The Second Act Comeback

The ideal MHD case is simpler and easier to visualize, as we can treat the field lines as being frozen to the plasma 26]. By frozen, we mean that the magnetic field must move with the plasma; as a consequence, the system can be decomposed into tubes of equal magnetic flux. If a flux tube moves radially outward, it moves into a region of lower magnetic field 27], which increases the volume of the flux tube and adiabatically cools it. For a flux tube to move outward, it must exchange places with another flux tube moving inward, whose volume will be reduced in the region of stronger magnetic field and lead to adiabatic heating. This swapping of flux tubes is known as an electrostatic interchange (or flute) instability 28]. Note that by definition there is no change to the magnetic topology and there are no parallel currents. This mode is the plasma equivalent of the Rayleigh-Taylor instability in neutral fluids, in which a heavy fluid is supported a against gravity by a lighter fluid. In neutral fluids the instability grows when the pressure gradient is anti-parallel to the gravitational force; in plasmas it grows when the pressure gradient is antiparallel to the radius of curvature of the magnetic field; such plasmas are often referred to as having bad curvature.