  We have measured signals induced in 5 Corbino electrodes by spontaneous
oscillations of 2D surface electrons on liquid helium at $\sim$0.3\,K, with a
perpendicular magnetic field and microwave radiation. Analysis using
multi-scale, time-resolved, methods yields results consistent with
magnetoplasmons modulated by slow surface gravity waves, with the latter
requiring consideration of the 3rd dimension. Calculation of phase differences
and phase coherences between signals from differently-positioned pairs of
electrodes enables reconstruction of the electron dynamics on the helium
surface.
