AbstractUsing data from low‐altitude polar orbit satellite NOAA/POES and ACE, we investigate the possible sources of the enhancements of relativistic electrons in the magnetosphere. The observations from NOAA/POES for the different geomagnetic activity periods show that (1) substorms injections provide seed electrons for MeV electron enhancement associated with geomagnetic storms; (2) energetic electrons in the solar wind provide an alternative source for the relativistic electrons in the magnetosphere during the SEP (Solar Energetic Proton) events.