Phenomena observed by HF long‐distance diagnostic tools in the HF modified auroral ionosphere during magnetospheric substorm

Abstract

The HF long‐distance diagnostic tools located in St. Petersburg combined with multi‐instrument observations at Tromsø have been operated in conjunction with the ionospheric heating facility near Tromsø to examine the heater‐induced phenomena in the auroral ionosphere. The distinctive feature of the heating experiment was the reflection of the pump wave from auroral sporadic Es‐layer. During substorm expansion, strong stimulated electromagnetic emission (SEE) at the third harmonic of the downshifted maximum frequency was found. It is believed that SEE is accompanied by excitation of the VLF waves penetrating into magnetosphere and stimulating the precipitation of the energetic electrons (10–40 keV) of ∼1‐min duration due to a cyclotron resonant interaction of the natural precipitating electrons (1–10 keV) with the heater‐induced whistler waves in the magnetosphere. In the recovery phase of the auroral substorm, emissions at the heater second harmonic frequency were accompanied by excitation of AFAI in the Es layer with the large decay time up to several minutes after the heater turn‐off. The possible explanation for these emissions can be found from the linear conversion process when the pump wave of O mode may directly convert to electrostatic waves in the region of the HF plasma resonances. The large decay time may result from electromagnetic instability growth in the heater‐modified ionosphere at the downward field‐aligned currents.