SPEAR-induced field-aligned irregularities observed from bi-static HF radio scattering in the polar ionosphere

Experimental results from SPEAR HF heating experiments in the polar ionosphere are examined. Bi-static scatter measurements of HF diagnostic signals were carried out on the Pori (Finland)–SPEAR–St. Petersburg path at operational frequencies of 11,755 and 15,400 kHz and the London–SPEAR–St. Petersburg path at frequencies of 12,095 and 17,700 kHz, using a Doppler spectral method. The SPEAR HF heating facility generates heater-induced artificial field-aligned small-scale irregularities (AFAIs), which can be detected by HF diagnostic bi-static radio scatter techniques at St. Petersburg at a distance of about 2000 km. In accordance with the Bragg condition, HF bi-static backscatters were sensitive to small-scale irregularities having spatial sizes of the order of 9–13 m across the geomagnetic field line. The properties and behaviour of AFAIs have been considered in the winter and summer seasons under quiet magnetic conditions and under various status of the polar ionosphere (the presence of “thick” and “thin” sporadic Es layers, different structures of the F2 layer). The experimental results obtained have shown that AFAIs can be excited in the F as well as in the E regions of the polar ionosphere. The excitation of a very intense wide-band spectral component with an abrupt increase in the spectral width up to 16–20 Hz has been found in the signals scattered from striations. Along with a wide-band component, a narrow-band spectral component can be also seen in the Doppler sonograms and in the average spectra of the signals scattered from the SPEAR-induced striations. AFAIs were excited even when the HF heater frequency was up to 0.5 MHz larger than the critical frequency. A simulation of the ray geometry for the diagnostic HF radio waves scattered from AFAIs in the polar ionosphere has been made for the geophysical conditions prevailing during experiments carried out in both the winter and summer seasons.     

Relation between Hertz Range Artificial Pulsations and the Dynamics of the Auroral Electrojet: Experiment at the SPEAR Facility

We have analyzed the artificial pulsations detected in several cases in the hertz range in terms of the eastern electrojet dynamics. The pulsations were detected in 30% cases of pulse HF wave impact on the ionosphere during an experiment on modulated ionosphere heating at the Space Plasma Exploration by Active Radar (SPEAR) facility (Spitsbergen Archipelago) in November 2013. The pulsations occur when the polar edge of the eastern electrojet approaches the assumed region of influence on the ionosphere due to the northward drift of the electrojet or its extension with increased intensity. The pulsations vanish when the electrojet moves away from the influenced region, drifting southward. The beginning of pulsations is delayed by several minutes with respect to the starting of the HF transmitter. The pulsations are observed only in one of the horizontal components of the geomagnetic field at a distance of 50 km from the SPEAR facility, and they vanished at a distance of about 1300 km.