The effect of the sea on lunar variations of the vertical component of the geomagnetic field

Phmt. Spa

Sd. 1969, Vol. 17, pp. 487 to 4%. Rxmumm Rem. Rinted in Northam Ireland

THE EFFECT OF THE SEA ON LUNAR VARIATIONS OF THE VERTICAL COMPONENT OF THE GEOMAGNETIC FIELD S. R. C. MALZN Institute of Geological Sciences, Royal Greenwich Observatory, Herstmonccux Castle, Hailsham, Sussex, England Ah&met-A number of observatories, mostly near the coast, show an anomalous value for L,(Z) which is probably caused by tides iu the sea. In au attempt to separatethe sea tide and ionospheric effects, data for the IGY have been analysed treating day and night separately. The night-time values are ascribed to non-ionospheric causes, and are used to produce a ‘correct.& L,(Z) of purely ionospheric origin. Although the ‘corrected’ values are much smaller than the uncorrected values, they show a much more consistent pattern. The nonionospheric effect is much larger thau had been suspected and appears to extend for a considerable distance inland. 1. INTRODUCTION

A number of determinations of the lunar semi-diurnal variations of the vertical component of the geomagnetic field, h(Z), have revealed anomalously large amplitudes, e.g. at Amberley, New Zealand (Bullen and Cummack, 1954), Watheroo, Western Australia (Malin and Winch, 1968) and at Hartland, Lerwick and Valentia in the British Isles (Malin, 1967). Since all of these observatories except Watheroo are within 5 km of the coast, it seems likely that the enhancement of the amplitude is associated with proximity to the sea. Although Watheroo is 90 km from the coast, the amplitude of ,5&Z) is exceptionally large. Unlike the coastal observatories, however, Watheroo has a similar anomalous amplitude in S(Z). It is possible that some of the coastal effect is due to anomalous induction effects resulting from the conductivity differences over land and sea. A more likely mechanism for the effect (Larsen and Cox, 1966) is the generation of electric currents in the sea due to tidal movements of the sea across the geomagnetic field. The study of L on a worldwide basis is valuable for investigating the current systems which cause the L variations and for testing models of velocity in the ionosphere, as well as for deducing information about the conductivity of the upper mantle. The value of such a study is greatly reduced if the L data are adulterated by effects which are not of ionospheric origin. To discard those observatories which lie near to the coast would remove more than half the observatories, including all those on mid-oceanic islands which form an essential part of the worldwide coverage, particularly in the Southern Hemisphere. It is therefore of great interest to attempt to separate any L effect due to ocean tides from the combined direct and induced effect resulting from lunar atmospheric tidal influence on the ionospheric current system. A method for doing this has been suggested by K. Weekes (private communication). The ocean tidal effect will be operative both by day and by night, whereas the overhead ionospheric currents become very small during the hours of darkness. Thus any L.&Z) effect in data for night hours only may be ascribed mainly to ocean tides, or to some other non-ionospheric cause. 2. DATA AND ANALYSIS

A simple analysis of IGY data has been carried out for the observatories already mentioned (Amberley, Hartland, Lerwick, Valentia and Watheroo) and for Eskdalemuir 487

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S. R. C. MALIN

(Scotland) and Irkutsk (Siberia). Although the latter two observatories have been shown by previous analyses (Malin, 1967; Malin, unpublished) to have small amplitudes for b(Z), it is of interest to include them since Eskdalemuir, which is 50 km from the sea, completes the set of British Isles observatories operating during the IGY, and Irkutsk is at a very great distance from the sea. Alternate mean hourly values of 2 from 1957 July 1 to 1958 December 30 were used. The five International Disturbed Daysof each month were omitted. As afirst approximation, day was taken from 06” to 18h LMT and night from 18h to OSh. Mean values were formed for alternate solar hours at 24 lunar phases and from these means La(Z) night and h(Z) day were deduced. The results are presented in Columns 1 and 2 of Table 1. These results may TABLE1

L,Q L*(Z) night

&(a

day

&Y (3)

0

(1)

ni$ht +

L,(z) day night

L(z) (5)

(4)

I,

2,

1,

1.

1,

1,

1,

1,

1,

A*

Observatory

w

(9

w

0

w

(“1

w

(“1

w

(“1

Eskdalemuir

0.87

93

1.23

0.71

-28

0.85 f 0.22

Hartland

164

5

3.20

-4

2.41

-1

0.80

-14

2.51 f 0.16

-1

2.91

-1

040

-79

10

Lerwick

2.85

7

3.02

Valentia

2.02

20

3.77

Amberley

3.06

55

280

53

Watheroo

2.14

41

3.91

78

Irkutsk

0.04

-42

0.43

13

-19

0.79

2.89

40

15

0.89

2.93

54

0.14

2.89

65

1.27

0.23

-14

0.21

40 -2

3.06 f 0.53

0

5

3.04 f 0.19

15

3.07 f 0.14

53

108

3.14 f 0.19

64

-104 -24

0.23 f 0.13

-17

be combined to give the values which would be obtained from an analysis ignoring the distinction between day and night (Column 3) and also to produce the values due to ionospheric causes only (Column 4) on the assumption that the non-ionospheric effect is thesame during the day as during the night. Results obtained from the formal method of analysis described by Leaton, Malin and Finch (1962), which is a development of that of Chapman and Miller (1940), are presented in Column 5. 3. DISCUSSION

A comparison of Columns 3 and 5 confirms that the present method, though crude, produces results which are very similar to those from a more rigorous analysis. Also, the vector probable errors in Column 5 give some indication of the vector probable errors which would be associated with the present results. The non-ionospheric results (Column 1) are surprisingly large, particularly for Eskdalemuir and Watheroo, which are inland observatories. This may be partly due to the simple day/night division used, which allows some of the daylight data to be included with the night data during Summer months. However, analyses of separate seasons for Hartland show only a 20 per cent increase in la(Z) night from Winter to Summer. In all cases except Irkutsk the ionospheric effect (Column 4) has a smaller amplitude than the non-ionospheric contribution.

EFFECT OF THE SEA ON GEOMAGNETIC LUNAR VARIATIONS

489

It remains to be seen whether the ionospheric and non-ionospheric effects deduced by this method are realistic. If the non-ionospheric effect is due to the sea, it extends much further inland than was suspected. The amplitude of the effect at Watheroo would appear to be improbably large except that independent evidence (Everett and Hyndman, 1967) shows that the coastal effect is particularly large in South Western Australia, and does appear to extend for a considerable distance inland. Unless the electric currents in the sea induce appreciable currents in the upper mantle, however, the results of Everett and Hyndman may not be relevant, since they arise from anomalous conductivity associated with the continental shield rather than from the direct influence of the sea. It was anticipated that the non-ionospheric effect at Eskdalemuir would be negligible, so that this observatory could be used as a purely ionospheric standard for comparison with the other British Isles observatories. It appears, however, that observatories further from the coast should have been used. Two such observatories at approximately the same latitude as the British Isles are Moscow and Irkutsk. The amplitude of L,(Z) at Moscow is 0.147 (Benkova et al., 1964), that for Irkutsk appears in Table 1. These amplitudes are of the same order as those ascribed to ionospheric causes in the British Isles. Removal of the non-ionospheric effect leads to a much more consistent pattern for L&Z) in the British Isles. The greatest deviation occurs at Lerwick, where the vector probable error suggests that the determination is weakest. Another approach to the problem is to consider the ratio of L&Z) to S,(Z), the solar semi-diurnal variation which is almost entirely due to the ionospheric current system. Since & and S, have very similar periods, any effects due to induction will affect both in nearly Also, those observatories which are at approximately the same the same proportion. latitude will experience atmospheric tides of approximately the same amplitude, so the ratio of L, to S, should be nearly constant for the Furely ionospheric effect. This is much more nearly true for the corrected value of L&7) (Column 4 of Table 1) than for the uncorrected L.&Y) (Column 5 of Table 1). The results are presented in Table 2. TABLE 2

Dip latitude (4

S*(Z) (Y)

Eskdalemuir

53.6

690

0.123

o-103

Hartland

49-4

8.65

0.291

0.092

Lerwick Valentia

58.5 50.5

8.48 9.71

0.361 o-313

o-047 o-092

Irkutsk

56-4

3.31

0.069

0.063

Observatory

&Q/&(z)

&(Z) ~~@VS*(Z)

4. CONCLUSIONS

It appears that the division of La(Z) into ionospheric and non-ionospheric parts may be successfully accomplished as described above, although it is clear that the method could be refined. It would also be desirable to deduce vector probable errors associated with the components of L&Z). Since the non-ionospheric contribution is so large, even for observatories relatively far from the coast, it would be desirable to evaluate the effect for all observatories before using their results to deduce ionospheric current systems. It should be borne

490

S. R. C. MALIN

in mind that this will necessarily reduce the current to zero during the night hours. Also it would be of importance to investigate elements other than 2 to see if they have nonionospheric components. Acknowledgements-The author is grateful to Mr. B. R. Leaton and Dr. D. E. Winch of the Institute of Geological Sciences and to Dr. K. Weekes and Dr. D. Schlapp of the University of Exeter for helpful discussions.

BBNKOVA,N. P., ZAOULYAYBVA,V. A., KATZLUHVILI,N. A., MARTINOVA,K. V. and MARDERPELD, B. E. (1964). Geomg. & Aeronomy, U.S.S.R. 4,611.

BULLEN,J. M. and CUMMACK,C. H. (1954). N.Z. Jf. Sci. Tech&. B. 35,371. CHAPMAN, S. and MILLMR,J. C. P. (1940). Mon. Not. R. astr. Sot. geophys. Suppl. 4,649. EVERETT,J. E. and HYNDMAN, R. D. (1967). Phys. Earth Planet. Interiors. 1,24. LARSEN, J. and Cox, C. (1966). J. geoplrys. Res. 71,4441. hATON, B. R., MALIN, S. R. C. and FINCH, H. F. (1962). R. Ohs. Bulls., No. 63. M,UIN, S. R. C. (1967). Geophys. J. R. astr. Sot. 13,397. MALIN, S. R. C. and WINCH, D. E. (1968). Nature, Land. 218,941.