Preliminary results of measurements made during 1979-1980 are discussed. Variability in the radius measurements of 0.4 pi is found, of unknown origin

Duvall, T. L., Jr.

Jones, H. P.

English

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SOLAR RADIUS MEASUREMENTS
T. L. Duvall, Jr. and H. P. Jones
NASA/GSFC, Laboratory for Astronomy
and Solar Physics - -
Solar Physics, Southwest Solar Station
Tucson, Arizona
ABSTRACT
Preliminary results of solar radius measurements made during 1979-1980
are discussed, variability in the radius measurements of 0.4w is found, of
unknown origin.
INTRODUCT ION
An observational program to search for variability of the solar radius was
begun in December 1979 at Kitt Peak. This program was initiated because of the
claims of a secular decrease in the solar radius of ~ l_/century (ref. I). A
searcL of the literature, however, also reveals claims of variability on
time scales such as a 0.i_ solar cycle variation (ref. 2). The first ten
months of observations are discussed in this preliminary report. Unexplained
variability at the level of 0.4_ is found. At this stage of the investigation,
all possible sources for this variability are being considered--instrumental,
atmospheric, and solar.
OBSERVATIONS
The technique used to measure the solar radius is the classical one of
obtaining the time interval between passage of opposite solar limbs across a
detector pointed at a fixed place in the sky. The known rotation rate of the
earth and declination of the sun are then used to derive an angular diameter.
In the present application of this technique a linear array of 512 detectors is
used. Each detector subtends an angle of i_ x i_. The detector array, aligned
at a 15° angle with respect to the solar drift direction, is used to measure
512 chord lengths in the neighborhood of the diameter of the circular solar
disk. A diameter is derived from the chord lengths by a least-squares fit.
The telescope used is the Vacuum Solar Telescope of the Kitt Peak National
Observatory. The detector array and associated electronics is part of the 512
channel magnetograph normally used with this telescope.
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An observation consists of recording the signals from the detector array
with a time constant of 0.3 s for the time it takes the sun to drift across
the entire array (~ 2 1/2 minutes). Four such observations were made on each
• of 18 da_s between 4 December 1979 and 30 September 1980. For each day's
observations radius values are computed. The mean value of each set of 4 and
also a statistical uncertainty are derived. The statistical imcertainties
are consistent with an uncertainty for each uf the daily mean values of 0.2_.
The 18 daily mean values are found to vary by more than this estimated un-
certainty and have a standard deviation of 0.4_. The source of this excess
variability is not known at this time.
As a preliminary search for secular variability, the 18 daily mean values
were fitted to a linear trend. The result is an apparent decrease in solar
radius of 0.2 ± 0.2 r/year. Because the size of the uncertainty, this result
is not statistically significant.
REFERENCES
i. Eddy, J. A.; and Boornazian, A. A.: Secular Decrease in the Solar Diamete_
1863-1953. Bull. Am. Astron. Soc. ii, 1979, p. 437.
2. Meyermann, B.: Zur Pulsation der Sonne, Astron. Nachr. 279, 1950, p. 45.
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Solar radius measurements

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