  A new method to quantify fluctuations in the initial state of heavy ion
collisions is presented. The initial state energy distribution is decomposed
with a set of orthogonal basis functions which include both angular and radial
variation. The resulting two dimensional Fourier coefficients provide
additional information about the nature of the initial state fluctuations
compared to a purely angular decomposition. We apply this method to ensembles
of initial states generated by both Glauber and Color Glass Condensate
Monte-Carlo codes. In addition initial state configurations with varying
amounts of fluctuations generated by a dynamic transport approach are analysed
to test the sensitivity of the procedure. The results allow for a full
characterization of the initial state structures that is useful to discriminate
the different initial state models currently in use.
