  Let G be an affine algebraic group and let R be an associative algebra with a
rational action of G by algebra automorphisms. We study the induced G-action on
the spectrum Spec R of all prime ideals of R, viewed as a topological space
with the Jacobson-Zariski topology, and on the subspace consisting of all
rational ideals of R. Here, a prime ideal P of R is said to be rational if the
extended centroid of R/P is equal to the base field. The main themes of the
article are local closedness of G-orbits in Spec R and the so-called
G-stratification of Spec R. This stratification plays a central role in the
recent investigation of algebraic quantum groups, in particular in the work of
Goodearl and Letzter. We describe the G-strata in terms of certain commutative
spectra. Our principal results are based on prior work of Moeglin & Rentschler
and Vonessen. We generalize the theory arbitrary associative algebras while
also simplifying some of the earlier proofs.
