Finding the sources of irradiance variation at sunspot minimum

In 2006-2007 the Solar Bolometric Imager (SBI) will operate in the polar stratosphere where near-space conditions can be attained for 10 to 30 days. The instrument will provide bolometric (wavelength-integrated light) and color temperature images of the Sun. At the upcoming sunspot minimum, SBI observations will be able to detect subtle sources of solar irradiance variation with the least confusion by signals from the magnetic fields. This is the best observational approach to characterizing potential causes of the long-term irradiance variations. Possible predicted sources of secular variability include torsional waves and meridional flow variations. SBI uses a 30-cm diameter F/12 Dall-Kirkham telescope with uncoated mirrors, and neutral density filters to provide broadband (bolometric) sensitivity that varies only by +/- 7% over the wavelengths from 0.31 mu m to 2.6 mu m. Inferred solar irradiance variations will be compared with space based full-disk radiometric measurements.