Observations of detached subauroral proton arcs by the FUV instrument on the IMAGE spacecraft have been recently reported and shown to be produced by ring current ions precipitating in the afternoon local time sector during geomagnetically disturbed periods. Data from June 18, 2001 show a direct link between a subauroral proton arc and a global observation of a plasmaspheric plume by the IMAGE EUV instrument. Using a T96 magnetic field model, the proton arc maps to a broad region of enhanced cold plasma density associated with the plume. The link between the proton arc and plume suggests that the precipitation may be due to pitch angle scattering of energetic protons by electromagnetic ion cyclotron (EMIC) waves, which may be preferentially generated within the plume since the presence of cold, dense ions lowers the threshold for the EMIC instability.