'Magnetohydrodynamics (MHD)' (''magnetofluiddynamics'' or ''hydromagnetics'') is the
academic discipline which studies the
dynamics of
electrically conducting fluids. Examples of such fluids include
plasmas, liquid metals, and
salt water. The word ''magnetohydrodynamics (MHD)'' is derived from ''magneto-'' meaning
magnetic field, and ''hydro-'' meaning
liquid, and -
dynamics meaning movement. The field of MHD was initiated by
Hannes Alfvén[1], for which he received the
Nobel Prize in Physics in
1970.
The idea of MHD is that magnetic fields can
induce currents in a moving conductive fluid, which create forces on the fluid, and also change the magnetic field itself. The set of equations which describe MHD are a combination of the
Navier-Stokes equations of
fluid dynamics and
Maxwell's equations of
electromagnetism. These
differential equations have to be solved
simultaneously, either analytically or
numerically. Because MHD is a ''fluid'' theory, it cannot treat
kinetic phenomena, i.e., those in which the existence of discrete particles, or of a non-thermal distribution of their velocities, is important.
Ideal and resistive MHD
MHD Simulation of the Solar Wind
The simplest form of MHD, Ideal MHD, assumes that the fluid has so little
resistivity that it can be treated as a
perfect conductor. (This is the limit of infinite
magnetic Reynolds number.) In ideal MHD,
Lenz's law dictates that the fluid is in a sense ''tied'' to the magnetic field lines. To be more precise, in ideal MHD, a small rope-like volume of fluid surrounding a field line will continue to lie along a magnetic field line,
even as it is twisted and distorted by fluid flows in the system.
The connection between magnetic field lines and fluid in ideal MHD fixes the
topology of the magnetic field in the fluid -- for example, if a set of magnetic field lines are tied into a knot, then they will remain so as long as the fluid/plasma has negligible resistivity. This difficulty in reconnecting magnetic field lines makes it possible to store energy by moving the fluid or the source of the magnetic field. The energy can then become available if the conditions for ideal MHD break down, allowing
magnetic reconnection that releases the stored energy from the magnetic field.
Ideal MHD equations
The ideal MHD equations consist of the
continuity equation (mass), the
momentum equation,
Ampere's Law in the limit of no electric field and no electron diffusivity, and a
temperature evolution equation. As with any fluid description to a kinetic system, a closure approximation must be applied to highest moment of the particle distribution equation. This is often accomplished with approximations to the heat flux through a condition of adiabaticity or isothermality.
Applicability of ideal MHD to plasmas
Ideal MHD is only strictly applicable when:
# The plasma is strongly collisional, so that the time scale of collisions is shorter than the other characteristic times in the system, and the particle distributions are therefore close to Maxwellian.
# The resistivity due to these collisions is small. In particular, the typical magnetic diffusion times over any scale length present in the system must be longer than any time scale of interest.
# We are interested in length scales much longer than the ion skin depth and Larmor radius perpendicular to the field, long enough along the field to ignore Landau damping, and time scales much longer than the ion gyration time (system is smooth and slowly evolving).
The importance of resistivity
In an imperfectly conducting fluid, the magnetic field can generally move through the fluid, following a
diffusion law with the resistivity of the plasma serving as a
diffusion constant. This means that solutions to the ideal MHD equations are only applicable for a limited time for a region of a given size before diffusion becomes too important to ignore. One can estimate the diffusion time across a Solar active region (from collisional resistivity) to be hundreds to thousands of years, much longer than the actual lifetime of a sunspot -- so it would seem reasonable to ignore the resistivity. By contrast, a meter-sized volume of seawater has a magnetic diffusion time measured in milliseconds.
Even in physical systems which are large and conductive enough that simple estimates suggest that we can ignore the resistivity,
resistivity may still be important: many
instabilities exist that can increase the 'effective' resistivity of the plasma by factors of more than a billion. The enhanced resistivity is usually the result of the formation of small scale structure like current sheets or fine scale magnetic turbulence, introducing small spatial scales into the system over which ideal MHD is broken and magnetic diffusion can occur quickly. When this happens,
Magnetic Reconnection may occur in the plasma to release stored magnetic
energy as
waves, bulk mechanical
acceleration of material,
particle acceleration, and
heat.
Magnetic reconnection in highly conductive systems is important because it concentrates energy in time and space, so that gentle forces applied to a plasma for long periods of time can cause violent explosions and bursts of radiation.
When the fluid cannot be considered as completely conductive, but the other conditions for ideal MHD are satisfied, it is possible to use an extended model called resistive MHD. This includes an extra term in
Ampere's Law which models the collisional resistivity. Generally MHD computer simulations are at least somewhat resistive because their computational grid introduces a numerical resistivity.
The importance of kinetic effects
Another limitation of MHD (and fluid theories in general) is that they depend on the assumption that the plasma is strongly collisional (this is the first criterion listed above), so that the time scale of collisions is shorter than the other characteristic times in the system, and the particle distributions are
Maxwellian. This is usually not the case in fusion, space and astrophysical plasmas. When this is not the case, or we are interested in smaller spatial scales, it may be necessary to use a kinetic model which properly accounts for the non-Maxwellian shape of the distribution function. However, because MHD is very simple, and captures many of the important properties of plasma dynamics, it is often qualitatively accurate, and is almost invariably the first model tried.
Effects which are essentially kinetic and not captured by fluid models include
double layers, a wide range of instabilities, chemical separation in space plasmas and electron runaway.
Structures in MHD systems
Schematic view of the different current systems which shape the Earth's magnetosphere
In many MHD systems, most of the electric current is compressed into thin, nearly-two-dimensional ribbons termed
current sheets. These can divide the fluid into magnetic domains, inside of which the currents are relatively weak. Current sheets in
the solar corona are thought to be between a few meters and a few kilometers in thickness, which is quite thin compared to the magnetic domains (which are thousands to hundreds of thousands of kilometers across). Another example is in the earth's
magnetosphere, where current sheets separate topologically distinct domains, isolating most of the earth's
ionosphere from the
solar wind.
Extensions to magnetohydrodynamics
Resistive MHD
Resistive MHD describes magnetized fluids with non-zero electron diffusivity. This diffusivity leads to a breaking in the magnetic topology.
Extended MHD
Extended MHD describes a class of phenomena in plasmas that are higher order than resistive MHD, but which can adequately be treated with a single fluid description. These include the effects of Hall physics, electron pressure gradients, finite Larmor Radii in the particle gyromotion, and electron inertia.
Two-Fluid MHD
Two-Fluid MHD describes plasmas that include a non-negligible electric field. As a result, the electron and ion momenta must be treated separately. This description is more closely tied to Maxwell's equations as an evolution equation for the electric field exists.
Hall MHD
In 1960, M. J. Lighthill criticized the applicability of ideal or resistive MHD theory for plasmas
[2]. It concerned the neglect of the "Hall current term", a frequent simplification made in magnetic fusion theory. Hall-magnetohydrodynamics (HMHD) takes into account this electric field description of magnetohydrodynamics. The most important difference is that in the absence of field line breaking, the magnetic field is tied to the electrons and not to the bulk fluid.
[3]
Applications
Geophysics
The fluid core of the
Earth and other planets is
theorized to be a huge MHD
dynamo that generates the
Earth's magnetic field due to the motion of the molten rock. Such dynamos work by stretching magnetic
field lines that thread through turbulent or sheared flows in a conductive fluid: the total length of magnetic field line in a particular volume determines the strength of the magnetic field, so stretching the field lines increases the magnetic field.
Astrophysics
MHD applies quite well to astrophysics since over 99% of
baryonic matter content of the Universe is made up of plasma, including
stars, the
interplanetary medium (space between the planets), the
interstellar medium (space between the stars),
nebulae and
jets. Many astrophysical systems are not in local thermal equilibrium, and therefore require an additional kinematic treatment to describe all the phenomena within the system (see
Astrophysical plasma).
Sunspots are caused by the
Sun's magnetic fields, as
Joseph Larmor theorized in
1919. The
solar wind is also governed by MHD. The differential
solar rotation may be the long term effect of magnetic drag at the poles of the Sun, an MHD phenomenon due to the
Parker spiral shape assumed by the extended magnetic field of the Sun.
Previously, theories describing the formation of the
Sun and
planets could not explain how the Sun has 99.87% of the mass, yet only 0.54% of the
angular momentum in the
solar system. In a
closed system such as the cloud of gas and dust from which the Sun was formed, mass and
angular momentum are both
conserved. That conservation would imply that as the mass concentrated in the center of the cloud to form the Sun, it would spin up, much like a skater pulling their arms in. The high speed of rotation predicted by early theories would have flung the proto-Sun apart before it could have formed. However, magnetohydrodynamic effects transfer the Sun's angular momentum into the outer solar system, slowing its rotation.
Breakdown of ideal MHD (in the form of
magnetic reconnection) is known to be the cause of
solar flares, the largest explosions in the solar system. The magnetic field in a solar
active region over a sunspot can become quite stressed over time, storing energy that is released suddenly as a burst of motion,
X-rays, and
radiation when the main current sheet collapses, reconnecting the field.
Engineering
MHD is related to
engineering problems such as
plasma confinement, liquid-metal cooling of
nuclear reactors, and
electromagnetic casting (among others).
In early 1990s,
Mitsubishi built a boat, the '
Yamato,' which uses a
magnetohydrodynamic drive, is driven by a liquid
helium-cooled
superconductor, and can travel at 15
km/h.
MHD power generation fueled by potassium-seeded coal combustion gas showed potential for more efficient energy conversion (the absence of solid moving parts allows operation at higher temperatures), but failed due to cost prohibitive technical difficulties.
[4]
Trivia
The ebbing salty water flowing past London's
Waterloo Bridge interacts with the
Earth's magnetic field to produce a potential difference between the two river-banks.
Michael Faraday tried this experiment in 1832 but the current was too small to measure with the equipment at the time,
[5] and the river bed contributed to short-circuiting the signal. However, by the same process, Dr.
William Hyde Wollaston was able to measure the voltage induced by the tide in the English Channel in 1851.
[6]
History
The first recorded use of the word ''magnetohydrodynamics'' is by
Hannes Alfvén in 1942:
:"At last some remarks are made about the transfer of momentum from the Sun to the planets, which is fundamental to the theory (§11). The importance of the magnetohydrodynamic waves in this respect is pointed out."
[7]
Notes
1. Alfven, H., "Existence of electromagnetic-hydrodynamic waves" (1942) ''Nature'', Vol. 150, pp. 405
2. M. J. Lighthill, "Studies on MHD waves and other anisotropic wave motion," ''Phil. Trans. Roy. Soc.'', London, vol. 252A, pp. 397-430, 1960.
3. E.A. Witalis, "Hall Magnetohydrodynamics and Its Applications to Laboratory and Cosmic Plasma", ''IEEE Transactions on Plasma Science'' (ISSN 0093-3813), vol. PS-14, Dec. 1986, p. 842-848.
4. http://navier.stanford.edu/PIG/C4_S9.pdf
5. Dynamos in Nature by David P. Stern
6. McKetta J McKetta, "Encyclopedia of Chemical Processing and Design: Volume 66" (1999)
7. Alfvén, H., "On the cosmogony of the solar system III", ''Stockholms Observatoriums Annaler'', vol. 14, pp.9.1-9.29
See also
★
Electrohydrodynamics
★
Plasma stability
★
Shocks and Discontinuities (MHD)
★
Computational Magnetohydrodynamics
★
Magnetic reconnection
★
Alfvén wave
★
Magnetosonic wave
★
Ferrofluid
★
MHD generator
★
MHD sensor
★
Magnetohydrodynamic drive
★
Magnetic flow meter
★
Magnetic Reynolds number
★
Nuclear reactor liquid-
metal cooling
MHD in fiction:
★
The Hunt for Red October/
Caterpillar drive
★
Clive Cussler's novel
Valhalla Rising involved two ships, one surface and one submarine, that used MHD drives. Also, his novel with
Brul,
Dark Watch which is part of the new
Oregon Files
★
Dale Brown's novel ''
Silver Tower'' (
1988) depicted Space Station Armstrong maintaining a small nuclear MHD reactor to power the Skybolt laser.
References
★ P. A. Davidson, "''An Introduction to Magnetohydrodynamics''", May 2001 452 p ISBN 0-521-79487-0
★ Jordan, R.,"''
A statistical equilibrium model of coherent structures in magnetohydrodynamics''". Nonlinearity 8 (July 1995) 585-613.
★ Hurricane, O. A., B. H. Fong, and S. C. Cowley, "''
Nonlinear magnetohydrodynamic detonation: Part I''". Physics of Plasmas Vol 4(10) pp. 3565-3580. October 1997.
★ Tabar, M. R. Rahimi, and S. Rouhani, "''
Turbulent Two Dimensional Magnetohydrodynamics and Conformal Field Theory''". Department of Physics, Sharif University of Technology. Institute for Studies in Theoretical Physics and Mathematics. Tehran, Iran. arXiv:hep-th/9503005 v1 1 Mar 1995.
★ Pai, Shih-I. "''Magnetogasdynamics and Plasma Dynamics''". Vienna: Springer-Verlag, 1962. 197 p. ISBN 0-387-80608-3
★ Biskamp, Dieter. "''Nonlinear Magnetohydrodynamics''". Cambridge, England: Cambridge University Press, 1993. 378 p. ISBN 0-521-59918-0
★ Ferraro, Vincenzo Consolato Antonio and Plumpton, Charles. "''An Introduction to Magneto-Fluid Mechanics''", 2nd ed.
★ Roberts, P.H. "''Introduction to Magnetohydrodynamics''". London: Longmans Green, 1967.
★ Kulikovskiy, A.G. & Lyubimov, G.A. "''1965 Magnetohydrodynamics''". Addison&Wesley, Massachusetts.
★ Sutton, G. W., and A. Sherman, "''Engineering Magnetohydrodynamics''", McGraw-Hill Book Company, New York, 1965.
★ "''Magnetohydrodynamic Generators with Nonequilibrium Ionization''", AIAA Journal, Vol. 3, April, 1965, p 591.
★ Hughes, W., and F. Young, "''The Electromagnetodynamics of Fluids''", New York, John Wiley & Sons Inc. 1966.
★ Dr. James B. Calvert, ''
"Magnetohydrodynamics"'',
2002-10-20
★ David P. Stern, NASA, ''
"The Sun's Magnetic Cycle"''
★ Jean-Pierre Petit, CNRS (France), ''
"Magnetohydrodynamics (MHD)"''
★ Bansal, J.L. "''Magneto Fluid Dynamics of Viscous Fluids''". (1994) Jaipur Publishing House, Jaipur, India.
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