Solar Radius Observations by the Michelson Doppler Imager

The Michelson Doppler Imager (MDI) instrument on the SOHO spacecraft is approaching the completion of six years of observing the Sun. During this period, full disk continuum images with 4 arc-second resolution have been taken at a cadence of at least four images per day. Because of the absence of atmospheric blurring and the stable environment of the L1 halo orbit, these images provide a long term measurement of the solar limb. The determination of changes in the solar radius from these images is affected by both an annual thermal variation in the MDI front entrance window and by a slow shift in the instrument focus due to aging of the entrance bandpass filter. These effects are being modeled in order to determine an estimate of the solar cycle variation in the solar radius. This research is supported by NASA grant NAG5-10483 at Stanford University.