  The LASCO C2 and C3 coronagraphs recorded a unique coronal mass ejection on
April 2, 1999. The event did not have the typical three-part CME structure and
involved a small filament eruption without any visibile overlying streamer
ejecta. The event exhibited an unusually clear signature of a wave propagating
at the CME flanks. The speed and density of the CME front and flanks were
consistent with the existence of a shock. To better establish the nature of the
white light wave signature, we employed a simple MHD simulation using the LASCO
measurements as constraints. Both the measurements and the simulation strongly
suggest that the white light feature is the density enhancement from a
fast-mode MHD shock. In addition, the LASCO images clearly show streamers being
deflected when the shock impinges on them. It is the first direct imaging of
this interaction.
