SPM-11, September 12, 2005

VARIATIONS OF SOLAR RADIUS: OBSERVATIONS

WITH RHESSI

M.D. Fivian, H.S. Hudson and R.P. Lin

Space Sciences Lab, UC Berkeley

SPM-11, September 12, 2005

RHESSI radius observations

• Solar aspect sensor (SAS) determines precise limb location for X-ray telescope rotating at 0.25 rps

• Spin stabilization requires rapid cadence of precise limb measures - some 6 x 109 obtained thus far

• Experiment resembles that of Dicke & Goldenberg (1974), but in space rather than in New Jersey

SPM-11, September 12, 2005

SPM-11, September 12, 2005

The SAS data are precise

• RMS error < 30 mas per point

• Data are abundant: current rate of collection is ~ 100 sec-1

• “Solar constant” provides a (statistically) standard candle for reference

• Rotation helps to control systematic errors

SPM-11, September 12, 2005

SPM-11, September 12, 2005

Oblateness

SPM-11, September 12, 2005

Recent solar oblateness results(Sofia et al., 1995)

SPM-11, September 12, 2005

SPM-11, September 12, 2005

Effects of solar activity

SPM-11, September 12, 2005

SPM-11, September 12, 2005

SPM-11, September 12, 2005

Dealing with systematic errors

yes

SPM-11, September 12, 2005

Potential applications of data

• Faculae• Facular drift• Sunspot Wilson

depression• Limb flares (“white

light prominences”)

• Oblateness• Higher-order figure• Long-period g-modes?• Variability

SPM-11, September 12, 2005

Conclusions

• RHESSI limb sensors have adequate precision and coverage for many applications

• Solar activity (faculae and spots) can easily be seen at the limb

• The oblateness signal is detectable in a relatively short integration

SPM-11, September 12, 2005

End