Continuous Measurements of the Solar UV have been made by an instrument on the Solar Mesosphere Explorer (SME) since October 1981. The results for the wavelength interval 200 to 300 nm show an irradiance decrease to a minimum in early 1987 and a subsequent increase to mid-April 1989. The observed UV changes during part of solar cycles 21 to 22 represent approx. 35 percent (during the decreasing phase) and 25 percent (during the increasing phase) of the observed variations of the solar constant for the same time period as the SME measurements

London, Julius

Pap, Judit

Rottman, Gary J.

English

NASA Technical Reports Server

N90-28163
OBSERVED SOLAR NEAR UV VARIABILITY: A CONTRIBUTION
TO VARIATIONS OF THE SOLAR CONSTANT
Julius London l, Judit Pap l, and Gary J. Rottman 2
University of Colorado, Boulder, CO 80309 USA
1Department of Astrophysical, Planetary and Atmospheric Sciences
2Laboratory for Atmospheric and Space Physics
Abstract
Continuous Measurements of the Solar UV have been made by an instrument
on the Solar Mesosphere Explorer (SME) since October 1981. The resultsfor
the wavelength interval200-300 nm show an irradiancedecrease to a minimum in
early 1987 and a subsequent increaseto mid-April 1989. The observed UV changes
during part of solarcycles 21-22 represent approximately 35 percent (during the
decreasing phase) and 25 percent (during the increasingphase I of the observed
variationsof the solarconstant forthe same time period as the SivIEmeasurements.
Solar irradiancereceived by the earth-atmosphere system and itslatitudedistribution
representsthe ultimate driving force for the ocean/atmosphere circulationand long-term
climate of the system. How the system generallyresponds to the imposed external driving
forceisreasonably well known. However, many of the detailsof this response are not as
well understood. They originate,from complex spatialinteractionsand multiprocess non-
linearfeedbacks, both positiveand negative (see,forinstance HANSEN et al.,1984). Some
theoriesof solarinduced climate variationsare based, in part,on whether perturbations of
the solar irradianceare effectiveprimarily at the earth'ssurface (sea surface) (e.g.,REID
and GAGE, 1988) or in the middle atmosphere (e.g.,RAMANATHAN, 1982).
Many models of climate variabilityinvolve changes of the totalsolar irradiance,the
solarconstant, without regard to the wavelength dependence of such changes. The atmo-
spheric penetration of solar radiation,however, isa slgn]ficantfunction of wavelength. It
is,therefore,important to determine the spectraldistributionofthe differentcontributions
to observed solarconstant variations.As far as we know, measurements of the time varia-
tions of these contributions are currently availableonly for UV wavelengths. Ifthe recent
suggestion (KUHN et al., 1988) of latitude-dependent solar surface temperature variations
contribute significantly to the observed solar constant changes, it should be possible to ver-
ify this contribution by monitoring the solar irradiance spectral distribution in the visible
and near-infrared. Although solar irradiance below 300 nm represents only 1.1 percent of
the solar constant, time variations of its relative energy are significantly larger than those
of the solar constant (see, for instance, LONDON et al., 1984).
Estimates of the solar constant and its temporal changes have been made using ground-
based, balloon, rocket, and satellite measurements (e.g., FRC)HLICH, 1977). The most
consistent set of time monitored measurements made so far are the results of satellite obser-
vations from two different cavity radiometer type experiments: the Earth Radiation Budget
MERB) Nimbus-7 (e.g., HICKEY et al., 1988) and the Active Cavity Radiometer Irradiance
onitor, Solar Maximum Mission (ACRIM I SMM) (e.g., WILLSON et al., 1986; WILL-
SON and HUDSON, 1988) programs. Each instrument is capable of observing the total
solar irradiance over a spectral range that extends from the short UV to the far IR. The
ERB Nimbus-7 measurements started in November 1978, the ACRIM I SMM measurements
started in February 1980. Both programs continue to function as of 1 March 1989. For the
time period 1980-1987 they each reported approximately 300 mean daily measurements per
year. Although there is an average 0.2 percent difference in reported solar constant values,
the data from the two systems track each other quite well over the long period of overlapped
observations. When the data were subjected to an 81-day smoothing filter, the correlation
between the two observation sets was +0.93 for the period Jan 1980-May 1986, largely as a
result of the consistent downward trend shown by the two data sets. For the early period of
solar cycle 22, (Jun 1986-Oct 1987) the correlation fell to +0.71. For the daily means, the
correlations were +0.79 and +0.22 for the two pcriods respcctivcly. The 81-day smoothcd
https://ntrs.nasa.gov/search.jsp?R=19900018847 2020-03-19T21:01:47+00:00Z
10
datasetsareshownfor the periodJan 1980-May1988in Fig. 1. AdditionalACRIMI
datato mid-1988(WILLSONandHUDSON,1988),notshownin Fig. 1,alsoindicatea
continuedsharpincreasein solarconstantvaluestartinginearly1987.
MeasurementsofthetimevariationsofsolarUVirradiancehavebeenrecentlyreviewed
by (LEAN,1987).Themostrelevantobservationsavailableforestimatingthecontribution
oflongperiodUVirradiancevariationsto thoseofthesolarconstantareobservationsderived
fromtheUVSSolarMesosphereExplorer(SME)instrument.TheseSMEmeasurements
coverthewavelengthinterval120-300nm. Theobservationsstartedin October1981and
wereterminatedin mid-April 1989. Detailsof the SMEexperiment,includingrelative
accuraciesat differentwavelengthsarediscussedinROTTMAN,1987andreferencestherein.
An estimateof UV (175--400nm) variations(HEATHandSCHLESSINGER,1984)is also
derivedfrommeasurementsfromthe SolarBackscatterUltraviolet(SBUV)instrumentonNimbus-7.
Theobserveday-to-daysolarconstantmeasurementsarequitenoisy,andwe,there-
fore,usedan81--dayrunningmeanasa filter for boththeSMEandERBNimbus-7data
setscoveringthetime interval1 Jan 1982to 30Apr 88, thecoincidentperiodfor whichdatafrombothprogramsareavailable.
The81-daysmoothedtimevariationsoftheSMEtotalsolarirradiancefor thespectral
interval200-300nmandtheERBNimbus-7solarconstantvaluesforthe6-1/3yearperiod
areshowninFig. 2. TheUV valueshowanaveragedecreasefromJan1982to aminimumin
mid-March1987.Duringthis periodthereisanindicatedoscillationwithanaverageperiod
of about230daysanddecreasingamplitudeup to thetime of minimum. This oscillation
is found at almost all of the 5 nm subintervals contributing to the total SME curve given
in Fig. 2. with very good coherence in phase. The ERB Nimbus-7 solar constant data
show long-term decreasing values up to early 1986, some increase to mid-1987 and then
a pronounced rise in the solar constant values consistent with the UV increase. There is,
however, only a loosely corresponding shorter period oscillation analogous to that seen with
the solar UV data.
Siop4 of Tr_x_d Lima for S)¢X UV Solar Irradlance( "10(-4) W/mA2 yr ):
WaTLl_ngth Slop_ S!op4
("_) (a) (b)
200-205 -4.9 10. g
205-210 -9.2 13.1
210-215 -10.7 31.2
215-220 -16.6 32.5
220-225 -37.5 26.3
225-230 -17 . 9 46 . 8
230-235 -29 .0 24 . 7
235-240 -25 .1 64 .5
240-245 -33.9 29.2
245-250 -16 . 8 30 .3
250-255 -17.8 35 . 5
255-260 -36.0 36.7
2{;0-265 -32.5 26.4
2_5-270 -60.5 76.8
270-275 6. O 117.5
275-280 -52.1 434.3
280-285 -38.6 42.1
2|S-290 -41.6 117.7
290-295 -I01.3 76.8
29s-300 -3e.6 lo5.3
200-300 -609.1 _406.e
,-aa (sc) -1_,_.0 6_a,.o
(SC) -_gs_._ s31.5
(a) l Jan 19|2 - 30 May 1986
(b} 1 Jan Ig|7 - 30 Ap_ 1988
The UV and solar constant data sets were divided into two intervals covering the joint
descending and ascending period of the observed irradiancc values in each set. SME observed
data were averaged over 5 nm intervals. The computed slope for each 5 nm interval for the
two different period as well as the total spectral variation (200-300 nm) and the associated
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ERB change For each period are given in Table 1. For the negative phase of solar cycle 21
the UV decrease is about -0.06 Wm-2yr -l. The observed solar constant decrease is about
0.180 Wm-2yr -_ (ERB) or 0.196 Wm-2yr -_ (ACRIM) giving an average UV contribution
to the solar constant change during this period of about 32 percent. For the relatively
short period shown for the increasing phase of cycle 22, the values are 0.14 Wm-2yr -1 and
0.62 Wm-2yr -l (ERB), giving a percent contribution of the UV irradiance to the observed
solar constant increase of about 22.7 percent. As can be seen from the results shown in
Table 1, most of the large contributions are contained in the wavelength region 265-300 nm,
although, for reasons unknown to us at present, there is a small increase in the spectral
interval 270-275 nm. Most of the UV energy beyond 265 nm is absorbed in the lower and
middle stratosphere (20--40 kin) (see, for instance, BRASSEUR AND SOLOMON, 1986).
Thus, we see that a significant portion (_-25-35 percent) of the observed solar constant
variation is absorbed at levels above the troposphere where there may be important effects
on the time varied radiation budget. These effects need to be included in models of solar
influences on climate variations.
Acknowledgements
We would like to thank H. Lee Kyle and R. C. Willson for making available to us the
solar constant data used in our analysis. We also thank Kristin Hoyer and Barry Knapp for
programming assistance. One of us (J.P.) acknowledges with appreciation partial support
as a grant-in-aid from the Graduate School (CRCW), University of Colorado. This study
was supported by research grants NSG 5153 and UARA/NAS5-27263 from the National
Aeronautics and Space Administration. The SME project is supported by NASA under
contract number NASW-4091.
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Observed solar near UV variability: A contribution to variations of the solar constant

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