Modelling solar irradiances using ground-based measurements

Adv. Space Res. Vol. 11,No.5, pp. (5)271-.(5)274, 1991 Printed in Great Britain. All rights reserved.

0273—1177/91 $0.00 + .50 Copyright © 1991 COSPAR

MODELLING SOLAR IRRADIANCES USING GROUND-BASED MEASUREMENTS J. M. Pap,* W. H. Marquette** and R. F. Donnelly*** *

University of Colorado (CIRES)-NOAA/SEL, C. Box 216, Boulder, CO 80309,

U.S.A. **CALTECH/Big Bear Solar Observatory, 40386 North Slwre Lane, Big Bear City, CA 92314, U.S.A. ***NOAAISpace Environmental Laboratory, 325 Broadway, Boulder, CO 80303, U.S.A.

ABSTRACT

The preliminary results of the photometry of Ca-K plage remnants show that during the fall of 1986 the remnants gave a significant contribution to the irradiance variations. The contribution of the plage remnants to the combined plage and remnant index was on average about 13 % and it chanqed with time. INTRODUCTION Earlier results demonstrate that changes of total irradiance on active region time scale arise from the combined effect of bright faculae and dark sun~pots/1/, while the UV irradiance is primarily modified by the plages /2/. However, a recent study of /3/ has shown that at the time of solar minimum the UV flux at Lyman alpha (121.6 nm) is primarily influenced by events other than plages, possibly by the remnants of plages /4/ which have been neglected in the calculation of the present Ca—K plage index /5/. In this paper we present the first, preliminary results of photometric measurements of the CaK plage remnants, which were made at the CALTECH/Big Bear Solar Observatory. Full-disk Ca-K spectroheliograms photographed with a 10 cm diameter image at the Sacramento Peak National Solar Observatory were digitized using an RCA newvicon video camera and Big Bear Observatory’s Eyecom III imaqe processor with a 480 x 640 video grid. The areas and intensities of the CaPlages and plaqe remnants were measured on the Eyecom III using software developed by Dr. A. Patterson. PHOTOMETRY OF THE CA-PLAGE REF1NANTS “Je examine the variation of total and UV irradiances in 1986, during the solar minimum conditions, when only a few active regions were present on the Sun and the spatial structure of solar activity was simpler than during high levels of activity. The dashed lines of Figure 1 show the daily values of the Lyman alpha flux (a) measured by the Solar Mesosphere Explorer satellite /6/, and the NOAA9 Mg II core-to-wing ratio (b) /4/, which is a measure of the UV irradiance /7/. The total irradiance, measured by the ACRIM radiometer on the Solar Maximum Mission satellite /l/~ has been corrected for the sunspot darkening by means of the “Photometric Sunspot Index /1/and is presented in Figure lc. The solid lines show the Ca-K plage index, which has been scaled to the actual solar irradiance by means of linear regression. As can be seen, in many cases the plage index underestimates the changes observed in both the total and UV irradiances. For the photometry we chose to study as an initial test case the fall of 1986, when a number of plages formed leading to the increases in solar. EJV and total irradiances. The plages appearing on the. Sun at the end of October are seen in Figure 2a, where the left side panel shows its computer plot produced at the Big Bear Solar Observatory. The plages are indicated by boxes enclosfnq them along with their Big Bear identification numbers which are published in the Solar Geophysical Data (SGD) catalogue. The listed plages were newly formed, except for the No. 833, which is a return of the plage No. 820 forming at the beginning of October. Figure 2b shows the photograph of the Sun for November 24, when large plage remnants appeared. These remnants are also enclosed by boxes, and numbered with one or two digit numbers. The

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J. M. Pap et al.

troheliograms is 25.4 % for plages and 21.7 % for plage remnants. Figure 2c shows the Sun in Ca-K line on December 15, when only two plages (No. 871 and 872) occurred but an extended remnant (No. 37) was seen in the northern hemisphere. This remnant was the return of plage !ki. 850, which formed at the end of October. 4.8 I

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We have measured the intensity and area of the remnants which are indicated by the boxes in Figure 2. These remnants do not include all the bright features which were observed on the Sun. We found that the error of the area and intensity ‘leasurements of small tiny network components can be as large as 50 %, while it is reduced to 10—20 % for the large remnants which can be identified easily. Because of this we selected which werethose 21.7 features % brighter as than remnants the background on the spectroheliograms. The measured area and intensity of remnants are shown in Figure 3, corn— paredtheto MgtheII plage and core—to—wing area, intensity ratio.

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Fig. 1. Dashed lines show the daily values of the SME/Lyman alpha irradiance (a), the Mg II core-to-wing ratio (b) and the SLiM/ACRIM total irradiance corrected for the sunspot darkening (c) during 1986. The solid lines show the Ca-K plage index, scaled to the irradiances by linear regression analysis.

Unfortunately, because of the large number data, we can not develop ofa missing statistically significant model of solar irradiance from these measurements. However, our results show that the neglected plage remnants give a significant contribu— tion to the irradiance We have calculated the variabilities. “remnant index” from the area and intensity of remnants shown 3b and in the same way inas Figures the plage index3d, was calculated, which is a measure of the projected plage areas weighted by their based averaged relative brightness on the formula by /8/. The plage index and the remnant index are shown in Figures 3f and 3g. In the second part of December, the UV irradiance increased in parallel with the appearance of the remnant No. 37, which had the largest area and intensity of the remnants shown in Figure 2c. Our measurements show that on average the remnant index, calculated for the time interval of

November to December, equals about 13 percent of the total plage + remnant index. On the other hand, when large plage remnants are on the Sun, as for example on December 19, the contribution of remnants to the plage + remnant index exceeds 50 percent. As can be seen from Figure 3, both the measured remnant area and intensity show sharp peaks at the end of November and December, while the UV irradiance shows a broad peak. This indicates that besides the remnants possibly other network components may also modify the solar irradiance. That is, one has to measure the area and intensity of all bright features existing on the Sun in order to predict the changes in solar irradiances observed from space. For this purpose it is necessary to increase the measuring precision and to extend our study for additional time intervals. CONCLUSIONS

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Modelling Solar Irradiances

(5)273

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Fig. 2. Solar pictures in Ca-K line (photographed at the Sacramento Peak National Soisr Observatory) are shown on the left side panels on October 28, November 24 and December 14, 1986. The right side panels show their computer plots produced at the Big Bear Solar Observatory. The plages and the largest remnants are numbered in the pictures according to the numbers given on the computer plots. tion changes with time. Our results also show that besides the plaqe remnants additional cot”ponents of the bright network play a role in the irradiance variabilities. These results stress the importance of direct photometric measurements of plages and their remnants as well as the remaining network components in order to improve the present irradiance models.

J. M. Pap et al.

(5)274

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plotted for the time interval of October to December 1986. The Mg core-

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to-wing ratio derived from the NOM9 !neasurements (e), the Ca-K plage index (f) and the calculated remnant index (g) are given, too.

respectively. The useful discussions with Dr. Gary Chapman and Dr. Alan Patterson on Ca-K line photometry are also approciated. This study was supported by a grant of the NOAA Space Environmental Laboratory. The contribution of WHM was supported by the NSF under ATM-88l 6007 and by NASA under NGL 05 0023 034. REFERENCES 1. R.C. Willsotj, 5. Gulkis, Fl. Janssen, H.S. Hudson and G.A. Chapman, Science, 211, 700

(1981). 2. J. Lean, 0.R. White, W.C. Livingston, D.F. Heath, R.F. Donnelly and A. Skumanich, J. Geophys. Res.,

87, 10,307 (1982).

3. J.M. Pap, H.S. Hudson, G.J. Rottman, R.C. 1Jillson, R.F. Donnelly and J. London, in: Climate Impact of Solar Variability, ed. K. Schatten, NASA Conference Publ., in press (1990). 4. R.F. Donnelly, in: Handbook for Middle Atmosphere Program (MAP), eds. J. Lastovicka, T. Miles and A. O’Neill, Vol. 29., p.1. (1989). 5. W.H. Marquette and S.F. Martin, Solar Phys., 117, 227 (1988). 6. G.J. Rottman, Mv. Space Res., Vol. 8., No. 7., p.(7)53 (1988). 7. D.F. Heath and B.14. Schlesinger, J. Geophys

Res., 91, 8672 (1986).

8. W.E. Swartz and R. Overbeck, Ionosphere Research Laboratory Report 373 (E), Pennsylvania State University (1971).