Observations of the ATS-6 radio beacon from Fairbanks show structured enhancements of the night-time electron content during substorms. The probability of their occurence increases with the level of magnetic disturbance, and they are most likely to be observed when the daily sum of K indices exceeds 20 in winter or 30 in summer. There is a remarkably close coincidence between these electron-constant enhancements and auroral absorption events detected by a riometer, though the relative magnitudes vary with the time of day. Electron density profiles measured with the Chatanika incoherent-scatter radar confirm that the electron density at 90 km is enhanced during auroral absorption events. However, D and E-region ionization is insufficient to account for the observed enhancement of the electron content, and an F-region effect is a more likely cause. During one such event the F-region critical frequency was 8 MHz.