Synoptic views of the solar limb: RHESSI radius and SOHO images

M.D. Fivian, H.S. Hudson, H.J. Zahid

The RHESSI mission incorporates precise astrometric measurements
of the solar limb shape via narrow-bandwidth optical continuum
observations designed for solar tracking with high time resolution.
These data reveal facular regions as increases, and sunspots as
decreases of apparent solar radius. We compare these signatures
with SOHO images synoptically 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. The current data reduction achieves random error
levels on the order of 0.1 mas. This work anticipates the use of
high-contrast coronal or chromospheric measurements to provide
masking functions to screen against magnetic features in determinations
of solar oblateness and higher-order permanent features at even
higher precision in the future.