Center-to-limb variation of facular contrast H. Jabran Zahid, Martin D. Fivian & Hugh S. Hudson Space Sciences Lab, UC Berkeley

RHESSI and the Solar Aspect System (SAS) SAS has 3 lens/sensor pairs used in reconstruction of spacecraft pointing Spacecraft in space and rotating at 15 RPM

SAS data and reconstruction Sensor: 2048-pixel linear CCD, 1.73 arc sec/pixel Spectral band: 670 nm x 12 nm FWHM Read-out: ~1000 chords/day per system

The RHESSI data There are two types of data: 1.Masked data has sunspots and faculae removed ~ Quiet-sun 2.Unmasked data has only sunspots removed Facular contrast function: C(μ) = (Unmasked – Masked)/Masked = Unmasked/Masked - 1

Masking with EIT 284Å Transform from image cartesian coordinates to polar coordinates (radius, position angle) Average ~4 images/day into 1 image/day Fill in any gaps by averaging neighboring pixels

Definition of sunspots and faculae and the RHESSI-EIT correlation Sunspots defined as negative residual greater than 3% of averaged limb darkening function (~0.2% of data). Faculae identified by setting threshold in EUV brightness (~12% of data)

Facular contrast as a function of latitude, 3yrs average As you increase in latitude you only have increasingly larger radii, resulting in a cone shape Each frame in the movie is an averaging of two degrees of latitude from plot above.

Fit to facular contrast function from -10 to -14 degrees Max of C(μ) at μ = 0.24

Fit to facular contrast function from -10 to -14 degrees latitude Local max of C(μ) at μ = 0.97

Conclusion First time an in-space rotating telescope has been used to investigate the photometric facular contrast function The results are consistent with the hot wall model for faculae At 670nm, facular contrast is positive everywhere and shows a peak at μ = 0.24 (76º normal to line- of-sight), similar to Ahearn & Chapmen, 1999 Future work will attempt to calibrate and normalize contrast by using EIT EUV images and MDI magnetograms and look for time variations in C(μ)