4. Discussion

[21]  The studies showed that the wide-band oblique sounding of the modified ionospheric region with artificially created small-scale irregularities is an effective tool for studies of mechanisms of propagation and formation of the HF signal field. For example, for the first time at a long-distant midlatitude path in the absence of considerable horizontal gradients of the electron concentration the determining influence of the sporadic E layer on formation in the upper ionosphere of the radio wave field propagating at frequencies exceeding MUF (of the standard hop mechanism via the F region) was shown. It is evident that to obtain more complete data on the AIT influence on the remote propagation of short radio waves a continuation of the studies in various geophysical conditions is needed.

[22]  As for the diagnostics of the artificial ionospheric turbulence by the method of aspect scattering of testing HF signals, the relation of the characteristics of the Doppler spectrum of the scattered signals to the magnetic activity level was established. According to the data obtained, in relatively quiet ionospheric conditions, DSF of SS is ~0 to 2 Hz, and there are no quasiperiodic variations in DSF. During the magnetic storm DSF reached values of ~8-10 Hz, and sporadic trains of quasiperiodic variations in DSF with the period of ~40-60 s and the amplitude modulation of ~0.5-2 Hz were registered.

Figure 8

[23]  To interpret the experimental data, the data on the solar wind and interplanetary magnetic field (IMF) were used because it is known [Nishida, 1980] that variations of these parameters play the key role in the development of a magnetic storm. Figure 8 shows for the period of the experiment the behavior of the solar wind V, B_z component, latitudinal ( q ) and azimuth ( j angles of IMF (in the solar-ecliptic coordinate system), and the electromagnetic power of the solar wind ( e ) incoming into the magnetosphere (the data were taken from the Advanced Composition Explorer (ACE) satellite Web site http://hiraiso.crl.go.jp/). Figure 8 shows that at approximately 1340 UT on 17 August, a jump of the wind velocity and the magnitude of the B_z component occurred. This sharp increase may be identified with the shock wave and its impact on the magnetosphere. At 0100 UT on 18 August, there occurred a sharp change of the orientation of the IMF B_z component from the northward to the southward and this level of the B_z magnitude (about -17 nT) stayed till 0600 UT. Simultaneously, the rate of the energy income into the magnetosphere e strongly increased up to 8 TW. The second increase of e but with lower values of ~1-3 TW occurred from 1600 UT on 20 August to 1600 UT on 22 August. It is known [Blagoveshchensky and Zherebtsov, 1987] that when the IMF B_z component turns southward, the large-scale convection electric field is intensified and this leads to a generation of magnetospheric substorms. The effect of the main phase of the magnetic storm was manifested in strong variations of the geomagnetic field in Tromsö and negative disturbance of the critical frequency in Khabarovsk (see Figures 2 and 3). After 0600 UT on 18 August the southward component of B_z smoothly decreased and since 0000 UT on 19 August changed sign to the northward direction ( B_z > 0 ). Approximately from 1920 UT on 19 August and later on 20, 21, and 22 August there occurred numerous changes of the B_z component sign from the northward to southward and back. This might have generated a sequence of magnetospheric substorms [Brunelly and Namgaladze, 1988]. The change of the B_z sign was also manifested in the behavior of the magnetic field in Tromsö and variations of the critical frequency in Khabarovsk. The variations of the IMF vector rotation southward and back northward and also of the magnitude of the southward component ( B_z < 0 ) itself were accompanied by variations of the value of the latitudinal angle q which on 20-22 August fluctuated within the 45^o interval with a small shift toward negative values. Comparing Figures 8 and 3, one can see a correlation of the variations in the geomagnetic field components in Tromsö to the changes of the solar wind electromagnetic energy power incoming into the magnetosphere and also to the behavior of the IMF B_z component. The southward direction of the latter ( B_z < 0 ) determines a development of magnetospheric disturbances. Figures 2c and 8 show that the negative disturbance of the electron concentration correlates well (but with some delay) to the same parameters.

[24]  During a magnetic storm a formation of the magnetosphere-ionosphere current systems occurs, causing an increase of the electric fields at ionospheric heights. It is worth noting here that according to the Basu Sun et al. [2001] data the magnetospheric electric fields penetrating into middle latitudes during a magnetic storm may be responsible for the ionospheric effects, leading to fluctuations of the total electron content and blinking of radio signals of navigation satellite systems. According to our data obtained on the basis of the measurements of HF signals scattered at artificial small-scale magnetically oriented irregularities the electric field at F -region heights increased during the magnetic storm up to the values of ~8.6 mV m ^-1 as compared with the value of ~1 mV m ^-1 in quiet conditions. This led to the increase of the irregularities drift velocity up to values of ~186 m s ^-1 typical for the high-latitude ionosphere. The trend (close to a linear one) in the form of the DSF increase (Figure 5b, left) and DSF decrease (Figure 5d, left) may be due to the variations in the electric field vector (both by the magnitude and direction) at F -region heights during the development and recovery phases of the ionospheric effects of the magnetic storm.

[25]  The occurrence of trains of quasiperiodic variations in DSF of the HF signals in the magnetically disturbed period manifests a presence of wave processes in the region containing artificial small-scale irregularities. Since at the scattering at strongly stretched magnetically oriented irregularities (L_| L, where L_| and L are the scales of the irregularities along and across the magnetic field, respectively) only the component of the drift velocity perpendicular to the magnetic field is measured [Nasyrov, 1991], one can conclude that the wave processes responsible for the DSF variations have the lateral to the geomagnetic field lines component of charged particle motion. This effect may be caused by the lateral magnetohydrodynamics waves playing the determining role in the activity of the natural pulsations of the geomagnetic field [Gul'el'mi and Troitskaya, 1973].

[26]  The problems of registration and interpretation of the quasiperiodic variations in DSF of the testing signal scattered at the artificial small-scale magnetically oriented irregularities were considered by Sinitsin et al. [1999] and Blagoveshchenskaya and Troshichev [1996]. Sinitsin et al. [1999] considered the relation of the DSF variations to the plasma drift in the electric field of the geomagnetic pulsations. Blagoveshchenskaya and Troshichev [1996] considered the influence of the traveling ionospheric disturbances generated by powerful decameter radioemission of the heating facility or caused by the natural acoustic gravity waves on the Doppler measurements results.

[27]  By they periodicity (~40-60 s) the observed variations in DSF are close to the geomagnetic pulsations of the Pc3-4 type and also to the irregular oscillations of the Pi2 type [Saito, 1969]. It should be noted that the quasiperiodic variations in DSF were not observed on 19 August at the recovery phase under positive value of the IMF B_z component. At the same time on 20, 21, and 22 August when IMF had almost radial direction (the azimuth angle j was 180^o and the latitudinal angle q fluctuated around 0^o, that is, numerous reorientation of the B_z component from the southward to the northward directions took place) quasiperiodic oscillations of DSF of the scattered signal were registered. In the same way the activity of trains of the Pi2 geomagnetic pulsations is manifested. The pulsations are usually generated during a magnetospheric substorm. Its expansion phase is preceded by reorientation of the IMF B_z component from the northward to southward direction [Gul'el'mi and Troitskaya, 1973]. The energy coming in from the solar wind in the reconnection process on the daytime side of the Earth and also the energy of the magnetic field in the magnetosphere tail released in the reconnection process may be the energy sources of the magnetic pulsations.

[28]  Assuming that the quasiperiodic pulsations of the DSF of the scattered signal are due to the wave processes related to the propagation of the lateral MHD waves generated during a magnetic substorm, we evaluate the magnetic field variations providing the observed values of the Doppler oscillations amplitudes. For the lateral oscillations of the field lines frozen into the plasma the relation between the plasma motion velocity and variations of the magnetic field in the linear approximation is given by the following expression [Frank-Kamenetsky, 1964]:

(2)

where r = S m_i N_i is the plasma density, and m_i and N_i are the ion mass and concentration, respectively.

[29]  At heights of the F region, ions of atomic oxygen predominate, i.e., n_e N_i N( O⁺) [Fatkullin et al., 1981]. From equations (1) and (2) for the values l = 3 10³ cm, d F_D = 1-2 Hz, and N( O⁺) = 5 10⁵ cm ^-3 we obtain d H (0.6-1.2)  nT. Such values of the amplitudes of the geomagnetic field pulsations agree with the experimental data [Gul'el'mi and Troitskaya, 1973]. Some trains of the quasiperiodic variations in the geomagnetic field with an amplitude of a few tens of nanoteslas registered in Tromsö one can see, for example, in Figure 3 on 22 August within the 0700-0800 UT interval. Comparing the experimental and calculated values of the geomagnetic field, one should introduce a correction for the different location of Tromsö (69.66^o N) and the SURA facility (56.1^o N). According to Saito [1969] the pulsation amplitudes increase with latitude. The performed calculations and the data comparison are of an estimation character. A detailed comparison requires magnetogram recordings with high amplitude and temporal resolution directly in the vicinity of the SURA facility. To confirm the mechanism of quasiperiodic oscillations of the Doppler spectrum of the scattered signal related to propagation of the MHD waves generated during a magnetic substorm, coordinated radiophysical studies of AIT combined with measurements of the geomagnetic pulsations in the place of location of the heating facility are needed.

[30]  At the same time, the data available on the relation of the variations in the DSF of the scattered signal to the geophysical situation in the period of the experiment indicate to the natural source of the MHD waves responsible for the quasiperiodic oscillations of the Doppler frequency shift of the scattered signal observed during the magnetic storm.