We report a new category of dayside auroral transients using auroral image data from the Polar spacecraft Ultraviolet Imager. The transients are characterized by a sudden brightening of the auroral patch in the midday sub-auroral zone, and thus are named midday sub-auroral patches (MSPs). MSPs were found to occur concurrently with storm sudden commencements in response to the compression of the magnetosphere by interplanetary (IP) shocks. After brightening, the MSP widened and/or moved equatorward. The latitudinal location of an MSP depends on the solar wind dynamic pressure in the shock downstream: a larger dynamic pressure corresponds to a lower latitude. The lifetime of an MSP is short (~5--6 min). The magnetospheric source region for the MSPs, as determined with the Defense Meteorological Satellite Program particle data from one fortuitous event, is the central plasma sheet (CPS). Sudden compression of the Earth's magnetic field due to the IP shock impact may change the pitch angles of a pre-existing population of CPS particles, which consequently fall into the loss cone and produce auroral emission.