3. Modeling and Discussion

[22]  The midlatitude ionosphere during magnetic storms gets features of the high-latitude ionosphere. This is manifested in appearance of rather strong anomalous signals related either to side reflection of radio waves from the northern wall of the ionospheric trough or from large-scale structures of the globules types with enhanced electron concentration or to the scatter at intense small-scale irregularities located at the southern boundary of the auroral oval, as well as to a combination of all this factors. The side signals were registered during geomagnetic disturbances at frequencies both below and above MOF of the hop propagation modes. The frequency range of SS and its position relative to the main mode depends on some factors and is determined by the geometry of the ionosonde location, ionospheric conditions at the propagation path, and geophysical situation.

[23]  In this paper we do not consider the problem of interpretation of all observed anomalous signals recorded at the paths considered, in particular the anomalous signals registered at frequencies above MOF of hop propagation at long midlatitude paths Khabarovsk-Rostov on Don and Irkutsk-Rostov on Don, and also quasi-multiple signals observed at the Magadan-Irkutsk path. This problem requires additional studies with use of direction finding measurements, detailed ionospheric and geophysical information, and also calculation of radio wave propagation in three-dimensional inhomogeneous medium. The main attention will be paid to interpretation of strongly spread signals observed at the Inskip-Rostov on Don path during extremely strong magnetic storm on 29-31 October 2003. We believe that the side spread signals are due to the scattering of radio waves at small-scale field-aligned irregularities. Such irregularities exist at the southern boundary of the auroral oval [Fejer and Kelley, 1980; Tsunoda, 1988]. They may serve as an indicator for determination of the position of the auroral oval during magnetic disturbances by registration of the side (scattered) signals at oblique sounding ionograms.

[24]  For the Inskip-Rostov on Don path where the spread side signals were observed during the magnetic storm, we performed calculations in order to localize the region with the irregularities responsible for the scatter. The IRI model and (superposed on it) empirical model of the main (midlatitude) ionospheric trough [Karpachev, 2003; Karpachev and Afonin, 2004] were used in the calculations. The concentration in the F -layer maximum on the trough walls coincide with the values given by the IRI model. Within the trough the interpolation was performed depending on the width, depth, and minimum concentration position. The correction of the electron concentration values using the data of Chilton station (51.6^oN, 1.3^oE) (http://www.wdc.rl.ac.uk) was also performed to provide the agreement between the calculated and experimental ionograms of the direct signal.

[25]  Evidently, the region with the irregularities should be located within the ellipse with the focal points in the reception and transmission stations. The position of the scattering region within the ellipse itself was fitted by calculations, comparing the model and experimental ionograms and using the aspect scatter condition at strongly stretched field-aligned irregularities [Ponyatov and Uryadov, 1996]. The location of such scattering regions was taken in the form of a latitude-longitude net in the vicinity of the southern boundary of the auroral oval. The position of the region containing irregularities in the form of a disk with a radius of 50 km varied within considerable limits both, in latitude (from 58^oN to 62^oN) and longitude (from 25^oE to 40^oE) in the height interval from 230 km to the F-layer maximum. The calculations showed that in principle the side signals caused by the radio wave scattering at small-scale field-aligned irregularities may come from different regions. The best coincidence of the experimental and calculated ionograms was a criterion of the choice.

[26]  According to the calculations for the regions with scattering irregularities in the vicinity of 25^oE (the distance from Inskip to the scattering region (SR) is of the order of 1800-1900 km) the aspect conditions at low frequencies 8-10 MHz are fulfilled for the second mode: the scattering occurs from the peak of the second hop. At frequencies above 12 MHz radio waves reach SR in the peak of the first hop. However, either the reception point Rostov on Don is located in the dead zone for the scattered ray or the latter goes away to the ricocheting mode. Therefore the scattered signal from this region is not received at high frequencies. For the eastern regions in the vicinity of 40^oE, the distance from Inskip to SR increases and the scattering from the peak of the first hop is observed only for high frequencies. This range is narrowing shifting to higher frequencies (above 15-16 MHz) and almost disappears for the regions with the longitude above 40^oE. Frequencies below 12 MHz are scattered from the peak of the second hop. For frequencies 12-16 MHz SR falls into the dead zone between the peaks of the first and second hops, so the scattered signal from this range is absent. For the regions with scattering irregularities in the vicinity of 28-32^oE an optimum case is realized under the given geometry of the position of the transmitter (Inskip) and receiver (Rostov on Don). The scattered ricocheted modes are transformed into hop modes and the reception station Rostov on Don falls into the reception zone. Therefore for these regions the maximum range of passing and reception of the scattered signal is observed.

[27]  Figure 8b shows the ionogram at the Inskip-Rostov on Don path calculated for the conditions of the experiment (1442 UT, 29 October 2003) taking into account radio wave scatter at field-aligned irregularities. The diffusive signal delayed relative the direct signal is marked by ScS (scattered signal). This signal is due to the scattering at irregularities located in the topside ionosphere. The subionospheric coordinates of the centers of the regions with irregularities providing the better agreement of the experimental and calculated data on the scattering lie within the latitude and longitude intervals 60-62^oN and 28-32^oE, respectively. Comparing Figures 8a and 8b, one can see a quite satisfactory agreement between the calculated and experimental ionograms. Figure 8c shows the ray trajectories at a frequency of 16 MHz realized at the direct path along the arc of a great circle Inskip-Rostov on Don and along the path with the deviation from the great circle arc at the aspect scattering at field-aligned irregularities located in the upper ionosphere at heights of about 270-300 km. The calculations were performed for the region with irregularities, the center of the region having subionospheric coordinates 60^oN, 28^oE. Figure 8d shows the geographical position in the map of the auroral oval of the scattering region which according to the propagation conditions and scattering geometry provides the main input to the spread (scattered) side signal in the oblique sounding ionogram for the given position of the transmitting and reception points. One can see in Figure 8d that the calculated region with irregularities responsible for the scattering and appearance in the OS ionogram of anomalous spread signal is well positioned to the location of the southern boundary of the auroral oval (according to the measurements on board the DMSP satellite (http://sd-www.jhuapl.edu/Aurora/ovation/ovation_display.html), the latter fact supporting the suggested interpretation of the nature of the anomalous signal.

[28]  It should be noted that during the registration of side spread signals in the OS ionograms in the period 1340-1530 UT on 29 October 2003 the B_z component of the interplanetary magnetic field (IMF) was northward ( B_z > 0 ), however the value of the IMF magnitude was high. It is known that magnetic storms well correlate to the southward component of IMF ( B_z < 0 ). The data on the dynamics of the auroral oval obtained on the basis of the oblique sounding of the disturbed ionosphere confirm a possibility of a substorm development and southward motion of the auroral oval also in the periods when B_z > 0. These data show also that with an increase of the magnitude of the interplanetary magnetic field the energy injected into the magnetosphere increases and this causes intensification of the magnetospheric and ionospheric current systems determining the dynamics of the high-latitude ionosphere and formation of irregularities. The equatorward motion of the auroral oval directly or indirectly manifests changes in the magnetospheric configuration as a result of the reconnection of field lines of the interplanetary and geomagnetic fields and intensification of the ring current. According to Yokoyama et al. [1998] the intensity of the ring current measured by Dst provides the main input into variation in the equatorial boundary of the auroral oval. Moreover, Yokoyama et al. [1998] suggested that the equatorial boundary of the oval moves southward due to the increase of the auroral electrojet activity. It is worth noting that during the extreme magnetic storm on 29-31 October 2003 an aurora was observed at the midlatitude station IZMIRAN (55^oN, 37^oE) [Panasyuk et al., 2004].

[29]  As for the traveling ionospheric disturbances (TID) observed at the midlatitude path Inskip-Rostov on Don during the magnetic storm on 29 October 2003 at 0800-1000 UT (see Figure 10), they are an ionospheric response to acoustic gravity waves (AGW). The mechanisms of generation of AGW in the auroral zone are known and related to the Joule heating, Lorenz force, and particle precipitation [Hocke and Schlegel, 1996; Hunsucker, 1982; Williams et al., 1988]. At high latitudes, TID are well detected by HF radars. For example, according to the data of the observations on the network of SuperDARN radars [Bristow et al., 1994; MacDougall et al., 2001], at high latitudes at backscatter oblique sounding (BOS) signals a modulation of the echo signal is often observed. This modulation is interpreted as a focusing at the radio waves reflection from the ionosphere modulated by traveling wave disturbances.

[30]  The traveling ionospheric disturbances registered in oblique sounding ionograms may serve as an indicator of manifestation of the magnetosphere-ionosphere relations during geomagnetic disturbances because of generation of AGW and their propagation from the high to middle latitudes. It is worth noting here that a good correlation between AGW and magnetic activity was observed also in the Worldwide Atmospheric Gravity-wave Study (WAGS) experiment [Williams et al., 1988]. In this experiment various technical means took part at high and middle latitudes including the incoherent scatter radar EISCAT, networks of magnetometers and riometers, vertical sounding ionosondes, and also the system of HF radars.

[31]  The comparison of the results of measurements of MOF variations during a storm and in quiet conditions shows that during a storm the amplitude of the MOF variations increases considerably, the amplitude of rapid variations (  15 min) intensifies, and the behavior of MOF with time becomes of a pulsating character, the latter manifesting wave processes occurring in the ionosphere in the periods of geomagnetic disturbances. The estimates made on the basis of the modeling of ionospheric HF propagation, taking into account TID by the method described by Erukhimov et al. [1998], show that the observed effect of the DFC modulation in the vicinity of MOF may be expected from wave-like disturbances of the electron concentration with the amplitude of about 20-30% of the background level. The large-scale variations in MOF observed approximately from 1100 to 1400 UT are evidently due to the passing of the terminator at the propagation path. Similar effect was observed in the dusk at the midlatitude Inskip-Moscow path in April 2002 in quiet geomagnetic conditions [Cherkashin et al., 2003].

[32]  Thus, according to the data obtained magnetic storms are accompanied by a generation of powerful TID and southward motion of the ionospheric trough and auroral oval. The combination of these factors provides (by refraction and radio wave scatter) formation of side signals recorded in oblique sounding ionograms at midlatitude paths during geomagnetic disturbances.