The 1908 discovery of magnetic fields in sunspots raised the question of how such magnetic fields arise in astronomical bodies. The realization that the material in the Sun is in the plasma state made Larmor (1919) speculate that motions in the plasma may generate or sustain such magnetic fields. The first steps towards magnetohydrodynamics (MHD), which combines electrodynamics and hydrodynamics, were taken by Cowling (1933) and Ferraro (1937). These first results raised serious doubts whether MHD can explain the origin of the magnetic fields of the earth and of sunspots. The MHD equations first appeared in their full form in the works of Alfvén (1942a, 1943c), who used these equations to postulate what he called ‘electromagnetic-hydrodynamic waves’ (now known as Alfvén waves), which were discovered in the laboratory only about a decade later. Alfvén’s work was initially met with skepticism (and his theory of sunspots was rejected), and MHD was taken up very slowly at first. With the astrophysical interest generated by the discovery of magnetic fields in other astronomical systems from around 1950, the methods of MHD started spreading rapidly, leading to first solutions of the so-called ‘dynamo problem’—to explain how magnetic fields arise in astrophysical systems. The concept of ‘flux freezing’, first put forth by Alfvén (1943c), led to a new viewpoint that magnetic fields can be considered more fundamental than electric currents, which made MHD a more coherent and accessible theory. Curiously, Alfvén himself opposed this viewpoint and, although he was given the 1970 Nobel Prize “for fundamental work and discoveries in magnetohydrodynamics”, he became a harsh critic of MHD in later life.
