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A survey of historical and recent solar diameter observations
Adv. Space Res. Vol. 8, No. 7, pp. (7)129.-(7)132, 1988 Printed in Great Britain. All rights reserved.
0273—1177/88 $0.00 + .50 Copyright © 1989 COSPAR
A SURVEY OF HISTORICAL AND RECENT SOLAR DIAMETER OBSERVATIONS E. Ribes,* J. C. Ribes,** I. Vince*** and Ph. Merlin** *Observatoire de Paris, Section d’Astrophysique, 92195 Meudon, France * * Observatoire de Lyon, 69230 Saint Genis-Laval, France * * *Astronomical Observatory, Volgina 7, 11050 Belgrade, Yugoslavia
ABSTRACT We report new observational evidence of periodic changes in the solar diameter. Progress has been made at two levels - Systematic measurements of the solar diameter (both horizontal and vertical) have been made using a Meridian Circle at the Belgrade Observatory, from 1974 to 1986. An oscillation (amplitude of ±0.20arcsec, period of about 900 days) is present in both diameters. Although the data are fewer and noisier than the astrolabe measurements made by Laclare (1987), they provide an independent confirmation of the presence of the damped oscillation reported by Delache et al. (1985), with the same phase, period and amplitude. A Fourier analysis of the solar diameters observed by La Hire (6,797 values), from 1683 to 1718, has been made. The analysis exhibits a number of peaks, all of which are present in the power spectrum of modern diameter data. In particular, a peak at 9,6 years is clearly visible. This result indicates that the solar cycle occured during the Maunder minimum, although the dearth of sunspots at the solar surface is confirmed. Some possible implications of these findings are suggested with respect to climatic variability on Earth. INTRODUCTION The solar diameter has been regularly measured from the seventeenth century, when precision astronomy became available /1/. Changes in the solar diameter were reported as early as the eighteenth century /2/. These changes have been challenged, however, as the definition of the solar limb is very much dependent on the technique used and on the personal bias of the observer. Thus, controversies about the reality of a modulation of the solar envelope have arisen from time to time /3/. Recent measurements of the solar diameter have been analyzed, using a Danjon-type astrolabe and covering an l1-y cycle /4/. The Fourier spectrum analysis exhibits a series of peaks, the most important corresponding to a 900-day periodicity, with a mean amplitude of ±0.15 arcsec on the solar radius. It is, therefore, important to analyse independent diameter observations, in order to confirm the periodicities present in the astrolabe data. Such time-series are available. Observations of the solar diameter, covering the same l1-y cycle as the one observed by Laclare, have been obtained at the Belgrade Astronomical Observatory. Also, 36 years of transit timings have been obtained by La Hire, during the seventeenth century, using the same 6-foot quadrant. The historical observations span the Maunder minimum, a period of anomalous sunspot activity. The existence of the modern and historical diameter series is of threefold interest. If the diameters observed at Belgrade exhibit the same periodicities as those found in the modern observations, it would mean that one could rule out personal bias as well as an instrumental effect. Moreover, if the periodicities are present in other solar activity indices, they are probably of solar origin. Finally, the comparison between the power spectrum of the historical data and the modern diameters measurements could be used to understand the low level of sunspot activity during the little Ice-Age. OBSERVATIONS AND RESULTS The modern series of solar diameters consists of two independent sets of data, obtained from visual observation of the Sun (in right ascension and in declination), with the Meridian Circle at the Belgrade Observatory, from 1977 to 1986. Horizontal and vertical diameters are thus obtained. A description of the instrument and the method of observation is described in /5/. The accuracy for each radius is ±0.27 arcsec for the polar radius, and
±0.5
arcsec for the equatorial radius. The numbers of independent determinations of the solar raidus over the cycle are 433 and 349 for the polar and equatorial radius respectively. Apparent diameters have been normalized to one astronomical unit. Both polar (vertical) and equatorial (horizontal) diameters exhibit a 900-day periodicity, the amplitude of which is about ±0.2 arcsec. Details are discussed elsewhere /6/.
(7)129
(7)130
E. Ribesetal.
solar envelope. It is of interest to note that some of the periodicities are also found in other solar activity indices, such as the H /9/ and -ray /10/ flaring activity.This indicates that the periodicities mentioned above are of solar origin. Their presence at the time of the Maunder minimum confirms that the solar cycle was active, although the sunspot level was reduced. A line is weak in the historical data, and corresponds to a 335 day periodicity. it may be seen in the astrolabe data, and is a prominent feature in the Zurich sunspot number. A line such as this has been associated with the active phase of the ll-y cycle, and interpreted by Delache et al. /7/, as the change of the solar limb due to the blocking flux by sunspots. Therefore, it is not surprising that the 335d-periodicity is missing at the time of a reduced sunspot activity. A strong half-year periodicity (above the 5 level) is present in the historical data and will be discussed in a forthcoming paper /11/. a
0 I-.
‘a I-’
U)
-
0..
-
0
0
~ 0
0.01
0.05
r
-
b
C
°~
R
___________-
0
-
0.01
0.00
t (period) of the historical
Figure 2 a) Fourier transform of the time-distribution meridian timings (from 1683 to 1718) versus the inverse of the period b) Amplitude of the Fourier spectrum of the same sampling versus the inverse of the period (in days). DISCUSSION AND CONCLUSION Some of the periodicities discussed here can be considered as absolute periodicities, as they are present in other solar activity indices /12/. It should be noted that we found an intriguing correlation between the maxima and minima of the solar diameter (at the 900-day period) during the last solar cycle, and the starting phases of the stratospheric winds, the quaai-biennal oscillation of the Earth’s atmosphere /131/61. The westerlies, at the 10 millibar pressure, begin when the solar diameter is a minimum (Fig.l). The westerlies also correspond to the onset of a giant convection, the rolls /14/, which seems to be the response of the expulsion of the deep magnetic fields. If the correlation is not accidental, the solar variability (magnetic or convective) could provide the forcing of the stratospheric circulation, although there is still no known mechanism available. It could be also the reason for measuring diameter variations due to changes in the terrestrial atmosphere. The presence of the absolute periodicities in the seventeenth century diameter data confirms that the solar cycle was operating during the Maunder minimum, even though we have indication that the magnetic fields were probably confined to deep layers. We interpret the increase of the solar diameter and the slower rotation of the sunspots within the Maunder minimum, as a global expansion and cooling off of the solar envelope, causing the little Ice-Age.
Historical and Recent Solar Diameter Observations
(7)131
The comparison of the diameter variation obtained with the meridian circle and the astrolabe data is shown in Fig.l. The observed variations are in good agreement with the oscillation reported by Delache et al. /7/, that is they are similar in phase and amplitude. The degree of correlation is 0.47 (with a confidence level of 98 per cent), when both polar and equatorial diameters are combined. The correlation may have reduced when we compare diameters at a given time corresponding to different obliquities, with respect to the solar equator. Both observations (the meridian transits at Belgrade as well as the astrolabe data from France) are visual and subject to various biases. These biases are different for each technique, and from one observer to another. Thus, there is no reason why the systematic variations within a series of measurements made by the astrolabe and the meridian circle should be the same. Therefore, this result confirms that the observed oscillation is not caused by personal and instrumental biases, but of solar (or terrestrial atmosphere).
RADIUS
961.50
961.00
+
~
I
959.50.
b
~1
~44 + ~‘4
E•
~{~L
f t
t
t
4 959.00.
E
W
,.
w I
1977
I
1978
w I
1979
I
1980
1981
1982
1983
1984
I
1985
1986
YEAR
Figure 1 Comparison of the global radius (polar and equatorial) observed with the meridian circle (a) and with the astrolabe (b).We have indicated the starting phases of the zonal stratospheric circulation, at the 10 millibar pressure : the Easterlies and Westerlies (denoted as E and w) correspond to the maxima and minima of the solar diameter. The historical data are the meridian transit timings made by a single observer (La Hire), from 1683 to 1718. The details of this instrument (a 6-ft. quadrant) and the method of determining the horizontal (equatorial) diameter have been published elsewhere /8/. The accuracy for each radius determination is smaller than ±3 arcsec. The large number of observations, involving approximately 150 to 200 yearly measurements regularly distributed over the 36 years, reduces the noise. A Fourier analysis of the data has been performed, and the results are shown in Fig. 2. Several peaks are present at the 2 to 3 level. A peak at 3505 days (corresponding to a 9.6y periodicity) can be interpreted as the signature of the solar cycle. The variation is anti-correlated with the sunspot number, as is the case for modern cycles. This result may indicate that the solar cycle was operating during the Maunder minimum, although the level of sunspot activity recorded at the solar surface was low. The amplitude of the line in the Fourier spectrum is ±0.45arcsec, which is larger than the present one (±0.18 arcsec). The ll-y solar cycles associated with the dearth of sunspots, manifest themselves as a super-oscillating envelope. Other peaks are present in the power spectrum, in particular around 2 to 3 years, 17 months and 155 days (denoted hereafter as P, H and R 1). The amplitude of these lines is larger than those reported by Delache et al. /7/ and Laclare /4/. This suggests that the characteristics of the solar cycles, during the Maunder minimum, was a super oscillation of the
(7)132
E. Ribes eta!.
REFERENCES 1. Ph. La Hire,
Archives Ohs. Paris, manuscripts D2, 1-10,
(1683-1718).
2. P.J.D. Gething, Monthly Notices 115, 558-570 (1955). 3. J. Lalande, Astronomie, Ed. Desaimp, Paris, 4. F. Laclare C.R. Acad.
2nd edn (1771)
Sci. (Paris), 305, s&rie II, 451 (1987)
5. S. Sadzakov, M. Dacie and D.
Saletic, Publ. Dept. Astron. Belgrade 6, 119 (1976).
6. E. Ribes, J.C. Ribes, I. Vince and Ph. Merlin, C.R. Acad. (1988).
Sci. (Paris), in the press
7. Ph. Delache, F. Laclare and H. Sadsaoud, Nature 317, 416 (1985). 8. E. Ribes, J.C. Ribes and R. Barthalot, 9. K. Ichimoto, J. Kubota, M. Suzuki,
Nature 326, 52 (1987).
I. Tohmura and H. Kurokawa, Nature 316, 422 (1985).
10. E. Rieger, G.H. Share, D.J. Forrest, C. Kanbach, C. Reppin and E.L. Chupp, Nature 312, 623 (1984). 11. E. Ribes, J.C. Ribes, Ph. Merlin and R. Barthalot,
submitted.
12. Ph. Delache, this issue. 13. B. Naujokat, Journal of Atmos. Sci. 43, 1873 (1986). 14. E. Ribes and F. Laclare, Ceophys. Astrophys. Fluid Dynamics 41, 171 (1988).
0273—1177/88 $0.00 + .50 Copyright © 1989 COSPAR
A SURVEY OF HISTORICAL AND RECENT SOLAR DIAMETER OBSERVATIONS E. Ribes,* J. C. Ribes,** I. Vince*** and Ph. Merlin** *Observatoire de Paris, Section d’Astrophysique, 92195 Meudon, France * * Observatoire de Lyon, 69230 Saint Genis-Laval, France * * *Astronomical Observatory, Volgina 7, 11050 Belgrade, Yugoslavia
ABSTRACT We report new observational evidence of periodic changes in the solar diameter. Progress has been made at two levels - Systematic measurements of the solar diameter (both horizontal and vertical) have been made using a Meridian Circle at the Belgrade Observatory, from 1974 to 1986. An oscillation (amplitude of ±0.20arcsec, period of about 900 days) is present in both diameters. Although the data are fewer and noisier than the astrolabe measurements made by Laclare (1987), they provide an independent confirmation of the presence of the damped oscillation reported by Delache et al. (1985), with the same phase, period and amplitude. A Fourier analysis of the solar diameters observed by La Hire (6,797 values), from 1683 to 1718, has been made. The analysis exhibits a number of peaks, all of which are present in the power spectrum of modern diameter data. In particular, a peak at 9,6 years is clearly visible. This result indicates that the solar cycle occured during the Maunder minimum, although the dearth of sunspots at the solar surface is confirmed. Some possible implications of these findings are suggested with respect to climatic variability on Earth. INTRODUCTION The solar diameter has been regularly measured from the seventeenth century, when precision astronomy became available /1/. Changes in the solar diameter were reported as early as the eighteenth century /2/. These changes have been challenged, however, as the definition of the solar limb is very much dependent on the technique used and on the personal bias of the observer. Thus, controversies about the reality of a modulation of the solar envelope have arisen from time to time /3/. Recent measurements of the solar diameter have been analyzed, using a Danjon-type astrolabe and covering an l1-y cycle /4/. The Fourier spectrum analysis exhibits a series of peaks, the most important corresponding to a 900-day periodicity, with a mean amplitude of ±0.15 arcsec on the solar radius. It is, therefore, important to analyse independent diameter observations, in order to confirm the periodicities present in the astrolabe data. Such time-series are available. Observations of the solar diameter, covering the same l1-y cycle as the one observed by Laclare, have been obtained at the Belgrade Astronomical Observatory. Also, 36 years of transit timings have been obtained by La Hire, during the seventeenth century, using the same 6-foot quadrant. The historical observations span the Maunder minimum, a period of anomalous sunspot activity. The existence of the modern and historical diameter series is of threefold interest. If the diameters observed at Belgrade exhibit the same periodicities as those found in the modern observations, it would mean that one could rule out personal bias as well as an instrumental effect. Moreover, if the periodicities are present in other solar activity indices, they are probably of solar origin. Finally, the comparison between the power spectrum of the historical data and the modern diameters measurements could be used to understand the low level of sunspot activity during the little Ice-Age. OBSERVATIONS AND RESULTS The modern series of solar diameters consists of two independent sets of data, obtained from visual observation of the Sun (in right ascension and in declination), with the Meridian Circle at the Belgrade Observatory, from 1977 to 1986. Horizontal and vertical diameters are thus obtained. A description of the instrument and the method of observation is described in /5/. The accuracy for each radius is ±0.27 arcsec for the polar radius, and
±0.5
arcsec for the equatorial radius. The numbers of independent determinations of the solar raidus over the cycle are 433 and 349 for the polar and equatorial radius respectively. Apparent diameters have been normalized to one astronomical unit. Both polar (vertical) and equatorial (horizontal) diameters exhibit a 900-day periodicity, the amplitude of which is about ±0.2 arcsec. Details are discussed elsewhere /6/.
(7)129
(7)130
E. Ribesetal.
solar envelope. It is of interest to note that some of the periodicities are also found in other solar activity indices, such as the H /9/ and -ray /10/ flaring activity.This indicates that the periodicities mentioned above are of solar origin. Their presence at the time of the Maunder minimum confirms that the solar cycle was active, although the sunspot level was reduced. A line is weak in the historical data, and corresponds to a 335 day periodicity. it may be seen in the astrolabe data, and is a prominent feature in the Zurich sunspot number. A line such as this has been associated with the active phase of the ll-y cycle, and interpreted by Delache et al. /7/, as the change of the solar limb due to the blocking flux by sunspots. Therefore, it is not surprising that the 335d-periodicity is missing at the time of a reduced sunspot activity. A strong half-year periodicity (above the 5 level) is present in the historical data and will be discussed in a forthcoming paper /11/. a
0 I-.
‘a I-’
U)
-
0..
-
0
0
~ 0
0.01
0.05
r
-
b
C
°~
R
___________-
0
-
0.01
0.00
t (period) of the historical
Figure 2 a) Fourier transform of the time-distribution meridian timings (from 1683 to 1718) versus the inverse of the period b) Amplitude of the Fourier spectrum of the same sampling versus the inverse of the period (in days). DISCUSSION AND CONCLUSION Some of the periodicities discussed here can be considered as absolute periodicities, as they are present in other solar activity indices /12/. It should be noted that we found an intriguing correlation between the maxima and minima of the solar diameter (at the 900-day period) during the last solar cycle, and the starting phases of the stratospheric winds, the quaai-biennal oscillation of the Earth’s atmosphere /131/61. The westerlies, at the 10 millibar pressure, begin when the solar diameter is a minimum (Fig.l). The westerlies also correspond to the onset of a giant convection, the rolls /14/, which seems to be the response of the expulsion of the deep magnetic fields. If the correlation is not accidental, the solar variability (magnetic or convective) could provide the forcing of the stratospheric circulation, although there is still no known mechanism available. It could be also the reason for measuring diameter variations due to changes in the terrestrial atmosphere. The presence of the absolute periodicities in the seventeenth century diameter data confirms that the solar cycle was operating during the Maunder minimum, even though we have indication that the magnetic fields were probably confined to deep layers. We interpret the increase of the solar diameter and the slower rotation of the sunspots within the Maunder minimum, as a global expansion and cooling off of the solar envelope, causing the little Ice-Age.
Historical and Recent Solar Diameter Observations
(7)131
The comparison of the diameter variation obtained with the meridian circle and the astrolabe data is shown in Fig.l. The observed variations are in good agreement with the oscillation reported by Delache et al. /7/, that is they are similar in phase and amplitude. The degree of correlation is 0.47 (with a confidence level of 98 per cent), when both polar and equatorial diameters are combined. The correlation may have reduced when we compare diameters at a given time corresponding to different obliquities, with respect to the solar equator. Both observations (the meridian transits at Belgrade as well as the astrolabe data from France) are visual and subject to various biases. These biases are different for each technique, and from one observer to another. Thus, there is no reason why the systematic variations within a series of measurements made by the astrolabe and the meridian circle should be the same. Therefore, this result confirms that the observed oscillation is not caused by personal and instrumental biases, but of solar (or terrestrial atmosphere).
RADIUS
961.50
961.00
+
~
I
959.50.
b
~1
~44 + ~‘4
E•
~{~L
f t
t
t
4 959.00.
E
W
,.
w I
1977
I
1978
w I
1979
I
1980
1981
1982
1983
1984
I
1985
1986
YEAR
Figure 1 Comparison of the global radius (polar and equatorial) observed with the meridian circle (a) and with the astrolabe (b).We have indicated the starting phases of the zonal stratospheric circulation, at the 10 millibar pressure : the Easterlies and Westerlies (denoted as E and w) correspond to the maxima and minima of the solar diameter. The historical data are the meridian transit timings made by a single observer (La Hire), from 1683 to 1718. The details of this instrument (a 6-ft. quadrant) and the method of determining the horizontal (equatorial) diameter have been published elsewhere /8/. The accuracy for each radius determination is smaller than ±3 arcsec. The large number of observations, involving approximately 150 to 200 yearly measurements regularly distributed over the 36 years, reduces the noise. A Fourier analysis of the data has been performed, and the results are shown in Fig. 2. Several peaks are present at the 2 to 3 level. A peak at 3505 days (corresponding to a 9.6y periodicity) can be interpreted as the signature of the solar cycle. The variation is anti-correlated with the sunspot number, as is the case for modern cycles. This result may indicate that the solar cycle was operating during the Maunder minimum, although the level of sunspot activity recorded at the solar surface was low. The amplitude of the line in the Fourier spectrum is ±0.45arcsec, which is larger than the present one (±0.18 arcsec). The ll-y solar cycles associated with the dearth of sunspots, manifest themselves as a super-oscillating envelope. Other peaks are present in the power spectrum, in particular around 2 to 3 years, 17 months and 155 days (denoted hereafter as P, H and R 1). The amplitude of these lines is larger than those reported by Delache et al. /7/ and Laclare /4/. This suggests that the characteristics of the solar cycles, during the Maunder minimum, was a super oscillation of the
(7)132
E. Ribes eta!.
REFERENCES 1. Ph. La Hire,
Archives Ohs. Paris, manuscripts D2, 1-10,
(1683-1718).
2. P.J.D. Gething, Monthly Notices 115, 558-570 (1955). 3. J. Lalande, Astronomie, Ed. Desaimp, Paris, 4. F. Laclare C.R. Acad.
2nd edn (1771)
Sci. (Paris), 305, s&rie II, 451 (1987)
5. S. Sadzakov, M. Dacie and D.
Saletic, Publ. Dept. Astron. Belgrade 6, 119 (1976).
6. E. Ribes, J.C. Ribes, I. Vince and Ph. Merlin, C.R. Acad. (1988).
Sci. (Paris), in the press
7. Ph. Delache, F. Laclare and H. Sadsaoud, Nature 317, 416 (1985). 8. E. Ribes, J.C. Ribes and R. Barthalot, 9. K. Ichimoto, J. Kubota, M. Suzuki,
Nature 326, 52 (1987).
I. Tohmura and H. Kurokawa, Nature 316, 422 (1985).
10. E. Rieger, G.H. Share, D.J. Forrest, C. Kanbach, C. Reppin and E.L. Chupp, Nature 312, 623 (1984). 11. E. Ribes, J.C. Ribes, Ph. Merlin and R. Barthalot,
submitted.
12. Ph. Delache, this issue. 13. B. Naujokat, Journal of Atmos. Sci. 43, 1873 (1986). 14. E. Ribes and F. Laclare, Ceophys. Astrophys. Fluid Dynamics 41, 171 (1988).