rhessi, the wilson effect, and solar oblateness h.s. hudson, m.d. fivian & h.j. zahid space...
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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
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
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
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
The oblateness signal
Current fit 9.72 +/- 0.19 mas(random error ~10-4 pixels)
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
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.