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Temporal variations of ionospheric waves in the D- and F-regions
Journal of Atmospheric and Terrestrial Physics,Vol.41, pp. @Pergamon Press Ltd. 1979. Printed in Northern Ireland
OOZl-9169/79/0901-1011$02.00/0
1011-1013
Temporal variationsof ionosphericwaves in the D- and F-regions V. I. DROBJEV, V. M. KRASNOV and N. M. SALIHOV Sector
of ionosphere
(Receioed
25 August
AN Kaz. SSR, Alma-Ata, 1978:
68 U.S.S.R.
in revised form 20 April
1979)
Abstract-It is shown that mesoscale ionospheric wave disturbances in the D- and F-regions regularly occur at all times of day, night and season (characteristic periods -100, 24, 12.6 min) and are a characteristic property of dynamic processes in the ionosphere.
The study of the spatial and temporal characteristics of mesoscale ionospheric disturbances plays an important role in understanding the mechanism of the generation and the location of the source of internal acoustic-gravity waves. These characteristics have been studied for many years (VASSEUR et al., 1971; FRANCES, 1974). However, little or no information has been published concerning the spectral content of D-region ionospheric waves or on the simultaneous study of the spectral content of D- and F-region ionospheric waves. This paper describes a spectral study of D- and F-region mesoscale ionospheric waves and considers the influence of the solar terminator on the ionosphere. The phase and amplitude of the HF signals at oblique (Tashkent-Alma-Ata, 700 km) and vertical (Alma-Ata) incidences were measured during 1977-78. The apparatus and technique of data treatment have been described by DROBJEV el al. (1977) and KRASNOV (1976). The oblique incidence frequency was 2.5 MHz, which means that the reflection height was 75-85 km. The vertical incidence frequencies were chosen to be 5 MHz for daytime and sunset hours and 3 MHz for nighttime, the reflection heights were 200-250 km. Single mode signals were selected from the available data records as previously described by KRASNOV (1976), and then the Doppler frequency shifts were calculated. Doppler shift fluctuations fd(f) were digitized at 1 min sampling intervals for vertical incidence data and at 2 min sampling intervals for oblique incidence data. The low frequency trend with periods more than 1 h was removed. The spectral analysis of the Doppler shift fluctuations was carried out according to the methods of Blackman and Tukey (JENKINS and WATTS, 1972) and calculated for record lengths of 4-5 h. For both the D- and F-region data the calculated 1011
power spectra showed several distinct peaks. Figure 1 shows daytime graphs obtained from averaging 45 spectra for the D-region (Fig. la) and of 40 spectra for the F-region (Fig. lb). It is seen that wave disturbances with average periods: 24, 12 and 6 min are characteristic for the D-region and 27, 12.5 min for the F-region. Spectral analysis of the filtered low frequency component of fd(t) showed the existence of a pronounced peak around 1.5 h for both the D- and F-regions. Comparison of experimental results taken on separate days showed that these peaks in the power spectrum at different ionospheric heights can occur on the same frequencies and on different frequencies. The analysis of night records showed the existence of frequency components with periods -20 and - 11 min. Figure 2 shows the month to month or seasonal variation in the power spectrum peaks for the D- and Fregions. The seasonal variations show that for all periods analysed the D-region spectra exhibit approximately the same periods (24, 12 and 6 min). However at the autumnal equinox there is some evidence for an increase in the 24 and 12 min periods in the D-region and of the 27 min period in the F-region. The results described above give an experimental estimate of the maximum cutoff frequency (w,, Brunt-Vaisala frequency) of atmospheric gravity waves. The minimum period of wave disturbances in the D-region of -6 min and in the F-region of 13 min correspond well with theoretical estimations of the magnitude of wg calculated for mid-latitudes and confirms the suggestion that the observed wave disturbances are caused by atmospheric gravity waves. The investigation of the diurnal variations in the frequency spectrum enabled the influence of the solar terminator on the ionosphere to be studied. In Fig. 3 (Fig. 3a-vertical incidence Doppler, Fig. 3b-oblique incidence Doppler) curve 1 shows the detailed fluctuations fd(f) and curve 2 shows the
1012
V. I. DROBJEV,V. M. KRASNOVand N. M. SALIHOV
3 8
10
f2
2
4
f97GOrJT” 1976 Fig. 2.
3
6
FREWENCY,
3
cyctespa
f2
houz
Fig. 1 I
204.78
5MHa
a)
t,
I
t,
0
LT, h
1
t3.10.77.
2,5MH2
Fig. 3.
b)
6
Temporal variations of ionospheric waves in the D- and F-regions low frequency component of curve 1. The beginnings of the terminator on the Earth and in the ionosphere are marked by arrows on the records. It is seen that from the beginning of the passing of the terminator a considerable increase in the amplitude of the low frequency component occurs in both the D- and F-regions. A similar situation
1013
was observed during analysis of the rest of the records for vertical incidence. Thus mesoscale wave disturbances in the D- and F-regions regularly occur at any time of day and night and season (characteristic periods -100, 24 and 12.5 min) and are a characteristic feature of dynamic processes in the ionosphere.
REFERENCES
DROBJEVV. I., KRASNOVV. M. and SALIHOVN. M. FRANCISS. H. KRASNOV V. M. JENKINSG. M. and WARS D. G.
1977
VASSEURG., TEZXUDJ.
1971
REDDY
C. and
1974 1976 1972
The Ionosphere and Solar Couplings, Alma-Ata, Nauka, p. 33. J. geophys. Res. 79, 5245. J. Radiotekhnika EIectronika 21, 608. Spectral Analysis and its Applications, Mir Press, Moscow, p. 7. Proc. of 14th Plenary Meeting of COSPAR, Seattle.
OOZl-9169/79/0901-1011$02.00/0
1011-1013
Temporal variationsof ionosphericwaves in the D- and F-regions V. I. DROBJEV, V. M. KRASNOV and N. M. SALIHOV Sector
of ionosphere
(Receioed
25 August
AN Kaz. SSR, Alma-Ata, 1978:
68 U.S.S.R.
in revised form 20 April
1979)
Abstract-It is shown that mesoscale ionospheric wave disturbances in the D- and F-regions regularly occur at all times of day, night and season (characteristic periods -100, 24, 12.6 min) and are a characteristic property of dynamic processes in the ionosphere.
The study of the spatial and temporal characteristics of mesoscale ionospheric disturbances plays an important role in understanding the mechanism of the generation and the location of the source of internal acoustic-gravity waves. These characteristics have been studied for many years (VASSEUR et al., 1971; FRANCES, 1974). However, little or no information has been published concerning the spectral content of D-region ionospheric waves or on the simultaneous study of the spectral content of D- and F-region ionospheric waves. This paper describes a spectral study of D- and F-region mesoscale ionospheric waves and considers the influence of the solar terminator on the ionosphere. The phase and amplitude of the HF signals at oblique (Tashkent-Alma-Ata, 700 km) and vertical (Alma-Ata) incidences were measured during 1977-78. The apparatus and technique of data treatment have been described by DROBJEV el al. (1977) and KRASNOV (1976). The oblique incidence frequency was 2.5 MHz, which means that the reflection height was 75-85 km. The vertical incidence frequencies were chosen to be 5 MHz for daytime and sunset hours and 3 MHz for nighttime, the reflection heights were 200-250 km. Single mode signals were selected from the available data records as previously described by KRASNOV (1976), and then the Doppler frequency shifts were calculated. Doppler shift fluctuations fd(f) were digitized at 1 min sampling intervals for vertical incidence data and at 2 min sampling intervals for oblique incidence data. The low frequency trend with periods more than 1 h was removed. The spectral analysis of the Doppler shift fluctuations was carried out according to the methods of Blackman and Tukey (JENKINS and WATTS, 1972) and calculated for record lengths of 4-5 h. For both the D- and F-region data the calculated 1011
power spectra showed several distinct peaks. Figure 1 shows daytime graphs obtained from averaging 45 spectra for the D-region (Fig. la) and of 40 spectra for the F-region (Fig. lb). It is seen that wave disturbances with average periods: 24, 12 and 6 min are characteristic for the D-region and 27, 12.5 min for the F-region. Spectral analysis of the filtered low frequency component of fd(t) showed the existence of a pronounced peak around 1.5 h for both the D- and F-regions. Comparison of experimental results taken on separate days showed that these peaks in the power spectrum at different ionospheric heights can occur on the same frequencies and on different frequencies. The analysis of night records showed the existence of frequency components with periods -20 and - 11 min. Figure 2 shows the month to month or seasonal variation in the power spectrum peaks for the D- and Fregions. The seasonal variations show that for all periods analysed the D-region spectra exhibit approximately the same periods (24, 12 and 6 min). However at the autumnal equinox there is some evidence for an increase in the 24 and 12 min periods in the D-region and of the 27 min period in the F-region. The results described above give an experimental estimate of the maximum cutoff frequency (w,, Brunt-Vaisala frequency) of atmospheric gravity waves. The minimum period of wave disturbances in the D-region of -6 min and in the F-region of 13 min correspond well with theoretical estimations of the magnitude of wg calculated for mid-latitudes and confirms the suggestion that the observed wave disturbances are caused by atmospheric gravity waves. The investigation of the diurnal variations in the frequency spectrum enabled the influence of the solar terminator on the ionosphere to be studied. In Fig. 3 (Fig. 3a-vertical incidence Doppler, Fig. 3b-oblique incidence Doppler) curve 1 shows the detailed fluctuations fd(f) and curve 2 shows the
1012
V. I. DROBJEV,V. M. KRASNOVand N. M. SALIHOV
3 8
10
f2
2
4
f97GOrJT” 1976 Fig. 2.
3
6
FREWENCY,
3
cyctespa
f2
houz
Fig. 1 I
204.78
5MHa
a)
t,
I
t,
0
LT, h
1
t3.10.77.
2,5MH2
Fig. 3.
b)
6
Temporal variations of ionospheric waves in the D- and F-regions low frequency component of curve 1. The beginnings of the terminator on the Earth and in the ionosphere are marked by arrows on the records. It is seen that from the beginning of the passing of the terminator a considerable increase in the amplitude of the low frequency component occurs in both the D- and F-regions. A similar situation
1013
was observed during analysis of the rest of the records for vertical incidence. Thus mesoscale wave disturbances in the D- and F-regions regularly occur at any time of day and night and season (characteristic periods -100, 24 and 12.5 min) and are a characteristic feature of dynamic processes in the ionosphere.
REFERENCES
DROBJEVV. I., KRASNOVV. M. and SALIHOVN. M. FRANCISS. H. KRASNOV V. M. JENKINSG. M. and WARS D. G.
1977
VASSEURG., TEZXUDJ.
1971
REDDY
C. and
1974 1976 1972
The Ionosphere and Solar Couplings, Alma-Ata, Nauka, p. 33. J. geophys. Res. 79, 5245. J. Radiotekhnika EIectronika 21, 608. Spectral Analysis and its Applications, Mir Press, Moscow, p. 7. Proc. of 14th Plenary Meeting of COSPAR, Seattle.