Global ULF disturbances during a stormtime substorm on 25 September 1998

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.