- Shocks and discontinuities (magnetohydrodynamics)
Shocks and discontinuities are transition layers where the plasma properties change from one equilibrium state to another. The relation between the plasma properties on both sides of a shock or a discontinuity can be obtained from the conservative form of the
magnetohydrodynamic(MHD) equations, assuming conservation of mass, momentum, energy and of .
Rankine-Hugoniot jump conditions for MHD
The jump conditions across an MHD shock or discontinuity are referred as the
Rankine-Hugoniot equations for MHD. In the frame moving with the shock/discontinuity, those jump conditions can be written:
where , v, p, B are the plasma
density, velocity, (thermal) pressureand magnetic fieldrespectively. The subscripts t and n refer to the tangential and normal componentsof a vector (with respect to the shock/discontinuity front). The subscripts 1 and 2 refer to the two states of the plasma on each side of the shock/discontinuity
Contact and tangential discontinuities
Contact and tangential discontinuities are transition layers across which there is no particle transport. Thus, in the frame moving with the discontinuity, .
Contact discontinuities are discontinuities for which the thermal pressure, the magnetic field and the velocity are continuous. Only the mass density and temperature change.
Tangential discontinuities are discontinuities for which the total pressure (sum of the thermal and
magnetic pressures) is conserved. The normal component of the magnetic field is identically zero. The density, thermal pressure and tangential component of the magnetic field vector can be discontinuous across the layer.
Shocks are transition layers across which there is a transport of particles. There are three types of shocks in MHD: slow-mode, intermediate and fast-mode shocks.
Intermediate shocks are non-compressive (meaning that the plasma density does not change across the shock). They are also
isentropic. They are sometimes referred as rotational discontinuities. All thermodynamicquantities are continuous across the shock, but the tangential component of the magnetic field can "rotate".
Slow-mode and fast-mode shocks are compressive and are associated with an increase in
entropy. Across slow-mode shock, the tangential component of the magnetic field decreases. Across fast-mode shock it increases.
The type of shocks depend on the relative magnitude of the upstream velocity in the frame moving with the shock with respect to some characteristic speed. Those characteristic speeds, the slow and fast magnetosonic speeds, are related to the Alfvén speed, and the sonic speed, as follows:::where is the Alfvén speed and is the angle between the incoming
magnetic fieldand the shock normalvector.
The normal component of the slow shock propagates with velocity in the frame moving with the upstream plasma, that of the intermediate shock with velocity and that of the fast shock with velocity . The fast mode waves have higher
phasevelocities than the slow mode waves because the densityand magnetic fieldare in phase, whereas the slow mode wave components are out of phase.
Example of shocks and discontinuities in space
bow shock, which is the boundary where the solar wind's speed drops due to the presence of Earth's magnetosphereis a fast mode shock. The termination shockis a fast-mode shock due to the interaction of the solar wind with the interstellar medium.
Magnetic reconnectioncan happen associated with a slow-mode shock (Petschek or fast magnetic reconnection) in the solar corona. [H. E. Petschek, "Magnetic Field Annihilation" in The Physics of Solar Flares, Proceedings of the AAS-NASA Symposium held 28-30 October, 1963 at the Goddard Space Flight Center, Greenbelt, MD. Edited by Wilmot N. Hess. Washington, DC: National Aeronautics and Space Administration, Science and Technical Information Division, 1964., p.425]
* The existence of intermediate shocks is still a matter of debate. They may form in MHD simulation, but their stability has not been proven.
* Discontinuities (both contact and tangential) are observed in the solar wind, behind astrophysical shock waves (
supernova remnant) or due to the interaction of multiple CME driven shock waves.
* The Earth's
magnetopauseis generally a tangential discontinuity. [ [http://www.oma.be/BIRA-IASB/Scientific/Topics/SpacePhysics/Magnetopause.html Magnetopause] Belgian Institute for Space Aeronomy ]
*E. Priest, "Solar magneto-hydrodynamics"(chapter 5), Dordrecht, 1987.
*T. Gombosi "Physics of the Space Environment" (chapter 6), Cambridge University Press, 1998.
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