Interchange or Flute Instabilities

Abstract

A simple kind of hydromagnetic instability occurs when a plasma, with no internal field, comes in contact with a magnetic field in free space. If the strength of the magnetic field decreases with distance from the boundary of the plasma (in which case the field lines must be convex outward), then it is energetically favorable for the plasma to change places with the magnetic field. This phenomenon is called the interchange instability because the motion can be described as an interchange of the lines of force in space. In this case the field lines move as a whole, conserving their form and direction. The surface perturbation takes the shape of flutes along the magnetic field lines. Therefore, this instability is also known as a flute instability. This terminology provides a clear description of a plasma which separates the magnetic field lines and leaks into the space between them.