  Several phenomena connected with the magnetic field of the Sun (the cool
sunspots, the hot corona, solar flares, the solar wind) are collectively known
as solar activity. This paper discusses how one uses the MHD equations to
understand how the magnetic field of the Sun is produced by the dynamo process
and then gives rise to these diverse activities, making the Sun the best
laboratory for plasma physics in the limit of high magnetic Reynolds number
(defined at the end of the Introduction).
