A transverse wave that occurs in a region containing a magnetic field and a plasma. The ionized and therefore highly conducting material of the plasma is said to be ‘frozen in’ to the magnetic field and is forced to take part in its wave motion. At least part of the energy heating the Sun’s corona is thought to be provided by Alfvén waves propagating from the outer layers of the Sun. The waves are named after H. O. G. Alfvén. See also Magnetohydrodynamics.