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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)