Advances in Space Research 34 (2004) 262–264

www.elsevier.com/locate/asr

Photometry of the full solar disk at the San Fernando Observatory

G.A. Chapman *, A.M. Cookson, J.J. Dobias, D.G. Preminger, S.R. Walton

Department of Physics and Astronomy, San Fernando Observatory, California State University, Science 1 Mail Drop 8268,

Northridge, CA 91330 8268, USA

Received 19 October 2002; received in revised form 27 November 2002; accepted 17 December 2002

Abstract

Daily photometry of the full solar disk began at the San Fernando Observatory in mid-1985. At present, observations with two

photometric telescopes produce images in the red, blue and CaII K-line. The smaller telescope obtains images that are 512� 512

pixels. The larger one obtains images that are 1024� 1024 pixels. In addition, the larger telescope produces images with a narrower

K-line and an IR filter. Images are processed to determine a number of photometric quantities including sunspot deficits and facular/

network excesses. These photometric quantities are highly correlated with fluctuations in the total solar irradiance (TSI) from

spacecraft experiments.

� 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Daily photometry; Full solar disk photometry; Total solar irradiance; San Fernando Observatory

1. Introduction

The solar irradiance is an important forcing function

in the earth’s climate. Although the spectral and total

irradiance affect different layers of the earth’s atmo-sphere, the total irradiance is important in determining

the earth’s mean temperature

Nimbus-7/ERB and SMM/ACRIM-I were the first

spaceborne experiments to clearly show that the total

solar irradiance (TSI) was variable (Hickey and Alton,

1983; Willson et al., 1981). ACRIM-I, especially,

showed that the TSI was variable on almost all time

scales. We now know that the TSI varies by 0.1% on thesolar cycle time scale having its maximum value at the

time of solar maximum (Fr€ohlich and Lean, 1997;

Fr€ohlich et al., 1994). Assuming that the quiet sun

output is constant, it appears that facular/network

emission outweighs the blocking of radiation by sun-

spots. The physics behind this is still unsolved. We be-

lieve that the quiet sun is largely constant because most

of the variation in TSI can be accounted for by modelsthat include only sunspots and facular/network. Re-

* Corresponding author. Tel.: +1-818-677-2775; fax: +1-818-677-

3234.

E-mail address: gary.chapman@csun.edu (G.A. Chapman).

0273-1177/$30 � 2004 COSPAR. Published by Elsevier Ltd. All rights reser

doi:10.1016/j.asr.2002.12.003

gression coefficients suggest that 10% or less of the

variance is unexplained by these features. Further work

to lower the noise in both ground-based and space-

based data is needed to confirm and refine these results.

2. The instruments

The two photometric telescopes in use at present areCFDT1 and CFDT2, where CFDT stands for Cartesian

Full Disk Telescope. CFDT1 produces images with

512� 512 pixels, each pixel being 5.12� 5.12 arc-secs.

CFDT2 produces images with 1024� 1024 pixels, each

pixel being approximately 2.5� 2.5 arc-secs. Both tele-

scopes have a filter wheel mounted before the detector

which is a linear diode array. An image of the Sun is

created by turning off the telescope track drive allowingthe Earth’s rotation to scan the solar image. Both tele-

scopes have interference filters to define the spectral

bandpass. The red and blue filters are at a wavelength of

672 and 473 nm, respectively, with a bandpass of 10 nm.

Both telescopes have a CaII K-line filter at 393 nm with

bandpass of 1 nm. In addition, CFDT2 has a 393 nm

K-line filter with a bandpass of 0.3 nm and an IR filter at

wavelength of 997 nm with a bandpass of 10 nm. Tocorrect for variations in transparency while an image is

ved.

G.A. Chapman et al. / Advances in Space Research 34 (2004) 262–264 263

being obtained, the photocurrent from a photodiode is

recorded for every line of the image. Data processing

has been described by Walton et al. (1998) and Prem-

inger et al. (2001).

3. Comparisons with spacecraft data

Red images are used to determine sunspot areas and

deficits whereas K-line images are used to determine

facular/network areas and excess. Sunspot photometric

quantities calculated from the red images are the PSI

(Photometric Sunspot Index), defined by Willson et al.

(1981) and the sunspot deficit, DEF, defined in Chap-man et al. (1992). The scaling coefficient used to deter-

mine the PSI was based on photometric observations of

individual sunspots using an extreme limb photometer

offset from the limb (Chapman and Meyer, 1986). The

resulting coefficient is very close to that determined in

Willson et al. (1981). Facular photometric quantities

calculated from the K-line images are several models for

a bolometric facular fluctuation called PhotometricFacular Indices (PFI) and a spectral irradiance fluctua-

tion in the K-line which is well correlated with the MgII

core-to-wing ratio from spacecraft.

Comparisons with the TSI from spacecraft use a

linear multiple regression analysis of the form

TSI ¼ S0 þ A� PSIþ B� PFI ð1Þwhere S0 represents the solar irradiance in the absence of

solar activity (deToma et al., 2001). Before carrying out

this regression, the values of PSI and PFI are converted

from dimensionless quantities by multiplication by anestimate of the quiet sun irradiance, typically 1367

W/m2. The quantity A should be near unity if the scaling

coefficient that determine PSI from earlier photometry

of sunspots is approximately correct and the quantity Bshould be near unity if the scaling coefficient from earlier

photometry of faculae is approximately correct. Using

the TSI from Nimbus-7, A and B were found to be

0.813� 0.013 and 0.990� 0.026, respectively (Chapmanet al., 1996, Table 6, column 1). Different indices give

somewhat different results.

From sunspots, we also calculate the quantity Dr,

which is the same as the earlier quantity, DEF. This

quantity sums the pixel by pixel sunspot deficit. It is highly

correlated with the commonly used PSI, but its value is

about 70% of PSI. From facular K-line data we calculatePK which is similar to PFI, but does not require identi-

ficationof features.P

K sums allpixels of aK-line contrastimageweighting eachpixel according to its locationon the

disk (Preminger et al., 2001). A recent regression of TSI

from the VIRGO/SOHO experiment using CFDT1/SFO

sunspot deficits,P

r, and facular excesses,P

K , gave an.

R2 of 0.91 (deToma et al., this volume).P

r is the sumover

all pixels in a red image and is very similar to Dr.

4. Comparisons with other solar data

4.1. Fits to PSPT data

A preliminary comparison has been carried out be-tween data from the SFO CFDTs and data from the

Precision Solar Photometric Telescope (PSPT) operated

on Mauna Loa (Lin and Kuhn, 1992). In order to make

the images closer in scale, the 2048� 2048 PSPT images

were binned to 1024� 1024, making them close to im-

ages from CFDT2. Images from seven days in 1998,

1999 and 2000 were chosen for comparison. Regressions

of sunspot area from blue CFDT2 images were wellcorrelated with sunspot area from binned red PSPT

images. The value of r2 was 0.9895 and the scale factor

was 1.060� 0.049. For the same seven days, the facular

area from the CFDT2 narrow K-line (0.3 nm) was

compared with the facular areas from the binned PSPT

K-line. The contrast criteria were different for the two

sets of images. For the CFDT2 narrow K-line, faculaewere identified as being brighter than 2.4%. For thebinned PSPT images, the contrast criterion was 4.8%.

The different criteria were adopted to compensate for

the very high contrast of K-line pixels in the PSPT im-

ages. The areas were fairly well correlated with an r2 of

0.898 and a scale factor of 1.047� 0.158. Obviously,

more work is needed to establish the best scaling.

4.2. Comparisons of CFDTl and SGDB sunspot data

Comparisons of CFDTl sunspot areas with those

published in the Solar Geophysical Data Bulletin

(SGDB) are ambiguous. Over long intervals and manysunspots, the SGDB sunspot areas are in approximate

agreement with those from the CFDT areas. However,

for individual sunspot regions, discrepancies can be

quite large. For example, for NOAA region 9115, the

regression of the SGD areas versus the CFDTl areas

gives the following result:

ASGD ¼ 101� 40þ ð0:28� 0:22Þ � ACFDT1: ð2ÞThe value of r2 is 0.15 for N ¼ 11 (eleven days of

data). For NOAA region 9504, the two sunspot areas

are in better agreement. For 11 days of data, the fit is:

ASGD ¼ �115� 115þ ð1:03� 0:27Þ � ACFDT1 ð3Þwith an r2 of 0.617. For NOAA region 8673, the fit gives

ASGD ¼ 78� 136þ ð0:81� 0:35Þ � ACFDT1 ð4Þwith an r2 of 0.43. For NOAA region 8742, the spot

areas are highly correlated as shown next.

ASGD ¼ �9� 27þ ð1:00� 0:079Þ � ACFDT1; ð5Þwhere the value of r2 is 0.96 for 9 days.

These results are fairly typical. Upon investigating the

areas reported by individual observing sites, there are

264 G.A. Chapman et al. / Advances in Space Research 34 (2004) 262–264

large discrepancies among their areas for the same re-

gion and the same day.

5. The decay rate of sunspots

The decay rates of 32 sunspots were studied between

the years 1988 and 2002 (Chapman et al., 2002). The

decay of each sunspot was fitted to a linear trend in day

number as it moved across the solar disk. A good fit, R2

¼ 0.87, was found for the following expression,

d ¼ aþ b� As þ c� Au=As; ð6Þ

where d is the decay rate in microhemispheres per day

(lhem/d), As is the corrected total sunspot area in lhemand Au=As is the ratio of umbral area to total area. The

coefficients were a ¼ 78� 25, b ¼ 0:095� 0:0069, and

c ¼ �423� 69. A decay rate of 1 lhem/d corresponds

to approximately 34 km2/s. For the smaller spots of

this study with As ¼ 800 lhem and Au=As ¼ 0:1, d ¼110 lhem/d which gives a linear diffusion coefficient of

approximately 3800 km2/s. For a subset of 13 spots, we

used the square root of As as an estimate of the cir-cumference of the spot, a regression of the decay rate

versusffiffiffiffiffiAs

pwas not significant (r2 of 0.03).

References

Chapman, G.A., Meyer, A.D. Solar irradiance variations from

photometry of active regions. Solar Phys. 103, 21–31, 1986.

Chapman, G.A., Herzog, A.D., Lawrence, J.K., Walton, S.R.,

Hudson, H.S., Fisher, B.M. Precise ground-based solar photom-

etry and variations of the total irradiance. J. Geophys. Res. 97,

8211–8219, 1992.

Chapman, G.A., Cookson, A.M., Dobias, J.J. Variations in total solar

irradiance during cycle 22. J. Geophys. Res. 101, 13,541–13,548,

1996.

Chapman, G.A., Dobias, J.J., Preminger, D.G., Walton, S.R. On the

decay rate of sunspots. Geophys. Res. Lett. 30, doi:10.1029/

2002GL016225, 2003.

deToma, G., White, O.R., Chapman, G.A., Walton, S.R., Preminger,

D.G., Cookson, A.M., Harvey, K.L. Differences in the Sun’s

radiative output in cycles 22 and 23. Astrophys. J. 549, L131–L134,

2001.

deToma, G., White, O.R., Chapman, G.A., Walton, S.R. Solar

irradiance variability: progress in measurement and empirical

analysis. Adv. Space Res., this issue, 2004 (doi: 10.1016/

j.asr.2003.03.040).

Fr€ohlich, C., Pap, J.M., Hudson, H.S. Improvement of the photo-

metric index and changes of the disk-integrated sunspot contrast

with time. Solar Phys. 152, 111–118, 1994.

Fr€ohlich, C., Lean, J.L. Total solar irradiance variations, in: Deubner,

F.-L., Christensen-Dalsgaard, J., Kurtz, D. (Eds.), IAU Symp.

185, New Eyes to See Inside the Sun and Stars. Kluwer, Dordrecht,

pp. 89–102, 1997.

Hickey, J.R., Alton, B.M. Status of solar measurements and data

reduction for ERB-Nimbus 7, in: Labonte, B.J., Chapman, G.A.,

HUdson, H.S., Willson, R.C. (Eds.), Solar Irradiance Variations

on Active Region Time Scales. NASA CP-2310, Washington, pp.

43–58, 1984.

Lin, H., Kuhn, J.R. Precision IR and visible solar photometry. Solar

Phys. 141, 1–26, 1992.

Preminger, D.G., Walton, S.R., Chapman, G.A. Solar feature iden-

tification using contrasts and contiguity. Solar Phys. 202, 53–62,

2001.

Walton, S.R., Chapman, G.A., Cookson, A.M., Dobias, J.J., Prem-

inger, D.G. Processing photometric full-disk solar images. Solar

Phys. 179, 31–42, 1998.

Willson, R.C., Gulkis, S., Janssen, M., Hudson, H.S., Chapman, G.A.

Observations of solar irradiance variability. Science 211, 700–702,

1981.