AAS 204th Meeting, June 2004
Session 37 Solar Magnetic Fields and the Photosphere
SPD Poster, Tuesday, June 1, 2004, 10:00am-7:00pm, Ballroom

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[37.19] Variations of Solar Radius: Observations with RHESSI

M. D. Fivian, H. S. Hudson, R. P. Lin (SSL/UCB)

The Solar Aspect System (SAS) of the rotating (at 15~rpm) RHESSI spacecraft has three subsystems. Each of these measures the position of the limb by sampling the full solar chord profile with a linear CCD using a narrow-band filter at 670~nm. With a CCD pixel size of 1.7~arcsec, the accuracy of each of the six limb positions is theoretically better than 50~mas using four pixels at each limb. Since the launch of RHESSI early 2002, solar limbs have been sampled with at least 100~Hz. That provides database currently containin 4~\times~109 single radius measurements. The main function of SAS is to determine the RHESSI pointing relative to Sun center. The observed precision of this determination has a typical instantaneous (16~Hz) value of the order of 50 mas (rms). We present initial RHESSI observations of the radius, including signatures of oblateness and of magnetic activity (spots and faculae).

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