Magnetohydrodynamics MHD

Most of the universe is in the plasma state, the fourth state of matter.

1. In a plasma the temperature, $T$, is so high that the gas becomes ionised and the atoms split into positive ions and negative electrons.

2. The plasma is treated as a continuous medium, a conducting gas through which eletric currents can flow and in which there exists a magnetic field, ${\bf B}$. The assumption of a continuous medium does require the typical lengthscales of the plasma to be sufficiently large. For example, the lengthscales must be larger than the typical distance an ion travels before a collision (the collision length).

3. All the phemonena observed on the Sun are created by the magnetic field and its interaction with the plasma.

In such situations the plasma may be described by the equations of MHD.