  This paper is concerned with intermediate range periodicity in the sunspot
area spectrum. An empirical model of sunspot area emergence based on Mercury
planet conjunctions was developed and the spectra of the model variation and
the sunspot area variation compared. By including solar cycle amplitude
modulation and the effect of solar magnetic field reversal the model was able
to predict fine detail in the sunspot area spectrum. As Mercury planet
conjunctions occur predictably it was possible to compare the time variation of
band limited components of sunspot area with the corresponding component
variations in the model. When the model component variation was stable
corresponding components of sunspot area lagged the model variation by a few
tens of days. When a 180 degree phase change occurred in the model variation
the corresponding component of sunspot area followed the change over an
interval of a few hundred days, first by decreasing to zero and then emerging
in phase with the model variation. Where periodicities in sunspot area did not
match periods of Fourier components of the model the autocorrelation function
of the model variation provided an indirect numerical link to these
periodicities. Included in these indirect periodicities were components at
periods of about 138, 235, 355, 472, 530, 605, 833, 1190 and 2020 days. Direct
periodicities in the model, those linked directly to Fourier components of
sunspot area, included components at about 88, 116, 176, 200, 292 and 405 days.
The model was developed without allowing for a variable threshold for sunspot
emergence. However, it was found that with a variable threshold above which the
model variation is effective in triggering sunspots, weak Fourier components at
the indirect periods were generated. The model with threshold then encompasses
all of the significant intermediate range periodicities in sunspot area.
