Solar Energy Vol. 28, No. 5, pp. 443--445, 1982 0038-.092X/82[050443-03503.0010 Printed in Great Britain. Pergamon Press Ltd.

TECHNICAL NOTE

Extraterrestrial solar irradianee variability: two and one-haft years of measurements from Nimbus 7

J. R. HICKEY, B. M. ALTON and F. J. GRIFFIN The Eppley Laboratory, Inc., Newport, RI, U.S.A.

H. JACOBOWITZ and P. PELLEGRINO NOAA/NESS, Washington, DC, U.S.A.

R. H. MASCHHOFF Gulton Industries, Inc., Albuquerque, NM, U.S.A.

and

E. A. SMITH and T. H. VONDER HAAR Colorado State University, Fort Collins, CO, U.S.A.

(Received 27 July 1981)

1. INTRODUCTION

The cavity pyrbetiometer of the Earth Radiation Budget Experi- ment (ERB) of the Nimbus 7 satellite has been performing solar irradiance measurements since 16 November 1978. Here, we present a preliminary analysis of the data obtained through 13 July 1981. Initial results were presented for the first 6 months of operation by Hickey et,~l.[1]. An announcement of the detection of solar variability derived from this data was made at the meeting of the American Geophysical Union[2] in May 1980. Delays in processing of the full data tapes have hampered complete analysis so that the "engineering" data set described in the references has been employed here. In this technical note, it is our purpose to present a much longer data set which indicates the nature and degree of solar variability and the trent in the long term regression. While time series analysis has been performed on the preliminary data[3], it will be mentioned only briefly here. It is noted that the solar variability found by the ERB data has since been confirmed for a period in 1980 by the ACRIM experiment of R. Willson aboard the Solar Maximum Mission (SMM) spacecraft[4].

2. INSTRUMENT STABILITY

The instrument is an H-F type cavity radiometer[5]. It has a self-calibrating heater which is activated to check the stability of reponsivity periodically. Fifty-one such calibration sequences over the period discussed here have been analysed. These analysis are performed on the highest quality data set which is not yet available for all of the orbits of measurements but is available for the calibration orbits. The stability of response has remained at the resolution level of the data system which is 0.05 per cent [1]. Thus, in the absence of any degradation of the radiometric response which does not affect the response to the electrical heater, the reliability of a single data point can be considered to be -+ 0.05 per cent based on digitization error only. Such long term degradation of the absorbtivity is possible but is unlikely for this cavity configura- tion with the prelaunch conditioning for space exposure. Com- parison with other sensors is the only true method of assessing this.

3. RESULTS The plot of 578 daily measurements for the first 971 days of the

Nimbus 7 solar mission is shown as Fig. 1. The range of the ordinate axis of the plot is only 0.73 per cent of the mean value in order to accentuate the variability. The spread of the data is from a low of 1369.0 Wm -2 to a high of 1375.1. This amounts to 6.1 Wm -2 or 0.444

per cent of the mean. The spread is about 6.5 times the standard deviation for the data set. A number of low points can be seen in the data which fall well below the mean or the regression line which is indicated on the plot. The lowest value is in August 1979. It is at the bottom of the dip with the longest half width on the time axis (about 12 days). This value is 0.27 per cent below the mean and 0.29 per cent below the regression estimate for the date. The highest value is only 0.17 per cent above the mean. There is indication of cyclic behavior in the data trace. The trend in the overall data set is downward at approximately 0.021 per cent per year with the regression yielding a 0.06 per cent decrease for the period.

4. COMPARISON WITH SOLAR INDICATORS Figure 2 is a comparison plot which shows values of the sunspot

number and solar 10 cm (2800 MHz) flux on the same time axis. Since the correlation of events is generally better with the latter, indicator vertical lines have been added for the peaks in the 2800 MHz flux which correspond to dips in the solar irradiance. Dashed lines are drawn where anti-corelation or questionable corelation is found. The solar indicator data does not extend to the end of the irradiance data set because of the delay in receiving this data. The irradiance data set has many missing points both because the ERB experiment is on a 3 day "on", 1 day "off" cycle and because the data for some "on" days is not yet available. This makes true time series analysis difficult.

In an event sense, the peaks in 2800 IdHz flux corresponds to dips in the irradiance data for all cases for which the former is greater than 250. This is also true for all sunspot numbers greater than 250. The vertical lines are drawn for all 2800 MHz peaks exceeding 200 for which there is also a discreet peaks in the sunspot number. There are 29 lines fo which 19 are solid and 10 are dashed. It is noted that the largest irradiance dip does not correspond to largest event of combined indicator values although the largest indicator event (about day 360) also corresponds to an irradiance dip. It is also noted that the next two events of high sunspot number after the largest do not correspond to dips in the irradiance while the sunspot number is larger for both of these events than it was for the major (August 1979) event. Further analysis of these events is in progress and will be pursued in greater detail when the higher quality irradiance data set is available. The ACRIM instrument aboard SMM also sensed the two dips shown at days 512 and 559 (early April and mid May 1980)[4].

443

444 J .R . HICKEY et al.

NIMBUS 7 ERB cavity Solar Irradiance (engineering level data) vs. time .

"~ " !~ :.. .ill -_:~:. ,..: ..-~./:. . . . . i . . i,:i . • . . . . . . . . .. ;.. • . . . . . . " . : . . ' . " :-.-. .. .. , . . .y--

f l /

+ " T • • ~ Cl "

I M e a n ~ n t s : first Nov. 16,1978 I S t d ~ ~ s t July 13,1981 1::111 Slope : -0.0008 W/mZ/day or approx. -0.021%/yr • "

1367 l T I~11'EI35 7 MI~SIi'N PFY

Fig. 1.

........ I" . . . . . . . . . . . . * . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . . ~l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . - I -+'- . . . . . . . . . . . . . . . 13~5

1370 Ch_ 10c Enginee ring Id atl a ~" I'[ [ [ , i i[ I ! I i i J l [ : I] : '[ [ ' 13o9 ~ . + , o ~ . . . . ~ . ~/m~l t ~ ] 1 ' .' I i l I [ I ' l J l t ; l l ' I I

.oo_ps0 _ .,, iLl ii J l Jl!lil! i L J _

"°l J ..... !.., I :! ,

°L--r Jl AA ~÷JL_~'A'.i ;~ ^ It ,,I ,~,, ', , -~

'78 '79 *79 '80 '80 '81

- ' ~ ,~ ~ " . . . . . . . . . . ~ - . . • _.

Mission Day: Day 1 = Nov. 16, ]-978

Fig. 2. Comparative plot of ERB/Nimbus 7 ch-10c solar irradiance and solar indicators Eppley Lab./NOAA-NESS update of 1 July 1981 for engineering level data.

S U M M A R Y

The preliminary solar irradiance measurements for more than two and one-half years of cavity radiometer measurements from the Nimbus 7 satellite have been presented. Initial analysis shows variability at the 0,44 per cent peak to peak level with a periodic dips of up to 0.27 per cent of the mean. The mean value for the period is 1372.7 Wm -2. A downward trent of about - 0.02 per cent per year is indicated by simple regression.

REFERENCES

I. J. R. Hickey, L. L. Stowe, H. Jacobowitz, P. Pellegrino, R. H. Maschhoff, F. House and T. H. Yonder Haar, Initial solar irradiance determinations from Nimbus 7 cavity radiometer measurements. Sci. 2~ , 281-283 (1980).

2. J. R. Hickey, F. J. Griffin, H. Jaco~witz, P. Pellegrino and R. H. Maschhoff, Comments on solar constant measurements

Extraterrestrial solar irradiance variability 445

from Nimbus 6 and 7. EOS Trans. Amer. Geophys. Union 61, SS-50 (1980).

3. E. A. Smith and J. R. Hickey, Solar variability analysis using Nimbus 7 ERB results. Proc. Filth Int. Pyrheliometer Com- parisons and Absolute Radiometer Comparisons (IPC V) 29 Sept.-17 Oct. 1980, Working Report No. 94, Swiss Meteoroloical Institute, Zurich, Switzerland (Feb. 1981).

4. R. C. Willson, S. Gulkis, M. Janssen, H. S. Hudson and G. A. Chapman, Observations of solar irradiance variability. Sci. 211,700-702 (1981).

5. J. R. Hickey, R. G. Frieden, F. J. Griffin, S. A. Cone, R. H. Maschhoff and J. Gniady, The self-calibrating sensor of the eclectic satellite pyrheliometer (ESP) program. Proc. 1977 Ann. Meeting of AS[ISES, pp. 15-1 to 15-4 (June 1977).