Photometric observations of the sun have been carried out at the San Fernando Observatory since early 1985. Since 1986, observations have been obtained at two wavelengths in order to separately measure the contributions of sunspots and bright facular to solar irradiance variations. Researchers believe that the contributions of sunspots can be measured to an accuracy of about plus or minus 30 ppm. The effect of faculae is much less certain, with uncertainties in the range of plus or minus 300 ppm. The larger uncertainty for faculae reflects both the greater difficulty in measuring the facular area, due to their lower contrast compared to sunspots, and the greater uncertainty in their contrast variation with viewing angle on the solar disk. Recent results from two separate photometric telescopes will be compared with bolometric observations from the active cavity radiometer irradiance monitor (ACRIM) that was on board the Solar Max satellite

Chapman, G. A.

Herzog, A. D.

Lawrence, J. K.

Walton, S. R.

English

NASA Technical Reports Server

A PROGRAM OF PHOTOMETRIC MEASUREMENTS OF SOLAR
IRRADIANCE FLUCTUATIONSFROM GROUND-BASED OBSERVATIONS
G.A.Chapman,A.D.Herzog,J.K.Lawrence,andS.R.Walton
San Fernando Observatory
Department of Physicsand Astronomy
California State University,Northridge
ABSTRACT
Photometric observations of the sun have been poles were not scanned (Lawrenceand
carried out at the San Fernando Observatory Chapman2).
since early 1985. Since 1986,observations have
been obtained at two wavelengthsin order to To reduce the volume of data, to greatly simplify
separately measure the contributions of sunspots the observational procedure, and to make the
and bright facular to solar irradiancevariations, observations less prone to bad weather, two new
We believe that the contributions of sunspots can telescope/photometer systemswere developed in
be measured to an accuracyof about __+30 ppm. 1984-1986. One of these systems, called the
The effect of faculae is much less certain, with Cartesian Full Disk Telescope (CFDT) is
uncertainties in the range of _ 300 ppm. The designedto scan the full solar disk in about three
larger uncertainty for faculae reflects both the minutes with a pixel spacing of 5 arc-sec. The
greater difficultyin measuring the facular area, CFDT produces a photometric image of the sun
due to their lower contrast compared to sunspots, in one of severalwavelengthswith each pixel
and the greater uncertainty in their contrast digitized to an accuracyof 12 bits. Since the
variation with viewingangle on the solar disk. sun's image is scanned by the earth's rotation, the
Recent results from two separate photometric image has virtually no distortion. The other
telescopes will be compared with bolometric photometric system is the Rotating Full Disk
observations from the ACRIM that was on board Photometer (RFDP). This system has a telescope
the Solar Max satellite, with an aperture of 7.5 cm and a photometer that
rotates about the disk center, producing an image
INTRODUCTION with the disk center at pixel 1 and the solar limb
near pixel 420. Due to limits on data rate, the
High precision, two-dimensionalphotometric data are averaged in real-time, 2 x 2, and written
mapping of solar active regions was begun in on magnetic tape. The effective pixel spacing is
1982,partly in response to the accurate, space- about 5 arc-see. The time to acquire a complete
based measurements of the total solar irradiance, image is approximately2½ minutes. These two
In the summer of 1982,observationswere carried photometric instruments are complementary to
out using a single linear diode array with the San each other in that they obtain photometric solar
Fernando Observatory 61/28cm vacuum images that are, on the one hand undistorted near
telescopes and vacuumspectroheliograph, the disk center and, on the other hand,
Beginning in 1983, two linear diode arrays were undistorted near the solar limb. These
operated, obtaining simultaneous photometric photometric systemsare used by undergraduate
images in nearbywavelengths. These data have a and graduate students on a daily basis. In
pixelspacing of 0.94 arc-see. Normally,data were addition, full disk photographs are obtained, as a
obtained only for 512 x 512 pixelregions centered back-up, in Hn and in a 3840)_filter. In this
on specific active regions or predicted returns of report, we describesome of the preliminary
active regions (Lawrence et al_). Occasionally, results of these observations and some plans for
the disk of the sun was scanned in four swaths, the near future.
each havinga height of 512 pixelsand a width of
2200. Only a small part of the north and south
16
https://ntrs.nasa.gov/search.jsp?R=19910003146 2020-03-24T06:53:04+00:00Z
APRIL 1985 SEPTEMBER198S
1 ii 11 le 11 ;_I 31 1 , . . . .
DAy 1 e 11 ii 21 21DAY
MAY 1985 OCTOBERIN$
1 i 11 1o 21 16 a_
JUNE19e5 NOVEMBER1985
DAY DAy
JULY19_5 DECEMBER1965
Fig. 1 Digital PSIdeterminedfromtheCFDTduring1985comparedto the fluctuationin the ACRIMsignalrelativeto 1367W/m2. The time
differencebetweentheACRIMsignaland theSan FernandoObservatoryPSIhasnotbeen removed.
17
SYNOPTIC FULL-DISK PHOTOMETRY
The CFDT began routine observationsusing a red ®. ,p.®,p.® ,p.®,p.,_'_i_ ,p.®v ...........
filterin the springof 1985(Chapman et al?).
Analysisof these data has resulted in a : : : ,.
determination of sunspot irradiance deficits and
areas (in units of millionths of a hemisphere) for
1985(Chapman and Davis4and DavisS).
Beginningin 1986,an intermediate band filter at
3920_ was added. This filter, with a bandpass of
100A,was intended to detect faculae. In 1988, a _"
10Anarrow band filter was added at a wavelength
of 3934_ This filter was intended to detect _
faculae more effectivelythan the 3920Afilter.
We present here some results for part of the
summer of 1988,as well as for most of the year
1985.
s
The RFDP was constructed and installed with the
s thelp of a NASA grant during 1984and 1985.
This photometer is fed by a 15cm aperture
achromat, stopped down to 7.5 cm, with a focal ._
length of 229 cm. The data are obtained at a
wavelengthof 5319Awith a bandpass of 100A.
The pointing of the RFDP is controlled by a Fig. 2 A contour plot of the wholesolar disk from the CFDT for
separate guider. Because of the limitations in 27 June 1988. Each pixel in this plot represents 20 arc-see.
data storage rate, pixelsare averaged,2 x 2, to 5
arc-sec,before being written to magnetic tape.
Thus, each image corresponds to 1320records,
each with a length of 256 pixels.
At the end of each record of a photometric
image, an accurate measurement of the sky
transparency is recorded. These data can be used
later to remove the effect of changes in sky
transparency that occur during the solar
observation. These "exposuremeterMobservations
are obtained at a wavelength of 0.531_m.
RECENT RESULTS
The observations from the CFDT for the year
1985have been processed (Chapman and Davis4
and DavisS). The Photometric Sunspot Index
(PSI) is compared with fluctuations in the
ACRIM signal in Fig. 1. The Photometric : : :
Sunspot Index is defined in Chapman and Meyer,_
based on the form first discussed in Willson et al.7 : : : : " :
In our work, the corrected sunspot area is
determined from the CFDT red image after
removingthe quiet sun limb darkening. This
results in a smoothed contrast map that is
searched for pixels that have a negative contrast Fig. 3 A contour plot of the active region at the SE limb, from theimagein Fib _ but at the full resolution,5 arc-seeper
that exceeds-8.5 percent. This criterion was pixeloftheCFDT.
18
determined by an examination of several sunspot
images obtained with one arc-sec pixels using the
28 cm vacuum telescope and vacuum P_
spectroheliograph. This same criterion is used to
define a sunspot in the higher resolution RFDP
data. We will see that there is a systematically
smaller sunspot area from CFDT data compared
with RFDP data that is probably due to the lower
spatial resolution of the CFDT optical system
(Chapman et al? and Herzog et al.8). A small
adjustment in the contrast criterion for CFDT
sunspots can bring these two systemsinto very
close agreement. Corrected sunspot areas can
also be determined from the CFDT K-line images
by changing the sunspot contrast criterion from -
8.5% to -9.6% (Wilsong'l°).
The Rotating Full Disk Photometer (RFDP) has
a 7.5 cm objective and a 2.5 arc-sec pixelat the
detector. Although the data are co-added,2 x 2,
in order to reduce the data rate, one expectsto
have larger sunspot signals from this system. This
appears to be the case.
The appearance of the sun as seen by these two Fig. 4 A contour plot of the active region shown in Fig. 3 from
the RFDP. The pixel size is nearly the same as for theinstruments is shown in Figs. 2-3. Fig. 2 shows CFDT.
the full disk seen by the CFDT at 6723_ In
order to plot the full disk on one plot, the data
have been averaged 4 x 4. Fig. 3 shows a sunspot
group from the same day at the full resolution of
the CFDT, approximately5 arc-sec per pixel. Fig.
4 shows the same sunspot group as seen by the
RFDP at a scale of 5 arc-sec per pixel. These
data are most useful in studying active regions
near the limb. An increasing geometric distortion RrDPCALIBRATION
that increases toward the disk center can be ,-_,Y-,To_-_-.3.4
removed. Except for some distortion, the sunspot - ._ °
images in Figs. 3 and 4 look quite similar. " "IL8-
2.6- 0D
Fig. 5 shows the relation of corrected sunspot _ "-2.2-
areas, in parts per million of the solar r_ ,- o ° ,
hemisphere, from the RFDP compared with _] ,.s_ ,
sunspot areas published in the Solar Geophysical _ TM- . o"_ _ sc_ a ad_
1.2. °
Bulletin. These data cover the time period from _ I. I_ 8 t_pO13 [?Oa
9 May to 22 August 1988(Herzog et al.8). _ 0.,. o_, ._ _ ° °0.6.
Although the correlation coefficientis high, r = _,- n
0.97, there is a significant difference from unity in ,J- _ _ °
r i i i ,
the slope of the relation. The published areas °. , _ ; , _ ,c,,.-_)
appear to underestimate the corrected area of _-**,,, _, ._,
sunspots, as measured by the RFDP, by
approximately 30 4-2 percent (one sigma error).
Fig. 5 A plot of correctedsunspot areas,in ppm,of the solar
The effects of bright faculae are determined by hemispheredetermined from the RFDP and compared withthose publishedin the SGD Bulletin. The valuesin the
searching a "flattened"K-line image for pixels S_D representaveragesfromeachstationreporting.
19
brighter than some criterion, usually 4% (about 
2.5 sigma), and calculating a Photometric Facular 
Index (PFI), a measure of the irradiance excess 
caused by faculae. 
The standard deviation in the Photometric 
Facular Index has been determined for several 
days when from 3 to 6 images have been obtained 
in the K-line filter. An analysis of these data 
suggests that the standard deviation in the PFI is 
about 200 millionths of the mean solar irradiance. 
If the mean solar irradiance is approximately 1367 
W/m2, then this photometric uncertainty 
corresponds to 0.27 W/mZ. This standard 
deviation includes noise from the possible 
evolution of active regions. 
A preliminary analysis for 25 days between 10 
June and 23 July 1988 shows that the CFDT and 
the RFDP corrected sunspot areas are highly 
correlated with a correlation coefficient of 0.990. 
The slope is not unity and it appears that the 
larger aperture RFDP obtains sunspot areas that 
are 9 percent larger than those from the CFDT. 
There may be a 200 millionth zero point offset, 
although its value is not statistically significant (1 
sigma). The analysis of more data will help to 
clarify this point (Herzog et al?). 
An analysis of ACRIM data and sunspot and 
facular photometry for a two-week period in June 
and July of 1988 shows that ground-based 
photometry can correlate with the spacecraft data 
with a multiple correlation coefficient of about 
0.98 for 21 data points. The quiet sun irradiance, 
determined from these regressions, is 
approximately 1367.0 W/m2. The details will be 
contained in a forthcoming paper (Chapman et 
al."). The largest source of systematic noise 
appears to be the time difference between the 
ground-based data and the daily values of 
irradiance available from the ACRIM group. A 
good fit to the ACRIM data is obtained using the 
Photometric Irradiance Fluctuation (PIF) (see 
Chapman et al? for further discussion of the PIF) 
to represent the sunspot signal but with a 
coefficient of 1.062. Basically, the PIF measures 
the photometric deficit of sunspot pixels relative 
to the mean solar irradiance. The facular signal 
used was based on the function given in Schatten 
et a~., '~ although the function given by Foukali3 
gave nearly as good a fit. The Schatten et a1.- 
function required a coefficient of 2.035. The 
squared partial regression coefficients for the 
CFDT signal and the Schatten et a1.-signal were 
0.96 and 0.69, respectively. 
In summary, the ground-based program described 
here appears to be able to match accurate space- 
based radiometry to within approximately 0.2 
W/m2, or  in the range of 100 to 200 parts per 
million, at least for the two-week interval 
described above. With further refinements in the 
image processing and improved averaging of the 
space-based data, this noise can be further 
reduced. Extending such an analysis over a 
longer time base should lead to some interesting 
improvements in our understanding of the sun. 
This research has been partly supported by NSF 
Grant ATM-8817634 and NASA Grant NAG-5- 
1219. 
REFERENCES 
1. Lawrence, J.K, Chapman, G.A., Herzog, 
AD., and Shelton, J.C. 1985, m, 292, 
297. 
2. Lawrence, 3.K and Chapman, G.A. 1990, 
&, Oct. 1990, in press. 
3. Chapman, G.A., Herzog, AD., Laico, D.E., 
Lawrence, J.K., and Templer, M.S. 1989, 
@, 343,547. 
4. Chapman, G.A. and G.A. Davis, 1990, in 
preparation. 
5.  Davis, G.A. 1989 M.S. Thesis, CSUN. 
6. Chapman, G . k  and Meyer, AD. 1986, Solar 
u, 103,21. 
7. Willson, R.C., Gulkis, S. Janssen, M., 
Hudson, H.S. and Chapman, G.A. 1981, 
Science, 211, 700. 
8. Herzog, AD., Chapman, G.A., Gluszczak, 
M. and Walton, S.R. 1990, BAAS, 21, 1110. 
9. Walton, S.R., Wilson, R and Chapman, G.A. 
1990, BAAS, 21, 1179. 
10. Wilson, R. 1990 M.S. Thesis, CSUN, in 
preparation. 
11. Chapman, G.A., Herzog, AD., Lawrence, 
J.K., Walton, S.R., Hudson, H.S. and Fisher, 
B., 1990, in preparation. 
12. Schatten, KH., Miller, N., Sofia, S., Endal, 
AS., Chapman, G.A., and Hickey, J. 1985, 
A&., 294,689. 
13. Foukal, P. 1981, in The Physics of Sunspots, 
L.E. Cram and J.H. Thomas, eds., Sunspot, 
NM, 391. 


A Program of Photometric Measurements of Solar Irradiance Fluctuations from Ground-based Observations

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