Measuring the Angular Size of the Sun
The Solar DIsk Sextant (SDS)
Measuring the solar diameter (and its variations) has a long and controversial history with results highly inconsistent with each other, not only in absolute values, but also in trends with time. The principal causes of the inconsistent results are the effects of the Earth's atmosphere, differences in the definition of the limb edge, the spectral range of the measurements, and instrumental variations that cannot be independently calibrated with a precision higher than the expected variations.
The SDS observes at an altitude of over 30 km, thus avoiding effects of the atmosphere. Angular calibration is achieved through the use of an optical wedge that splits the Sun's image, allowing opposite sides of the solar limb to be imaged and measured in a small area of the instrument's focal plane. The optical wedge is contact-bonded, ensuring long-term geometric stability.
The balloon-borne SDS experiment has measured the angular size of the Sun seven times over the period 1992 to 2011. The half-diameter is found to change over that time by up to 200 mas, whereas the estimated uncertainties of the measures, random plus systematic, are typically 20 mas. The variation appears as if it might be cyclic, but if so, it is not in phase with the solar activity. Thus, the measured variation is not an artifact of observational contamination by surface activity. While the SDS measures span 19 years they are sparse, making it impossible to say with any certainty that the observed variation is cyclic.The results shown in the figure below call out for additional observations and future (yearly) SDS flights are planned.
Simultaneous measurements of the photospheric and seismic radii are of great interest since they will allow us to calibrate solar models better. Differences and similarities of the temporal behavior of these radii should contain information on how and where magnetic fields linked to activity
change solar structure. The temporal behavior of the Sun's photospheric radius provides a key constraint on models of solar structure, particularly with regard to the location, geometry, and evolution of subsurface magnetic fields.
Image Credits: (header) NASA, ESA, J. Hester and A. Loll (Arizona State University)