Magnetohydrodynamics

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 fluid, and -dynamics meaning movement. The field of MHD was initiated by Hannes Alfvén, for which he received the Nobel Prize in 1970.

Applications

Geophysics

The fluid mantle 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.

Related Topics:
Earth - Theorized - Dynamo - Earth's magnetic field - Field lines

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Astrophysics

MHD applies quite well to astrophysics since over 99% of the matter content of the Universe is made up of plasma, including stars, interplanetary medium (the space between the planets), interstellar medium (space between the stars), nebulae and jets; however MHD is lacking when electric currents flows through these plasmas and produces filaments, double layers and plasma instabilities.

Related Topics:
Stars - Interplanetary medium - Interstellar medium - Nebulae - Jets - Double layer

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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.

Related Topics:
Sunspots - Sun's - Joseph Larmor - 1919 - Solar wind - Solar rotation - Parker spiral

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Previously, theories describing the creation of the Sun and planets could not explain how the Sun has 99% of the mass, yet only 1% 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.

Related Topics:
Sun - Planet - Angular momentum - Solar system - Closed system - Conserved

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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.

Related Topics:
Magnetic reconnection - Solar flares - Active region - X-rays - Radiation

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Engineering

MHD is related to engineering problems such as plasma confinement, liquid-metal cooling of nuclear reactors, and electromagnetic casting (among others).

Related Topics:
Engineering - Plasma confinement - Nuclear reactors - Electromagnetic

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In early 1990s, Mitsubishi built a boat, the 'Yamoto', which uses a magnetohydrodynamic drive, is driven by a liquid helium-cooled superconductor, and can travel at 15 km/h.

Related Topics:
Mitsubishi - Yamoto - Magnetohydrodynamic drive - Helium - Superconductor - Km/h

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