1. RHESSI and the solar radius H.S. Hudson, M.D. Fivian & H.J. Zahid Space Sciences Lab, UC Berkeley First report of our semi-intentional optical limb astrometry

2. GSFC, May 18 2006 Solar limb astrometry from the Reuven Ramaty High-Energy Solar Spectroscopic Imager RHESSI

3. GSFC, May 18 2006

5. Historical background The properties L 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 O, the “solar constant,” include ~ half a dozen known components The solar shape and the total irradiance are strongly linked: d(ln L O )/d(ln R O ) = w

6. GSFC, May 18 2006 Historical background The properties L 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 O, the “solar constant,” include ~ half a dozen known terms The solar shape and the total irradiance are strongly linked: d(ln L O )/d(ln R O ) = w … but we are not sure about the magnitude, or sign, of w!

7. GSFC, May 18 2006 Total solar irradiance variations - highlights

8. GSFC, May 18 2006 ARAA 26, 473 (1988) Frohlich, 2005 * *

9. GSFC, May 18 2006 ARAA 26, 473 (1988) Frohlich, 2005 Flares Minutes 200 ppm at most Woods et al. 2004

10. GSFC, May 18 2006 Two recent-suggested possible additional components R.C. Willson & A. Mordvinov, “Secular total irradiance trend during solar cycles 21-23,” GRL (2003) G. Withbroe, “Quiet Sun contributions to the total solar irradiance,” Solar Phys. 235, 369 (2006)

11. GSFC, May 18 2006 Two recent-suggested possible additional components R.C. Willson & A. Mordvinov, “Secular total irradiance trend during solar cycles 21-23,” GRL 2003 (controversial) G. Withbroe, “Quiet Sun contributions to the total solar irradiance,” Solar Phys. 235, 369 (2006)

12. GSFC, May 18 2006 Two recent-suggested possible additional components R.C. Willson & A. Mordvinov, “Secular total irradiance trend during solar cycles 21-23,” GRL 2003 (controversial) G. Withbroe, “Quiet Sun contributions to the total solar irradiance,” Solar Phys. 235, 369 (2006) (identifiable with the “active network”?)

13. RHESSI and the solar radius H.S. Hudson, M.D. Fivian & H.J. Zahid Space Sciences Lab, UC Berkeley First report of our semi-intentional optical limb astrometry

14. GSFC, May 18 2006 Solar limb astrometry from the Reuven Ramaty High-Energy Solar Spectroscopic Imager RHESSI

15. GSFC, May 18 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

16. GSFC, May 18 2006 SAS reduced data Four limb pixels * 6 Normalization to R 0 Deviations from mean profile

17. GSFC, May 18 2006 FWHM ≈ 4.5 arcsec ≈ 2.6 pixels

18. GSFC, May 18 2006 Data

19. GSFC, May 18 2006 The p-modes explain some “random” noise August 2004: 57-orbit incoherent sum spectrum

20. GSFC, May 18 2006 August 2004: 57-orbit incoherent sum spectrum

21. GSFC, May 18 2006 SOHO/MDI view of limb p-modes: “Rossby hills”? Kuhn et al. 2000

22. GSFC, May 18 2006 The oblateness signal Oblateness 9.72 +/- 0.19 mas (random error ~10 -4 pixels)

23. GSFC, May 18 2006 Very early oblateness measurements Auwers, 1891

24. GSFC, May 18 2006 Very early oblateness measurements Auwers, 1891 P - A > 0 => prolate!

25. GSFC, May 18 2006 Solar Disk Sextant (1992, 1994, 1995, 1996 flights) Egidi et al., Solar Phys. 236, 407 (2006)

26. GSFC, May 18 2006 SOHO/MDI Emilio et al., 2000

27. GSFC, May 18 2006 Oblateness results SDS8.21 +- 0.84 mas MDI10.76 +- 0.78 mas RHESSI9.72 +- 0.19 mas

28. GSFC, May 18 2006 Oblateness results SDS8.21 +- 0.84 mas MDI10.76 +- 0.78 mas RHESSI9.72 +- 0.19 mas

29. GSFC, May 18 2006 The oblateness signal II 9.80 +- 0.10 mas

30. GSFC, May 18 2006 Synoptic-chart representation of RHESSI (early reductions)

31. GSFC, May 18 2006 Potentially observable limb features p-modes ! g-modes r-modes ? Granulation Other convective motions Sunspots ! Faculae ! Active network Flares Prominences Coronal holes Oblateness ! Higher-order shape terms Gravitational moments J2, J4… Global temperature variation Limb-darkening function Planetary tides

32. GSFC, May 18 2006 Wilson effect: historical background Alexander Wilson, local Glasgow font designer, physician, meteorologist and astronomer de La Lande, Maskelyne… ADS 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?)

33. GSFC, May 18 2006 On three fine days in 18th-century Glasgow…

34. GSFC, May 18 2006 Tbe Wilson effect: observation and modeling 1,000 x

35. GSFC, May 18 2006 Timewise modeling

36. GSFC, May 18 2006 The problem of faculae Granulation, solfläckar och facklor nära solranden (from the Swedish Solar Telescope homepage)

37. GSFC, May 18 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 time-variable 3-dimensional structure It seems hopeless to understand their structure in simple terms, and the modeling needs validation Faculae may therefore be noise from RHESSI’s point of view

38. GSFC, May 18 2006 What about g-modes? Rogers & Glatzmaier 2005 Toner et al. 1999

39. GSFC, May 18 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 expect to measure higher-order shape terms Sunspots, faculae, and p-modes show clear signals We dare to think about g-modes, but the analysis will be very challenging

40. GSFC, May 18 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?