Solar Photometry at the San Fernando Observatory
Dora Preminger [dora.preminger@csun.edu], Gary Chapman, and Angela Cookson;
San Fernando Observatory, California State University, Northridge.
The solar photometry program at the San Fernando Observatory, now in its 26th year,
records daily full-disk photometric images, with specially selected filters. Each image is
processed to produce photometric quantities that measure the relative contributions of solar
features to the image. We find that on solar cycle timescales, the net effect of solar activity is to
strongly increase disk-integrated Ca II K-line intensity, while weakly decreasing red and blue
continuum intensity. This result is surprising; however it does unite observations of the Sun and
highly active Sun-like stars. It is also qualitatively in agreement with the SORCE/SIM
observations between 2004 and 2009, although the magnitude of the solar-cycle trend we
observe is much smaller than that reported by SIM. We can use SFO photometric parameters to
model up to 95% of TSI variability. Our most successful TSI model attributes changes in TSI to
two distinct components: the activity-induced changes in the visible continuum flux and in the
Ca II K spectral line flux. From this model we infer that the +0.1% increase in TSI between
solar minimum and maximum includes an increase in spectral line flux of order +1% and a
decrease in continuum flux of order -0.02%. Spectral lines are concentrated primarily in the UV,
therefore, if high solar activity warms the climate, this must be entirely due to the increased UV
radiation that is absorbed in the Earth’s atmosphere.