This paper studies dayside shock aurora forms and their variations observed by the ground-based all-sky imager (ASI) in Svalbard on 30 November 1997. The interplanetary shock arrived at Earth when Svalbard was at ∼1120 magnetic local time. The ASI detected an auroral intensification by a factor of 2 or more in both green and red line emissions within 5 min after the shock arrival. The intensified green emissions were mainly diffuse aurora on closed field lines. They were latitudinally below and adjacent to the red aurora, which was mainly in the form of arcs and beams along the magnetic east-west direction. The diffuse aurora expanded equatorward and eastward, and its intensity exceeded the red arcs that were at ∼5 kR. We confirmed that the eastward propagating diffuse aurora was actually moved antisunward along the oval, which suggests that the antisunward propagating shock aurora seen in space is mainly diffuse aurora. The intense diffuse aurora could be caused by wave instabilities led by a temperature anisotropy and/or caused by an enlarged loss cone. After the shock arrival, the detected low-latitude boundary of the cusp moved equatorward at a speed of ∼18 km min⁻¹. As a result, the cusp meridional width was doubled from ∼0.8° to 1.6° in latitude in 10 min. This finding implies that a low-latitude reconnection occurred during the compression. In this study the auroral signatures and speculated mechanisms are consistent with those revealed by in situ particle and wave observations from FAST and DMSP.