The study of the interaction between a magnetic field and Fourth state of matter: a gas in which many or most of the atoms are ionized. In the plasma state the atoms have split into positive ions and negative electrons, which can flow freely, so the gas becomes electrically conducting and a current can flow. treated as a continuous medium. Most of the That which contains and subsumes all the laws of nature, and everything subject to those laws; the sum of all that exists physically, including matter, energy, physical laws, space, and time. Also, a cosmological model of the universe. contains not normal gas, but instead plasma. Many dynamical astronomical processes are caused by the subtle nonlinear relationship between a magnetic field and plasma. There is virtually no interaction with a magnetic field in a neutral gas, but in a plasma extremely close coupling with the magnetic field means that whatever the plasma is doing intimately affects the magnetic field and vice versa. Magnetohydrodynamics builds on the tools of both fluid dynamics and electromagnetism, but it possesses many new features that are present in neither. It does consider individual particles but, instead, treats plasma as a continuous medium. The assumption of a continuous medium is valid for length-scales much larger than the mean-free path for particle collisions, which is typically 3 cm in the solar Region immediately above a star’s photosphere. and 30 km in the solar Extended outer atmosphere of the Sun. The glow of the corona is a million times less bright than that of the photosphere; it can only be seen when the disk of the Sun is blocked during a total solar eclipse, or by using a coronagraph, which artificially blocks the disk.