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Long-period fluctuations of meteorological origin observed in the lower ionosphere
OlJ21~9169/89 $3.00+.oO Maxwell Pergamon Macmillan plc
Journal of Armospheric and Terrestrial Physics, Vol. 51, No. 5, pp. 381-388, 1989. Printed Great
in
Britain.
Long-period fluctuations of meteorological origin observed in the lower ionosphere D. PANCHEVA,E. APOSTOLOV Geophysical Institute, Bulgarian Academy of Science, ‘Academy G. Bonchev’ 3, 1113 Sofia, Bulgaria and
J. LASTOVICKAand J. BOSKA Geophysical Institute, Czechoslovakian Academy of Science, Bocni II, 141 3 1 Prague 4, Czechoslovakia (Received in$nalform
16 November 1988)
Abstract-Application of a correloperiodogram analysis to day and night-time radio wave absorption values for five LF radio paths (164, 155, 185,218 and 272 kHz) in central and southern Europe during the interval 1 June 1979-30 June 1980, fluctuations with the following quasi-periods were found: 2.3-3.2, 46 and 10.5-12 days. They exist in all the time series investigated. Other fluctuations with periods of 7-9 days are observed mainly in central Europe. Shorter period fluctuations are most active during the solstices and especially during winter, while the longer period fluctuations (10.5-12 days) have significant amplitudes also in spring. The analysis of satellite data on the intensity of ionizing solar H Lyman-alpha and X-ray (0.14.8 nm) radiation, of the variations of the geomagnetic activity (au(N)-index) and of lower thermospheric wind (m 95 km) in central Europe, shows that the fluctuations observed in absorption are caused not by fluctuations in the solar ionizing flux, but probably by fluctuations in the neutral atmosphere. This could offer a possibility of monitoring fluctuations of the planetary wave period range in the upper mesosphere and lower thermosphere by means of radio-wave absorption measurements.
1. INTRODUCTION Many studies on the variations of the upper atmosphere parameters concern fluctuations with time scales of one day to a few weeks. These fluctuations are planetary oscillations which play an important role, not only in the lower atmosphere, where they significantly contribute to the atmospheric weather formation, but also in the upper atmosphere, where their effect on the electron concentration and on radiowave absorption is important. Many authors found various planetary waves with periods of about 5 days-two weeks in the D-region electron density (e.g. LAUTER,1974; LASTOVICKA,1976; AP~~TOLOVet al., 1976), or in winds at 8&100 km (e.g. GLASS and SPIZZICHINO, 1974; TEPTIN et al., 1970; SALBY and ROPER, 1980 ; MANS~N et al., 198 1; CHERNOBROVKINA,1985 ; MAN~~Nand MEEK, 1986 ; VINCENT and CRAIG, 1985) and/or simultaneously in radiowave absorption in the lower ionosphere and in tropospheric and stratospheric parameters, particularly for the winter period (e.g. SCHWENTEK,1974; DELAND and FRIEDMAN, 1972 ; FRASER and THORPE, 1976 ; FRASER, 1977; PANCHEVAet al., 1985). A survey of
experimental and theoretical findings on atmospheric planetary-scale travelling waves was given by SALBY (1984). The object of the present paper is to investigate the periodic fluctuations for the 2-l 5 day range in the day and night-time radio-wave ionospheric absorption in central and southern Europe during the interval 1 June 1979-30 June 1980. In order to interpret the results, we investigate periodic fluctuations in the lower thermospheric wind, the solar ionizing flux and geomagnetic activity during the same interval. The investigated period (1979/1980) corresponds to the near-maximum part of the 21st sunspot cycle.
2. ABSORPTION DATA AND METHOD
We use averaged night- and day time (cos x = 0.2) absorption data obtained by the A3 method (oblique incidence) for five LF radio paths. Their characteristics are presented in Table 1. For all five radio paths, the height of reflection at cos x = 0.2, is within the 75-85 km interval, while during the night it varies from 85 to 100 km for different paths. From Table 1
381
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PANCHEVA et al.
Table 1. Parameters
Transmitterreceiver
Frequency-distance equivalent frequency
Allouiss Sofia Brasov-
164 kHz-1720 km 25 kHz 155 kHz-370 km 75 kHz 185 kHz-182 km 120 kHz 218 kHz-75 km 200 kHz 272 kHz-236 km 170 kHz
Sofia
BerlinKtihlungsbom Monte CarloRoburent TopolnaPruhonice
we can see that the data used provides information on a region over Europe with a latitudinal range of 44”N53S”N and within the longitudinal interval 7S”E24.4”E. The time series considered (without the 272 kHz radio path) have 396 values each. The data from the 272 kHz path are only available within the interval 1 September 1979-23 January 1980, and this time series (day and night-time) only contains 145 values each. To detect the periodicities of the time series under investigation, we use the method of correloperiodogram analysis (KOPECK~ and KUKLIN, 1971) as a special case of the classical periodogram analysis. The presence of periodic fluctuations in the parameters considered is expressed by maxima in the amplitude spectrum.
of A3 measuring
paths
Reflection point 4
Y
Reflection altitude night day
45S”N,
13.2”E
75 km
44.2”N,
24.4”E
53.5”N,
12.6”E
44”N,
7.5”E
85 km
100 km
49.3”N,
16”E
85 km
100 km
85-90 km 95 km 955100 km
level 0.9 is selected, marked by a horizontal line in all spectra. Arrows in the right hand side of the figure above and under this line denote confidence levels 0.99 and 0.75, respectively. From Fig. 1 it can be seen that within the periodic range of 2-15 days in which we are interested, the highest amplitudes are exhibited by the spectral lines at about 11 days for 164 kHz and at about 12 days for 218 kHz. Besides these, there are weaker spectral lines around confidence level 0.75, namely those corresponding to periods of 5.6 days in the two spectra and 8.6 days for a frequency of 218 kHz. Figure 2 presents the amplitude spectra of the variations of night-time absorption at frequencies 164, 155, 185 and 218 kHz. The spectral lines corresponding to periods of about 10-l 1 days have the highest amplitudes for all frequencies. There are also
3. FLUCL’UATIONS IN RADIO-WAVE ABSORPTION Figure 1 presents the amplitude spectra of the day time radio-wave absorption variations of the 164 kHz and 218 kHz signals. The test period is given within the periodic range 2-30 days with a step of 0.2 days. As a criterion for statistical significance a confidence
30
I
I
I
I
I
I
25
20
15
IO
5
2
PERIOD
Fig. 1. Amplitude
NIGHT
-
I 15 (DAYS)
spectra, day time, 164 and 218 kHz radiowave absorption.
10
1
I
5
2 PERIOD
0.9
16LkHz
0.9
185 kHz
(DAK)
Fig. 2. Amplitude spectra, night-time, 164, 155, 185 and 218 kHz radio-wave absorption.
383
Fluctuations observed in the lower ionosphere
\A
&j’--
A
-0.9
NIGHT
2
x0 2
5
_
i$Z03 DAY
l“f-
L
I 30
1
I
I
I
I
I
25
20
15
10
5
2 PERIOD
I (DAYS)
Fig. 3. Amplitude spectra, day and night-time, 272 kHz radio-wave absorption, 1 September 1979-23 January 1980.
spectral lines with a lower amplitude, grouped around periods 3-5 and 8-9 days. In the shorter time series at a frequency of 272 kHz (Fig. 3), both for day and night-time conditions, the correloperiodogram analysis also shows intensive fluctuations with a period of 11 days and weaker 3- and &day fluctuations. All these periodic fluctuations in the time variations of the day and night-time ionospheric absorption can be generally grouped in the periodic ranges 2.5-3,46, 8-9 and 10.5-12 days; and the analysis made by PANCHEVAet al. (19X7), on the basis of the same absorption data, by a filter technique and autocorrelation analysis, showed their fundamental character. They are not a noise characteristic in the spectra or harmonics of a higher order. In order to investigate the development of these fluctuations in time, we apply the correloperiodogram analysis to different temporal segments. The length of each segment is 64 days and they are shifted by 20 days (10 days at 272 kHz). For each temporal segment only those periodic fluctuations are taken into account whose spectral lines have an amplitude around and above the confidence level 0.9. The results for the day time series of the absorption data are given in Table 2, and for the night-time data-in Table 3. The data are grouped in seasons. The amplitudes indicated in the tables correspond to the mean value of the observed amplitudes in the respective periodic ranges. The fluctuations with quasi-periods of 2.3-3.2, 4-6 and 10.5-12 days are observed in all five LF radio paths for central and southern Europe. Other fluctuations with periods about 7-9 days are observed mainly in the northern radio paths (185 kHz and 272 kHz). In the lower D-region (75-85 km), i.e. the day time series, the 2.>3.2-day ~u~uations have their main maximum in winter and almost the same sec-
ondary maxima in summer and spring, while in the higher D-region (85-100 km), i.e. the night-time series, the maxims of amplitudes in spring and summer becomes the main maximum, with a secondary maximum in winter. The 4-6day and 7-9-day fluctuations are most intensive in winter both in the lower and higher D-region. The most clear-cut fluctuations, in the time variations of ionospheric absorption during the whole interval under investigation June 1979June 1980, is the 10.5-12-day one. In the lower D-region its amplitudes are maximum during the equinox months while in the higher D-region the fluctuation has its main maximum in winter and secondary in summer and spring.
4. FLUCTUATIONS IN IONIZINGRADIATiONAND GEOMAGNETIC ACTIVITY We now try to clarify the nature of the observed fluctuations in absorption. The height region responsible for the LF radio wave absorption studied is ionized mainly by the H-Lyman-alpha (121.6 nm) solar radiation (direct in the day time, geocoronally scattered at night). The solar X-ray radiation within the wave-range 0.1-0.8 nm also exerts an important influence on day time absorption. In the analysis of the time vacations of solar X-ray radiation for the period 1969-1972 (APOS~LO~ and LEWS, 1975), an intensive period of 11.8 days was found, which was not observed for the same time interval in the generally used solar index F,0,7. In our study, we use the average day time X-ray data (0.14.8 nm) published by BOUWERet al. (1982). We use the Lyman-alpha data measured by the AEE satellite. It is generally accepted that these data need some correction in the years 1979-1980 (e.g. LEAN, 1987). The correction method (developed by BOSKA, 1986) is based on the F,*,,-dependent ratio of FJF# and on the Lyman-beta llux measurements performed aboard the AE-E satellite. Because of a large gap in the Lyman-alpha data during February and March 1980. they are divided into two time-series of different lengths: 1 June 1979-31 January 1980 (245 values) and 1 April-30 June 1980 (91 values). To cover the gap, we use the solar radio flux at 2800 MHz as a parameter describing generally the solar wave radiation. Fluxes of high energy particles are an important ionization source of the night-time D-region but their precipitation can only play a dominant role during geoma~eti~ storms, or several days after, and can hardly introduce the observed periodic ~uctuations into absorption.
However,
as a check, we analyzed the
D. PANCHFVA et al.
384
Table 2. Seasonal distribution of the basic fluctuations and their amplitudes in day time, 164 and 218 kHz radio-wave absorption 164 kHz
Season Summer
Period (days)
212 kHz
Amplitude (dB)
3.3-3.8 4.k6.2
1.2 1.6
11.1-11.8
1.8
4.14.8 10.7~11.3
1.9 2.6
4.5-5.7 7.2-8.0 10.5~11.4
2.3 2.3 2.5
4.3-5.8 7.2-1.7 ll.(tl1.7
2.2 1.8 3.0
Autumn
Period (days)
2.8 6.5 10.9-I 1.2
218 kHz Amplitude Cd@
1.6 2.8 2.9
Winter 6.M.7
2.2
10.0-10.9
3.1
Spring
geomagnetic au(N)-index as an indirect characteristic of particle precipitation in middle latitudes of the Northern Hemisphere. The amplitude spectra of Lyman-alpha and solar flux at 2800 MHz for the time-interval 1 June 197931 January 1980 are presented in Fig. 4a, and the amplitude spectra of solar flux at 2800 MHz, X-rays and au(N)-index for the whole period investigated are presented in Fig. 4b. The almost perfect coincidence between the amplitude spectra of Lyman-alpha and solar-flux at 2800 MHz is evident from Fig. 4a. The statistically significant spectral lines (above significant level 0.9) are only those with periods of 26.8 days (solar rotation) and of 15.8 and 13.2 days. The statistically significant periods observed in ionospheric absorption are not observed in the Lyman-alpha flux. The statistically significant spectral lines for the Xray flux are those corresponding to periods of 12.1 and 13.2 days and for the aa(index : 5.5, 6.2, 7.4, 8.2, 9.4 and 10.6 days. In order to investigate the seasonal variations of the amplitude and the characteristic periods of these fluctuations, we again apply the correloperiodogram analysis to temporal segments, identical with those used for the absorption analysis. The results for the X-ray flux and au(N)-index are presented in Table 4. The 1Zday fluctuation in the X-ray flux in June 1979June 1980 has a changeable period : longer in autumn and winter (12.8-15.8 days) with the shortest one in spring (10-12.2 days). This fluctuation has its maximum amplitude in winter, but its period is considerably longer than the observed lO_lZday fluctuation in the day time ionospheric absorption. In the
Period (days) 2.4-2.8 5.5-5.9 7.8-8.2 ll.lL11.7 2.8-3.2 5.0-5.1 12.0-12.6 2.1-2.8 4.3-5.5 8.0-9.0 9.8-l 1.8 2.3 5.9-6.5 8.2-9.2 10.8-I 1.8
Amplitude (dB) 2.8 2.4 2.5 2. 2.2 1.8 5.4 4.0 4.6 3.0 3.5 2.8 3.8 3.1 5.0
X-ray flux there exist other spectral peaks with smaller amplitudes, such as a 6.2-7-day fluctuation in summer and autumn (which completely vanishes in winter and spring), and a 8.4-lo-day fluctuation in winter and spring with a small amplitude. These probably cannot influence the ionization state of the ionosphere. Compare the results from Tables 2 and 4, we see that : (a) there is no correspondence between the statistically significant periods in day time ionospheric absorption and solar X-ray flux during the seasons and (b) the fluctuations in ionospheric absorption have maximum amplitudes in winter, while those in X-rays have maximum amplitudes in summer and autumn. This shows that the fluctuations in the day time ionospheric absorption are not generated by similar fluctuations in the solar X-ray radiation. The same conclusion can be made for the fluctuations in the Lyman-alpha flux and in the solar flux at 2800 MHz. Their statistically significant peaks have periods longer than 13.2 days in all seasons. Comparing the statistically significant periods during the seasons for the night-time absorption and the geomagnetic au(N)-index (Tables 3 and 4), we notice a comparatively good coincidence in the simultaneous appearance of fluctuations in the period intervals : 46.5, 7-9 and l&12 days (except for winter). 4-6.5and 7-9-day fluctuations in the geomagnetic activity are most intensive during summer, while the l&12day fluctuation has its maximum amplitude in summer and autumn. Fluctuations in absorption exhibit a different seasonal dependence of their amplitude. The fluctuations investigated in the geomagnetic activity could slightly stimulate analogous fluctuations
spring
Winter
Autumn
Summer
Season ~__lll_
1.8
11.5-11.7
1.2 1.1 1.8
10.0-12.0 2.1-2.4 5.0-6.0 8.0-6.6 11.0
1.4
10.0-12.0
5.5-5.9 8.2-9.1 10.7-12.8
4.5-6.0 7.0-8.0 10.9-11.3 2.4-2.8 4.2-5.5
0.9 0.9 1.2 1.25 1.2
7.0-8.0 10.6-I 1.9
Period (days)
1.4 1.0 1.1 0.8 1.1
1.0 1.2 1.2 1.1 1.2
0.9 1.2
1.2 1.6 1.2 1.1 1.3 1.3 1.2 1.3 1.5 1.8 1.35 1.1 1.2 1.6
1.4
6.1-6.8 8.0-9.2 IO.%-1 1.0
6.5-7.5 10&l 1.2
(days)
Period
1.4 1.3 2.0
1.35 1.7
(dB)
Amplitude
272 kHz --~
164, 155, 185, 272 and 218 kHz radio-wave
Amplitude (dB)
185 kHz
in night-time,
7.5-9.0 10.2-12.0 2.2-2.7 4.5-5.1 7.3-8.5 11.5-12.4 2.2-3.0 4.3-5.7 8.0-9.5 11.1-12.7 2.3-3.2 4.5-6.2 8G9.6 11.0-12.0
2.4-3.6
Period (days)
and their amplitudes
Amplitude (dB)
155 kHz
of the basic fluctuations
4.5-6.0 7.8-8.1 10.4-11.0 2.1-3.4 4.0-5.5
5.4-5.8
1.15 1.1
Amp~tud~ (dB) ___________
2.3-3.3
Period (days)
..-
distribution
164 kHz
Table 3. Seasonal
10.6-12.2 2.2-2.8 4.7-6.0 7.5-8.5 10.5-12.5 2.4.--3.2 4.M.2 8.CL9.8
10.7-12.3 2.3-3.7 5.5
2.1-3.5 4.5-6.0
Period (days)
1.4 1.1 1.3 1.0 2.0 1.25 1.2 1.6
1.8 0.8 0.9
1.35 1.2
Amplitude (dB)
218 kHz
absorption
FZ 6 g 2 6’ g B a ;t:
s 8 f.
? F 1 B ‘-. E :
386
D. PANCHEVA et al. I
Lag
2 z z w
o_
r
Q
I
v L”--.----
I
I
/
I
I
30
25
20
15
10
I
-
0.9
2800MHz
I
2 PERIODiDAYS) 5
J
Fig. 4a. Amplitude spectra, corrected AE-E Loan-alpha data and solar flux at 2800 MHz, 1 June 1979-31 January 1980.
Z~NAL
-
$C c_
*
a9
2-
a’2-
A
1
L-
-/v o-
-
n
1
I
I
I
I
30
25
20
15
10
5 PERIOD
2
-.
(DAYS)
Fig. 5. Amplitude spectra for zonal and meridional wind (9&100 km) from Collm, Central Europe (52”N, 15”E).
Figure 5 presents the amplitude spectra of zonal and meridional winds (night-time) for the entire period investigated. Above the confidence level 0.9 for the zonal wind there are fluctuations with periods 7.4 and 20 A 9.4 days, and above the confidence level 0.75-, 3- and 5-day fluctuations. In the meridional wind, spectral peaks are observed above the 0.9 confidence level with periods of 2.4 days with a series of peaks in the interval 5-7 days, 9.8 and 12 days. These periods coincide fairly well with the periods established in the radiowave absorption. In order to investigate the developCl9X-RAY E 5ment of these fluctuations in time, we again use the 5 correloperiodogram analysis on temporal segments. : o2 LThe results for the different seasons with spectral 1 peaks above confidence level 0.9 are presented in 8 2 2 Cl9 aa Table 5. The IO-12-day ~uctuation in wind has its Q OrLi i i I I basic amplitude maximum in winter with a secondary 30 25 20 15 10 5 2 one in summer. This result coincides with the seasonal PERIOD (DAYS) variation of the IO-12-day fluctuation in the nightFig. 4b. Amplitude spectra, solar flux at 2800 MHz, X-ray time ionospheric absorption, for the altitude interval flux and geomagnetic aa(index. of 85-100 km, at which level the wind is measured. The 7-9 day fluctuation in the zonal and meridional wind has its basic maximum also in winter and a secondary one in summer. As this fluctuation is very in the night-time ionospheric absorption, but they strong, especially in the zonal wind, it is probably are not a dominant factor. connected with the analogous fluctuation in the nighttime absorption. In contrast to the above mentioned 5. FLUCTUATIONS IN WIND fluctuations, the 4.5-6-day fluctuation has a different seasonal behaviour. In the zonal wind the basic ampliVariations of the prevailing wind in the lower thertude maximum is observed in winter and a secondary mosphere play a very important role in the meteoroone in summer, while in the meridional wind it is logical control of the lower ionosphere as demonthe other way around, the basic maximum being in strated, for example, by the lower ionosphere response to sudden stratospheric warmings (e.g. LASTOVICKA, summer with a secondary one in winter. In the nighttime as well as in the day time ionospheric absorption, 1984).Hence, we used the prevailing wind measured the basic maximum of this fluctuation is in winter continuously at Collm (GDR) by the Dl method and a secondary one in summer, but with increasing (ionospheric drift) at * 90-100 km, as a neutral atmoaltitude the secondary summer maximum grows sphere parameter representing the effect of dynamics larger. The fluctuations in the zonal wind are likely to on the LF radio wave absorption. The Collm data exert a stronger influence on the variations in ionowell represents the wind for the two paths in central spheric absorption, but with increasing altitude the Europe (185 and 272 kHz). I,1
Fluctuations observed in the lower ionosphere
387
Table 4. Seasonal distribution of the basic fluctuations and their amplitudes in X-ray flux and geomagnetic au(iV)-index X-ray Season
Period (days)
aa(l\r)-index
Amplitude (w m-‘)
Summer
Autumn
7.0 12.613.4 6.2-6.6
13.0 14.5 13.5
12.8-14.8
14.0
9.2-10.0
9.0
13&15.8
17.0
8.4-8.6 10.612.2
9.9 10.5
Amplitude (nT)
5.862 7.2-7.8 11.6-12.8 6.4 8.C8.6 9.8-l 1.2 6.c7.2 9.49.8 10.8%12.6 14.0 4.k6.2 8.2 ll.Gll.2
Winter
Spring
influence of the meridional wind becomes greater. The most interesting fluctuation is the shortest periodic one of 2.3-3.2 days observed in the zonal, as well as in the meridional wind. The basic maximum of this fluctuation is in summer and a secondary one in winter and spring. The same pattern is observed in the analogous fluctuation in ionospheric absorption. Since the possible ionizing sources [Lyman-a, X-rays, au(N)] do not display such a high frequency fluctuation, we can assert that the neutral wind generates this fluctuation in ionospheric absorption.
Period (days)
6.5 9.0 7.0 4.8 5.5 5.3 5.2 7.5 4.7 8.5 4.8 4.1 4.2
6.
CONCLUSION
Strictly speaking, the fluctuations in the period ranges 2.5-3, 4-6, 7-9 and 10.5-12 days are quasiperiodic rather than periodic, since both the amplitude and period vary with season, and this is particularly true for the fluctuations with periods shorter than 10 days. Summarizing, we may say that the fluctuations with periods of 2.3-3.2, 4-6 and 10.5-12 days found to exist in the ionospheric absorption of radio waves
Table 5. Seasonal distribution of the basic fluctuations and their amplitudes in zonal and meridional wind (9&100 km) from Collm Zonal wind
Season Summer
Autumn
Winter
Spring
Meridional wind
Period (days)
Amplitude (m ss’)
Period (days)
Amplitude (m s-l)
2.1-3.5 4.2-5.8 7.c9.0 1l.gl2.8 2.2-2.7 5.0-6.0 7.0-9.0 10.0-l 1.5 2.43.2 4.8-6.0 7.2-9.6 10.3-I 1.7 13.5-14.3 2.43.2 4.G5.4 7.0-9.0
11.0 4.8 7.0 6.0 4.0 4.0 6.5 5.0 5.8 8.5 7.8 7.2 4.8 5.8 6.0 6.5
2.1-3.1 4.1-5.8 6.6-7.5 10.9-12.4 3.&3.5 4.1.-5.8
6.0 7.5 4.0 4.0 3.5 4.4
10.&12.8 2.2-3.0 4.5-6.0 7.s9.0 lO.t%l2.5
3.0 5.0 5.2 4.5 4.8
2.1-3.5 4.560 11.1-12.4
4.0 3.8 4.2
388
D. PANCHEVA et al.
in central and southern Europe, and with periods between 7 and 9 days found in central Europe seem to reflect fluctuations in winds in the lower thermosphere (i.e. in the parameters of the neutral atmosphere). They are not caused by fluctuations in the solar ionizing flux (Lyman-cr, X-rays). The dominant periods in various parameters are known to change to some extent interannually. The probable meteorological origin of the observed fluc-
tuations in radio wave absorption seems to offer a possibility of monitoring fluctuations in the upper mesosphere and lower thermosphere in the planetary wave period range by means of radio-wave absorption measurements. However, to establish a meteorological origin and to clarify the question of the stability of periods, further examination is necessary. An investigation of the same parameters over an extended period (July 198CJuly 1985) is underway.
REFERENCES A~~STOLOVE., LETFUSV. and NESTOROV G. APOSTOLOVE. and LETFUSV. BOUWERS. D. BOUWERS., DONNELLYR. F., FALCONJ., QUINTANAA. and CALDWELL G.
1916 1975 1983 1982
CHERNOBROVKINAN. A. DELANDR. and FRIEDMANR. FRASERG. FRASERG. and THORPEM. GLASSM. and SPIZZICHINO A. KOPECK~M. and KUKLING. LASTOVIEKA J. LASTOVIEKA J. LAUTERE. A.
1985 1972 1911 1976 1974 1971 1916 1984 1974
LEANJ. MAN~~NA. and MEEKC. MAN~~NA., MEEKC. and GREGORYJ. PANCHEVA D., Apos~o~ov E., NE~T~ROVG. and LASTOVICKA J. SALBYM. SALBYM. and ROPERR. SCHWENTEK H. TEPTING., KAZAKOVAT. and NAZARENKOV.
1987 1986 1981 1987
J. geophys. Res. 92,839. J. atmos. terr. Phys. 48, 1039. Handbook for MAP 2, 101. Studia geophs. geod. 31, 301.
1984 1980 1974 1970
Rev. Geophys. Space Phys. 22,209. J. atmos. Sci. 37, 231. J. atmos. terr. Phys. 36, 1113. Wind, Drift and Inhomogenities in the Ionosphere, p. 104. Novosibirsk (in Russian).
E%OSKAJ.
1986
PANCHEVAD. and APOSTOLOVE. VINCENTR. and CRAIG R.
1985 1985
PhD. Thesis, Geophysical Institute, Prague (in Czechoslovakian). Paper 11.02.10, 5th Sci. Ass. IAGA/IAMAP, Prague. Paper 11.03.03, 5th Sci. Ass. IAGA/IAMAP, Prague.
C. r. Acad Bulg. Sci. 29, 3. Bull. astr. Inst. Csl. 26, 193. J. geophys. Res. 88,7823. A Summary of Solar l-8 A. Measurements from the SMS and GOES satellite 1977-1981, NOAA Tech. Memor. ERL SEL-62. Issled. geom. aeronom. Fiz. Solnca 71, 34 (in Russian). J. atmos. terr. Phys. 34,295. J. atmos. terr. Phys. 39, 121. J. atmos. terr. Phys. 38, 1003. J. atmos. terr. Phys. 36, 1825. Issled. geom. aeronom. Fiz. Solnca 2, 167 (in Russian). Beitr. Geophys. 85, 276. Adv. Space Res. 4, 61. Proc. COSPAR Symp. Methods of Measurements and Results of Lower Ionosphere Structure, 377. Akad.
Vlg., Berlin, GDR.
Reference is also made to the following unpublished material:
Journal of Armospheric and Terrestrial Physics, Vol. 51, No. 5, pp. 381-388, 1989. Printed Great
in
Britain.
Long-period fluctuations of meteorological origin observed in the lower ionosphere D. PANCHEVA,E. APOSTOLOV Geophysical Institute, Bulgarian Academy of Science, ‘Academy G. Bonchev’ 3, 1113 Sofia, Bulgaria and
J. LASTOVICKAand J. BOSKA Geophysical Institute, Czechoslovakian Academy of Science, Bocni II, 141 3 1 Prague 4, Czechoslovakia (Received in$nalform
16 November 1988)
Abstract-Application of a correloperiodogram analysis to day and night-time radio wave absorption values for five LF radio paths (164, 155, 185,218 and 272 kHz) in central and southern Europe during the interval 1 June 1979-30 June 1980, fluctuations with the following quasi-periods were found: 2.3-3.2, 46 and 10.5-12 days. They exist in all the time series investigated. Other fluctuations with periods of 7-9 days are observed mainly in central Europe. Shorter period fluctuations are most active during the solstices and especially during winter, while the longer period fluctuations (10.5-12 days) have significant amplitudes also in spring. The analysis of satellite data on the intensity of ionizing solar H Lyman-alpha and X-ray (0.14.8 nm) radiation, of the variations of the geomagnetic activity (au(N)-index) and of lower thermospheric wind (m 95 km) in central Europe, shows that the fluctuations observed in absorption are caused not by fluctuations in the solar ionizing flux, but probably by fluctuations in the neutral atmosphere. This could offer a possibility of monitoring fluctuations of the planetary wave period range in the upper mesosphere and lower thermosphere by means of radio-wave absorption measurements.
1. INTRODUCTION Many studies on the variations of the upper atmosphere parameters concern fluctuations with time scales of one day to a few weeks. These fluctuations are planetary oscillations which play an important role, not only in the lower atmosphere, where they significantly contribute to the atmospheric weather formation, but also in the upper atmosphere, where their effect on the electron concentration and on radiowave absorption is important. Many authors found various planetary waves with periods of about 5 days-two weeks in the D-region electron density (e.g. LAUTER,1974; LASTOVICKA,1976; AP~~TOLOVet al., 1976), or in winds at 8&100 km (e.g. GLASS and SPIZZICHINO, 1974; TEPTIN et al., 1970; SALBY and ROPER, 1980 ; MANS~N et al., 198 1; CHERNOBROVKINA,1985 ; MAN~~Nand MEEK, 1986 ; VINCENT and CRAIG, 1985) and/or simultaneously in radiowave absorption in the lower ionosphere and in tropospheric and stratospheric parameters, particularly for the winter period (e.g. SCHWENTEK,1974; DELAND and FRIEDMAN, 1972 ; FRASER and THORPE, 1976 ; FRASER, 1977; PANCHEVAet al., 1985). A survey of
experimental and theoretical findings on atmospheric planetary-scale travelling waves was given by SALBY (1984). The object of the present paper is to investigate the periodic fluctuations for the 2-l 5 day range in the day and night-time radio-wave ionospheric absorption in central and southern Europe during the interval 1 June 1979-30 June 1980. In order to interpret the results, we investigate periodic fluctuations in the lower thermospheric wind, the solar ionizing flux and geomagnetic activity during the same interval. The investigated period (1979/1980) corresponds to the near-maximum part of the 21st sunspot cycle.
2. ABSORPTION DATA AND METHOD
We use averaged night- and day time (cos x = 0.2) absorption data obtained by the A3 method (oblique incidence) for five LF radio paths. Their characteristics are presented in Table 1. For all five radio paths, the height of reflection at cos x = 0.2, is within the 75-85 km interval, while during the night it varies from 85 to 100 km for different paths. From Table 1
381
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PANCHEVA et al.
Table 1. Parameters
Transmitterreceiver
Frequency-distance equivalent frequency
Allouiss Sofia Brasov-
164 kHz-1720 km 25 kHz 155 kHz-370 km 75 kHz 185 kHz-182 km 120 kHz 218 kHz-75 km 200 kHz 272 kHz-236 km 170 kHz
Sofia
BerlinKtihlungsbom Monte CarloRoburent TopolnaPruhonice
we can see that the data used provides information on a region over Europe with a latitudinal range of 44”N53S”N and within the longitudinal interval 7S”E24.4”E. The time series considered (without the 272 kHz radio path) have 396 values each. The data from the 272 kHz path are only available within the interval 1 September 1979-23 January 1980, and this time series (day and night-time) only contains 145 values each. To detect the periodicities of the time series under investigation, we use the method of correloperiodogram analysis (KOPECK~ and KUKLIN, 1971) as a special case of the classical periodogram analysis. The presence of periodic fluctuations in the parameters considered is expressed by maxima in the amplitude spectrum.
of A3 measuring
paths
Reflection point 4
Y
Reflection altitude night day
45S”N,
13.2”E
75 km
44.2”N,
24.4”E
53.5”N,
12.6”E
44”N,
7.5”E
85 km
100 km
49.3”N,
16”E
85 km
100 km
85-90 km 95 km 955100 km
level 0.9 is selected, marked by a horizontal line in all spectra. Arrows in the right hand side of the figure above and under this line denote confidence levels 0.99 and 0.75, respectively. From Fig. 1 it can be seen that within the periodic range of 2-15 days in which we are interested, the highest amplitudes are exhibited by the spectral lines at about 11 days for 164 kHz and at about 12 days for 218 kHz. Besides these, there are weaker spectral lines around confidence level 0.75, namely those corresponding to periods of 5.6 days in the two spectra and 8.6 days for a frequency of 218 kHz. Figure 2 presents the amplitude spectra of the variations of night-time absorption at frequencies 164, 155, 185 and 218 kHz. The spectral lines corresponding to periods of about 10-l 1 days have the highest amplitudes for all frequencies. There are also
3. FLUCL’UATIONS IN RADIO-WAVE ABSORPTION Figure 1 presents the amplitude spectra of the day time radio-wave absorption variations of the 164 kHz and 218 kHz signals. The test period is given within the periodic range 2-30 days with a step of 0.2 days. As a criterion for statistical significance a confidence
30
I
I
I
I
I
I
25
20
15
IO
5
2
PERIOD
Fig. 1. Amplitude
NIGHT
-
I 15 (DAYS)
spectra, day time, 164 and 218 kHz radiowave absorption.
10
1
I
5
2 PERIOD
0.9
16LkHz
0.9
185 kHz
(DAK)
Fig. 2. Amplitude spectra, night-time, 164, 155, 185 and 218 kHz radio-wave absorption.
383
Fluctuations observed in the lower ionosphere
\A
&j’--
A
-0.9
NIGHT
2
x0 2
5
_
i$Z03 DAY
l“f-
L
I 30
1
I
I
I
I
I
25
20
15
10
5
2 PERIOD
I (DAYS)
Fig. 3. Amplitude spectra, day and night-time, 272 kHz radio-wave absorption, 1 September 1979-23 January 1980.
spectral lines with a lower amplitude, grouped around periods 3-5 and 8-9 days. In the shorter time series at a frequency of 272 kHz (Fig. 3), both for day and night-time conditions, the correloperiodogram analysis also shows intensive fluctuations with a period of 11 days and weaker 3- and &day fluctuations. All these periodic fluctuations in the time variations of the day and night-time ionospheric absorption can be generally grouped in the periodic ranges 2.5-3,46, 8-9 and 10.5-12 days; and the analysis made by PANCHEVAet al. (19X7), on the basis of the same absorption data, by a filter technique and autocorrelation analysis, showed their fundamental character. They are not a noise characteristic in the spectra or harmonics of a higher order. In order to investigate the development of these fluctuations in time, we apply the correloperiodogram analysis to different temporal segments. The length of each segment is 64 days and they are shifted by 20 days (10 days at 272 kHz). For each temporal segment only those periodic fluctuations are taken into account whose spectral lines have an amplitude around and above the confidence level 0.9. The results for the day time series of the absorption data are given in Table 2, and for the night-time data-in Table 3. The data are grouped in seasons. The amplitudes indicated in the tables correspond to the mean value of the observed amplitudes in the respective periodic ranges. The fluctuations with quasi-periods of 2.3-3.2, 4-6 and 10.5-12 days are observed in all five LF radio paths for central and southern Europe. Other fluctuations with periods about 7-9 days are observed mainly in the northern radio paths (185 kHz and 272 kHz). In the lower D-region (75-85 km), i.e. the day time series, the 2.>3.2-day ~u~uations have their main maximum in winter and almost the same sec-
ondary maxima in summer and spring, while in the higher D-region (85-100 km), i.e. the night-time series, the maxims of amplitudes in spring and summer becomes the main maximum, with a secondary maximum in winter. The 4-6day and 7-9-day fluctuations are most intensive in winter both in the lower and higher D-region. The most clear-cut fluctuations, in the time variations of ionospheric absorption during the whole interval under investigation June 1979June 1980, is the 10.5-12-day one. In the lower D-region its amplitudes are maximum during the equinox months while in the higher D-region the fluctuation has its main maximum in winter and secondary in summer and spring.
4. FLUCTUATIONS IN IONIZINGRADIATiONAND GEOMAGNETIC ACTIVITY We now try to clarify the nature of the observed fluctuations in absorption. The height region responsible for the LF radio wave absorption studied is ionized mainly by the H-Lyman-alpha (121.6 nm) solar radiation (direct in the day time, geocoronally scattered at night). The solar X-ray radiation within the wave-range 0.1-0.8 nm also exerts an important influence on day time absorption. In the analysis of the time vacations of solar X-ray radiation for the period 1969-1972 (APOS~LO~ and LEWS, 1975), an intensive period of 11.8 days was found, which was not observed for the same time interval in the generally used solar index F,0,7. In our study, we use the average day time X-ray data (0.14.8 nm) published by BOUWERet al. (1982). We use the Lyman-alpha data measured by the AEE satellite. It is generally accepted that these data need some correction in the years 1979-1980 (e.g. LEAN, 1987). The correction method (developed by BOSKA, 1986) is based on the F,*,,-dependent ratio of FJF# and on the Lyman-beta llux measurements performed aboard the AE-E satellite. Because of a large gap in the Lyman-alpha data during February and March 1980. they are divided into two time-series of different lengths: 1 June 1979-31 January 1980 (245 values) and 1 April-30 June 1980 (91 values). To cover the gap, we use the solar radio flux at 2800 MHz as a parameter describing generally the solar wave radiation. Fluxes of high energy particles are an important ionization source of the night-time D-region but their precipitation can only play a dominant role during geoma~eti~ storms, or several days after, and can hardly introduce the observed periodic ~uctuations into absorption.
However,
as a check, we analyzed the
D. PANCHFVA et al.
384
Table 2. Seasonal distribution of the basic fluctuations and their amplitudes in day time, 164 and 218 kHz radio-wave absorption 164 kHz
Season Summer
Period (days)
212 kHz
Amplitude (dB)
3.3-3.8 4.k6.2
1.2 1.6
11.1-11.8
1.8
4.14.8 10.7~11.3
1.9 2.6
4.5-5.7 7.2-8.0 10.5~11.4
2.3 2.3 2.5
4.3-5.8 7.2-1.7 ll.(tl1.7
2.2 1.8 3.0
Autumn
Period (days)
2.8 6.5 10.9-I 1.2
218 kHz Amplitude Cd@
1.6 2.8 2.9
Winter 6.M.7
2.2
10.0-10.9
3.1
Spring
geomagnetic au(N)-index as an indirect characteristic of particle precipitation in middle latitudes of the Northern Hemisphere. The amplitude spectra of Lyman-alpha and solar flux at 2800 MHz for the time-interval 1 June 197931 January 1980 are presented in Fig. 4a, and the amplitude spectra of solar flux at 2800 MHz, X-rays and au(N)-index for the whole period investigated are presented in Fig. 4b. The almost perfect coincidence between the amplitude spectra of Lyman-alpha and solar-flux at 2800 MHz is evident from Fig. 4a. The statistically significant spectral lines (above significant level 0.9) are only those with periods of 26.8 days (solar rotation) and of 15.8 and 13.2 days. The statistically significant periods observed in ionospheric absorption are not observed in the Lyman-alpha flux. The statistically significant spectral lines for the Xray flux are those corresponding to periods of 12.1 and 13.2 days and for the aa(index : 5.5, 6.2, 7.4, 8.2, 9.4 and 10.6 days. In order to investigate the seasonal variations of the amplitude and the characteristic periods of these fluctuations, we again apply the correloperiodogram analysis to temporal segments, identical with those used for the absorption analysis. The results for the X-ray flux and au(N)-index are presented in Table 4. The 1Zday fluctuation in the X-ray flux in June 1979June 1980 has a changeable period : longer in autumn and winter (12.8-15.8 days) with the shortest one in spring (10-12.2 days). This fluctuation has its maximum amplitude in winter, but its period is considerably longer than the observed lO_lZday fluctuation in the day time ionospheric absorption. In the
Period (days) 2.4-2.8 5.5-5.9 7.8-8.2 ll.lL11.7 2.8-3.2 5.0-5.1 12.0-12.6 2.1-2.8 4.3-5.5 8.0-9.0 9.8-l 1.8 2.3 5.9-6.5 8.2-9.2 10.8-I 1.8
Amplitude (dB) 2.8 2.4 2.5 2. 2.2 1.8 5.4 4.0 4.6 3.0 3.5 2.8 3.8 3.1 5.0
X-ray flux there exist other spectral peaks with smaller amplitudes, such as a 6.2-7-day fluctuation in summer and autumn (which completely vanishes in winter and spring), and a 8.4-lo-day fluctuation in winter and spring with a small amplitude. These probably cannot influence the ionization state of the ionosphere. Compare the results from Tables 2 and 4, we see that : (a) there is no correspondence between the statistically significant periods in day time ionospheric absorption and solar X-ray flux during the seasons and (b) the fluctuations in ionospheric absorption have maximum amplitudes in winter, while those in X-rays have maximum amplitudes in summer and autumn. This shows that the fluctuations in the day time ionospheric absorption are not generated by similar fluctuations in the solar X-ray radiation. The same conclusion can be made for the fluctuations in the Lyman-alpha flux and in the solar flux at 2800 MHz. Their statistically significant peaks have periods longer than 13.2 days in all seasons. Comparing the statistically significant periods during the seasons for the night-time absorption and the geomagnetic au(N)-index (Tables 3 and 4), we notice a comparatively good coincidence in the simultaneous appearance of fluctuations in the period intervals : 46.5, 7-9 and l&12 days (except for winter). 4-6.5and 7-9-day fluctuations in the geomagnetic activity are most intensive during summer, while the l&12day fluctuation has its maximum amplitude in summer and autumn. Fluctuations in absorption exhibit a different seasonal dependence of their amplitude. The fluctuations investigated in the geomagnetic activity could slightly stimulate analogous fluctuations
spring
Winter
Autumn
Summer
Season ~__lll_
1.8
11.5-11.7
1.2 1.1 1.8
10.0-12.0 2.1-2.4 5.0-6.0 8.0-6.6 11.0
1.4
10.0-12.0
5.5-5.9 8.2-9.1 10.7-12.8
4.5-6.0 7.0-8.0 10.9-11.3 2.4-2.8 4.2-5.5
0.9 0.9 1.2 1.25 1.2
7.0-8.0 10.6-I 1.9
Period (days)
1.4 1.0 1.1 0.8 1.1
1.0 1.2 1.2 1.1 1.2
0.9 1.2
1.2 1.6 1.2 1.1 1.3 1.3 1.2 1.3 1.5 1.8 1.35 1.1 1.2 1.6
1.4
6.1-6.8 8.0-9.2 IO.%-1 1.0
6.5-7.5 10&l 1.2
(days)
Period
1.4 1.3 2.0
1.35 1.7
(dB)
Amplitude
272 kHz --~
164, 155, 185, 272 and 218 kHz radio-wave
Amplitude (dB)
185 kHz
in night-time,
7.5-9.0 10.2-12.0 2.2-2.7 4.5-5.1 7.3-8.5 11.5-12.4 2.2-3.0 4.3-5.7 8.0-9.5 11.1-12.7 2.3-3.2 4.5-6.2 8G9.6 11.0-12.0
2.4-3.6
Period (days)
and their amplitudes
Amplitude (dB)
155 kHz
of the basic fluctuations
4.5-6.0 7.8-8.1 10.4-11.0 2.1-3.4 4.0-5.5
5.4-5.8
1.15 1.1
Amp~tud~ (dB) ___________
2.3-3.3
Period (days)
..-
distribution
164 kHz
Table 3. Seasonal
10.6-12.2 2.2-2.8 4.7-6.0 7.5-8.5 10.5-12.5 2.4.--3.2 4.M.2 8.CL9.8
10.7-12.3 2.3-3.7 5.5
2.1-3.5 4.5-6.0
Period (days)
1.4 1.1 1.3 1.0 2.0 1.25 1.2 1.6
1.8 0.8 0.9
1.35 1.2
Amplitude (dB)
218 kHz
absorption
FZ 6 g 2 6’ g B a ;t:
s 8 f.
? F 1 B ‘-. E :
386
D. PANCHEVA et al. I
Lag
2 z z w
o_
r
Q
I
v L”--.----
I
I
/
I
I
30
25
20
15
10
I
-
0.9
2800MHz
I
2 PERIODiDAYS) 5
J
Fig. 4a. Amplitude spectra, corrected AE-E Loan-alpha data and solar flux at 2800 MHz, 1 June 1979-31 January 1980.
Z~NAL
-
$C c_
*
a9
2-
a’2-
A
1
L-
-/v o-
-
n
1
I
I
I
I
30
25
20
15
10
5 PERIOD
2
-.
(DAYS)
Fig. 5. Amplitude spectra for zonal and meridional wind (9&100 km) from Collm, Central Europe (52”N, 15”E).
Figure 5 presents the amplitude spectra of zonal and meridional winds (night-time) for the entire period investigated. Above the confidence level 0.9 for the zonal wind there are fluctuations with periods 7.4 and 20 A 9.4 days, and above the confidence level 0.75-, 3- and 5-day fluctuations. In the meridional wind, spectral peaks are observed above the 0.9 confidence level with periods of 2.4 days with a series of peaks in the interval 5-7 days, 9.8 and 12 days. These periods coincide fairly well with the periods established in the radiowave absorption. In order to investigate the developCl9X-RAY E 5ment of these fluctuations in time, we again use the 5 correloperiodogram analysis on temporal segments. : o2 LThe results for the different seasons with spectral 1 peaks above confidence level 0.9 are presented in 8 2 2 Cl9 aa Table 5. The IO-12-day ~uctuation in wind has its Q OrLi i i I I basic amplitude maximum in winter with a secondary 30 25 20 15 10 5 2 one in summer. This result coincides with the seasonal PERIOD (DAYS) variation of the IO-12-day fluctuation in the nightFig. 4b. Amplitude spectra, solar flux at 2800 MHz, X-ray time ionospheric absorption, for the altitude interval flux and geomagnetic aa(index. of 85-100 km, at which level the wind is measured. The 7-9 day fluctuation in the zonal and meridional wind has its basic maximum also in winter and a secondary one in summer. As this fluctuation is very in the night-time ionospheric absorption, but they strong, especially in the zonal wind, it is probably are not a dominant factor. connected with the analogous fluctuation in the nighttime absorption. In contrast to the above mentioned 5. FLUCTUATIONS IN WIND fluctuations, the 4.5-6-day fluctuation has a different seasonal behaviour. In the zonal wind the basic ampliVariations of the prevailing wind in the lower thertude maximum is observed in winter and a secondary mosphere play a very important role in the meteoroone in summer, while in the meridional wind it is logical control of the lower ionosphere as demonthe other way around, the basic maximum being in strated, for example, by the lower ionosphere response to sudden stratospheric warmings (e.g. LASTOVICKA, summer with a secondary one in winter. In the nighttime as well as in the day time ionospheric absorption, 1984).Hence, we used the prevailing wind measured the basic maximum of this fluctuation is in winter continuously at Collm (GDR) by the Dl method and a secondary one in summer, but with increasing (ionospheric drift) at * 90-100 km, as a neutral atmoaltitude the secondary summer maximum grows sphere parameter representing the effect of dynamics larger. The fluctuations in the zonal wind are likely to on the LF radio wave absorption. The Collm data exert a stronger influence on the variations in ionowell represents the wind for the two paths in central spheric absorption, but with increasing altitude the Europe (185 and 272 kHz). I,1
Fluctuations observed in the lower ionosphere
387
Table 4. Seasonal distribution of the basic fluctuations and their amplitudes in X-ray flux and geomagnetic au(iV)-index X-ray Season
Period (days)
aa(l\r)-index
Amplitude (w m-‘)
Summer
Autumn
7.0 12.613.4 6.2-6.6
13.0 14.5 13.5
12.8-14.8
14.0
9.2-10.0
9.0
13&15.8
17.0
8.4-8.6 10.612.2
9.9 10.5
Amplitude (nT)
5.862 7.2-7.8 11.6-12.8 6.4 8.C8.6 9.8-l 1.2 6.c7.2 9.49.8 10.8%12.6 14.0 4.k6.2 8.2 ll.Gll.2
Winter
Spring
influence of the meridional wind becomes greater. The most interesting fluctuation is the shortest periodic one of 2.3-3.2 days observed in the zonal, as well as in the meridional wind. The basic maximum of this fluctuation is in summer and a secondary one in winter and spring. The same pattern is observed in the analogous fluctuation in ionospheric absorption. Since the possible ionizing sources [Lyman-a, X-rays, au(N)] do not display such a high frequency fluctuation, we can assert that the neutral wind generates this fluctuation in ionospheric absorption.
Period (days)
6.5 9.0 7.0 4.8 5.5 5.3 5.2 7.5 4.7 8.5 4.8 4.1 4.2
6.
CONCLUSION
Strictly speaking, the fluctuations in the period ranges 2.5-3, 4-6, 7-9 and 10.5-12 days are quasiperiodic rather than periodic, since both the amplitude and period vary with season, and this is particularly true for the fluctuations with periods shorter than 10 days. Summarizing, we may say that the fluctuations with periods of 2.3-3.2, 4-6 and 10.5-12 days found to exist in the ionospheric absorption of radio waves
Table 5. Seasonal distribution of the basic fluctuations and their amplitudes in zonal and meridional wind (9&100 km) from Collm Zonal wind
Season Summer
Autumn
Winter
Spring
Meridional wind
Period (days)
Amplitude (m ss’)
Period (days)
Amplitude (m s-l)
2.1-3.5 4.2-5.8 7.c9.0 1l.gl2.8 2.2-2.7 5.0-6.0 7.0-9.0 10.0-l 1.5 2.43.2 4.8-6.0 7.2-9.6 10.3-I 1.7 13.5-14.3 2.43.2 4.G5.4 7.0-9.0
11.0 4.8 7.0 6.0 4.0 4.0 6.5 5.0 5.8 8.5 7.8 7.2 4.8 5.8 6.0 6.5
2.1-3.1 4.1-5.8 6.6-7.5 10.9-12.4 3.&3.5 4.1.-5.8
6.0 7.5 4.0 4.0 3.5 4.4
10.&12.8 2.2-3.0 4.5-6.0 7.s9.0 lO.t%l2.5
3.0 5.0 5.2 4.5 4.8
2.1-3.5 4.560 11.1-12.4
4.0 3.8 4.2
388
D. PANCHEVA et al.
in central and southern Europe, and with periods between 7 and 9 days found in central Europe seem to reflect fluctuations in winds in the lower thermosphere (i.e. in the parameters of the neutral atmosphere). They are not caused by fluctuations in the solar ionizing flux (Lyman-cr, X-rays). The dominant periods in various parameters are known to change to some extent interannually. The probable meteorological origin of the observed fluc-
tuations in radio wave absorption seems to offer a possibility of monitoring fluctuations in the upper mesosphere and lower thermosphere in the planetary wave period range by means of radio-wave absorption measurements. However, to establish a meteorological origin and to clarify the question of the stability of periods, further examination is necessary. An investigation of the same parameters over an extended period (July 198CJuly 1985) is underway.
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1916 1975 1983 1982
CHERNOBROVKINAN. A. DELANDR. and FRIEDMANR. FRASERG. FRASERG. and THORPEM. GLASSM. and SPIZZICHINO A. KOPECK~M. and KUKLING. LASTOVIEKA J. LASTOVIEKA J. LAUTERE. A.
1985 1972 1911 1976 1974 1971 1916 1984 1974
LEANJ. MAN~~NA. and MEEKC. MAN~~NA., MEEKC. and GREGORYJ. PANCHEVA D., Apos~o~ov E., NE~T~ROVG. and LASTOVICKA J. SALBYM. SALBYM. and ROPERR. SCHWENTEK H. TEPTING., KAZAKOVAT. and NAZARENKOV.
1987 1986 1981 1987
J. geophys. Res. 92,839. J. atmos. terr. Phys. 48, 1039. Handbook for MAP 2, 101. Studia geophs. geod. 31, 301.
1984 1980 1974 1970
Rev. Geophys. Space Phys. 22,209. J. atmos. Sci. 37, 231. J. atmos. terr. Phys. 36, 1113. Wind, Drift and Inhomogenities in the Ionosphere, p. 104. Novosibirsk (in Russian).
E%OSKAJ.
1986
PANCHEVAD. and APOSTOLOVE. VINCENTR. and CRAIG R.
1985 1985
PhD. Thesis, Geophysical Institute, Prague (in Czechoslovakian). Paper 11.02.10, 5th Sci. Ass. IAGA/IAMAP, Prague. Paper 11.03.03, 5th Sci. Ass. IAGA/IAMAP, Prague.
C. r. Acad Bulg. Sci. 29, 3. Bull. astr. Inst. Csl. 26, 193. J. geophys. Res. 88,7823. A Summary of Solar l-8 A. Measurements from the SMS and GOES satellite 1977-1981, NOAA Tech. Memor. ERL SEL-62. Issled. geom. aeronom. Fiz. Solnca 71, 34 (in Russian). J. atmos. terr. Phys. 34,295. J. atmos. terr. Phys. 39, 121. J. atmos. terr. Phys. 38, 1003. J. atmos. terr. Phys. 36, 1825. Issled. geom. aeronom. Fiz. Solnca 2, 167 (in Russian). Beitr. Geophys. 85, 276. Adv. Space Res. 4, 61. Proc. COSPAR Symp. Methods of Measurements and Results of Lower Ionosphere Structure, 377. Akad.
Vlg., Berlin, GDR.
Reference is also made to the following unpublished material: