20-30 % of the electron
concentration background level.[33] The main results of the paper may be formulated as follows:
[34] 1. It is demonstrated that the system of the oblique sounding paths covering a vast Eurasian longitudinal sector from England to Magadan may serve as an effective tool for monitoring of ionospheric effects of geomagnetic disturbances. The network of LFM ionosondes may be used as bistatic HF radars sensitive to both large-scale structures (the main ionospheric trough, auroral oval, traveling ionospheric disturbances, patches with increased electron concentration) and small-scale irregularities accompanying such large-scale formations. The network of LFM ionosondes for oblique sounding may serve as an addition to the existing ground-based and satellite systems of ionospheric monitoring in various geophysical conditions, the latter being especially important for the regions weakly equipped by diagnostics means.
[35] 2. The joint analysis of the VS and OS of the ionosphere and satellite data shows that the appearance of the additional signals during geomagnetic disturbances may be due to the refraction of radio waves in the region of the auroral oval and main ionospheric trough, and also by the scatter at irregularities of different scales in the vicinity of the equatorial boundary of the auroral oval and the northern wall of the trough. First of all this is confirmed by the data obtained at the Magadan-Irkutsk path in the conditions of moderate magnetic storm when the distance-frequency characteristics (DFC) of the anomalous signal repeated the shape of the DFC of the standard mode.
[36] 3. On the basis of numerical simulations and comparison to the experimental data of oblique sounding the identification is performed of the additional strongly spread signals registered during the magnetic storm at the midlatitude Inskip (England) to Rostov on Don path as signals scattered at small-scale field-aligned irregularities located at the southern boundary of the auroral oval. It is shown that during the main phase of the magnetic storm on 29 October 2003, there occurred a shift of the southern boundary of the auroral oval from high to middle latitudes with a mean velocity of about 2.5o per hour. It is worth noting that the ionospheric effects observed at the midlatitude path of oblique LFM sounding Inskip-Rostov on Don during the prominent magnetic storm on 29-31 October 2003 are more typical for the auroral zone of the Arctic. In such situation the radio wave propagation conditions at midlatitude paths become similar to the corresponding conditions in the high-latitude ionosphere. Fairly intense signals with the propagation outside a great circle arc are registered and this fact should be taken into account while organizing the operation of various radio electronic systems (HF radiocommunication, over horizon HF radiolocation, radio direction finding etc.)
[37] 4. It is shown that during the main phase of a strong magnetic
storm on 29 October 2004 a modulation of DFC in the vicinity of MOF
related to the generation of acoustic gravity waves and their response in
the form of traveling ionospheric disturbances was observed at the
midlatitude Inskip-Rostov on Don path. According to the estimates the
amplitude of the wave disturbances was
20-30 % of the electron
concentration background level.
[38] Concluding we note that the results of the performed studies of shortwave propagation manifest a complicated picture of physical processes in the magnetosphere-ionosphere system during geomagnetic disturbances. These processes involve the structure and dynamics of both large-scale formations (ionospheric trough, auroral oval) and accompanying these formations small-scale irregularities. During strong disturbances these factors are able to impact considerably the HF signal characteristics not only in the high-latitude ionosphere, but at middle latitudes as well, where the main part of decameter radio lines passes. It is evident that for further progress in this region and obtaining of more complete data on the physics of ionospheric processes and mechanisms of HF propagation in the conditions of geomagnetic disturbances coordinated studies with use of modern radiophysical and geophysical methods of measurements are needed. The experimental study of the ionosphere by the oblique sounding method at paths of different length and orientation in the region of the subauroral ionosphere may serve as an important tool of diagnosis of the position and structure of the main ionospheric trough. We plan in the future to carry out such studies attracting both national and foreign systems of diagnostics of the ionosphere and magnetosphere, including direction finding measurements [Vertogradov et al., 2004].
