We have compared three years of daily CaII K-line images from the Big Bear Solar Observatory (BBSO) with HI Lymanα irradiance data from the Upper Atmosphere Research Satellite (UARS). The daily full-disk CaII K-line images are reduced to a new index of integrated excess emission, which reproduces both the 27 day rotational modulation and the solar cycle decrease in Lyα irradiance. Our analysis shows that while plages reproduce the 27-day variation quite well, the total K-line emission excess above the quiet background is needed to reproduce the secular solar cycle trend in the Lyα irradiance. The resulting K-line index exhibits a high degree of correlation (0.9) with the time series of measured Lyα flux

Johannesson, A.

Marquette, W.

Zirin, H.

English

Caltech Authors - Main

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REPRODUCTION OF THE LYMAN a IRRADIANCE VARIABILITY 
FROM ANALYSIS 
OF FULL-DISK IMAGES IN THE Call K-LINE 
A. JOHANNESSON, W. MARQUETTE, and H. ZIRIN 
Big Bear Solar Observatory, California Institute of Technology 
Pasadena, CA 91125 
(Received 10 April, 1995; in revised form 10 July, 1995) 
Abstract. We have compared three years of daily Call K-line images from the Big Bear Solar 
Observatory (BBSO) with HI Lyman a irradiance data from the Upper Atmosphere Research 
Satellite (UARS). The daily full-disk Call K-line images are reduced to a new index of integrated 
excess emission, which reproduces both the 27 day rotational modulation and the solar cycle decrease 
in Ly a irradiance. Our analysis shows that while plages reproduce the 27-day variation quite well, 
the total K-line emission excess above the quiet background is needed to reproduce the secular solar 
cycle trend in the Ly a irradiance. The resulting K-line index exhibits a high degree of correlation 
(0.9) with the time series of measured Ly a flux. 
1. Introduction 
Spatially resolved images of the Sun from the ground in the Call K-line (3933 A) 
and from space in the Ly a line (1250 A) appear quite similar (Bonnet et al., 
1980), so it is plausible that Call K-line observations could be used to estimate 
the irradiance of the strongest UV line. Various measurements of Call indices 
(Lean et al., 1982; Pap et al., 1991a; Pap et al., 1991b) have shown that this is 
possible, but that the enhanced network must be included in such measurements. 
Call K-line plage indices can reproduce the rotational modulation (Donnelly et 
al., 1986) quite well, but they generally underestimate the longer-term variations 
due to the 11-year cycle (Lean, 1991). A longer, more accurate database of UV 
irradiance and digital K-line measurements with resolution and contrast adequate 
to measure the enhanced network were needed. 
The SOLSTICE and SUSIM experiments on UARS (Rottman, 1988; Reber, 
1993; London et al., 1993; Brueckner et al., 1993) have provided high-quality 
irradiance measurements in the wavelength region 1100-4200 A since October 
1991. Full-disk Call K-line images (BBSO data) have been recorded daily at the 
Big Bear Solar Observatory (BBSO) since October 1981 (digitally since October 
1982), with only a short break due to the 1992 earthquake. Call K-line plages were 
systematically measured to produce an index which was published until November 
1987 when this work had to be discontinued for lack of funds. Because of the 
long-term significance of these observations, we continued a low-level program 
of simply recording and digitizing the images (Marquette, 1992) without analysis. 
The UARS guest investigator program made it possible to measure the BBSO data 
Solar Physics 161: 201-204, 1995. 
© 1995 Kluwer Academic Publishers. Printed in Belgium. 
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202 A. JOHANNESSON, W. MARQUETI'E, AND H. ZIRIN 
and compare it with the new long database of accurate Lya irradiance measure-
ments, and the digitized data made it possible to evaluate possible contributions to 
the Lya irradiance. 
2. BBSO Call K-line Data and Data Reduction 
The BBSO data are taken with a Daystar 1.0 A filter centered on the Call K-line 
at 3933 A. We currently use a Cohu video camera and an 8-bit Aspro digitizer to 
obtain the full-disk Call K-line images. The scale of the digitized images is 4 arc 
seconds per pixel. 
Fig. l. Call K-line images (reduced for limb darkening) showing the Sun with different activity 
features (Aug. 2 [middle] and Sep. 7 [right], 1994) producing the same UARS Ly a irradiance and 
about the same K-line indices. To the left is a detail of the Aug. 2 image which displays the Call 
K-line network present in the BBSO images. 
The earlier digital data had not been analyzed so there had been no quality 
feedback, and images were somewhat irregular and variable. Since mid-year 
1994, a fixed black level has been used, and the gain is more uniform. As a result, 
the quality of results of the image analysis have improved. 
We used an IDL routine to remove the effects of limb darkening and instru-
mental non-uniformity. Because the software cannot assume a symmetrical limb 
darkening function, the quiet-sun brightness is estimated in each image from the 
areas between bright magnetic features. The gain is adjusted for constant contrast 
of quiet-sun features. Bright magnetic features are found by a combination of 
an edge detector and a median filter. The pixels closest to the limb are treated 
separately using the assumption that solar activity and the quiet-sun background 
have well-separated spatial frequencies in circular tracings at large radii. 
We subtract the estimated background from the original image to get a residual 
image without limb darkening or brightness enhancements (Fig. 1). An activity 
index is then calculated as: 
Inde1: = c L fij + m 
i.i 
fo = { 
R'f!. 
lJ 
0 
if Rij > T 
if R· < T iJ -
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LYMAN a IRRADIANCE VARIABILITY 203 
where R is the residual image, T is a threshhold, and c and m are the coefficients 
of a fit to the UARS data. T is set to 2 er above the background. By setting T 
and the free parameter p we found the best fit to the data with p = 0.5. Because 
we do not have spatially resolved photometric Ly a data, we do not know the 
contribution from individual features. 
3. Results 
We analyzed 750 Call K-line images which represent the part oftheBBSO database 
covering the available UARS Ly a data (October 1991 - September 1994). The 
comparison (Fig. 2) between the UARS SOLSTICE Ly a irradiance (London et 
al., 1993) and the BBSO index shows a correlation coefficient of 0.9 for the whole 
period, 1991 to 1994, with no phase lag. The correlation is somewhat lower after 
a detrend (0.8 for 1994). Due to the combination of a lack of dynamic range in the 
digital images and low frequency flat fielding errors, we cannot accurately measure 
the network features with excess brightness below 3% of the disk center intensity. 
10 
I~ 8 
E 
3' 
E 
t3 6 
z 
<( 
6 
<( 
"' '!: 
1992 1993 1994 
DATE OF OBSERVATION 
Fig. 2. UARS SOLSTICE HI Lyman a irradiance (upper graph) and BBSO Call K-line index 
(lower graph). The BBSO data have been fitted to the UARS plot by linear regression. Both plots 
are seven-day running averages. The gap in 1992 is due to an earthquake. 
Both data sets are filtered with a seven-day running average. The BBSO data 
are fitted to the UARS data by linear regression and is shifted for clarity. Notice 
that both the short-term modulation, mostly due to the rotation of plages, and 
the long-term decrease, due to the overall decrease in solar activity, are fairly well 
represented in the BBSO data. If the network is omitted the long-term trend cannot 
be fitted. 
© Kluwer Academic Publishers • Provided by the NASA Astrophysics Data System 
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204 A. JOHANNESSON, W. MARQUETIE, AND H. ZIRIN 
There are exceptions to the good time correlation. At the end of 1992 and 
the middle of 1993, the BBSO data have about twice the amplitude of rotational 
modulation of the UARS SOLSTICE Ly a irradiance. At this point we cannot 
distinguish real differences from the effects of data gaps and gain variations. 
The excess K-line emission is due to a range of sources. Fig. 1 shows two quite 
different distributions that give the same resulting K-line and Ly a indices. Thus 
it seems that there are no special sources, all excess K-line was measured equally. 
Fig. 2 shows that the Ly o variation can not be explained without the contribution 
of the widespread Call K-line network. The good fit of the Ly a variation by the 
K-Iine excess suggests that the background Sun does not change with the cycle. 
4. Conclusion 
We have empirically fitted the Ly a irradiance measured by the UARS satellite 
using ground-based Call K-line images from BBSO. By including everything 
above 20' above the background, we can fairly well reproduce the rotational 
modulation as well as the secular decline at the end of Solar Cycle 22. We plan to 
expand the comparison to other UARS UV wavelengths and to BBSO Ha images. 
We have recently ordered a digital CCD camera, which will provide much higher 
photometric quality and stability. 
Acknowledgements 
The SOLSTICE and SUSIM data were provided courtesy of the NASA UARS 
project by Drs. Gary Rottman and Guenter Brueckner, respectively. We thank 
Dr. Judit Pap and Dr. Oran White for helpful discussions. This research is funded 
by the UARS Guest Investigater Program of NASA under grant NAG5-2782. 
References 
Bonnet, R. M., Bruner, E. C.. Acton. L. W., Brown, W. A., Decaudin. M.: 1980, Astrophys.J. 237, 
L47. 
Brueckner. G. E .. fallow. K. L.. Floyd, L. E., Lean. J. L. and VanHoosier, M. E.: 1993, Journal of 
Geophvs. /<..es. 98, 695. 
Donnelly, R. F.. Puga, L. C., and Busby, W. S.: 1986. NOAA Technical MemorandumERLARL-146 
Lean.J.: 1991. Revs. ofGeophvs. 29, 505. 
Lean, J. et al.: 1982, J. Geophvs. Res. 87, 307. 
London, J ., Rottman, G. J .. Woods, T. N.: l 993, Geophys. Res. Let. 20, 1315 . 
.,...,.. Marquette, W. H.: l 992. in the Proceedings of"the Workshop on the Solar Electromagnetic Radiation 
Stulfrfor Solar Crc/e 22. 
--.- Pap, J.M., London. J., Rottman. G. J.: l991a,Astron. Astrophys. 245, 648. 
Pap, J. M., Marquetle. W. H., Donnelly, R. F.: 1991 b, Adv. Space Res. 8, 271. 
.-.-.11 Reber, C. A.: l 993. Geophys. Res. let. 20, 1215. 
Rottman, G. J .: l 988, Adv. Space Res. 8, 31. 
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Reproduction of the Lyman α irradiance variability from analysis of full-disk images in the CaII K-line

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Reproduction of the Lyman α irradiance variability from analysis of full-disk images in the CaII K-line

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