Changes in solar shape and diameter were detected as a possible probe of variability in solar luminosity, an important climatic driving function. A technique was designed which will allow the calibration of the telescope field, providing a scale for long-term comparison of these and future measurements

Hill, H. A.

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https://ntrs.nasa.gov/search.jsp?R=19830011119 2020-03-21T03:55:03+00:00Z
(NASA-CR-169898) SOLAR DTANITIR
KJASURTHENTS FOR STUDY OF SUN Ctl"All
COUPLING Final Report, Jun, 19 RI Nov, 1982
(Arizona Univ. . Tucson.)	 10 p HC A02/flF A01
csCl 04B G3/47
V83-19390
Unclas
08 74 5
SOLAR DIAMETER MEASUREMENTS FOR STUDY OF
SUN CLIMATE COWLING
Henry A. Hill
Department of Physics
University of Arizona
Tucson, Arizona 85721
January 1983
F inal Report for Period June 1981 through November 1932
Prepared for
GODDARD SPACE FLIGHT CENTER
Greenbelt, Maryland 20771
ORIGINAL PAGE 13
OF POOR QUALITY.
This project was initiated to detect changes in solar shape and
diameter as a possible probe of an important climatic driving function,
variability in the solar luminosity. These changes are to be monitored
using the techniques and facilities developed at SCLERA I during the
1970's f:,, measuring the solar diameter. During the contract period,
this project's observing program on the SCLERA instrument was continued,
as was the requisite data reduction. A technique has been designed
which will allow the calibration of the telescope field, providing a
scale for long-term comparison of these and future measurements; con-
struction of this device has begun.
1 SCLERA is an acronym for the Santa Catalina Laboratory for
Experimental Relativity by Astrometry, a facility jointly operated by
Wesleyan University arid the University of Arizona.
PRECEDING PAGE BLANK NOT FILMED
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 	 I
2. Summary of Work Performed 	 . . . . . . . . . . . . . . . . . . . 	 1
3. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . .	 3
Appendix A: Evidence for Long-Term Variations in the
Solar Limb Darkening Function; Implications
	
towards Indirect Luminosity Diagnostics . . . . . .	 5
	
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 	 7
iv
ORIGINAL PAGE 18
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1. Introduction
Many researchers have attempted to detect and monitor climatically
significant changes in the solar constant (for review, see White 1977;
Pepin, Eddy and Merrill 1980; Sofia et al. 1979; Gough 1980; Kandel
1981). One indirect diagnostic of luminosity variability utilizes
changes in the solar shape and diameter. Because of the advances made
at SCLERA during the 1?70's in the accurate measurement of solar diam-
eters, studies of this type may offer a more sensitive and cost-
e ffective probe than direct measurements.
Improvements in solar diameter measurements have been made possible
by an astrometric telescope designed solely for work relating to the sun
and by the introduction of a new technique for the definition of an edge
on the solar limb. Oleson et al. (1974) give a description of the
telescope used in SCLERA's work; discussions of the edge definition may
be found in papers by Hill, Stebbins and Oleson (1975) and Hiil (1978).
A program to monitor the diameter and shape of the sun as an
indirect diagnostic of the solar luminosity is ongoing at SCLERA. Dur-
ing the period of 1 June 1981 through 30 November 1982, this program was
partially supported by NASA contract NAGS-143. In the following report
we will summarize the work completed and the problems encountered under
this contract.
2• Summary of Work Performed
At the beginning of the contract period, observations of the solar
diameter were in progress. As discussed in previous reports (Hill
1981), the telescope detectors were operating in a "frozen" mode (i.e.,
no change or alterations), allowing the measurement of day-to-day
1
ORIGINAL PAGE 13
OF POOR QUALn
changes in the solar shapE. The primary measuring engine was a combined
phase-encoded Michelson He-Ne laser and double pass white light inter-
ferometer, the latter prodi.A ng a zero-point fiducial for the former.
Only thirteen acceptable daily data segments were acquired over a
41-day period which commenced before the contract starting date. The
seasonal rains and thunderstorms which prevail in the Southwest began
during the latter part of June. However, the year previous to this had
seen long patterns o,' exceptional cloudiness unusual for this area.
W -g ather during the summer of 1981 continued this trend, with severe
lightning storms in the mountains and around the SCLERA telescope site.
Unfortunately, during August of 1981 several surges passed through the
existing lightning protection, severely damaging the telescope computer
system and electronics. For the remainder of the contract period, the
activity at the telescope site centered around efforts to restore the
old system and, finally, its complete replacement with a new computer
system. New and more thorough lightning protection has been installed
at the site to prevent accidents of this type from recurring.
Although no new data were taken after the storm damage, the analy-
sis of the existing 1981 data has proceeded to the preliminary level.
These new results were presented at the X`/III General Assembly of the
International Astronomical Union in August 1982 (Caudell et al. 1982);
the associated abstract is included as an Appendix in this report.
Briefly, this data consisted of thirteen daily time strings of solar
diameters, measured cyclically at seven position angles on the solar
disk, with each string lasting from four to six hours. After the larger
systematic errors were modeled and removed and the white light inter-
ferometer fiducial incorporated, each daily string was averaged over
2
ORIGINAL PACE i$
Of POOR QUAL'TY
Position angle and time to produce a mean daily solar diameter. When
this mean was plotted versus calendar date, the gross signature of the
variation in the earth-sun distance due to the eccentricity of the
earth's orbit was clear. Residuals about this smooth trend were ob-
served and have been attributed to changes in the solar limb darkening
function. Work on the understanding of this effect is proceeding at
this time, both in theory and data analysis which involves further
quantification of the systematic errors. It is anticipated that a
measure of the solar flux will be developed from this analysis.
Theoretical work is also proceeding which will form ti'e basis for
interpretation of changes in the limb  darkening function in terms of
changes in flux. One paper on this topic is currently in the final
stage of preparation (Hill and Logan 1983).
Development of the device which will provide the calibration of the
telescope scale in the foct.l plane has progressed to the stage at which
capital equipment will be ordered. The telescope itself is being modi-
fied to allow the easy incorporation of the optical grating which con-
stitutes the heart of the calibration system. Although this device was
not operational at the time and thus will have no effect on the 1981
data set, new data sets will have the cal-ibration information available.
1.	 Concl L!sion
For the contract period indicated, many of the goals set for this
project in previous reports have been accomplished. This primarily
involves the demonstration that the SCLERA instrument can be used to
monitor long-term variations in the solar diameter. This has been shown
at both the scientific and administrative levels. The frozen mode of
operation required for this program was new to this instrument and hF s
3
proved feasible. Although the unforeseen lightning damage reduced the
actual amount of diameter data collected, it is anticipated that the
existing 1981 data will yield some useful information about the use o4
solar diameter measurements as a probe of luminosity.
ORIGINAL PKLI"18
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4
ORKIINAL PAGE 13
Appendix A	
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Evidence for Long-Term Variations in the Solar L'mb Darkening Function;
Implications towards Indirect Luminosity Diagnostics
T. P. Caudell, B. Beardsley, L. Yi, H. A. Hill, and R. J. Bos
Department of Physics, University of Arizona, Tucson, AZ
For many years now, it has been clear in the solar diameter
measurements at SCLERA that changes in the extreme limb darkening
function are responsible for most of the time varying signals. These are
manifest through the properties of the FFTD edge definition and have time
scales ranging from minutes to hours and to several days. A nF!w set of
measurements was taken in the early summer 1981, containing several
unique advances over previous data sets, which provide further proof that
these changes at the limb are observable and display significant
variations over a time scale of several days. This was made possible by
the implementation of a "white light" interferometer which yields a zero
point fiducial against which the diameter measuring interferometer may be
calibrated. Although thi s 's not an absol ute measurement of the sol ar
diameter, it allows the long-term variations in diameter to be studied
within a single data set.
	 In addition, this new data set sampled seven
equally spaced position angles from pole to pole, recording one diameter
and six extreme-limb intensity profiles at each location. From this
information, an average daily relative diameter was derived as well as
three average daily measures of shape of the limb darkening function
using off-line application of the FFTD with different aperture sizes.
The relative diameters plotted against calendar date clearly display the
geometric variation expected Crum the elliptical motion of the earth.
When this and other major systematics are modeled and removed, variations
of a large fraction of an aresecond still remain. A linear correlation
was performed between these variations and the measures of changes in the
limb darkening function. A high degree of correlation was found.
Although a linear function is not the exact functional relation between
these two quantities, the degree of correlation is sufficient to prove
that significant variations in the observed relative diameter are
produced by changes in the extreme limb darkening with several day time
scales. Therefore extreme care must be taken when analyzing the solar
diameter as an indirect diagnostic of luminosity, using models which do
not include accurate treatment of the solar atmosphere. Conversely,
monitoring the shape of the extreme limb darkening function will supply a
sensitive measure of the atmospheric temperature gradient and therefore
the solar flux.
This work was partially supported by the D.O.E. and N.A.S.A.
ORIGINAL PAGE 19
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6
Caudel 1 , T. P., Beardsl ey, B., Yi , L., Hi 11 , H. A., and Bos, R. J.
1982, XVIII General Assembly of the International Astronomical
Union, Patras, Greece, August 1982.
Gough, D. 0. 1980, Nature, 288, 639.
Hill, H. A.	 1978, The New Solar Physics, ed. J. A. Eddy (Westview
Press, Boulder), p. 135.
Hill , H. A. 1981, NASA Final Report, "Solar Diameter Measurements for
Study of Sun Climate Coupling," September 1981.
Hill, H. A., Stebbins, R. T. and Oleson, J. R. 	 1975, Astrophysical
Journal, 200, 487.
Kandel, R., ed. 1981, Proceedings of workshop on "Sun and Climate,"
Toulouse, France, September 1980.
O1 eson, J. R., Zanoni , C. A., Hill, H. A., Healy, A. W., Clayton, P. D.,
and Patz, D. L. 1974, Applied Optics, 13, 206.
Pepi n, R. 0., Eddy, J. A. and Merri 11 , R. B., eds. 1980, "The Ancient
Sun," Proceedings of conference held in Boulder, Colorado, October
1979.
Sofia, S., O'Keefe, J., Lesh, J. R. and Endal, A. S.
	
1979, Science,
204, 1306.
White, 0. R., ed. 1977, "The Solar Output and its Variation" (Univer-
sity of Colorado Press, Boulder).
7


Solar diameter measurements for study of Sun climate coupling

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