1. SPM-11, September 12, 2005 VARIATIONS OF SOLAR RADIUS: OBSERVATIONS WITH RHESSI M.D. Fivian, H.S. Hudson and R.P. Lin Space Sciences Lab, UC Berkeley

2. SPM-11, September 12, 2005 RHESSI radius observations Solar aspect sensor (SAS) determines precise limb location for X-ray telescope rotating at 0.25 rps Spin stabilization requires rapid cadence of precise limb measures - some 6 x 10 9 obtained thus far Experiment resembles that of Dicke & Goldenberg (1974), but in space rather than in New Jersey

3. SPM-11, September 12, 2005

4. The SAS data are precise RMS error < 30 mas per point Data are abundant: current rate of collection is ~ 100 sec -1 “Solar constant” provides a (statistically) standard candle for reference Rotation helps to control systematic errors

5. SPM-11, September 12, 2005

6. Oblateness

7. SPM-11, September 12, 2005 Recent solar oblateness results (Sofia et al., 1995)

8. SPM-11, September 12, 2005

9. Effects of solar activity

10. SPM-11, September 12, 2005

12. Dealing with systematic errors yes

13. SPM-11, September 12, 2005 Potential applications of data Faculae Facular drift Sunspot Wilson depression Limb flares (“white light prominences”) Oblateness Higher-order figure Long-period g-modes? Variability

14. SPM-11, September 12, 2005 Conclusions RHESSI limb sensors have adequate precision and coverage for many applications Solar activity (faculae and spots) can easily be seen at the limb The oblateness signal is detectable in a relatively short integration

15. SPM-11, September 12, 2005 End