Lunar tidal oscillations in the horizontal ionospheric drift at the equator

Planet. Space Sci. 1973, Vol. 21, pp. 1109 to 1114. Peramon Press. printed in Northertt Ireland

LUNAR TIDAL OSCILLATIONS IN THE HORIZONTAL IONOSPHERIC DRIFT AT THE EQUATOR R. K. MISRA Remote Sensing Division, Indian Space Research Organization, Research Laboratory, Ahmedabad-9, India (Received injnalform

c/o Physical

30 January 1973)

Abstract-The effect of lunar tides on the apparent ionospheric drift velocity (v) for an equatorial station Thumba (0.6’S dip) is computed by using nearly six years of data at tixed solar hours. Significant tides are observed in the E-region drifts, particularly around 12.00 hr and in the F-region drifts around WOO hr. A good correlation in the phases of the lunar tides in H and V is found to exist, suggesting a strong electrojet control of the horizontal ionospheric drift around these hours. INTRODUCTION

The periodic variations of electron density and height of the ionospheric layers are known to exist with respect to both solar and lunar time and these are known as solar and lunar tides respectively. These ionospheric variations should therefore be closely related to the corresponding geomagnetic variations through the electromagnetic drift effects. The lunar variations are purely due to the gravitational effect of the Moon on the ionosphere and its study in the horizontal ionospheric drift velocities can prove of importance to evaluate this correlation and thereby separate out the gravitational and thermal or other ionizing effects. Lunar tidal variations off& and h’E, were first studied by Matsushita (1953) using routine published data at a few stations. Thomas and Svenson (1955) have examined the lunar tides in h’E, for Brisbane using special night-time measurement. Wright and Skinner (1959) computed tides in h’E, using daytime data at Ibadan. The results of all these workers show that the phase of semi-diurnal tidal variations in h’_E,is about 6-7 lunar hr at stations in the middle as well as the equatorial zone. The amplitude is, however, smaller in the equatorial zone. The phenomenon of appearance and disappearance of sporadic E-layer at the Equator (Misra, 1971) has also been found to be dependent on lunar time (Knetch, 1959; Bhargava, 1963). The first evidence of the lunar tide in the Fs-region came when Martyn (1947) found a large semi-diurnal variation in h,F,, h’F2 and foFz at Huancayo. He showed that the tidal amplitudes are greater in the winter than in the summer season. Later studies of lunar variation in foFz at other stations showed that there were large changes in both the phase and amplitude with the position of the station (Appleton and Beynon, 1948; Martyn, 1948, 1949; McNish and Gauter, 1949). Further analysis of lunar variations led Rastogi (1961a) to conclude that the phase of the M2 tide in noon&F, is governed by the magnetic latitude rather than by the geomagnetic latitude and the phase reversal takes place around magnetic latitude of loo. The present paper describes the broad features of the lunar tidal variations in the horizontal ionospheric drift for both these regions, namely E, and F, over the equatorial station Thumba, situated at 0-6”s dip. Very few such studies have been made earlier and for an equatorial station the lunar tides in horizontal ionospheric drifts have probably not been reported on so far. The method of analysis, used and its application to Thumba drift results, is first discussed. 1109

R. K. MISRA

1110

METHOD OF ANALYSIS In

order to compute the lunar daily variations at a fixed lunar age (LY), a method basically similar to that of Chapman and Miller (1940) is followed. Firstly the data on magnetically disturbed days (Cp > 1.2) are excluded to remove the irregular disturbance effects. The monthly mean value of the element at each hour is calculated and these are taken to represent average solar daily variation S, during the month. In order to remove the S variation, the monthly mean value of the element for each hour is substracted from all the values at that hour during the month. This is done for all the months for which the data is utilized. One is thus left with a series of hourly deviations for each day, which are free from the solar variations. The entire data is then divided into three seasons, namely winter, equinox and summer, according to the following criterion: winter consisting of four continuous months from November of one year to February of the next year; summer consisting of months from May to August of the same year and equinoxes consisting of March, April, September and October of the same year. For each day of a particular season, a series of hourly deviations is thus available i.e. ai for the@ day of the month at ith hour. LUNAR MONTHLY

VARIATIONS AT FIXED SOLAR HOUR

For a particular solar hour, the deviations of all the days are grouped according to lunar ages 00, 01,02, . . . ,21, 22 and 23 and the average value of all the deviations for a particular lunar age Y, is obtained. Thus a sequence of deviations in terms of lunar age is obtained for a particular solar hour, which is resolved into first and second harmonics according to the equation

Mt = i W&v n=1

+ A>

(1)

where R, and Jbn are the amplitude and phase angle respectively of the nth component. Local time and longitude corrections are now applied to the lunar age, as the v values are referred to Greenwich noon. Thus the lunar age at local solar time is given by v* = vo + 0*0339((t - 12) + 2)

(2)

where iz = longitude in hours east of Greenwich. R, can also be expressed in terms of the lunar time T, using the equation t = T + v and the lunar phase ,Uis defined as ,U= 24 - v. The probable error P.E. in the amplitude of lunar daily variation at a fixed v value is calculated according to the following relation (Rastogi, 1962b): P.E. = 0*27%(v).

(3)

The magnitude of P.E. determines the significance of the harmonic coefficients of the lunar variation in drift. It is considered to be significant only if a coefficient exceeds two or three times the value of its probable error. ANALYSIS OF THUMBA DRET

DATA

Misra et al. (1971) have summarized the complete six years apparent drift results for Thumba, i.e. from 1964 to 1969. These results were obtained by the spaced receiver technique of Mitra (1949) and Krautkramer (1950). The same six years E- and F-region apparent drift data was analyzed by the above method for each of the three seasons as well as the whole year, for different solar hours. In order to increase the amount of data and thus reduce the statistical errors, drift velocities were grouped for 08-10 hr, 11-13 hr and

LUNAR TIDAL OSCILLATIONS IN THE HORIZONTAL IONOSP~~~

DRIFT

1111

14-16 hr and the average values were used to determine the lunar tide at each of the three central hours, namely 0900,120O and 1500 hr LT. E- and F-region tides in drift were also computed separately for the 1100 hr, when the electrojet is strongest at Thumba. All the computations were carried out by IBM 1620 machine. RESULTS

Figures 1 and 2 give the seasonal as well as the annual lunar tidal variations in drift speed for E- and F-regions respectively for the forenoon, noon and afternoon periods. The curves are the two harmonic built up curves. Figure 3 gives the lunar tide in drift at 1100 hr LUNAR

TIDE IN E-REGION

HORIZONTAL

DRIFTS

AT

THUMP

(1%4-1969)

e I

0600-1000

HR IS1

,,oo-,240

04

HR

IST

WO-1600

HR IST

EIl2162024 LUNAR AGE Y

FIG. 1. LUNARTIDEIN E-REGIONHO~O~ALD~AT~~A GROUPS (19644969).

AT DWFZRENT SOLARHOUR

alone. The probable errors and the amplitude of semi-diurnal component for each of the seasons are listed in Table 1. The values which are below the significance limit are bracketed. The following features are prominent from these figures: (i) The E-region drifts exhibit lunar tidal variations in all the three seasons as well as the whole year for the 11-13 hr group. The phases are the same for winter and summer while it is later by about two lunar hours in the equinox. This correlates well with the shift of the entire daily variation curve towards late evening during equinoxes (Misra and Rastogi, 19171). (ii) The solar hour group 08-10 for the E-region also shows the existence of a fairly significant lunar tide during winter and summer but not in equinoxes and annual curves. (iii) The solar hour group 14-16 for the E-region shows very little tide in the annual and almost none in different seasons. (iv) F-region shows the existence of a significant lunar tide in the winter and summer seasons for the 08-10 and 14-16 solar hour groups. (v) The F-region annual tide is seen only in 14-16 hr group, but of a much reduced amplitude. (vi) The lunar tide at 1100 hr is most clear during winter and equinoxes for the Eregion and for all the seasons as well as annual for the F-region.

R. K. MISRA

1112

LUNAR

TIDE

IN

F-REGION

kIORIZONTAC

DRIFTS AT

THUMBA

(1964-1969) ,,m-,300HR 1ST

moo--1000 HR IST

Mm--1wo HR IST l20 +15 +10 +5 0 3 +I0 +5 -': -10 +I0 +5 -5" +5 -i -10

0

4

8

12 16 20 240

4

6

12 16 20 24 o

LUNAR

FrG.2. LUNAR

4

TIDE IN F-REGION HORIZONTAL DRIIV AT THUMBA GROUPS (1964-1969).

LUNAR

TIDE

(1964-1969)

IN AT

THUMBA II00

8

12 16 20 24

AGE 3

AT DIFFERENT SOLAR HOUR

DRIFTS

HR

IST F-REGION

E-REGION

W

lYli?zi

*IO 0

E

HO = a 0

0 - -10

L!iiif S

PI0 0

-10 0

A

-IO

0 FIG. 3. LUNAR

4

2

I2

I6 20 24 LUNAR

0 4 AGE y

I

I2

I6 20

24

TIDE IN E- AND F-REGION HORIZONTAL DRIFTS AT THUMBA FOR THE PERIOD 1964-1969AT 1100 hr IST.

LUNAR

TIDAL

OSCILLATIONS

IN THE HORIZONTAL

IONOSPHERIC

DRIFT

1113

TABLET. A~~DEOFTHELUNAR~DALOSCILLA~ONSMTHEHORIZONTALDR~ATTHUMBAANDTHEIR PROBABLEERRORSINlll/S

08-10 hr

P.E.

11-13 hr

Winter Equinox Summer Annual

11.3 (ii)

2.8 29 3.0 3.0

6.1

Winter Equinox Summer Annual

12.6 8.9

fki3SOll

(210)

6.6

P.E.

14-16 hr

P.E.

11.00 hr

P.E.

(:&

3.4 3.2

10.7 (6.4)

4.3 4.7

18.8

3.7

16.3 11.5

3.6 3.1

9.0 8.4

::;

F-REOION 2.1 2,2 F-REGION

3.6 3.0 3.2 3.5

DISCUSSION

Lunar tides in H were calculated by Rastogi and Trivedi (1970) for Trivandrum (close to Thumba) at different solar hours. It was also shown that the mean lunar tidal oscillations in H near the magnetic equator have a strong longitudinal effect caused by the longitudinal variations of the equatorial electrojet currents. The E-region lunar tidal oscillations in drift speed (Fig. 1) during the winter at 09 hr, during winter and summer at 12 hr and in the annual at 15 hr coincide very well in their phases with those in H at the same solar hours. E-region curves at 1100 hr (Fig. 3) during winter and equinoxes are also similar. This would be expected as the tides in H are linked to the electrojet, which in turn controls the ionospheric drift at E- as well as F-region levels as the E-region electric field is transferred to the F-region along the magnetic line of force. The F-region results for 1100 hr for winter and equinoxes correlate well in their phases with the lunar tides in H at that solar hour for Thumba. The correlation is not good for other hours. Sharma and Rastogi (1970) have discussed in detail the inter-relation between geomagnetic and ionospheric lunar tides at the Equator and shown that a good correlation exists. A strong control of ionospheric drifts by electrojet at Thumba has already been shown by Chandra et al. (1971). Very few workers have studied the lunar tides in drift and a detailed comparison is not possible at this stage. Phillips (1952) and Chapman (1953) have studied the lunar tidal variations in E-region horizontal drifts for Cambridge and Ottawa respectively. Their results show a good agreement between the 12 hourly horizontal drift with regard to phase, if there is no phase reversal of the barometric pressure oscillations at the E-region level. Ramanna and Rao (1962) have discussed the lunar daily variations in horizontal drifts at Waltair and have shown it to be related to the lunar barometric pressure oscillations at a similar latitude (Chapman and Westfold, 1956). Acknowledgements-1 am grateful to Professor R. G. Rastogi under whose supervision the entire drift data was collected and also to Dr. M. R. Deshpande and Dr. Harish Chandra, who were associated with the data collection in the earlier years. Thanks are also due to the late Professor V. Sarabhai whose continuing interest in the subject enabled the use of Thumba Rocket Launching Station for the experiments. The help rendered by Sri. H. G. S. Murthy, Director, Thumba Rocket Launching Station and the staff of the range, during the course of data collection, is also acknowledged with thanks. REFERENCES APPLETON,E.V. and BEYNON, W.S. G.(1948). Nature162,486. BHARGAVA,B. N. (1963). J. atmos. terr. Phys. 25,367. CHAPMAN, J. H. (1953). Can.J. Phys. 31,120.

Y

1114

R. K. MISRA

CHAPMAN,S. and MILLER,J. C. P. (1940). Mon. Not. R. astr. Sot. geophys. SuppI. 4,649. CHAPMAN,S. and WESTFOLD,K. C. (1956). J. atmos. terr. Phys. 8,l. CHANDRA, H., Mrslu, R. K. and RASTOGI,R. G. (1971). Planet. Space Sci. 19,1497. KRALITKRAMER, J. (1950). Arch. elekt. #bertr. 4,133. KNETCH,R. W. (1959). J. atmos. terr. Phys. 14,348. MARTYN, D. F. (1947). Proc. R. Sot. 190A, 273. MARTYN, D. F. (1948). Proc. R. Sot. 194A, 429. MARTYN, D. F. (1949). Nature 163,34. MCNISH, A. G. and GALITIER, T. N. (1949). J. geophys. Res. 54,303. MATSUSHITA,S. (1953). Rep. Zonosph. Res. Japan 7,45. MISRA, R. K. and RAS~~;I, R. G. (1971). J. Znstn. Telecommun. Engrs, New Dehli 17,406. MISRA, R. K., CI-IANDRA, H. and RASTOGI,R. G. (1971). J. Geomagn. Geoelect. 23,181. MISRA, R. K. (1971). Ph.D. Thesis, Gujarat University, India. P. 154. MITRA, S. N. (1949). Proc. Z.E.E. 96, Part III, 441. PHILLIPS,G. J. (1952). J. atmos. terr. Phys. 2,14. RASTOGI,R. G. (1962). J. Res. Natn. Bur. Stand. 66D, 601. RASTOGI,R. G. (1961). Nature 189,214. RASTOGI,R. G. and TRMZDI, N. B. (1970). Planet. Space Sci. 18,367. RAMANNA, K. V. V. and RAO, B. R. (1962). J. atmos. terr. Phys. 24,220. SHARMA, R. P. and RAS~OCI,R. G. (1970). Znd. J. pure appl. Phys. 8,853. THOMAS,J. A. and SVENSON,A. C. (1955). Aust. J. Phys. 8,554. WRIGHT, R. W. and SKINNER,N. J. (1959). J. atmos. terr. Phys. 13,217.