Geomagnetic effects of the solar eclipse, 12 October 1958, at Apia, Western Samoa

Transcription

1 New Zealand Journal of Geology and Geophysics ISSN: (Print) (Online) Journal homepage: Geomagnetic effects of the solar eclipse, 12 October 1958, at Apia, Western Samoa A. L. Cullington To cite this article: A. L. Cullington (1962) Geomagnetic effects of the solar eclipse, 12 October 1958, at Apia, Western Samoa, New Zealand Journal of Geology and Geophysics, 5:3, , DOI: / To link to this article: Published online: 21 Dec Submit your article to this journal Article views: 333 View related articles Citing articles: 2 View citing articles Full Terms & Conditions of access and use can be found at

2 1962] 499 GEOMAGNETIC EFFECTS OF THE SOLAR ECLIPSE, 12 OCTOBER 1958, AT APIA, WESTERN SAMOA A. L. CULLINGTON Magnetic Survey, Geophysics Division, Department of Scientific and Industrial Research, Christchurch (Received for publicaiiou, 20 March 1962) ABSTRACT Small geomagnetic effects in accord with Chapman's theoretical considerations were clearly observed due to the solar eclipse on 12 October 1958, at Apia, Western Samoa, which was not in the path of totality. In Hand D the effects were diminutions in the departures of the daily variation from the night-time values. The maximum effects in Hand D occurred a short time after mid-eclipse. The effect in Z changed sign passing through zero a few minutes after maximum obscuration of the sun. A comparison was made of the maximum reduction of the conductivity in the ionosphere at Apia with observation at Suwarrow and Rarotonga, and the results from these three stations were chronologically consistent with the passage of the solar eclipse. INTRODUCTION Since 1900 many attempts have been made to ascertain whether or not a solar eclipse has any effect on the geomagnetic field. Much work has been carried out on this problem by Bauer (1900, 1920), Chree (1915), Nordman (1907), Gama (1948), and others. On the theoretical side Chapman (1933) estimated the order of the effect that should be expected to result from a solar eclipse. More recently Egedal and Ambolt (1955) studied the effects of the solar eclipse of 30 June 1954 on the magnetic declination recorded at several observatories. Malurkar (1954) studied the records from Indian magnetic observatories for days of solar eclipse and stated that the problem had to be classed as yet unsolved. Kato (1%0) observed the changes of the Earth's magnetic field at Suwarrow Island in the South Pacific Ocean at the time of the total solar eclipse of 12 October Suwarrow Island was on the path of totality, and Kato observed the eclipse effect very distinctly in the horizontal magnetic component. Apia, Western Samoa, was not on the path of totality for the eclipse of the SUIl of 12 October 1958, but a partial eclipse with considerable obscuration of the Sun would have occurred there. Chapman and Bartels (1940, p. 354) stated: "According to Chapman's theoretical discussion, partial eclipses (if, say, 70 per cent or more of the Sun is obscured at maximum phase) may be little inferior, in their magnetic effects, to total eclipses." A study was therefore made of the magnetic changes at the time of this eclipse as recorded at the Apia Magnetic Observatory, which is operated by the Magnetic Survey, Geophysics Division, Department of Scientific and Industrial Research. N.Z. J. Geol. Geopbys. 5:

3 500 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS [AUG. GEOMAGNETIC EFFECTS Fig. 1 shows the path of totality during the solar eclipse of 12 October 1958, and the situation of Apia relative to it. L-- l l---io ~\ " -J"O 40 FIG. I-The path of totality of the solar eclipse of 12 October The circumstances for this eclipse at Apia ( ' S, ' W) were computed as follows: UT LMT First Contact 12 October 1958 Mid-Eclipse Last Contact 18h 39m 19h 46m 21h 02m 07h 12m 08h 19m 09h 35m Fig. 2 shows the magnetogram traces recorded at Apia Observatory on the day preceding the eclipse, the day of the eclipse, and the day succeeding the eclipse. At the time of the eclipse there appears to have been a distinct decrease in the horizontal component. Estimation of the amount of change. due to the eclipse is not easy because of the difficulty of deciding where the normal trace would have been if there had been no eclipse effect. Experience at Apia has shown that there is much variability in 5q and this has to be allowed for. For this reason comparison with the traces of the preceding and succeeding days was not considered to be sufficient. In the

4 1962] CULLINGTON - GEOMAGNETIC EFFECTS 501 APIA MAGNETIC OBSERVATORY OCT T...~ <::::::: -::-: Ht ;. I50r'... : : : :!te H BASE 110' Dt DBASE 21 G.M.T I50r T,...-- APIA MAGNETIC OBSERVATORY OCT 1958 Ht==;!f ~t ~ ~. ~ _' II.:: ISOr.. H BASE-.. Tlo' 21 G.M. T z BASE T I50r zt ===~;;============~=:==~= : APIA MAGNETIC OBSERVATORY OCT 19S8 T ~ H?.'. -=::::::::::::: -----= : 15~r H BASE ]10' DB~tE--' 21 G.M.T 'I50r z BASE zt :~~~=::=:================ T Sig.11 FIG. 2-Apia magnetograms for 11, 12, and 13 October 1958.

5 502 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS [AUG. light of knowledge of the variability of Sq at Apia, the normal traces were drawn for D, H, and Z, and measurements of the departures in the recorded traces and of the normal curves were carried out for every 10 minutes. The departures were measured, not from the mean for the day, but from the mean of the values for a few hours from to hours GMT, as it was considered that the ionisation and presumably the currents or the diamagnetism of the E layer would be least then. The mean values of the elements for this period were: H = 34874y : Z = 20376y : D = "S' E. Fig. 3 shows the deviation of the magnetic elements H, D, and Z at the time of the eclipse from the normal diurnal variation. In Hand D the effect has been a decrease in the amount of diurnal variation. The maximum effect in the horizontal component was a decrease of about 10 gammas and occurred about 15 minutes after mid-eclipse at Apia. In D the changes were not so well defined but the maximum deviation occurred shortly after mid-eclipse. In Z the deviation changed sign and was zero about the time that the deviation of H was ;l maximum, indicating that the disturbance electric current system of the Chapman model was overhead about 15 minutes after the time of mid-eclipse at Apia. In this investigation no account was taken of the effect of induced earth currents. The deviation of the horizontal component at the time of the eclipse was not symmetrical. The rate of decrease before the maximum was different from the rate of recovery after the maximum. Kato (1960) found a similar asymmetry in the deviation of the horizontal component during the time of the eclipse at Suwarrow. He attributed the cause of this asymmetry to the return current of the distorted Sq current produced by the decrease of conductivity of the ionosphere in the eclipsed area. Ngata et al. (1955) studied the critical frequencies of the E layer at Japanese stations at the time of the solar eclipse on 9 May They found that the maximum decrease of the critical frequency occurred a short time after the maximum phase of the eclipse. They also found that the rate of decrease was different from the rate of recovery, and this feature of the asymmetry they thought to be caused by an extraordinarily active region on the Sun at the time. The lack of symmetry in the H curve shown in Fig. 3 was probably caused by the passage of the shadow and its direction of movement across the Sq current system over Apia. Also, the time from commencement of eclipse to the maximum obscuration, of the Sun was about 9 minutes less than the time from maximum obscuration to the end of the eclipse. It is possible that ionising radiation may not have been emitted uniformly from the visible surface of the Sun, and this might have contributed to the observed asymmetry. Kato adjusted his curve and made it symmetrical, but this does not appear to have been necessary. In the past many claims have been made that magnetic effects due to solar eclipse have been found, but most of these claims cannot be supported because of the occurrence of other magnetic disturbance that would obscure the small eclipse effects. Magnetic conditions on 12 October 1958, however, were quiet, enabling the magnetic effects due to the eclipse at Apia to be found beyond all doubt. At Apia the effects in D were not as clearcut as in H because the sensitivity ofthe D variorneter was much less than

6 1962] CULLINGTON - GEOMAGNETIC EFFECTS Or I ~... I I :;0 I H -s r -10Y +1'.. D 0' -1/ OY~." I I 'V I j -51" GJ cd ~ 18h U T III U C W FIG. 3-Deviation of H, D, and Z from the normal diurnal variation at the time of eclipse.

7 504 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS [AUG. that of the H variometer. It appears that the D and Z variometers employed by Kato were too insensitive to detect the eclipse effect in these elements. At tropical stations suitably situated with respect to the path of totality, magnetographs should be used in which the scale values are not more than 3 to 4 gammas per millimetre in all elements. CONDUCTIVITY OF THE IONOSPHERE According to Chapman (1933) the eclipse effect may be considered as equivalent to the superposition upon the normal current system in the ionosphere of an additional system due to the change in the conductivity in the area of the shadow. He took the simple case of steady uniform current flow I in an infinite plane sheet of uniform conductivity K, when a circle was cut out of the sheet and replaced by an equal circle of uniform but different conductivity K'. The current I' across the circle will be uniform and of magnitude 2K'l/(K+K'). Let b.h be the change in H due to normal current I and b.h' the change due to the current 1', then b.h = 27T1 and b.h' = 2'iT1'. Let 8H = b.h - b.h'. Since I' = 2K'I/(K+K') then K'/K = (b.h-8h)/(b.h+8h). TABLE 1-Variation of Electrical Conductivity of E Layer (K'/K) at Apia, Expressed as Percentage DT ilh sn' 8H K'/K y y y % t

8 1962] CULLINGTON - GEOMAGNETIC EFFECTS 505 Table 1 shows the changes 6.H, 6.H', and 8H, as well as the computed values of the ratio K'/K expressed as a percentage. Chapman (1933) showed that if the diurnal variation current flowed in the :E layer the maximal diminution due to the eclipse in the departures in H from the night-time value would be 28%. Table 1 shows that the maximum diminution in the H departures occured at 2000 hr UT, and this amounted to a decrease of 26% in the H departure of the normal curve at this time from the nighttime value. This lends support to the idea that the E layer is the seat of the 5q current system. / ~,'" 80 ~ I III ~ ~I,I] I 18h. U.T FIG. 4-Variation of the electrical conductivity of the E layer at the time of eclipse. Fig 4 shows the variation of the electrical conductivity of the ionosphere E layer by means of a smoothed curve drawn through the plotted values of K'/K. The results show that the conductivity was decreased to about 59% of that of the normal day at a time a few minutes after mid-eclipse. At Suwarrow ( ' 40" S, ' 13" W), Kato found that the conductivity was decreased to about 58'6% that of the normal day at the time of maximum eclipse. He also found from the ionospheric data from Rarotonga, ( ' S, ' W), where the eclipse was partial with about 80% obscuration at maximum, that the maximum decrease in conductivity K'/ K to 59'4% occurred a few minutes after the maximum phase there. In connection with the study of ionospheric measurements made at Calcutta during the solar eclipse of 20 July 1944, Mitra (1948) reported that as the eclipse progressed the ionisation decreased smoothly, the minimum

9 506 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS [AUG. being reached about 10 minutes after occurrence of the maximum obscuration and being about 43% below the normal value as obtained from the control observations. The times of maximum solar obscuration and of maximum decrease in conductivity at Apia, Suwarrow, and Rarotonga were as shown in Table 2. TABLE 2-0bscuration and Decrease in Conductivity Time of Time of Max. Deer. Max. Deer. in Station Max. Phase in Conductivity Conductivity of Eclipse Obscuration K'/K K'/K UT % UT % Apia '0 Suwarrow Rarotonga The fact that the time of maximum decrease in conductivity progresses chronologically with the progression of time of mid-eclipse, with a small time lag, from Apia to Suwarrow and to Rarotonga, as well as the fact that the magnetic changes on the H magnetograms at Apia and Suwarrow are not coincident in time, points to a local cause. Also, there is no change on the Amberley (43 09' S; ' E) magnetograms that could be attributed to an eclipse-indeed at this time the magnitude of the solar eclipse visible at Amberley would have been very small. Generally speaking, magnetic disturbances shown on the Apia records are more dearly shown on the Amberley records and with much greater amplitude. CONCLUSIONS The magnetic changes recorded at Apia at the time of the solar eclipse on 12 October 1958 were due to a local cause, because they occurred at a different time from those recorded at Suwarrow, and the changes are not shown on the Amberley records. The conductivity of the ionosphere (E layer) has been shown to decrease as the area of shadow increases and to increase as the area of shadow decreases. The times at which this decrease and increase in the conductivity occur march in progression with the times of waxing and waning of the Moon's shadow at Apia, Suwarrow, and Rarotonga. The effects described above are in accord with the theory that, when during the eclipse the Moon hides the Sun, the ionisation of the E layer is reduced and there is thus a decrease in the daily magnetic variation. The maximum decrease in the conductivity K'/K at Apia, Suwarrow, and Rarotonga is remarkably constant at 41%, although the maximum obscuration varied from 80% to 100%. This supports the suggestion of Chapman that partial eclipses, if 70% or more of the Sun is obscured, may be little inferior in their magnetic effects to total eclipses. It is thus not necessary for the observation of the geomagnetic effects of solar eclipses for the magnetic observatory to lie within the path of totality.

10 1962] CULLINGTON - GEOMAGNETIC EFFECTS 507 There are clearly geomagnetic effects due to solar eclipses, but the effects are small, the magnetic effects at Apia having maximum amplitudes of loy in H, 0 6' in D, and 5y in Z. Magnetic conditions on the eclipse day were very quiet; otherwise these small changes could have been easily obscured by other magnetic fluctuations. REFERENCES BAUER, L. A. 1900: Resume of Magnetic Observations'Made Chiefly by the United States Coast and Geodetic Survey on the Day of the Total Solar Eclipse, May 28, Terr. Magn. Atmos, Elect. 5: BAUER, L. A. 1920: Results and Analysis of Magnetic Observations during the Solar Eclipse of May Terr. Magn. Atmos. Elect. 25: CHAPMAN, S. 1933: The Effect of a Solar Eclipse on the Earth's Magnetic Field. Terr. Magn. Atmos. Elect. 38: CHAPMAN, S.; BARTELS, ]. 1940' "Geomagnetism", Vols 1 and 2, pp. 354, O.D.P., 1049 pp. CHREE, C. 1915: Magnetic and Electric Observations at Kew Observatory Relating to the Solar Eclipse of August 21, Terr. Magn. Atmos, Elect. 20: EGEDAL, ].; AMBOLT, N. 1955: The Effect on Geomagnetism of the Solar Eclipse of 30 June J. Aimos, Te'Y. PhyJ. 7: GAMA, L : Magnetic Effects Observed at Vassouras, Bazil, during the Solar Eclipse of May 20, Terr. Magn. Atmos. Elect. 53: KATO, Y. 1960: The Effect on the Geomagnetic Field of the Solar Eclipse of October 12, Sci. Rep. Tohoku Univ., 5th s«, Geopbys. 12: 1-9. MALURKAR, S. L. 1954: Geomagnetic Records at Colaba and Alibag on Days of Solar Eclipse. Indian J. Met. Geopbys. Suppl. 5: MITRA, S. K. 1948: "The Upper Atmosphere", pp Roy. Asiatic Soc. Bengal, Calcutta. 616 pp. NAGATA, T.; NAKATA, Y.; RIKITAKE, T.; YOKOYAMA, I. 1955: Effect of the Solar Eclipse on the Lower Parts of the Ionosphere and the Geomagnetic Field. Rep. lonospb. Res. Japan 9: NORDMAN, C. 1907: Contribution to the Study of the Effects Produced on the Magnetic Declination by the Total Solar Eclipse of August 30, Terr. Magn. Atmos. Elect. 12: