Center-to-limb variation of facular contrast

H. Jabran Zahid, Martin D. Fivian

& Hugh S. HudsonSpace Sciences Lab, UC Berkeley

RHESSI and theSolar 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 functionfrom -10 to -14 degrees

5

0

)(n

n

p

52 3 4 5

0 1 2 3 4 50

( ) ( )n

n

C p A A A A A A

Max of C(μ) at μ = 0.24

52 3 4 5

0 1 2 3 4 50

( )n

nn

n

C A A A A A A A

40

21

2

3

4

25

( 9.6 0.86) 10

(3.5 0.12) 10

0.13 0.0054

0.20 0.011

0.14 0.011

(4.2 0.38) 10

A x

A x

A

A

A

A x

Fit to facular contrast functionfrom -10 to -14 degrees latitude

5

0

)(n

n

p

52 3 4 5

0 1 2 3 4 50

( ) ( )n

n

C p A A A A A A

Local max of C(μ) at μ = 0.97

52 3 4 5

0 1 2 3 4 50

( )n

nn

n

C A A A A A A A

40

21

2

3

4

25

( 9.6 0.86) 10

(3.5 0.12) 10

0.13 0.0054

0.20 0.011

0.14 0.011

(4.2 0.38) 10

A x

A x

A

A

A

A x

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)