June 29, 2006, SPD - AAS, New Hampshire 1

The Optical Solar Limbas Observed by RHESSI

‘New Oblateness Measurement’

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

Space Sciences Laboratory, UC Berkeley

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RHESSI/SAS Instrument

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RHESSI: Solar Aspect System

Sensor CalibrationLimb position

Geometrical CalibrationSun center and Radii

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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 functions• Planetary tides• Dynamo signatures• ?

Local Features Global Features

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Historical Results

First Oblateness Measurement(Auwers, 1891)

Absolute Radius Measurements(Kuhn, ApJ 613)

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RHESSI/SAS Radius Measurements

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Confusion/Signal of Magnetic Activity

N

W

S

E

N

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Sensitivity of Oblateness Measurement

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Oblateness Results

SDS 8.21 ± 0.84 mas

MDI 10.76 ± 0.78 mas

RHESSI 10.84 ± 0.11 mas

SDSMDIRHESSI

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Conclusions

• The RHESSI aspect sensors provide radius determinations of  unprecedented precision

• At this new level of precision, we find strong signatures of solar  magnetism

• The data will allow us to constrain solar interior models,  supplementing helioseismology  in a fundamentally new manner