magnetohydrodynamiska

Magnetohydrodynamics (MHD) is the study of the dynamics of electrically conducting fluids—such as plasmas, liquid metals, and saltwater—in the presence of magnetic and electric fields. It describes how fluid motion and magnetic fields influence each other, combining fluid dynamics with Maxwell's equations. In Swedish terminology, the field is magnetohydrodynamik, and the adjectival form magnetohydrodynamiska is used to describe related phenomena.

Its governing equations include the continuity equation, the momentum equation with the Lorentz force, the induction equation for the magnetic field, and an appropriate energy equation. In the ideal MHD approximation, finite electrical resistivity is neglected and the magnetic field is frozen into the fluid; in resistive MHD, diffusion and magnetic reconnection are allowed. The magnetic Reynolds number, ratio of advective to diffusive transport of magnetic field, is a key parameter.

Important phenomena include Alfven waves, which propagate along field lines, magnetic pressure and tension shaping flows, and magnetic reconnection, which rapidly converts magnetic energy into kinetic and thermal energy. MHD models are applied across astrophysics (solar and stellar plasmas, accretion disks), geophysics (the Earth's core and geomagnetic field), and laboratory plasma research (tokamaks and liquid-metal experiments). Numerical simulations are widely used to study nonlinear MHD behavior and turbulence.

History and status: magnetohydrodynamics emerged from work by Hannes Alfven in the mid-20th century; he shared the Nobel Prize in Physics in 1970 for contributions to MHD and space plasma physics.