3. Results of Measurements

[7]  During the period of measurements from 31 March to 14 April 2004 several chromospheric flares occurred on the Sun (the data are taken from ftp://ftp.ngdc.gov/stp/solar.data). The strongest of them were observed in the following moments and were characterized by the following fluxes of the X-ray radiation in the 1-8 Å range: (1) beginning on 5 April at 0533 UT, maximum at 0555 UT, end at 0614 UT, the flux 0.028 erg cm-2 s-1; (2) beginning on 6 April at 1230 UT, maximum at 1328 UT, end at 1344 UT, the flux 0.032 erg cm-2 s-1; (3) beginning on 8 April at 0953 UT, maximum at 1019 UT, end at 1047 UT, the flux 0.015 erg cm-2 s-1; and (4) beginning on 11 April at 0354 UT, maximum at 0419 UT, end at 0435 UT, the flux 0.013 erg cm-2 s-1.

Figure 1
[8]  Figure 1 shows time-altitude dependence of the amplitude of the reflected extraordinary wave Ax and electron concentration Ne in the ionospheric D region during the flare on 5 April 2004. The flare duration is shown in Figure 1 by the segment of direct line. The values of the electron concentration were found using 5-min averaging and the inverse function. Figure 1 shows that during the flare at heights of the D and E regions there occurs a considerable depletion of the intensity of the radio echo of extraordinary polarization. The electron concentration in the lower ionosphere increases.

[9]  The measurements period was characterized by fairly high solar and geomagnetic activity. Because of this the majority of days in Tumanny town was characterized by auroral disturbances. Only 2 April and 14 April were quiet days and that made it possible to use these days for the comparison.

Figure 2
[10]  Figure 2 illustrates the influence of the M-class solar flare on the structure of the D region of the polar ionosphere. The values of the electron concentration were obtained with 1-min averaging. Dashed curves in Figure 2 show the time profile of the solar X-ray radiation (according to the data of the GOES 10 satellite). The maximum values of the flux on 5 April were 2.9 times 10-6  W m -2 and 1.63 times 10-5 W m-2 in the ranges 0.5-3 Å and 1-8 Å, respectively. Figure 2 shows that the flare produced a considerable increase in the electron concentration at altitudes below 85 km. The increase of the electron concentration was accompanied by changes in the lower ionosphere structure, in particular, by appearance of a two-layer region of additional ionization observed during 40 min. The time behavior of the additional ionization corresponded to the variations in the intensity of the X-ray radiation of the flare in the ranges 0.5-3 Å or 1-8 Å. Calculations of the electron concentration profiles based on theoretical models of the ionization [Smirnova et al., 1988] before and during the flare in question confirm the statement that the effect of sudden ionospheric disturbances in the D region is actually caused by the solar hard X-ray radiation.

Figure 3
[11]  Figure 3 shows vertical profiles of the electron concentration averaged over 10 min in quiet conditions (dashed curves) and at the maximum intensity of the solar flares on 5, 6 and 8 April (solid curves). The flares on 5 and 6 April occurred in the conditions of relatively weak auroral disturbance. On the background of this disturbance the additional ionization produced by the flare was distinctly seen. The maximum electron concentration was (0.7-1.0) times 103 cm-3 and (1.5-2.7) times 103 cm-3 in the height ranges 64-70 km and 77-79 km, respectively. During the flares on 8 and 11 April, an auroral disturbance occurred in Tumanny. The electron concentration in the lower D region reached values of 1.7times 103 cm -3 and the additional ionization of the flares almost was not seen.

[12]  The increase of the ionization in the lower D region is in a good qualitative agreement with the well-known effect of the increase of the amplitude of long radio waves [Al'pert, 1972; Belikovich et al., 1975; Mitra, 1974] and of the changes in their phase height [Davies, 1990; Hargreaves, 1995].

Figure 4
[13]  Figure 4 shows the power spectrum of the fluctuations of the electron concentration during the flare on 5 April 2004. The power spectrum of the fluctuations was calculated using the direct Fourier transformation of the autocorrelation function for the 60-min series of the data and was smoothed using the Tiuki spectral window [Jenkins and Watts, 1969, 1970]. One can see a manifestation of wave-like variations in the isolines of the electron concentration in Figure 2. Variations of the ionization in the polar lower ionosphere during the flares were accompanied by generation of atmospheric waves with periods longer than 3 min.

[14]  According to the data of space diversity reception of the scattered radio waves during the flare a drift of ionospheric irregularities with a velocity of not more than 100 m s -1 was detected. The presence of the horizontal velocity shear and the change of the direction of its azimuthal component to the opposite one was an interesting feature of the observed drifts in the moment of the flare maximum intensity.

Figure 5
[15]  Figure 5 shows the riometer recordings at the frequency of 32 MHz containing splashes of the space radiation during solar flares. The vertical segment in Figure 5 shows the scale of the relative measurements of the cosmic noise level. It is worth noting that similar recordings of "interferences" to riometer operation in a wide frequency range were detected also at the Finnish chain of riometer stations [University of Oulu, 2004]. The comparison of the splashes in the riometer recordings at remote stations at several frequencies with the solar phenomena occurring during this time makes it possibly to conclude that the "interferences" to riometers are not of a local origin but are generated by solar sporadic radiation. In the considered cases the splashes occurred at the phase of development of a solar flare and lasted about 10-15 min. (0540-0550 UT on 5 April, 1230-1245 UT and 1310-1325 UT on 6 April). Such splashes of the space radioemission in the meter and decameter ranges were numerously observed at high-latitude riometer stations [Brunelli and Los', 1973; Hunsucker and Hargreaves, 2003]. It is assumed [Akasofu and Chapman, 1975] that these splashes are produced by synchrotron radiation of high-energy particles emitted by the disturbed regions on the Sun. Therefore such parts of the recordings may be used for obtaining of additional information on the fluxes of high-energy solar particles.


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