  In order to explore a recent proposal that the solar core may contain a
component that varies periodically with a period in the range 21.0 - 22.4 days,
due either to rotation or to some form of oscillation, we have examined the
time series formed from measurements of the solar neutrino flux by means of the
GALLEX, Homestake and Kamiokande experiments. Direct Fourier transform analysis
of the Homestake data shows that the most prominent peak in the entire spectrum
(examined down to 5 days period) is found at a frequency of approximately 17.2
y-1 corresponding to a period of approximately 21.3 days. According to the
"shuffle test," the probability of finding this large a peak in the prescribed
search band is about 0.03%, if it is assumed that there is no correlation
between count rate and time. The GALLEX and Kamiokande data are examined in a
way that searches for similarity in the shapes of the two spectra in sliding
windows in frequency. We find that the "spectral correlation measure" peaks at
17.2 y-1, and the shuffle test indicates that the probability of finding this
large a peak at a specified frequency is 2%, if it is again assumed that for
each time series there is no correlation between count rate and time. The
combined significance estimate is of order 1 part in 105 that the results are
due to chance, on the assumption that there is no real structure to the
count-rate time series.
