Pergamon Adv. Space R~. Vol. 14, No. 9, pp. (9)33-(9)39, 1994

Copyright © 1994 COSPAR Printed in Great Britain. All rights reserved.

0273-1177/94 $6.00 + 0.00

OBSERVED SOLAR UV IRRADIANCE VARIATIONS OF IMPORTANCE TO MIDDLE ATMOSPHERE ENERGETICS AND PHOTOCHEMISTRY

Julius London

Department of Astrophysical, Planetary, and Atmospheric Sciences, University of Colorado, Boulder, CO 80309-0391, U.S.A.

ABSTRACT

Absorption of solar UV irradiance in the spectral interval 120-420 nm is chiefly responsible for radiative heating and photodissociation of important atmospheric constituents (e.g., 02, Oa, H2 O, NO2, etc.) in the stratosphere, mesosphere, and lower thermosphere. Thus, the absolute value and time perturbations of the UV irradiance could significantly affect the energetics, photochemistry, and subsequent dynamics of these regions. Analysis of preliminary data from the SOLSTICE (UARS) observations for a period of 244 days (3 Oct 1991-2 Jun 1992) is discussed in this paper. The data provide mean daily values of the spectral distribution of the observed irradiances at l-rim resolution and their solar rotation and semirotation variations. The average amplitudes of the 27-day irradiance oscillations for the 244-day data period were 5.7% at Lyman-alpha (121 nm), i% at 200 nm, 0.5% at 210 nm, and generally less than 0.2% at wavelengths longer than 280 nm. The average amplitudes of 13.5-day oscillations were, by and large, about half of these values. Solar irradiance variations at 10.7 cm are highly correlated with those at Ly-a and other chromospheric emission lines (r = 0.7 to 0.8) and only moderately correlated with irradiances at wavelengths of 180-208 nm (r = 0.5). The correlation decreases as the source region of the irra~Uance gets closer to the base of the photosphere. At the 2-rim interval 279-281 nm, however, which contains the cores of the Mg IIh and k lines, the correlation is again approximately 0.8.

INTRODUCTION

The solar energy input to the upper atmosphere, i.e., between 20 and 100 kin, is contained dom- inantly in the wavelength interval of about 120-350 nm. This energy originates from different levels in the solar atmosphere from the mid to lower chromosphere, where strong line emission appears superimposed over the underlying continuum, to the level of the temperature minimum of about 4550 K. The sharp transition in the spectral interval 200-208 nm is associated with the aluminum edge where the emission originates in the upper photosphere. At longer wavelengths the continuum radiation is formed at lower photospheric levels and has a brightness temperature of almost 6000 K at 400 nm. For an overhead Sun, the level in the Earth's atmosphere at which the solar UV irradiance has an optical depth of unity varies with wavelength. This level is approx- imately 65-75 km for Ly-a, over 100 km for the Schumann-Runge oxygen continuum, from about 35 to 85 km for the Schumann-Runge oxygen bands, about 40 km for the center of the Hartley ozone bands, and the Earth's surface for about 300 nm. Thus, solar energy absorbed at various layers in the stratosphere, mesosphere, and lower thermosphere depends on the combination of the source strength of the emitted solar energy and the abundance and absorption cross sections of the atmospheric constituents at these levels. Effective strength of the time perturbations of the absorbed energy will vary according to the spectral dependence of the perturbation source. Although major periodic variations (i.e., solar cycle periods) are generally wavelength consistent, there are important spectral differences, for short-term changes, that indicate that time variations originating in the lower corona, chromosphere, or lower to upper photosphere show much weaker interrelationships (e.g. , /1/) . JASR 14:9-D

(9)33

(9)34 J. London

104

..-., 10 3 E t'-

E - . , . . .

E

uJ (b Z <

E3 ,< r r r r

r r < _.1 O ¢./)

102

101

10"1

[ G Ch

[

N Ch F C h i i

10"2 100 140 180 220 260 300 340 380 420

W A V E L E N G T H (nm)

Fig. I. Spectral distribution of average solar UV irradiance from SOLSTICE observations for the period 12-21 Oct 1991/4/. Ch = SOLSTICE spectrometer channel.

The Upper Atmosphere Research Satellite (UARS) was launched on 12 Sep 1991. Two of the instruments on the satellite measure the spectral distribution of solar irradiance in the wavelength interval 115-420 nm. The absolute magnitude as well as the temporal periodic and aperiodic irra- diance variations as observed by one of these instruments, the Solar Stellar Irradiance Comparison Experiment (SOLSTICE), is discussed below. The observations discussed here cover a period of 244 days from 3 Oct 1991 (91276), 22 days after launch, to 2 Jun 1992 (92154) at which time they were temporarily suspended as a result of difficulties with operation of the solar panels on the satellite. These observations were restarted on 18 Jul 1992 (92200), and are continuing as of the present time.

SOLSTICE OBSERVATIONS

Full-disk solar irradiance observations with a spectral resolution of 0.15 nm are made approxi- mately once during each satellite orbit and reported as a daily mean with 1-nm resolution over the spectral interval 115-420 nm. Details of the observing instrument and method of degrada- tion corrections using concomitant stellar observations are discussed by Rottman et al. /2 / and Woods et al. /3/ . A brief preliminary presentation of the data derived from these observations is contained in London et al. /4/ .

DATA ANALYSIS

The ten-day average of the observed solar irradiance (12-21 Oct 1992) for the spectral distribu- tion (115-420 nm) as derived from SOLSTICE observations and corrected to 1 A.U. is shown in Figure 1. The SOLSTICE spectrometer employs three channels for spectral observations at three overlapping wavelength intervals (see Figure 1). It is estimated that the accuracy of the irradiance

Observed Solar UV Irradiance Variations (9)35

values shown in Figure 1 is better than 3% and the precision is about 1%. A set of equivalent brightness curves associated with temperatures appropriate to the solar photosphere and lower chromosphere is also given in Figure 1.

The general pattern of the solar irradiance curve, including the individual emission and absorption features, is well known (e.g. , /5/) . For the time period covered by the averaged observations given in Figure 1, the equivalent brightness temperature for Ly-a is approximately 6800 K. The emission lines at 130, 134, and 139 nm (O I, C II, and Si IV) and the underlying continuum at the base of the chromosphere (T ~ 4550 K) are clearly evident. The rapid increased emission in the wavelength interval 200-208 nm is associated with the A1 I ionization edge where the source level of the emission decreases sharply in the photosphere. Beyond 210 nm, the solar irradiance increases to match the equivalent brightness temperature (T ~ 5800 K) of the base of the photosphere at about 400 nm. The dominant absorption lines in this part of the observed spectrum are those due to the Mg II doublet (,,~280 nm) and Ca II (,,~393, 397 nm).

Time series of the daily observed solar irradiances for the period 3 Oct 1991 to 2 Jun 1992 are given in Figures 2a-2d for different spectral intervals: Ly-a, 200-205, 210-215, and 250-255 nm. These show irradiance variations whose origins are at various representative levels in the chromosphere and photosphere and are effective in different regions of the Earth's atmosphere from the lower thermosphere down to the mid-stratosphere. Included in each of these diagrams are dashed lines that show 27-day running means over the 244 days of the data sets to indicate average irradiance changes over periods longer than 27 days. It should be noted that these data are preliminary and that further corrections for instrument degradation and improvements to the processing precedures are still being made. However, the general patterns of variations shown in the diagrams in Figures 2a-2d are reasonably correct.

The daily solar irradiance distribution for Ly-a during the period discussed here is given in Figure 2a. It is obvious that there was a persistent major solar disturbance that lasted over at least nine rotations.

The 27-day averaged irradiance at Ly-a increased to a maximum around day 92045 and t h e n

decreased through the remainder of the period. The highest irradiance shown in Figure 2a occurred on day 92035 (10.3 mW.m -2- nm -1) at the peak of the rotational oscillation at that time. The relative range for the 16 days (trough to peak) was slightly more than 20% of the mean value for these 16 days. Over the 244-day interval of the observations shown here, the 27-day range increased through about half of the observation period and then decreased slowly. At the same time, the apparent, slower-varying oscillation showed an averaged irradiance increase from day 91330 to day 92045 of about 12% followed by a decrease to day 92154 of about 20%. This long-period pattern was present at all wavelengths below 195 nm and in diminished form for the spectral interval 200-205 nm (Figure 2b), but was not observed at wavelengths beyond 210 nm (see, for instance, Figures 2c-2d). The smoothed time pattern for irradiances at the longer wavelengths suggests an averaged decrease of about 2.5% at 250-255 nm. To the extent that these longer-term wavelength differences in the irradiance pattern are real--and not the result of the applied, inadequate preliminary corrections for degradation, etc.--it is suggested that the longer- period spectral differences in irradiance variations are the result of photospheric and chromospheric differences in their manifestations of solar activity (see also/6/) . These differences are also found for short-period variations (i.e., 27- and 13.5-day) as discussed below. The spectral disparity in both short- and intermediate-term irradiance perturbations would certainly affect varied responses to solar activity at different levels in the upper atmosphere.

Time variations of the irradiances just short of the aluminum edge at 208 nm and originating in the upper photosphere (T ~, 4850 K) are given in Figure 2b. The irradiances at wavelengths from 210 to 215 nm, shown in Figure 2c, come from a slightly lower, and thus warmer, photo- spheric level. Both distributions display strong, synchronous 27-day variations through the entire reported period with notable solar semirotation perturbations up to at least day 92110. The appar- ent longer-period smoothed variations, however, are slightly different. Whereas the peak irradiance

(9)36 J. London

9.8

9.4

9.0

8.6

8.2

7.8

7.4

7.0

!llllllllllilllllll l l l l l [ l l l l l l l l l l l i l ~ l l l l l l l l l l L

(a) 121"122nmt" i

I I I I I I I I I I I I I I I I I I I l l l l

(b) 200-205nm 9.80

9.70

9.60 i ~

9.50 : "" ,. . .'" "'!

~ 9.40

9.30 "

~ 6.6 36.5

~ 36.4 36.3

C/) 36.2 36.1 36.0 35.9 35.8 35.7 35.6

53.2 53.0 52.8 52.6 52.4 52.2 52.0 51.8 51.6 51.4 51.2

91275 91325 92010 92060 92110 92160 DAY SINCE 1900001

Fig. 2. The observed solar irradiance for various wavelength intervals; daily val- ues ( ) and 27-day moving averages ( - - - ) . Day 91276 = 3 Oct 1991 /4 / .

Observed Solar UV Irradiance Variations

. 0 6 0 i t I i i i i i i i i i i i i i i i ~ i t i i i i i i i i i i

.o55 (a) T H E 2 7 - D A Y O S C I L L A T I O N

.050

.045 -_

.040

2 : . 0 3 5

.o3o

~" .o25 ~ _

o2o fl:

.o15

.010 -_

.oo5 -_-

0 I-

• 026 (b) THE 1 3 . 5 - D A Y O S C I L L A T I O N - .024

.022

.020

.018

~ . . 0 1 6 ~ -

. 0 1 4

• ~ .012 - -

~ - .010 -- _

n- .008

.006

t - . 0 0 4

.002

0 , l l t- 100 140 180 220 260 300 340 380 420

WAVELENGTH (nm) Fig. 3. The spectral distribution of the relative amplitudes of the 27-day and the 13.5-day oscillations over the 244-day period 3 Oct 91-2 Jun 92 from SOLSTICE observations.

(9)37

(9)38 J. London

for A = 200 to 205 nm occurs on day 92060 (one rotation after that for Ly-a), the peak irradiance for A = 210 to 215 nm occurs on day 92115. On average, the ratio of the irradiance at 200-205 nm to that at 210-215 nm is about 0.27, close to, but smaller than, the values given by Hall and Anderson / 7 / derived from stratosphere balloon observations. That ratio varies in parallel with solar activity. However, as will be shown below, the observed 27- and 13.5-day amplitudes, when averaged over the 244 days of the data set, are higher, by a factor of two, for the 200-205 nm interval than those for 210-215 nm. In addition, the day-to-day correlation between irradiances at Ly-a and 200-205 nm is 0.75 and that for Ly-a and 210-215 nm is 0.53, again indicating reduced linkage between chromospheric and photospheric short-term disturbances.

The spectral distribution of the relative amplitudes of the 27- and 13.5-day variations, as derived from a Fourier decomposition of the entire data set, are shown in Figures 3a and 3b. The mean relative amplitude of the 27-day harmonic is a maximum of 5.7% (range of ,,~11.5%) at Ly-a with significantly large values shown for the various chromospheric emission lines. There is a decrease of relative amplitude to 1% at 208 nm with a more abrupt decrease to 0.5% at 210 nm. The 27- day relative amplitude has a distinct maximum near 280 nm equal to that found in emission from the upper photosphere. Beyond 280 nm the relative amplitude of the 27-day oscillation does not appear to be significantly different from the precision of the obserwtions. The spectral pattern of the semi-solar rotation harmonic shown in Figure 3b is like that of the rotational harmonic with relative amplitudes of about one half of those given in Figure 3a. Similarly, Ly-a and the other chromospheric emission lines have maximum relative amplitudes, and these decrease to 0.8% at the aluminum edge at 208 nm. Again, an abrupt decrease to approximately 0.3% occurs at 210 nm, and, except for the peak near 280 nm, there is no significant value beyond 280 nm.

Models used to calculate the atmospheric response to solar activity frequently employ proxy data to represent time variations of the forcing function of the model. Some of the ground-observed data are highly correlated with irradiance variations originating in the upper chromosphere-lower corona with those originating in the lower chromosphere (e.g., 10.7-cm radio emittance and that from Ly-a at 121.6 nm; e.g., /8 ,9/ . Ground-based observed variation of 10.7-cm radiation for the time period discussed here does, indeed, closely resemble the irradiance variations of Ly-~ (r ~ 0.8). However, no single proxy can adequately replicate the time and spectral distribution of the solar energy input at different atmospheric levels.

CONCLUSIONS

A preliminary analysis of the SOLSTICE (UARS) irradiance observations over the spectral in- terval 115-420 nm gives an average spectrum during a period of high to moderate solar activity with an estimated accuracy of the observed data of about 3%. The spectral distributions of time variations show strong solar rotation and semirotation periods in the chromospheric emission lines at Ly-a and, up to about 180 nm, moderate variations to the edge of A1 I ionization (208 nm); there are weak but detectable variations up to about 290 nm but not beyond. In the 2-nm interval encompassing the Mg II h and k lines, 279-281 nm, the solar rotational and semirotational oscil- lations were as strong as those found for the lower chromosphere and upper photosphere source irra~liances. During the reported period, the 27-day smoothed data set indicated an irradiance increase through the middle of this period and a decrease toward the end for wavelengths up to about 210 nm, but, perhaps, an almost steady decline throughout this period at longer wave- lengths. It is clear, from a comparison of the short- and moderate-period variations at different wavelengths, that models involved in studies of the response of different upper atmospheric'levels to solar irra~llance fluctuations will need to make use of the results of direct observations of these irra~liances over long time periods. Indeed, such models are now being based on SME and SOL- STICE irra£1iance observations (e .g . , /10 / ) . At present, the SOLSTICE data axe being observed in a reasonably routine fashion, which, it is hoped, will continue through the current solar cycle.

Observed Solar UV Irradiance Variations (9)39

ACKNOWLEDGMENTS

Thanks are due to the SOLSTICE team of the Laboratory for Atmospheric and Space Physics, at the University of Colorado, for providing the preliminary level 3AS data used in this current analysis and to Fei Wu for providing important programming assistance. Support from the Up- per Atmosphere Research Program of the National Aeronautics and Space Administration under Contract No. NAS5-27263 is acknowledged and appreciated.

REFERENCES

1. J.L. Lean, Solar ultraviolet irradlance variations: A review, J. Geophys. Res., 92, 839- 868 (1987).

2. G.J. Rottman, T.N. Woods and T.P. Sparn, Solar stellar irradiance comparison experi- ment I: 1. Instrument design and operation, J. Geophys. Res., in press (1993).

3. T.N. Woods, G. Ucker and G.J. Rottman, UARS Solar stellar irradiance comparison experiment h 2. Instrument calibrations, J. Geophys. Res., in press (1993).

4. J. London, G.J. Rottman and T.N. Woods, Variations of solar ultraviolet irradiance de- rived from SOLSTICE (UARS) observations, in: Proceedings, International Radiation Symposium, TaUinn, Estonia, 1993, in press.

5. G.J. Rottman and T.N. Woods, In-flight calibration of solar irradiance measurements by direct comparison with stellar observations, SPIE Proceedings, 924, 136-143 (1988).

6. J.L. Lean and T.P. Repoff, A statistical analysis of solar flux variations over time scales of solar rotation: 1978-1982, J. Geophys. Res., 92, 5555-5563 (1987).

7. L.A. Hall and G.P. Anderson, Solar irradiance variation near 2075/~, J. Geophys. Res., 89, 9677-9678 (1984).

8. J. London, G.G. Bjarnason and G.J. Rottman, 18 months of UV irradiance observations from the Solar Mesosphere Explorer, Geophys. Res. Lett., 11, 54-56 (1984).

9. C.A. Barth, W.K. Tobiska, G.J. Rottman and O.R. White, Comparison of 10.7-cm radio flux with SME Solar Lyman-alpha flux, Geophys. Res. Lett., 17, 571-574 (1990).

10. L. Chen, G. Brasseur and J. London, The response of middle atmospheric ozone to solar UV irradiance variations with a period of 27 days, in: Proceedings, Quadrennial Ozone Symposium, Charlottesville, Virginia, 1993, in press.