GEOPHYSICAL RESEARCH LETTERS, VOL. 11, NO. 1, PAGES 54-56, JANUARY 1984

18 MONTHS OF UV IRRADIANCE OBSERVATIONS FROM THE SOLAR MESOSPHERE EXPLORER

Julius London 1 1,2 , Gudmundur G. Bjarnason , and Gary J. Rottman 2

1Department of Astrophysical, Planetary and Atmospheric Sciences University of Colorado, Boulder, Colorado 80309

2Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Colorado 80309

Abstract. Daily solar irradiance measurements in the spectral interval 120-305 nm have been made since 6 October 1981 with an instrument on

the Solar Mesosphere Explorer. The instrument operates with a spectral resolution of about 0.75 nm. Analysis of the observed data for the period 6 December 1981 to 3 June 1983 (20 solar rota- tions) shows that during this period there was an apparent decrease in irradiance at all wave- lengths observed (-19.7% + 9.7% at Ly-•) but the

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decrease was not significantly different from zero at wavelengths longer than 210 nm. The cross correlations between daily values of the solar irradiance and 10.7 cm flux varied from

0.7 (Ly-•) to 0.5 (210-215 nm) and •0 (290-295 nm). Calculations of the % range (i•e., highest to lowest value) of the irradiance within each solar rotation showed that for Ly-• the range varied between 6% and 30% over the 20 solar rota-

tions studied. At longer wavelengths the % range was smaller--about 7% at 180 nm and about 2%

beyond 240 nm. The percent range values indicate representative variations useful as input data for model calculations of stratosphere/mesosphere responses to short period solar variability.

Solar radiation in the ultraviolet spectral interval (120-310 nm) is largely responsible for the photochemical interactions and radiative heating of the stratosphere, mesosphere and lower thermosphere. For instance, irradiance at Ly-• (121.6 nm) dissociates water vapor whose products interact catalytically with ozone to reduce the ozone concentration in the mesosphere and lower thermosphere; solar irradiance in the interval 175-200 nm photodissociates molecular oxygen (Schumann-Runge bands) which then results in ozone production in the stratosphere, mesosphere and lower thermosphere; radiation at 210-310 nm is absorbed by ozone and is primarily responsible for heating the middle atmosphere; and excited atomic oxygen produced by photodissociation of ozone through absorption of solar radiation below 310 nm is effective in dissociation of otherwise

stable H-O and N 0 in the stratosphere, Signifi- 2 2 cant variations of the solar irradiance at any or all of these spectral regions could thus effect photochemical, radiative and subsequent dynamic perturbations in the middle atmosphere.

The Solar Mesosphere Explorer (SME) was launched on 6 October 1981 into a nearly-circu- lar, sun-synchronous orbit. An instrument on board the SME provides a daily measurement of the spectral solar irradiance in the interval 120-305 nm with a spectral resolution of about

Copyright 1984 by the American Geophysical Union.

Paper number 3L1967. 0094-827 6/84 / 003L- 19 67 $03.00

0.75 nm (Thomas et al., 1980; Rottman et al., 1982). The instrument is still operative. The data were somewhat noisy for a short period after launch and we here discuss variations of

the solar irradiance observations for the period 6 December 1981 to 3 June 1983 covering 20 solar rotations during the declining phase of solar cycle 21.

The basic data set for the present study consists of daily values of the solar irradiance for Ly-• and 5 nm averages over 4 different wavelength intervals: 185-].90 nm; 200-205 nm; 210-215 nm; and 290-295 nm. The use of 5 nm intervals averages is to smooth the effect of different solar line emissions which are of

relatively little geophysical importance. The data were calibrated to a rocket flight on 17 May 1982 (Mount and Rottman, 1983), corrected for instrumental drift and degradation (further corrections may be needed as additional rocket data become available), adjusted to I AU, and linearly interpolated for days of missing data. When a single observation exceeded 2• of the daily values, the datum was omitted and an inter- polated value was used. For the period discussed here about 2% of the observations were missing at wavelengths below 200 nm, increasing to about 7% at wavelengths above 240 nm. Except for one brief period, these were all single missing dayst

The daily irradiance for Ly-• is shown in Fig. 1 for the 18 month period. The computed long-term linear slope for the Ly-• irradiance over the 545 days is-19.7% + 9.7%. As can be

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seen, the data show a reasonably regular varia- tion, of about 27 days during the first 4 months followed by a 3 month noisy interval and then a recurrent 27 day oscillation, with decreasing amplitude for the remaining 11 months of analyzed data. Although the irradiance observations at other spectral intervals all indicate decreasing values during this period, none was statistically significantly different from zero at the 10 level• These results may be modified slightly as addi- tional calibration and instrument degradation corrections are applied. Auto correlations of the daily detrended irradiances were calculated for Ly-• and for four spectral intervals listed above. The strong 27 day variation shown here for Ly-• (• = 0.73) was also moderately present for the wavelength span 180-215 nm (•0.5) b•t was not evident beyond 240 nm. There was no evidence of any other dominant period (e.g•, 13 days) except during solar rotations 1721-23 when the Ly-• irradiance values were rather noisy and showed little range during each rotation (see Figs. 1 and 2A)• The lack of significant 27 day periodicity in the wavelength interval 240- 300 nm indicates relatively low persistence in photospheric and lower chromospheric perturba-

London et al.: 18 Months of UV Irradiance Observations 55

5.0

2.6

N J II R J S , N J Pi

"1 ' ' ' ' ' "' ' ' ' ' I '' "' '

,--

--

-

• I I I I • I ! I I I I ! I I I I ! I

DAY NUMBER

Fig. 1. Daily solar irradiance at Ly-• for period 6 December 1981 to 3 June 1983 (545 days).

tions. This does not, of course, preclude the existence of 27 day irradiance variability in relatively narrow solar emission or absorption lines. However, as indicated above, these variations have minimal geophysical influence.

Proxy data that could be used to explain UV solar radiation variations (e.g., 10.7 cm irradiance, Zurich sunspot number, etc., ) are particularly useful because there exist rela- tively long series of routine observations for these data. Linear correlations were calculated

between the detrended solar irradiance daily values at each spectral interval and detrended 10.7 cm irradiance, and Zurich sunspot numbers. The results are given in Table 1. As can be seen, the computed correlation between solar irradiance at 10.7 cm and Ly-• (0.70) is reason- ably high. This value is close to that derived by Vidal-Madjar (1977) based on 0SO-5 Ly-• bb- servations. For the period studied here, the 10.7 cm data can explain about 25% of the variance even at 210-215 nm. As also noted in

Rottman et al., 19.82, the correlations using sunspot numbers as the proxy parameters are slightly lowe r at all UV wavelengths. However, both fall to near zero at the longer wavelen•gth where the solar emission is no longer predomi-

T/eLE 1. STATISTICAL OWaALTERISTICS • SOLAR IFI:•AOI&NCE FOR TFE

m•,• 10,7c• s,s• (SOL, e0T.) O' CD LY.-• ).qC) B-11 0.22 0.62 21, q + .13.) •.7

-

.185-• • 2.5q B-l/ 0.58 0,50 5,2 + 1.5 13 • m -2 SF.c -1 el-1 -

2QD-2• •,t 8.5• B-11 0.57 0,q7 q.2 + 1.q 1.0 e CM-2 m:-I e-1

210-• • •,• B-12 O.Z!6 0.•9 2,q +_ 0.6 0,6 • c•"2 SF.C-1 m --I

2•0.-Z5 m 8,• B-i• --0,09 -0.0q 2,8 + 0,8 0,75 • cm-2 SF.C-1 m-1 -

Ly.

'I ß lit ! ,,, , I , , I , ! '11 I I I !

CARRINGTON ROTATION NUMBER

Fig. 2A. Percent range of solar irradiance at Ly-• for each:solar rotation.

nantly chromospheric in origin. Daily observed values of the total solar irradiance have

recently been reported by Hickey et al., 1980; Willson et al., 1981. These values rare nega- tively correlated with 10.7 cm irradiance (-0.56) and Zurich sunspot number (-0.36) (Willson, 1982). These negative correlations clearly arise from the overall lower photospheric bright- ness temperatures associated with active regions on the sun (e.g., Smith and Gottlieb, 1974; Willson, 1982).

Of particular interest in studies of possible middle atmosphere responses to solar radiation variations is the range of these variations on different time scales. The SME observations do

not cover a sufficiently long period to provide reliable values for the solar cycle irradiance changes at different UV wavelengths. However,

- m

,.--

..

•.

CARRINGTON ROTATION NUMBER

Fig. 2B. Percent range of sola•r irradiance for each solar rotation for four different spectral intervals.

56 London et al.: 18 Months of UV Irradiance Observations

WAYt:'I.I•I•(•TI-I (rim)

we can calculate variations of the irradiance

range for different solar rotation periods. For this purpose we used a 1-2-] daily f•lter to remove some of the day-to-day noise. The results showing the % irradia, ce range are given in Figs. 2A and B for Ly-• and four other wave- length intervals, as a function of Carrington Rotation Number.

The % range for Ly-• decreases from about 28% during the first period to a minimum of about 6% after 7 rotations. Solar activity then increased, became very reg•]ar, and the range of Ly-• irra- diation jumped to a maximum of 30% before it gradually decreased to half this value in May- June 1983. The average solar rotation variation for the entire period was slightly less than that given by Vida]-Madjar (1977) for a similar declining phase of the solar cycle. The varia- tion in the interval 185-190 nm also decreased

somewhat, from 7% to 3% in the first 7 months of the period studied. The irradiance range rose slowly during the next few months and then decreased through the remaining period• It should be noted that in this geophys•cally im- portant interval the range of solar irradiance never exceeded 10% during any solar rotation and was, on average, only s•ightly more than 5%. Beyond 210 nm the irradiance range during a solar rotation was about 2-B%, a value that could very well represent a combination of the day-to-day variation (see last column of Table 1) and •nstr•- ment noise at these wavelengths. The mean % range for the 20 solar rotations in the ]8 month period is given as a f•nction of the different 5 nm wavelength intervals in the fonrth of Table 1, and for 2 um intervals for the entire wavelength span 180-B00 nm in Fig. • The average % range for the 20 solar rotations cover- ing the data set decreases from about 7% at •80 nm to about 2% beyond 220 nm. The apparent increased range near 260 nm and 280 nm is associsted with s decreased pulse count at those wavelengths, a characteristic of the phototube used for the wavelength interval discussed here•

The solar irradiance range as observed in th_ data set provides an estimate for a reasonable limit to the average 27 day variations of solar ultraviolet radiation that can be used as input

to studies of stratospheric and mesospheric responses to solar irradiance variability. Since the 27 day variability has been shown to be wave- length dependent, its direct influence will be a strong function of height (see, for instance, Frederick, 1977). In addition, where the variability is relatively large ( i•e., below 200 nm) there seems to be large changes in the 27 day oscillation from one solar rotation to the next and during the solar cycle. Better definition of both the short and long period solar variations will be forthcoming as the basic data set is extended in time•

Acknowledgements. We would like to thank Kerry Spear fo r help in processing the irradiance data. We would also like to thank the reviewer

of this paper for very helpful comments. This work was supported in part by research grants NSG 5153 and UARS/NAS5-27263 from the National Aeronautics and Space Administration as•well as the SME project supported by the NASA Office of Space Science and Applications, contract number JPL 955357.

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(Received October 27, 1983; accepted November 16, 1983.)