Alfvén wave

An Alfvén wave is a transverse magnetohydrodynamic wave in which the plasma velocity and magnetic field perturbations oscillate perpendicular to the direction of propagation and to each other. First predicted by Hannes Alfvén in 1942, these waves propagate at the Alfvén speed and represent the fundamental oscillatory mode of a magnetized plasma. Unlike sound waves, Alfvén waves are incompressible in the linear regime and do not steepen into shocks, making them the primary mechanism for transporting energy and angular momentum across astrophysical plasmas without thermal dissipation. In nonlinear regimes, Alfvén waves can couple to compressive modes and drive magnetohydrodynamic turbulence, a process central to the heating of the solar corona and the accretion dynamics of accretion disks.

The Alfvén wave is not merely a perturbation; it is the propagating edge of magnetic topology, carrying the causal structure of the field through the plasma that cannot exist without it.