RHESSI, the Wilson effect, and solar oblateness

H.S. Hudson, M.D. Fivian & H.J. Zahid

Space Sciences Lab, UC Berkeley

Glasgow, 27 April 2006

Historical background I

• The properties LO, RO, MO, 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

Glasgow, 27 April 2006

Total solar irradiance variations - highlights

Glasgow, 27 April 2006

ARAA 26, 473 (1988)

Frohlich, 2005

Glasgow, 27 April 2006

ARAA 26, 473 (1988)

Frohlich, 2005

Flares Minutes 200 ppm at most Woods et al. 2004

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?)

Glasgow, 27 April 2006

On three fine days in 18th-century Glasgow…

Glasgow, 27 April 2006

An observatory at a site with better seeing…

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

Glasgow, 27 April 2006

SAS reduced data

• Four limb pixels * 6• Normalization to R0

• Deviations from mean profile

Glasgow, 27 April 2006

Data

Glasgow, 27 April 2006

The p-modes explain excess “random” noise

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

August 2004: 57-orbit incoherent sum spectrum

Glasgow, 27 April 2006

The p-modes explain excess “random” noise

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

August 2004: 57-orbit incoherent sum spectrum

Glasgow, 27 April 2006

SOHO/MDI view of limb p-modes: “Rossby Hills”?

Kuhn et al. 2000

Glasgow, 27 April 2006

The oblateness signal

Current fit 9.72 +/- 0.19 mas(random error ~10-4 pixels)

Glasgow, 27 April 2006

Solar Disk Sextant radius results

Egidi et al. 2006

Glasgow, 27 April 2006

Further in the past…

Auwers, 1891

Glasgow, 27 April 2006

Synoptic-chart representation of SAS data

Glasgow, 27 April 2006

Test of limb sunspots as a roll reference

Glasgow, 27 April 2006

More spatial modeling

Glasgow, 27 April 2006

More timewise modeling

Glasgow, 27 April 2006

The problem of faculae

Granulation, solfläckar och facklor nära solranden(from the Swedish Solar Telescope homepage)

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.

Glasgow, 27 April 2006

What about g-modes?

Rogers & Glatzmaier 2005

Toner et al. 1999

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.

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?