Variations of Solar Radius: Observations with Rhessi

The Solar Aspect System (SAS) of the rotating (15 rpm) RHESSI spacecraft has three subsystems. Each of these measures the position of the limb by sampling the solar chord profile with a linear CCD and a narrow-bandwidth filter at 670 nm. With a plate scale of 1.7 arcsec/pixel, the precision of each of the six simultaneously observed limb positions is observed to be better than 50 mas, using four pixels at each limb. Since the launch of RHESSI early 2002, solar limbs have been measured at about 100 samples/sec. This has provided a database of 7 × 10 single radius measurements at the time of writing. The main function of SAS is to determine the RHESSI pointing relative to Sun center. Using these data we find that we can readily measure the solar oblateness at a magnitude consistent with other recent measurements. We can also see clear signatures of sunspots and faculae near the limb.