The effects of an annular solar eclipse on the phase of vlf transmissions

Journalof Atmospheric and Terrestrial Physics, 1970, Vol. 32, pp. 1835-1838. Pergamon Press. Printed in Northern Ireland

SHORT PAPER

The effects of an annular solar eclipse on the phase of VLF transmissions R. D. HOY Mount St,romlo and Siding Spring Observatories, Research School of Physical Sciences, The Australian National University Canberra, Australia (Received 29 January

1970; in revised fom 20 March 1970)

Abs~r~ct-~he effects of the annular solar eclipses of March 18 and September 11, 1969 on VLF transmissions were studied. Phase changes attributable to the March 18 eclipse were found corresponding to an increased time of propagation over the paths of 2.0 ,usec from NWC (22.3 kHz) and 10 psec from GBR (16.0 kHz). Similarities were noted between the observed phase delays and calculated eclipse obscuration along the great circle paths. 1. INTRODUCTION CONTINUOUS phase comparisons are made of stabilized very low frequency (VLE’) transmitters against Mount Stromlo Observatory’s caesium beam frequency standard. The phase records for NWC and GBR on March 18, 1969 and NSS and NAA on September 11, 1969 were examined for phase deviations attributable to the annular solar eclipses of those dates.

2. ANNULAE SOLAR ECLIPSEOF STARCH (a) NE%‘,

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Phase comparison readings from NWC for March 13, 14, 17, 20 and 22 were averaged to produce a b-day mean phase pattern from 01.30 UT to 07.30 UT. The eclipse occurred while the whole path was in daylight. Thus, together with the moderate path length of 3700 km and high transmitter power, enabled the mean phase pattern to be determined with a standard error of O-2 psec. The difference between the phase pattern for March 18 and the B-day mean result was calculated and is shown in Fig. 1. A maximum phase delay of 2-O ysee was observed at 04.46 UT. The eclipse ma~mum at NWC occurred at about 04.35 UT, the obscuration of the disc being about 0.88 and the altitude about 71’. The path of annularity passed to the west of NWC in the Indian Ocean, see Fig. 3, and so 04.35 UT is the time of eclipse maximum on the path. Two other significant disturbances were noted on March 18. The first began about 01.15 UT and is shown in Fig. 1. No readings were obtained from 02.54 UT to 03.38 UT owing to difficulties with the chart drive mechanism which meant the exact beginning of the eclipse effect was not determined. A sudden ionospheric disturbance (S.I.D.) occurred immediately after the eclipse effect, but has not been shown in Fig. 1. It began at 06.31 UT, reached a maximum phase advance 1835

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Fig. 1. Time delay produced by March 18, 1969 annular solar eclipse on VLF signal from NWC. Closed circles are the recorded time delays. Open circles were interpolated across the ionospheric disturbance. The curve is the normalised obscuration of the Sun as calculated.

of 7.0 ,usec at 06.39 UT and then slowly decayed, ending at 08.10 UT. Open circles shown in Fig. 1 were obtained by interpolation between periods of stable phase immediately before and after the disturbance. (b) GBR

16.0 kHz

Phase readings from GBR for March 16, 17, 20, 21 and 23 were averaged to produce a &day mean phase pattern from 02.30 UT to 08.30 UT. Because of the low transmitter power and very long path length (17,000 km), the mean phase pattern for the 5 days has a standard error of about 2 ,usec. The difference between the phase pattern for March 18 and the B-day mean result was calculated and is shown in Fig. 2. The signal of March 18 faded out a number of times, but this effect is common for this time of day. A maximum phase delay of 10 ,usec was observed at 05.00 UT. The eclipse began in the southern Indian Ocean when only a short section of the western end of the path was dark and ended before sunset at the eastern end. The path of annularity crossed the great circle propagation path at 05.35 UT when the Sun’s altitude was about 63”. The S.I.D. mentioned in (a) was also seen on GBR. It began about 06.48 UT, reached a maximum phase advance of 9 ,usec at 07.06 UT and ended about 08.30 UT.

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Annular solar eclipse on VLF transmissions

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Fig. 2. Time delay produced by March 18, 1969 annular solar eclipse on VLF signal from GBR. Closed circles sre the recorded time delays. The curve is the normalised obscuration of the Sun as c&x&ated.

Figure 3 shows the geometry of the event as predicted by DUNCOMBE (1968) and the great circle paths from Mount Stromlo to GBR and NWC. 3. ANNULAR SOLAR ECLIPSE OF SEPTEMBER 11, 1969 The phase of VLF signals from NSS 21.4 kHz and NAA 17.8 kHz on September 11, were compared with a 6-day mean daily phase patterns for each. No significant phase delays attributable to the eclipse were observed. This could be explained by assuming at this time of day the signals propagate along the long great circle path as described by THOMPSON, ARCHER and HARVEY (1963) for NBA. 4. DISCUSSION OF RESULTS The decrease in ionizing radiation reaching the earth because of the obscuring of the sun by the moon would produce a decrease in the ion density in the D-region and so produce a phase delay similar to that produced at night. With this assumption, the obscurations at twenty-one points along the great circle propagation path including the transmitter and receiver were calculated and summed at 5-min intervals during the eclipse. The calculation of obscuration was based on the method given in the Explanatory Supplement to the Astronomical Ephemeris (1961) using the elements published in the Astronomical Ephemeris for the appropriate year. No allowance was made for variations of solar radiation intensity with zenith distance. The altitude for the calculations was 80 km and sunset occurred when the solar zenith distance was 99.5”. This allows 9” for the effect of altitude and 0.5’ for atmospheric refraction. The obscuration curves are superimposed on the experimental results after normalisation to the maximum observed phase delay.

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R. D. HOY

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Fig. 3. The situation of the great circle paths and the movement of the eclipse annularity at 80 km altitude (time in UT) on March 18, I,969.

Figure 1 shows good agreement was obtsined between observed phase delay and the obscuration curve for NWC with regard to the time of maximum eclipse effect, the duration and shape of the phase delay. The observed phase delay and calculated obscuration were averaged every 10 min. A correlation coefficient of 0.92 was obtained between the obscuration and the phase delay for the 15 mean values from 03.42 UT to 06.02 UT. Correlation coefficients were also calculated for the seven results before the mean eclipse maximum at 04.42 UT and the eight results from 04.52 UT to 06.02 UT giving 0.99 and 0.97 respectively. If the obscuration curve is retarded 10 min, a correlation is then obtained of 0.98 over 16 results. These results show a strong relationship between the obscuration and the observed phase delay. In the case of GBR, see Fig. 2, the observed effect occurred some 30 min earlier than predicted. A correlation coefficient of 0.90 was calculated between the observations and the predicted curve advance by 30 min using 10 min means as before. The maximum phase delay at 05.02 UT could be explained by some ionospheric disturbance superimposing a phase advance on the phase delay due to the eclipse after 06.02 UT, thus decreasing the observed phase delay. However the high correlation coefficient of 0.90 suggests that the eclipse produced the observed phase delay and that some of the assumptions used to produce the theoretical

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Annular solar eclipse on VLF transmissions

curve are incorrect. A maximum phase delay at 05.02 UT could occur if the propagation path was situated considerably south of the great circle path and thus crossed the path of annularity earlier than the great circle path. The phase delay observed on GBR during the September 22, 1968, total eclipse by the author (HoY, 1969) was not explained by a similar calculation of obscuration along the great circle propagation path. However, a propagation path to the south of the great circle path would give better agreement. The results available are not su~~iently accurate to allow any reliable conclusion to be drawn and the applic&ion of the mode theory of VLF propagation may give better agreement where long propagation paths are considered. REFERENCES DUNCOMBE S. S. HOY R. D. TEIOMFSON ARCHER and HARVEY

1968 1969 1963

US NavaI Uberva.t~

Circ. No. 122. 31, 1027.

J. Atmoq&. !l’wr. Php. Proc. IEEE 51, 1487.