  Over 24 years of synoptic data from the NSO Kitt Peak Vacuum Telescope is
used to investigate the coherency and source of the 27-day (synodic)
periodicity that is observed over multiple solar cycles in various
solar-related time series. A strong 27.03-day period signal, recently reported
by Neugebauer et al. (2000), is clearly detected in power spectra of time
series from integrated full-disk measurements of the magnetic flux in the 868.8
nm Fe I line and the line equivalent width in the 1083.0 nm He I line. Using
spectral analysis of synoptic maps of photospheric magnetic fields, in addition
to constructing maps of the surface distribution of activity, we find that the
origin of the 27.03-day signal is long-lived complexes of active regions in the
northern hemisphere at a latitude of approximately 18 degrees. In addition,
using a new time series analysis technique which utilizes the phase variance of
a signal, the coherency of the 27.03-day period signal is found to be
significant for the past two decades. However, using the past 120 years of the
sunspot number time series, the 27.03-day period signal is found to be a
short-lived, no longer than two 11-year solar cycles, quasi-stationary signal.
