  We review in a tutorial fashion some of the causes of impurity density
variations along field lines and radial impurity transport in the moment
approach framework. An explicit and compact form of the parallel inertia force
valid for arbitrary toroidal geometry and magnetic coordinates is derived and
shown to be non-negligible for typical TJ-II plasma conditions. In the second
part of the article, we apply the fluid model including main ion-impurity
friction and inertia to observations of asymmetric emissivity patterns in
neutral beam heated plasmas of the TJ-II stellarator. The model is able to
explain qualitatively several features of the radiation asymmetry, both in
stationary and transient conditions, based on the calculated in-surface
variations of the impurity density.
