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.
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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 10-6 W m
-2 and
1.63 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) 103 cm-3 and
(1.5-2.7) 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
1.7 103 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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