[2] Effects of space weather provide different and considerable impact on various sides of human activity and environment. First, these effects are manifested at high latitudes as a result of magnetosphere-ionosphere interaction of electric and magnetic fields at conditions of intensification of the solar wind and changes in the configuration of the interplanetary magnetic field. Geomagnetic disturbances being a determining factor for HF propagation at polar latitudes [Blagoveshchensky and Zherebtsov, 1987; Goodman and Aarons, 1990; Hunsucker and Bates, 1969; Zhulina, 1978] exert considerably influence on signal characteristics at midlatitude and subpolar paths [Blagoveshchensky and Borisova, 2000; Kurkin et al., 2004; Lockwood, 1981; Siddle et al., 2004; Stocker et al., 2003; Uryadov and Ponyatov, 2001; Uryadov et al., 2002; Zaalov et al., 2003].
[3] Speaking on the structure of the high-latitude ionosphere, its significant features are the main ionospheric trough and the auroral oval. The interest to them is due to the fact that the trough borders on the midlatitude ionosphere where the main channels of the decameter wave (DCMW) communication pass and trough shift to lower latitudes during magnetic storms can lead to undesirable effects of decreasing the maximum observed frequency (MOF) and variation of the signal mode structure. The midlatitude ionospheric trough (MIT) shape and dynamics depend on many factors including longitude, local time, season, and solar and geomagnetic activity levels. Various parts of the trough are different by the electron concentration, ionization gradient, and presence of irregularities [Deminov et al., 1996; Karpachev, 2003; Karpachev and Afonin, 2004; Karpachev et al., 1996; Rodger et al., 1992].
[4] The ionosphere irregularities of various scales generated in the vicinity of the southern boundary of the auroral oval lead to an increase of the signal spread of standard modes of propagation and to appearance of anomalous signals with delays increasing considerably the delays of the main modes propagating along the arc of a large circle. The level of such signals is rather high and they influence the work of radioelectronic systems of various application. On the other hand, the presence of additional signals on ionograms of the oblique sounding in the period of disturbances may be used for determination of the boundaries of the main ionosphere trough and diagnostic of small-scale ionosphere irregularities located in the vicinity of the southern boundary of the auroral oval.
[5] Many papers are dedicated to studies of the auroral oval [see, e.g., Yokoyama et al., 1998, and references therein]. Currently, studies of the subpolar and midlatitude ionosphere (including the auroral oval) are intensely developed using the data of vertical sounding, incoherent scatter radars, and meteorological and navigation satellite systems DMSP and GPS [Afraimovich et al., 2004; Potekhin et al., 1999; Vo and Foster, 2001; Zherebtsov et al., 1997]. The data obtained convincingly manifest dynamic of the auroral oval during a magnetic storm and its shift toward middle latitudes when the midlatitude ionosphere demonstrates the properties typical for the high-latitude ionosphere. Therefore taking into account the effects of space weather becomes very important for provision of reliable functioning of radioelectronic systems not only at high but at middle latitudes as well.
[6] Among the complex of means aimed at studying the ionosphere and its influence on radio wave propagation the technical means of the oblique sounding (OS) occupy a special place. First, if there exists and functions a developed network of the OS stations, simultaneous sounding at the paths in a different way oriented relative the auroral oval makes it possible to perform the diagnosis of the propagation environment and control the dynamics of the oval in various longitudinal and latitudinal sectors. Second, real-time sounding makes it possible to obtain the information on the real-time state of the HF channel. The latter makes it possible to adapt the systems of HF radio communication and over-the-horizon radiolocation to the current state of the ionosphere, the latter being especially important for high-latitude regions of Arctic and North Atlantics.
[7] For study of irregularities in the high-latitude ionosphere the HF radars of the SuperDARN [Greenwald et al., 1995] and CUTLASS [Yeoman and Luhr, 1997] are successfully used. However, it is known that a motion from high to middle latitudes of the southern boundary of the region with irregularities responsible for the scatter is observed during geomagnetic storms. In this situation due to the conditions of propagation and geometry of the scatter the irregularities are found outside the "visibility" zone of the high-latitude HF radars. The latter fact makes difficult a detailed study by these radars of the dynamics of small-scale irregularities localized at the southern boundary of the auroral oval at all stages of a magnetic storm. So to obtain a complete picture of the ionospheric irregularity dynamics during a magnetic storm it is of a considerable interest to use together with high-latitude radars also radars located at middle latitudes. For this task the Russian and global network of the LFM ionosondes [Ivanov et al., 2003] located at middle latitudes may be used as HF radars in a bistatic configuration. The experiments carried out during the recent years on the basis of the LFM ionosondes network demonstrated a perspective of such approach for studying the dynamics of ionospheric irregularities and wave processes in the ionosphere and magnetosphere during magnetic storms [Kurkin et al., 2004; Uryadov et al., 2002, 2004a, 2004b; Zherebtsov et al., 1997].
[8] There are two goals of this paper. The first one is to present the results of experimental studies and modeling of the peculiarities of HF signal propagation during geomagnetic disturbances obtained on the basis of the network of midlatitude circuits of oblique LFM sounding. The second goal is to demonstrate that the use of oblique sounding makes it possible to complete considerably the picture of midlatitude ionosphere behavior including the periods of magnetic storms in the Eurasian longitudinal sector poorly equipped by observational means and characterized by the maximum excess of geographical latitudes over geomagnetic latitudes.