The Satellite Total Solar Irradiance Database
Richard Willson [rwillson@acrim.com], ACRIM Experiments, Coronado, California
A precise knowledge of the total solar irradiance (TSI) over time is essential to
understanding the physics of solar luminosity variation and its impact on the Earth’s climate.
A National Research Council study found that sustained trends as small as 0.25% per century
were the most likely forcing for ‘little ice age’ climate minima during the 12th –19th
centuries. Recent analyses of TSI observations and proxies indicate TSI variation is an
important climate change forcing on many timescales including the industrial era. The
profound sociological and economic implications of understanding the relative climate
change contributions of natural and anthropogenic forcings makes it essential that the satellite
TSI database be precisely sustained into the foreseeable future.
There are currently three satellite TSI monitoring experiments in operation:
SOHO/VIRGO, ACRIMSAT/ACRIM3 and SORCE/TIM, in order of deployment (1996,
2000 and 2003, resp.). Results reported on their ‘native scales show the same basic
variations in TSI over time, yet some smaller variations detected by ACRIM3 are less well
defined or absent in the results of VIRGO and TIM. There is also a scale difference issue:
TIM results are 0.35% lower than those of ACRIM3 and VIRGO, outside the ± 0.1%
uncertainty bounds predicted for ACRIM3 and VIRGO, and well outside TIM’s ± 0.01%
uncertainty design goal. Ongoing experiments may resolve the scale difference but the
redundant/overlap TSI monitoring paradigm that has produced a useful 32-year database of
satellite TSI observations is the only demonstrated viable approach.