Modification of the local substorm ionospheric and field-aligned currents produced by the Tromso heating facility

Coordinated observations of ionospheric and magnetic pulsations in the 100– 120 s range observed in the high frequency (HF) modified ionosphere during the onset of an auroral substorm activation in the area of the upward field-aligned current (FAC) on 17 February 1996 are reported. The experiment was unusual in that during the near-vertical HF pumping an overhead strong sporadic E layer was present. The Doppler shift of HF signals on the London–Tromsö–St. Petersburg path showed wave-like variations during the HF pumping. There is an association between these signals, scattered from the HF pump-induced E region irregularities, and magnetic pulsations in the east-west magnetic field component measured at Tromsö. Magnetic field oscillations from all other IMAGE magnetometers in Scandinavia have also been analyzed. The data suggested that under the initiation stage of the FAC system, HF pumping of the ionospheric E region can modify the compressional mode of the Pi2 magnetic pulsations by modulating of the field-aligned currents associated with the substorm. In particular, the mechanism which we postulate is a modulation of background field-aligned currents by HF pump-generated direct and reflected Alfvén waves.