The two-fluid theory of inertial Alfv¿n waves predicts that wave energy spreads azimuthally across the background magnetic field, and that phase- and group- velocities in the B$perp$ plane are oppositely directed. These features have been observed in laboratory plasmas but have yet to be identified in the natural auroral plasma. Using narrow-field video recorded at 50 images/s, we have identified a time dependent morphology that is consistent with these predictions. The aurora at issue is best modeled as an expanding amplitude envelope within which periodic auroral structure evolves. If luminous structure is interpreted as a projection of the parallel wave electric field into the B$perp$ plane, then successive images reveal the cross-sectional structure of the resonant cone at increasing distance from the wave source. Phase- and group-velocities extracted under this interpretation were found to be consistent with theoretical predictions for a dispersing inertial Alfv¿n wave packet.