Synoptic Views of the Solar Limb: RHESSI Radius and SOHO Images

The RHESSI mission includes precise astrometric measurements of the solar limb shape at optical wavelengths as a part of its aspect-determination system. These data have precisions below 1 mas and extend over the full lifetime of the mission (from February, 2002). Synoptic maps of the limb shape reveal facular regions as increases, and sunspots as decreases, in the apparent radius. We compare these signatures with synoptic SOHO images for a 3-month period in 2004. The patterns are strongly similar, but the EUV synoptic maps have contributions from features not at the exact limb, which dominates the RHESSI data. This study anticipates making use of such high-contrast coronal or chromospheric measurements to provide a masking function to screen against these features in determinations of the true solar oblateness and higher-order permanent shape features. We also explore the possibility of cross-correlating RHESSI sunspot images against those of other optical telescopes, such as MDI, as a means of calibrating the roll coordinate of the telescope pointing.