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“Lunar tide in D-region” of the ionosphere near the magnetic equator
Journalof Atmospheric and TerrestrialPhysics,1970,Vol. 32,pp. 945-948.PergamonPress. Printedin NorthernIreland
SHORT PAPER
“Lu.nartide in D-region” of the ionosphere near the magnetic equator S. C. Physical (Received
CEAKRAVUTY Research
and R. G. RASTOCX
Laboratory,
Ahmedabad-9,
1969; in revised form
3 September
India
15 October 1969)
Abstract-The phase of the lunar tide in the ionospheric absorption at Colombo does not change appreciably with season during low sunspot years but has a semi-annual variation during the maximum sunspot years. Lunar tide in f-min for any particular season of the period IGY/IGC has a phase identical to that of absorption; the amplitude does not show any marked enhancement close to the magnetic equator. 1. INTRODUCTION
tides in the ionospheric absorption at equatorial stations have been computed by SKINNER and WRIGHT (1964) for Ibadan, and by RASTOGI (1969) for Colombo. The present analysis describes lunar tides in absorption at Colombo and in the minimum frequency reflected from the ionosphere (f-min) at Huancayo, Chimbote and Talara for IGY-IGC period. The noon values of absorption (L) and midday (lo-14 hr mean) values of f-min are utilised for the present study. The L or f-min values affected by solar flares, sudden commencement etc. are excluded from the data. The method of analysis, is essentially similar to that described by RASTOGI (1969). The deviations of individual observations from the corresponding monthly mean values are arranged according to the lunar age (Q).
LUNAR
2. LUNAR
TIDE
IN
ABSORPTION
AT COLOMBO
Figure 1 shows the variation of average AL at Colombo as the function of yO, separately for the three seasons of IGY/IGC. During any season lunar oscillations are seen to be almost identical for the two frequencies. Table 1 gives the amplitudes and phases (times of maximum positive deviation) of lunar monthly (M,) and lunar semi-monthly (M,) oscillations derived from the curves averaged for both the frequencies. During the E-months (March, April, September and October) the tide is predominantly semi-monthly in character, the amplitude (Pa) being about 1.2 dB, with the time of maximum positive deviation (tz) at 8.9 lunar hr. From a comparison with the corresponding values for the period 1964-68 (Table 1) it can be noted that the amplitude ( P2) has increased by a factor of three while the phase (tz) of the tide has changed by 4.3 lunar hr. During the D-months (November, December, January and February) the amplitude and phase of the M, tide are similar to that observed for the period 1964-65. During the J-months (May, June, July and August), the variations are rather irregular giving the amplitudes of M, and M, tides almost of the same magnitude. Thus the lunar tidal amplitudes in the D-region absorption at Colombo during 14
945
S. C. CH~KRAVARTYand R. G. RASTOGI
946
2.55
‘MHZ
COLOMBO 1957-w
oOVHHR“
24 00 LUNAR
2.85
I
AGE
I
06
I
blHz
I
12
I
I
Is
I
I
24
y0
Fig. 1. The variation of mid-day absorption of the radio waves at Colombo with the lunar age for different seasons of the year. Table 1. Harmonic coefficients of seasonal lunar variations at Colombo Ampl. Season
Period
Frequencies (MHz)
II-months
1964-68 1957-59 1964-68 1957-59 1964-68 1957-59
2.0, 2.2 and 2.6 2.55 and 2.85 2.0, 2.2 and 2.6 2.55 and 2.85 2.0, 2.2 and 2.6 2.55 and 2.85
E-months J-months
Time of max.
Ampl.
Time of max.
($6)
(lunar hr)
(%,
t2 (lunar hr)
O-38
23.4 21.3 8.0
0.78 0.67 0.42 1.17 0.38 0.61
2.0 2.5 4.6 Y.9 4.9 3.i
0.13 0.57
4
E and J-months
are larger during the years of high solar activity than during the low sunspot years. During the high sunspot years the phase of the tide experiences large variation with seasons such that the oscillations would cancel each other when averaged over the whole year, giving insignificant harmonic coefficients. 3. LUNAR TIDE IN f-min
AT SOUTH AMERICAN STATIONS
The measurements of absorption has not been undertaken at most of the ionospheric stations, hence a detailed study of the lunar tides in the lower ionosphere has not been possible. However, most of the ionospheric stations tabulate f-min; the minimum frequency reflected from the ionosphere. An attempt has been made in the present article to study the latitudinal variations of the D-region tide near the equator using the observations of f-min at the South American Stations, Huancayo Chimbote and Talara.
947
Lunar tide in D-region
Figure 2 shows the variation of the average Af-min with lunar age for different seasons and Table 2 gives the harmonic coefficients of the curves. For any particular season, the variations at different stations are strikingly similar and are mainly semimonthly in character. The time of maximum positive displacement show a large variation with season similar to that for the absorption at Colombo during 1957-59. HUANCAYO
TALARA
CHIWOTE IGY-IGC
ICY- ICC
00
06
u
16
24
00
I-UN
06
A
12
R
16
24
00
06
I2
AGE
I8
a4
%
Fig. 2. The varia;tion of mid-day f-min values at Huancayo, Chimbote T&m, with the lunar age for different seasons of the year.
and
Table 2. Summary of seasonal lunar variations inf-min Yhcbse
Amplitude
Phase
(k%z)
t1 (lunar hr)
(lk)
% (lunar hr)
2.0%
106
12.2
6*4’N 12.6ON -
16.1 21.8 18.3 21.5 21% 36-l 19.5 21.4
11.4 6.9 3.9 10.2 17.9 8.1 11.3 9.5
Amplitude Sensnn
St&ion
D-months
Huancayo Chimbote Tslara Huaneayo Chimbote Talarrt Huancayo Chimbote Talara
E-months
J-months
Magnetic dip
-
__~-
54.4 58.8 43.4 679 41.8 53.2 41.3 39.8 40.0
2.2 2.9 3.7 9.8 11-O 10% 1.1 2.8 0.5
S. C. CEAICRAVARTY snd R. G. RASTOGI
948
For a better understanding of this phase variation, the f-min data at the three stations were combined and lunar oscillations deduced for each month of the year. Figure 3 shows the variation of the time of maximum positive displacement of M, oscillation for each month of the year, being about 10 lunar hr during either of the equinoxes and around 01 lunar hr during the solstices showing a semi-annual variation of the phase. BROWN (1967) has shown similar type of phase variation in the lunar semi-monthly oscillation in f,,E at Ibadan. LUNAR SEMI-MONTHLY TIDE IN f-MIN HUANCAYO,CHIMBOTE
AND TALARA
:
IO-14 HRS
ii I*_ 2:: rz 3 I 9$5
kX6 F ma =s Fw 0
-
5-
a
-
z
4
1,
,
I
,
(
1,
,
,
I
JFMAHJJASOND
MONTHS
(IGY-ICC)
Fig. 3. The annual variation of the phase of the lunar semi-monthly tide avemged for the three equatorial &&ions, Huttncayo, Chimbote and Talara. The amplitude of M, oscillation in f-min does not show any systematic variation with the latitude of the station during any season (Table 2). Lunar tides in the magnetic field and the P-region have been shown to be enhanced at Huancayo, decreasing uniformly with the increase in latitude within the equatorial region (RASTOGI, 1962, 1963). These have been suggested to be related with the presence of the equatorial electrojet. The absence of any such variation of M, amplitude in f-min tide indicates that the lunar tide in the D-region does not seem to be affected by the equatorial electrojet currents. This may be due to the reason that the electrojet currents flow in the E-region level which is above the height where the maximum absorption of radio waves takes place.
REFERENCES BROWN R. A.
RASTOG~ R. G. RASTOGIR. G. RASTOGIR. G. SKIXNERN. J. and WRIGHTR. W.
1967 1962 1963 1969 1964
J. Atmosph. J. J. J. J.
Terr. Phys. 29, 1087. Res. NBS. Radio Prop. BBD, 601. geophys. Res. 69, 2445. Atmosph. Tern. Phys. 31, 759. Atmosph. Terr. Phys. 26, 122.
SHORT PAPER
“Lu.nartide in D-region” of the ionosphere near the magnetic equator S. C. Physical (Received
CEAKRAVUTY Research
and R. G. RASTOCX
Laboratory,
Ahmedabad-9,
1969; in revised form
3 September
India
15 October 1969)
Abstract-The phase of the lunar tide in the ionospheric absorption at Colombo does not change appreciably with season during low sunspot years but has a semi-annual variation during the maximum sunspot years. Lunar tide in f-min for any particular season of the period IGY/IGC has a phase identical to that of absorption; the amplitude does not show any marked enhancement close to the magnetic equator. 1. INTRODUCTION
tides in the ionospheric absorption at equatorial stations have been computed by SKINNER and WRIGHT (1964) for Ibadan, and by RASTOGI (1969) for Colombo. The present analysis describes lunar tides in absorption at Colombo and in the minimum frequency reflected from the ionosphere (f-min) at Huancayo, Chimbote and Talara for IGY-IGC period. The noon values of absorption (L) and midday (lo-14 hr mean) values of f-min are utilised for the present study. The L or f-min values affected by solar flares, sudden commencement etc. are excluded from the data. The method of analysis, is essentially similar to that described by RASTOGI (1969). The deviations of individual observations from the corresponding monthly mean values are arranged according to the lunar age (Q).
LUNAR
2. LUNAR
TIDE
IN
ABSORPTION
AT COLOMBO
Figure 1 shows the variation of average AL at Colombo as the function of yO, separately for the three seasons of IGY/IGC. During any season lunar oscillations are seen to be almost identical for the two frequencies. Table 1 gives the amplitudes and phases (times of maximum positive deviation) of lunar monthly (M,) and lunar semi-monthly (M,) oscillations derived from the curves averaged for both the frequencies. During the E-months (March, April, September and October) the tide is predominantly semi-monthly in character, the amplitude (Pa) being about 1.2 dB, with the time of maximum positive deviation (tz) at 8.9 lunar hr. From a comparison with the corresponding values for the period 1964-68 (Table 1) it can be noted that the amplitude ( P2) has increased by a factor of three while the phase (tz) of the tide has changed by 4.3 lunar hr. During the D-months (November, December, January and February) the amplitude and phase of the M, tide are similar to that observed for the period 1964-65. During the J-months (May, June, July and August), the variations are rather irregular giving the amplitudes of M, and M, tides almost of the same magnitude. Thus the lunar tidal amplitudes in the D-region absorption at Colombo during 14
945
S. C. CH~KRAVARTYand R. G. RASTOGI
946
2.55
‘MHZ
COLOMBO 1957-w
oOVHHR“
24 00 LUNAR
2.85
I
AGE
I
06
I
blHz
I
12
I
I
Is
I
I
24
y0
Fig. 1. The variation of mid-day absorption of the radio waves at Colombo with the lunar age for different seasons of the year. Table 1. Harmonic coefficients of seasonal lunar variations at Colombo Ampl. Season
Period
Frequencies (MHz)
II-months
1964-68 1957-59 1964-68 1957-59 1964-68 1957-59
2.0, 2.2 and 2.6 2.55 and 2.85 2.0, 2.2 and 2.6 2.55 and 2.85 2.0, 2.2 and 2.6 2.55 and 2.85
E-months J-months
Time of max.
Ampl.
Time of max.
($6)
(lunar hr)
(%,
t2 (lunar hr)
O-38
23.4 21.3 8.0
0.78 0.67 0.42 1.17 0.38 0.61
2.0 2.5 4.6 Y.9 4.9 3.i
0.13 0.57
4
E and J-months
are larger during the years of high solar activity than during the low sunspot years. During the high sunspot years the phase of the tide experiences large variation with seasons such that the oscillations would cancel each other when averaged over the whole year, giving insignificant harmonic coefficients. 3. LUNAR TIDE IN f-min
AT SOUTH AMERICAN STATIONS
The measurements of absorption has not been undertaken at most of the ionospheric stations, hence a detailed study of the lunar tides in the lower ionosphere has not been possible. However, most of the ionospheric stations tabulate f-min; the minimum frequency reflected from the ionosphere. An attempt has been made in the present article to study the latitudinal variations of the D-region tide near the equator using the observations of f-min at the South American Stations, Huancayo Chimbote and Talara.
947
Lunar tide in D-region
Figure 2 shows the variation of the average Af-min with lunar age for different seasons and Table 2 gives the harmonic coefficients of the curves. For any particular season, the variations at different stations are strikingly similar and are mainly semimonthly in character. The time of maximum positive displacement show a large variation with season similar to that for the absorption at Colombo during 1957-59. HUANCAYO
TALARA
CHIWOTE IGY-IGC
ICY- ICC
00
06
u
16
24
00
I-UN
06
A
12
R
16
24
00
06
I2
AGE
I8
a4
%
Fig. 2. The varia;tion of mid-day f-min values at Huancayo, Chimbote T&m, with the lunar age for different seasons of the year.
and
Table 2. Summary of seasonal lunar variations inf-min Yhcbse
Amplitude
Phase
(k%z)
t1 (lunar hr)
(lk)
% (lunar hr)
2.0%
106
12.2
6*4’N 12.6ON -
16.1 21.8 18.3 21.5 21% 36-l 19.5 21.4
11.4 6.9 3.9 10.2 17.9 8.1 11.3 9.5
Amplitude Sensnn
St&ion
D-months
Huancayo Chimbote Tslara Huaneayo Chimbote Talarrt Huancayo Chimbote Talara
E-months
J-months
Magnetic dip
-
__~-
54.4 58.8 43.4 679 41.8 53.2 41.3 39.8 40.0
2.2 2.9 3.7 9.8 11-O 10% 1.1 2.8 0.5
S. C. CEAICRAVARTY snd R. G. RASTOGI
948
For a better understanding of this phase variation, the f-min data at the three stations were combined and lunar oscillations deduced for each month of the year. Figure 3 shows the variation of the time of maximum positive displacement of M, oscillation for each month of the year, being about 10 lunar hr during either of the equinoxes and around 01 lunar hr during the solstices showing a semi-annual variation of the phase. BROWN (1967) has shown similar type of phase variation in the lunar semi-monthly oscillation in f,,E at Ibadan. LUNAR SEMI-MONTHLY TIDE IN f-MIN HUANCAYO,CHIMBOTE
AND TALARA
:
IO-14 HRS
ii I*_ 2:: rz 3 I 9$5
kX6 F ma =s Fw 0
-
5-
a
-
z
4
1,
,
I
,
(
1,
,
,
I
JFMAHJJASOND
MONTHS
(IGY-ICC)
Fig. 3. The annual variation of the phase of the lunar semi-monthly tide avemged for the three equatorial &&ions, Huttncayo, Chimbote and Talara. The amplitude of M, oscillation in f-min does not show any systematic variation with the latitude of the station during any season (Table 2). Lunar tides in the magnetic field and the P-region have been shown to be enhanced at Huancayo, decreasing uniformly with the increase in latitude within the equatorial region (RASTOGI, 1962, 1963). These have been suggested to be related with the presence of the equatorial electrojet. The absence of any such variation of M, amplitude in f-min tide indicates that the lunar tide in the D-region does not seem to be affected by the equatorial electrojet currents. This may be due to the reason that the electrojet currents flow in the E-region level which is above the height where the maximum absorption of radio waves takes place.
REFERENCES BROWN R. A.
RASTOG~ R. G. RASTOGIR. G. RASTOGIR. G. SKIXNERN. J. and WRIGHTR. W.
1967 1962 1963 1969 1964
J. Atmosph. J. J. J. J.
Terr. Phys. 29, 1087. Res. NBS. Radio Prop. BBD, 601. geophys. Res. 69, 2445. Atmosph. Tern. Phys. 31, 759. Atmosph. Terr. Phys. 26, 122.