The optical polarization of the Sun measured at a sensitivity of parts in ten million

Abstract

The integrated light of the Sun, an essentially spherical star with only slight asymmetries (small oblateness, weak overall magnetic field), would normally be found to be unpolarized, as observed in broad spectral bands with common instrumental sensitivities, 10⁻⁴−10⁻⁵ fractional polarization^1–4. Defining the Sun's intrinsic linear (LP) and circular (CP) polarizations down to much lower levels (10⁻⁷ or even 10⁻⁸) would have consequences not only in solar physics but in other areas, setting, for example, a new standard for stellar polarimetry. We report measurements of the general polarization of the Sun, both CP and LP, over the whole disk and over large sectors, at an absolute sensitivity level of ≲3×10⁻⁷, carried out during August–September 1986. Upper limits for the intrinsic whole-Sun LP from the best data (minimum Earth-atmospheric contamination) were 0.2×10⁻⁶ in the V (yellow) band and 0.8×10^–6 in B (blue). Definite CP was discovered. (1) The north and south polar zones showed values of V (normalized or fractional CP) of −1 to −6×10⁻⁶ for the north, and 0 to +2×10⁻⁶ for the south zones. The spectral CP rises steeply toward the blue. (2) The whole disk had a net CP of −0.1 to −1.0×10⁻⁶ (from red to blue), negative as with the magnetically dominant north pole. The spectral dependence of the global broadband CP resembles that of sunspots^5,6 and of local non-spot regions with magnetic flux tubes⁷.