We saw in Chapter 6 that a linear tearing mode is unstable when
the (real dimensionless) tearing stability index, (see Section 3.8), is positive, and is stable
otherwise. In the linear regime, an unstable tearing mode grows exponentially in time on a timescale that
is intermediate between the short hydromagnetic time, [see Equation (5.43)],
and the much longer resistive diffusion time, [see Equation (5.49)].
The mode reconnects magnetic flux at the socalled rational surface to produce a helical magnetic island chain. (See Section 5.16.)
The rational surface is the magnetic fluxsurface at which the tearing mode resonates with the equilibrium magnetic
field (i.e., at which
, where is the wavevector of the mode, and the equilibrium magnetic field).
The
tearing mode also rotates in the laboratory frame at the angular frequency

(9.1) 
where and
are the EcrossB and electron diamagnetic frequencies, respectively, at the
rational surface.
[See Equations (5.21), (5.29), (5.44), and (5.45).]
In fact, the previous equation implies that the tearing mode corotates with the electron fluid at the rational surface.
We saw in Section 5.16 that linear tearing mode theory breaks down as soon as the radial
width of the magnetic island chain exceeds the linear layer width, and must be replaced by nonlinear theory. The aim of this chapter is to employ the analysis of Chapter 8 to determine the nonlinear time evolution of
an unstable tearing mode.