Simulations of strongly magnetized slab jets in uniform expansion in a
n external atmosphere have been performed and compared to an unmagneti
zed expanding slab jet. A strongly magnetized super-Alfvenic jet with
lower magnetosonic Mach number but with sonic Mach number equal to a p
urely fluid jet destabilizes in a much shorter distance than the fluid
jet. A strongly magnetized sub-Alfvenic jet remains stable until the
jet becomes super-Alfvenic. At the Alfven point the jet is submagnetos
onic and destabilizes abruptly. Results from a spatial stability analy
sis of the magnetized slab jet are compared to the numerical simulatio
ns. In general, predictions based on the theory are borne out by the s
imulations. In particular, the stability properties of a super-Alfveni
c jet are confirmed to be a function of the magnetosonic Mach number,
and the theoretical prediction of rapid jet destabilization at the Alf
ven point transition between sub-Alfvenic and super-Alfvenic flow is c
onfirmed. However, nonlinear effects associated with decreasing the je
t density relative to the external density and with increasing the mag
netic field strength can increase the destabilization-decollimation le
ngth and reduce decollimation, respectively. Some of the implications
for three-dimensional jets are considered.