Scientific Objectives of the Solar Bolometric Imager Flight 1 (SBI-1)

Two 11-year cycles of space-borne radiometry have demonstrated that the total solar irradiance (and luminosity) increases by about 0.1% around activity maximum (Fröhlich and Lean, 1998). Analyses of this total irradiance record have shown that almost 90% of its variation is well correlated with the changing of the projected areas of dark sunspots and bright faculae and network (Fröhlich and Lean, 1998; Chapman et al., 1996). This strongly suggests that the measured luminosity variation can be attributed to the net effect on photospheric heat flow of the compensating contributions of these bright and dark magnetic structures (Foukal and Lean, 1988). However, it is still not clear whether the photometric effect of sunspots, faculae and network is actually equal or simply proportional to the measured radiometric fluctuations. Uncertain broad-band photometric contrasts of spots, and especially faculae and network, currently present the main obstacle to improved modeling of total irradiance fluctuations. The bolometric contribution of faculae is currently uncertain by as much as a factor of two. Until this uncertainty is removed it cannot be considered proven that the compensating effects of bright and dark photospheric magnetic structures account entirely for measured solar luminosity variation. Demonstration of this equality is critical in determining whether the thermal blocking model (Foukal el al., 1983) provides an adequate physical explanation of solar irradiance variation, or whether more complex processes such as magnetic storage or enthalpy advection (Chapman,1984; Schatten and Mayr, 1985) play a significant role. In addition, the possible existence of global changes that might dominate solar luminosity variation over climatologically important time scales is the most important unsolved problem in studies of solar luminosity variation (Shindell et al., Science 284, p. 305, 1999).

Here, we present a balloon-borne solar telescope equipped with an innovative bolometric detector (Foukal and Libonate 2001), capable of recording images with an angular resolution of about 5 arcsec in essentially total photospheric light. The Solar Bolometric Imager (SBI) provides the first opportunity to bolometrically image brightness variations at the solar photosphere. Its flat spectral response from the ultraviolet to the infrared (like that of ACRIM) directly provides the facular and network contribution to the total irradiance, and complements the non-imaging space-borne radiometer measurements.

The 3 main objectives of the balloon-borne SBI are:

To accurately measure (better than 10% per pixel) the bolometric contribution to the total solar irradiance of sunspots, faculae and enhanced network. This will help determine whether these structures can account for the rotational and 11-yr variability of the total irradiance, or whether other mechanisms highly correlated with their area variation might contribute significantly. We note that the ±10% precision in photometry and the comparable accuracy in photometric contrast refer to the errors relative to the amplitude of the fluctuations in total irradiance caused by spots, faculae and network. Since these fluctuations are themselves of 0.1% amplitude relative to the total irradiance signal, our ±10% photometric goal represents ±0.01% precision relative to the total irradiance. This is similar to the precision achieved by the space borne radiometers.

To search for other lower level inhomogeneities in photospheric heat flux uncorrelated with the photospheric magnetic structures themselves, and possibly associated with large-scale convective cells, meridional circulations, etc. Such inhomogeneities might prove more important over time scales longer than the 11-yr cycle.

To provide important engineering data to validate the space flight-reliability of the novel gold-blackened thermal array detector and to verify the thermal performance of the SBI uncoated optics in a vacuum environment.