More than 99% of the known matter in the universe is ionized into the plasma state. This cosmical plasma is generally permeated by magnetic fields. Hence, magnetic fields, and the concepts of magnetic field lines and magnetic field line motion, are of fundamental interest in the physics of space plasmas, including those that constitute the near environment of the Earth and other planets.
W. Belcher discussed the concept of magnetic field line motion as a useful tool in teaching electromagnetic theory in an article in Eos [Belcher, 2005]. However, there are some important limitations and pitfalls to this concept, none of which is mentioned in the article by Belcher.