Quasiperiodic perturbations of ionospheric plasma with ~10 min occurrence periods were observed by incoherent scatter radars (ISR) in high-latitude and midlatitude ionosphere after the onset of a stormtime substorm. The latter occurred during a period of complex dynamical activity and had a number of unusual characteristics. The perturbations indicate ultralow frequency (ULF) electric field enhancements over the Northern Hemisphere. Specifically, strong subauroral electric fields are associated with the ring current-related polarization jet (PJ). These perturbations correlate with magnetic variations in high-latitude and midlatitude regions spanning ~21 hours in magnetic local time (MLT). The radar and ground magnetometer data are supported by simultaneous observations from the DMSP and geostationary satellites. As a result of the onset, the preexisting PJ shifted equatorward to L ~ 2.5. Similar periodicities in the solar wind dynamic pressure and interplanetary magnetic field (IMF) appear to be natural sources of the ULF perturbations observed at high and middle latitudes.