Data from the Far Ultraviolet Imager (FUV) on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite show that subauroral proton arcs appear in the afternoon sector during geomagnetically disturbed periods when the interplanetary magnetic field rotates either from south to north or from west to east and when the magnetosphere is moderately compressed. Time series of proton aurora images show that the proton emissions are generally aligned along the equatorward part of the auroral oval. However, when interplanetary magnetic field (IMF) Bz changes from negative to positive the auroral oval contracts toward higher latitudes while the ring current proton precipitation remains stationary, resulting in a separation of several degrees between the latitude of the new oval position and a subauroral proton arc in the afternoon sector. A similar effect occurs when IMF By rotates from negative to positive, in which case the oval in the afternoon sector retreats toward higher latitudes, again leaving a separation between the oval and the subauroral proton arc of several degrees. Comparisons with low-altitude and geosynchronous satellite data show that the subauroral proton arc is caused by the precipitation of protons with energies from several keV to 30 keV and is likely associated with the existence of a plasmaspheric drainage plume. In contrast, the proton emissions along the main oval are caused by protons with energies generally less than 10 keV.