Adv.SpaceRes.Vol. 8, No. 7, pp. (7)5—(7)10, 1988 0273—1177/88$0.00 + .50Printed in Great Britain. All rights reserved. Copyright© 1989 COSPAR

OBSERVATIONOFTOTAL IRRADIANCEVARIABILITY FROM NIMBUS SATELLITES

J. R. Hickey,* B. M. Alton,* H. L. Kyle** andE. R. Major****The EppleyLaboratoryinc., Box419,Newport,RI02840,U.S.A.**NASA/GoddardSpaceFlight Center,Greenbelt,MD20771, U.S.A.** *ScienceSystemsandApplicationsinc., Lanham,MD20706, U.S.A.

ABSTRACT

The cavity pyrheliometer sensor aboard Nimbus 7 is now in its tenth year ofoperation. Results for 9 1/2 years show a decreasing trend through the endof solar cycle 21 of approximately -0.015% per year, a levelling off nearsolar minimum and an increasing irradiance for the beginning of cycle 22.Through April 1988 the irradiance has increased about 0.06% above theminimum value which was about 0.08% below the highest values obtainednear solar maximum. The rise appears more rapid than the decline.

INTRODUCTION

Total solar irradiance measurements from Nimbus satellites began with theNimbus 6 mission /1,2/ in July 1975 and continued on the Nimbus 7 mission/3,4/ in November 1978 as part of the Earth Radiation Budget (ERB) experi-ment. The latter mission included a cavity radiometer which was not amongthe sensors of the Nimbus 6 ERB. The cavity radiometer of Nimbus 7 isstill operational at this time (July, 1988), well into its tenth year.In this presentation we will discuss nine and one half (9 1/2) years ofresults from November 1978 through April 1988. The 100 month data set/5/, through February 1987, was discussed at the NSF workshop at NCAR inNovember 1987. The 9 year data set (missing the months of May and June1987) has been submitted for publication /6/. There had been indicationsof an increasing trend in the solar irradiance in the nine year set ofabout 0.02% in the annual mean above the previous year. Willson has alsoreported /7/ the onset of increasing irradiance for the SMM/ACRIM sensor.The ERBE type sensors are also measuring solar irradiance on a less frequentbasis. The ERBS sensor of this group is reported to also be indicating arise in irradiance /8/ of about 0.02% per year from June 1986 throughNovember 1987.

CAVITY SENSORRESULTS

The cavity pyrheliometer of Nimbus 7 is referred to as channel 1OC. Thedaily mean irradiance, processed to SEFDT level /9/, is plotted for the nineand one half year period in Figure 1. The sunspot number is plotted belowso the reader can orient the data in the cycle. The greater dispersion inthe data during solar active periods and the downward trend with the decayof cycle 21 are visible on the plot. At the end of the plot there isevidence of an increasing irradiance as the new cycle, 22, is entered. Thestart of the cycle has been placed at September 1986 by the Space Environ-ment Center. ERB solar mission years run from 1 Nov to 31 Oct. with thevertical lines at 1 Nov. of each year. The pertinent results are given inTable 1 for each mission year. The tenth year includes the 6 months fromNov 87 through Apr 88. The tabulated values are also shown in Figure 2.with the 1 sigma value indicated by triangles and the maximum and minimumdaily mean values by X and ~ respectively. The percentage difference fromthe first annual mean (running % change) is shown in Figure 3. The annualmeans decrease to 0.08% below year 1 for years 6, 7 and 8. Year 7 was themost stable year, not only because of low solar activity but also becausethe most stable Nimbus operating schedule for ERB was in effect for thewhole period. For year 7 the range of daily mean values was within 0.13%with a standard deviation of only 0.02%. There is a “bump“ in the downtrend

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(7)6 J. R. Hickey eta!.

TABLE 1. TOTAL IRRADIANCE RESULTS FOR 9 1/2 YEARS: ERB/NIMBUS 7DAILY MEAN SEFDT LEVEL DATA FROMCH 1OC

MISSION DATA ANNUAL STANDARD ANNUAL ANNUAL RUNNINGYEAR DAYS MEAN DEVIATION RANGE CHANGE CHANGE

Nov-Oct W/mA2 W/mA2 % W/mA2 % % %.1 78—79 265 1371.30 0.84 0.06 5.23 0.38 N/A N/A2 79—80 274 1371.11 0.63 0.04 3.82 0.28 —0.014 —0.0143 80—81 285 1370.80 0.73 0.05 4.57 0.33 —0.023 —0.0364 81—82 275 1370.62 0.69 0.05 3.33 0.24 —0.013 —0.0505 82—83 292 1370.60 0.50 0.04 2.88 0.21 —0.001 —0.0516 83—84 318 1370.18 0.64 0.05 4.03 0.29 —0.031 —0.0827 84—85 365 1370.16 0.29 0.02 1.75 0.13 —0.001 —0.0838 85—86 365 1370.17 0.30 0.02 1.81 0.13 0.001 —0.0829 86—87 340 1370.54 0.38 0,03 1.92 0.14 0.027 —0.055lOth* 182 1370.97 0.36 0.03 1.77 0.13 0.031 —0.024

WHOLE RANGE

114 mo. 2961 1370.59 0.68 0.05 5.95 0.43

The data set covers the period Nov. 16, 1978 to Apr. 30, 1988* 6 months of tenth year are included here: Nov. 87-Apr.88The maximum spread in annual means is 0.083 % (1.14 W/m~2)

SOLAR IRRADIANCE and SUNSPOTNo.ERS Nimbus 7 channel iOC 9 1/2 years

1.374

1.373

1.372 I

1.371 i T1.370 ii

~1.369 Iut 1.366 —__________________________________________

o

300.0

200.0

100.0

0.0Nov—75Nov—79N~v—6QNov—81N~v—82Nov—$3Noy—64Nov—65Nav—66Nov—87P4ov--BBNay—89

DATE— daily mean values

Figure 1. Daily Mean Solar Irradiance from Nov. 16, 1978 to Apr. 30,1988. Lower plot is Sunspot Number for the same period.

ObservationofTotalIrradianceVariability (7)7

ANNUAL MEAN IRRADIANCEERS NIMBUS 7 CH 1OC: 9 1/2 year.

1,374———

1.373- X

XX

1.372X Xr

E V XV X X V

1.371. y x ~— — —s-- —e— —r-- — —

I~Jv~1.370.

Ao

o1,369 . *

o

1.368.O

1.367-

1 .366 rnyr,yyri ‚TrryTrTT rTyn,r,r, rrrrrTTrrT ~Trfl1~TTT rrrrrrrrrT ~rrTTn‘rr, rry,,TITTV ‚r,,‘,,-r,T ~rTTTTT1TTrrrrrrrrr IrrTyrrrr

Nov—78 Nov— 79Nov— 8ONov— 81 Nov—82 Nov—83 Nov— 84Nov—85Nov— 86 Nov—87 Nov— 88Nov— 8GNov— 90

MISSION DATES

Figure 2. Annual Mean Solar Irradiance with standard deviation andrange of Daily Values.

RUNNING % CHANGEOF SOLAR IRRADIANCEERS / NIMBUS 7 CHANNEL 10C: 9 1/2 YRS.

0-

—0.01

—0.02 -

» —0.03zo

~ —0.04 -

—0.05 -

Ui—0.06

o~ —0.07 -

—0.08 -

009 I

1 3 5 7 9 11

MISSION YEARFigure 3. Percentage difference of Annual Mean Solar Irradiance fromthe mean of the first mission year (N0V78-0CT79)

(7)8 J. R. Hickeyetal.

ERB NIMBUS 7 SOLAR MEASUREMENTSOPERATING MODES y. TIME

C‘ItLE 21 MA~ I rCLE 2 start

V

3 lays o 1 off +

N FUL TIME I FULL IME

SPEd L OPEF TIONS

1 2 3 4 5 6 7 8 9 10 11 12

- nrrrrrrrr. ryrrrnrrr. rnTflrrT. . rTnntrn .Tn-rnrrn Ffl,rrTrn -yrnTrrn, TrrTrrrn, mrTtrrr ‚rr,trTn, rrrrrn,r.

Nov— 7SNov—79Nov— 8OF‘4ov—8 1 Nov— 82Nov—B3Nov— B4Nav—85Nov—S6Nov—87Nov— 88N~v—l39Nov—90

MISION DATES; Nimbus 7 ERS solara update 9 1/2 yrs + end of dota

Figure 4. Operating Modes of ERB, Nimbus 7 shown over the missionduration

SOLAR IRRADIANCE and SUNSPOT No.Nov 85 — .~pr88: ERB N7 Ch 10C

1.3720- — ______~—____ _______ _____—

S sCiai OpS.1.3715 - ___________ ___________ __________- Il —

S~ aiops.

1.3710-

_______ ____ ____ ______ Î ___— ______

13695z

_________ Cycie22S ~RTi:o.o-o _______

11/01/65 05/02/86 11/01/66 05/02/67 11/01/87 05/02/88

DATE: Time of Solar Cycle 22 Rising— Daily Mean Rz through May 66

Figure 5. ERB Nimbus 7 Daily Mean Irradiance and Sunspot Number for thestart of Solar Cycle 22 with special operations periods identified.

Observationof TotalIrradianceVariability (7)9

for year 5. During the period April-June 86 ERB was placed in a “specialoperations“ mode /5/ which required a different processing algorithm sincethe instrument was turned on and off every orbit and was in a thermaltransient state. The tabulated data include the correction. Near theonset of cycle 22, from late April to late August 1987, the ERB was againput in a “special operations“ mode. The operational modes for the missionare shown in Figure 4. After that period operations returned to full timeon. An increase in irradiance was confirmed with the return of stableoperations. Figure 5 is a plot of the daily mean data from 1 Nov 85 to theend of the processed data set and shows in more detail the onset of cycle22. Sunspot number is included at the bottom for the orientation. Thedates of special operations periods are marked above the irradiance data.We are restudying the corrections to be applied in these periods in accor-dance with modelling as previously described /5/. September 15, 1986 hasbeen marked as the start of cycle 22. It can be seen that the preliminarysunspot numbers have reached 90 on three occasions and above 100 for aprolonged period near the end of the data set. It appears that the charac-teristic sunspot blocking is returning while the total irradiance increases.The rise is close to 1 Wm

2 (0.07%) over the period of 2 and one half years.

OTHER COMMENTS

There was an event near the end of October and beginning of November 1987which caused 3 of the 4 interference channels (6,8 and 9) to exhibit rapidlyincreasing values. There was a Long Duration X-ray Event followed by aproton event. Since the increases indicate larger irradiance changes thanthe increase in the total, as discussed above, and as measured by the otherbroadband solar channels, the event while solar related may not indicatespectral irradiance changes only. It should also be noted that channel 3has been showing an increasing trend. This channel is the matching channelto the total irradiance monitor on Nimbus 6. The fact that it has notcontinued to decay, as had been expected may add to the understanding of theNimbus 6 record which extends back to 1975. Since much of this data hasonly recently been available for study, further discussion must be postponedto another presentation.

SUMMARY

We have been fortunate to have highly stable radiometers monitoring thesolar irradiance at the change of the solar cycles. Indications are thatthe increasing levels are rising faster than the downtrend declined. Aspread of 1 Wm2 in a period of two and one half years, is indicated fromthe ERB results. Other events associated with the onset of the new cycleare also evident in the ERB solar data. It is hoped that the sensor willmeasure through the next maximiim so that the results from the sameradiometer will be available for one complete cycle. Further modellingof the cavity and other ERB solar sensor results is presently in progress.

REFERENCES

1. W.L. Smith, J.R. Hickey, H.B. Howell, H. Jacobowitz, D.T. Hillearyand A.J. Drummond, Nimbus 6 earth radiation budget experiment, AppliedOptics, 16, 2, 306—317 (1977)

2. J.R. Hickey and D.T. Hilleary, Solar Radiation measurements from theNimbus 6 ERB experiment, COSPARNineteenth Plenary Meeting, paperC.4a.4, (1976)

3. J.R. Hickey, L.L. Stowe, H. Jacobowitz, P. Pellegrino, R.H. Maschhoff,F. House, and T.H. Vonder Haar, Initial Solar Irradiance Determinationsfrom Nimbus 7 Cavity Radiometer Measurements, SCIENCE, 208, 281-283(1980)

4. J.R. Hickey, B.M. Alton, F.J. Griffin, H. Jacobowitz, P. Pellegrino,E.A. Smith, T.H. yonder Haar and R. M. Maschhoff, Solar VariabilityIndications from Nimbus 7 Satellite Data, in S. Sofia (ed.), NASAConference Proceedings 2191, Variations of the Solar Constant,NASA/GSFC, Greenbelt, MD 20771 (1981)

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5. J.R. Hickey, B.M. Alton, H.L. Kyle and E.R. Major, Solar IrradiariceMeasurements by the Nimbus 7 ERB Experiment: An Update of 100 months,in Solar Radiative Output Variation, (ed.) P. Foukal, CambridgeResearch Instrumentation, Cambridge, MA 1988, p189.

6. J.R. Hickey, B.M. Alton, H.L. Kyle and D. Hoyt, Total IrradianceMeasurements by ERB/Nimbus 7: A Review of Nine Years, submitted toSpace Science Reviews (1988)

7. R.C. Willson and H.S. Hudson, Solar luminosity variations in solarcycle 21, Nature, 322, 810—812 (1988)

8. R.B. Lee, B.R. Barkstom, E.F. Harrison, M.A. Gobson, S.M. Natarajan,W.L. Edmonds, A.T. Mecherikunnel and H.L. Kyle, this issue.

9. SEFDT is Solar and Earth Flux Data Tape