where the magnetic field is in the x-direction.
A possible solution to the equation is the plane solitary wave
that propagates along the magnetic field. We show that this
solution is unstable to small transverse sinusoidal
perturbations of
wavenumber k such that 0<k<3. Following the method
of Allen & Rowlands (1993) we use a multiple scale perturbation method
to determine consistent expansions for the growth rate 
about k=0 and k=3 respectively. By considering a two-point
Padé approximant, we obtain an analytical expression for
valid for 
.
We also calculate numerically.
The Padé approximant is in very
good agreement with the numerical result.