1. RHESSI, the Wilson effect, and solar oblateness H.S. Hudson, M.D. Fivian & H.J. Zahid Space Sciences Lab, UC Berkeley

2. Glasgow, 27 April 2006 Historical background I The properties L O, R O, M O, and   (and X, Y, and Z) more or less characterize a star, but none of them are really constant The observed variations of L, the “solar constant,” include terms due to magnetic activity (spots and faculae), oscillations (p-modes) and convective flows (see ARAA 26, 473, 1988) Radius variability, often “discovered,” has really never been observed directly

3. Glasgow, 27 April 2006 Total solar irradiance variations - highlights

4. Glasgow, 27 April 2006 ARAA 26, 473 (1988) Frohlich, 2005

5. Glasgow, 27 April 2006 ARAA 26, 473 (1988) Frohlich, 2005 Flares Minutes 200 ppm at most Woods et al. 2004

6. Glasgow, 27 April 2006 Historical background II Alexander Wilson, local Glasgow font designer, physician, meteorologist and astronomer de La Lande, Maskelyne… Sunspot umbrae appear as depressions of the photosphere (but are they really, or is it just a trick of opacity? Or is it the same thing?) Limb darkening ~ height variation (limb at  5000 =.005?)

7. Glasgow, 27 April 2006 On three fine days in 18th-century Glasgow…

8. Glasgow, 27 April 2006 An observatory at a site with better seeing…

9. Glasgow, 27 April 2006 Operating principle of the RHESSI Solar Aspect Sensor (SAS Sensor: 1024-pixel linear CCD, 1.73 arc sec/pixel Spectral band: 670 nm x 12 nm FWHM Readout: limbs ~100 sec -1, chords ~1 min -1

10. Glasgow, 27 April 2006 SAS reduced data Four limb pixels * 6 Normalization to R 0 Deviations from mean profile

11. Glasgow, 27 April 2006 Data

12. Glasgow, 27 April 2006 The p-modes explain excess “random” noise August 2004: 57-orbit incoherent sum spectrum

13. Glasgow, 27 April 2006 The p-modes explain excess “random” noise August 2004: 57-orbit incoherent sum spectrum

14. Glasgow, 27 April 2006 SOHO/MDI view of limb p-modes: “Rossby Hills”? Kuhn et al. 2000

15. Glasgow, 27 April 2006 The oblateness signal Current fit 9.72 +/- 0.19 mas (random error ~10 -4 pixels)

16. Glasgow, 27 April 2006 Solar Disk Sextant radius results Egidi et al. 2006

17. Glasgow, 27 April 2006 Further in the past… Auwers, 1891

18. Glasgow, 27 April 2006 Synoptic-chart representation of SAS data

19. Glasgow, 27 April 2006 Test of limb sunspots as a roll reference

20. Glasgow, 27 April 2006 More spatial modeling

21. Glasgow, 27 April 2006 More timewise modeling

22. Glasgow, 27 April 2006 The problem of faculae Granulation, solfläckar och facklor nära solranden (from the Swedish Solar Telescope homepage)

23. Glasgow, 27 April 2006 The problem of faculae Faculae are now recognized in white light to be ordinary facular granules see behind magnetic flux tubes The structures are on small scales and have drastically time-variable 3-dimensional structure It seems hopeless to understand their structure in simple terms, but modeling (even with the redoubtable Carlsson involved) is far from perfect. Faculae are therefore noise from RHESSI’s point of view now.

24. Glasgow, 27 April 2006 What about g-modes? Rogers & Glatzmaier 2005 Toner et al. 1999

25. Glasgow, 27 April 2006 Conclusions The RHESSI/SAS data provide (by far) the best measures of solar limb-shape variations, an independent window on the solar interior We can see oblateness and higher-order stationary terms, we can see sunspots, faculae, and p-modes We dare to think about g-modes, but there are unknown questions about the stability time scales of the instrument parameters.

26. Glasgow, 27 April 2006 Radius quiz questions What is the scale of the solar radius at 30 kHz? At what wavelength is the minimum opacity of the solar atmosphere? How large are the planetary tides on the Sun?