  I present some simple exactly solvable models of spin diffusion caused by
synchrotron radiation noise in storage rings. I am able to use standard
stochastic differential equation and Fokker-Planck methods and I thereby
introduce, and exploit, the polarization density. This quantity obeys a linear
evolution equation of the Bloch type, which is, like the Fokker-Planck
equation, universal in the sense that it is independent of the state of the
system. I also briefly consider Bloch equations for other local polarization
quantities derived from the polarization density. One of the models chosen is
of relevance for some existing and proposed low energy electron (positron)
storage rings which need polarization. I present numerical results for a ring
with parameters typical of HERA and show that, where applicable, the results of
my approach are in satisfactory agreement with calculations using SLIM. These
calculations provide a numerical check of a basic tenet of the conventional
method of calculating depolarization using the n-axis. I also investigate the
equilibrium behaviour of the spin ensemble when there is no synchrotron
radiation. Finally, I summarize other results which I have obtained using the
polarization density and which will be published separately.
