Comparison of SORCE/TIM with SFO* Ground­based Photometry
Gary Chapman, Angela Cookson, and Dora Preminger *San Fernando Observatory
Department of Physics & Astronomy
California State University, Northridge
gary.chapman@csun.edu; angie.cookson@csun.edu; dora.premingerg@csun.edu San Fernando Observatory (SFO), Cal State Northridge
18111 Nordhoff St, Northridge, CA 91330­8268
This work is supported in part by NSF grant ATM­
0848518
Why Ground­Based
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Photometric Images can identify sources of TSI variation.
Ground­based data can identify and help correct short­term upsets in spacecraft data.
Ground­based data can help bridge the different native scales of spacecraft data.
SORCE/TIM TSI Data
TIM information, specifications, and data products available at http://lasp.colorado.edu/sorce
Measures TSI with 50­second cadence
6­hr and 24­hr averaged values available
SFO Photometric Data
CFDT1 & 2 (Walton et al (1998,Sol.Phys. 179, 31; ●
www.csun.edu/sfo)
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512 x 512 CFDT1 images, 5” square pixels
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1024 x 1024 CFDT2 images, 2.5” square pixels
Images produced by 512 or 1024 scans of a 512­ or 1024­ linear diode array
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672.3 nm red, 10 nm bandpass
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393.4 nm Ca II K, 1 nm bandpass
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Single red image, Ca II K image from two co­added scans
CFDT2
Sample Images
The next 6 images correspond to the following wavelengths:
997 nm, BP = 10 nm (IR)
780 nm, BP = 10 nm (NIR)
672 nm, BP = 10 nm (red)
472 nm, BP = 10 nm (blue)
393 nm, BP = 1 nm (wide K)
393 nm, BP = 0.3 nm (narrow K)
SFO Image Processing Photometric images produced in several wavelengths: CFDT1 since 1988; CFDT2 since 1992.
Robust algorithms developed for producing photometric contrast images and determining relative irradiance contributions of solar surface features (sunspots, faculae, and plage) from these images. (Walton et al (1998 Sol.Phys. 179 31))
Solar indices computed, including photometric sums (∑), sunspot areas and deficits, and facular areas and excesses, for the purpose of TSI modeling. (Preminger, Walton, & Chapman 2001, Sol.Phys. 202 53)
SFO Photometric Sum (∑) Index Photometric Sum (∑) has been one of the most successful photometric indices produced. (Preminger, Walton, & Chapman 2002, JGR, 107 6)
∑r and ∑K, used in multi­variable linear regressions against space­
based TSI, produce the best results.
∑r and ∑K are disk­integrated sums determined from red and Ca II K­line contrast­image pixels, respectively; each pixel is weighted by the appropriate limb­darkening.
∑r and ∑K do not require feature identification, i.e., determining whether a pixel belongs to a sunspot, facula, or network.
SFO Photometric Sum (∑), continued
∑
measures the relative change in spectral irradiance in filter passband due to all features assumes image noise is symmetric around zero, causing bright and dark_ noise_ pixels to cancel, leaving only contributions from real features
∑r measures irradiance contributions from photospheric structures seen in red continuum images
∑K measures variability of the upper photosphere/lower chromosphere seen in Ca II K images
Figure 1: TIM TSI and SFO model with 2
residuals (R = 0.9495)
Results and Discussion
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1253 days of data from 2003 March 02 to 2010 May 05.
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A two­parameter fit of SFO ∑r and ∑K to TIM TSI gives
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R2=0.9495 with the following equation TIM TSI = S0 + a1∑r + a2∑K where S0 = 1360.810 +/­ 0.004 Wm­2 These results provide further support for the hypothesis that the quiet Sun is constant over time (at least over the solar cycle).
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Figure 2: A linear regression of ∑K from CFDT1 against ∑K from CFDT2 for a quiet period in summer 2007.
Figure 3: A linear regression of ∑r from CFDT1 against ∑r from CFDT2 for a period in June 2004. (r2 = 0.979)
Previous two regressions
The K­line regression had an r2 = 0.964.
The red regression had an r2 = 0.979.
Interesting Historical Note
Abbot (1958) determined the solar constant was 1.9458 cal./cm2 min. This converts to 1360 W/m2 at low sunspot number (SN). He also found the solar constant varied with sunspot number as 0.0189 W/m2 per unit SN.
A modern value might be 0.01 W/m2 per unit SN.
More results (same time period)
Using sunspot deficits (DEF) and a facular model (PFIFA) we find:
TIM TSI = So + a1 x DEF + a2 x PFIFA
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With R = 0.9114, So = 1360.817 +/­ 0.004 and a1 = 1.170 +/­ 0.0115 and a2 = 1.350 +/­ 0.0136
Summary
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7 years of SORCE and SFO data
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R2 = 0.95
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TIM fit well with two SFO photometric indices
Acknowledgements
This work was partially supported by grants from NASA and the NSF most recently NSF ATM­
0848518 and NASA NNX11AB51G. Many of the observations were obtained by students, both graduate and undergraduate, at CSU, Northridge.
Abstract
Total Solar Irradiance (TSI) measurements have been available from the TIM instrument on the SORCE spacecraft since 2003.
We compare SORCE/TIM TSI data, both 24­hour and 6­hour averages, with photometric indices from red and Ca II K­line images obtained on a daily basis at the San Fernando Observatory (SFO). For 1375 days, from 2003 March 02 to 2010 May 05, we compare the data in linear multiple regression analyses. The best results come from using two photometric indices, the red and K­line photometric sums (∑), and SORCE TSI 6­hour averages interpolated to the SFO time of observation.
For this case, we obtain a coefficient of multiple correlation, R2, of 0.9495 and a quiet­Sun irradiance of S0 = ~1360.810 +/­ 0.004 Wm­
2.
These results provide further support for the hypothesis that the quiet Sun is constant over solar cycle time intervals.