  We study the dynamics of a trapped spin-1 condensate in a magnetic field.
First, we analyze the homogeneous system, for which the dynamics can be
understood in terms of orbits in phase space. We analytically solve for the
dynamical evolution of the populations of the various Zeeman components of the
homogeneous system. This result is then applied via a local density
approximation to trapped quasi-1D condensates. Our analysis of the trapped
system in a magnetic field shows that both the mean-field and Zeeman regimes
are simultaneously realized, and we argue that the border between these two
regions is where spin domains and phase defects are generated. We propose a
method to experimentally tune the position of this border.
