We here present the first results of fully three-dimensional MHD simulation
s of radiative cooling pulsed (time-variable) jets for a set of parameters
that are suitable for protostellar outflows. Considering different initial
magnetic field topologies in approximate equipartition with the thermal gas
, i.e., (1) a longitudinal and (2) a helical field, both of which permeate
the jet and the ambient medium, and (3) a purely toroidal field permeating
only the jet, we find that the overall morphology of the pulsed jet is not
very much affected by the presence of the different magnetic field geometri
es in comparison with a nonmagnetic calculation. Instead, the magnetic fiel
ds tend to affect essentially the detailed structure and emission propertie
s behind the shocks at the head and at the pulse-induced internal knots, pa
rticularly for the helical and toroidal geometries. In these cases, we find
, for example, that the H alpha emissivity behind the internal knots can be
about 3-4 times larger than that of the purely hydrodynamical jet. We also
find that some features, like the nose cones that often develop at the jet
head in two-dimensional calculations involving toroidal magnetic fields, a
re smoothed out or absent in the three-dimensional calculations.