Abstract We present a case study of a moderate geomagnetic storm, which exhibits an abrupt relativistic enhancement in the heart of the radiation belt accompanied by low‐latitude purple auroral emission. The relativistic enhancement occurs on a timescale of tens of minutes and reaches MeV energies deep within the inner magnetosphere. The enhancement is concurrent with mesoscale purple patches which protrude below the pre‐existing auroral oval. We analyze the event using a novel combination of observational data sets and physics‐based modeling. This includes multispectral auroral imaging, dosimeter data from the Global Positioning System constellation, auroral transport modeling, and global geospace modeling. Our analysis reveals the cause of both the radiation belt enhancement and purple emission to be direct, mesoscale injections of electrons reaching relativistic energies. Finally, we show that these injections are most likely caused by near‐Earth reconnection which produces deeply depleted entropy bubbles capable of trapping and transporting energetic particles into the inner magnetosphere.