun. The core is spun down by ~30% within 500ms after bounce, and the rotational energy extracted from the core is channeled into magnetic energy that generates a strong magnetically-driven wind, rather than a weak neutrino-driven wind. Baryon loading of the ejecta, while this wind prevails, precludes it from becoming relativistic. This suggests that a GRB is not expected to emerge from such AICs during the early protoneutron star phase, except in the unlikely event that the massive white dwarf has sufficient mass to lead to black hole formation. In addition, we predict both negligible 56Ni-production (that should result in an optically-dark, adiabatically-cooled explosion) and the ejection of 0.1Msun of material with an electron fraction of 0.1-0.2. Such pollution by neutron-rich nuclei puts strong constraints on the possible rate of such AICs. Moreover, being free from ``fallback,'' such highly-magnetized millisecond-period protoneutron stars may later become magnetars, and the magnetically-driven winds may later transition to Poynting-flux-dominated, relativistic winds, eventually detectable as GRBs at cosmological distances. However, the low expected event rate of AICs will constrain them to be, at best, a small subset of GRB and/or magnetar progenitors.

A new generation of convection-driven spherical dynamos using EBE finite element method

Intrinsic planetary magnetic fields are believed to be generated by convection-driven magnetohydrodynamic processes taking place in their fluid cores that are rotating and have spherical geometry. We present a new generation of multi-layered spherical dynamo models driven by thermal convection and based on an EBE (element-by-element) finite element method taking the full advantage of modern massively parallel computers. It is demonstrated that the fully nonlinear spherical dynamo can be effectively parallelized with nearly linear scalability. The results of the new EBE finite element dynamo are then compared with the well-known benchmark dynamo based on spectral methods, showing a satisfactory agreement between two fundamentally different models.