SUMMARY

SOLAR ULTRAVIOLET IRRADIANCE 1982 AND 1983

G. J. Rottman Laboratory for Atmospheric and Space Physics

University of Colorado Boulder, Colorado 80309 USA

The Solar Mesosphere Explorer (SME) has been in operation since October 6th 1981. In addition to making measurements of ozone and other trace const i tuents of the ea rth' s atmosphere the observatory i ncl udes a small spectrometer to make daily measurements of the solar ultraviolet i rradi ance in the spect ra 1 range 115 to 305 nm. The solar spectra are obtained with 0.75 nm spectral resolution. Examination of the data show a strong signature of the 27-day solar rotation displaying a time varing modulation of incoming solar radiation exceeding ±15% near Lyman-a, de­creasing to a few percent at 200 nm, and less than one percent near 300 nm. Long term trends in the data, due both to changes in the instrument response and to true long term solar variations, are removed in order to obtain quantitative measure of the intermediate term variation with time periods of a few days to weeks.

1. INTRODUCTION

The solar ultraviolet radiation below 300 nm is almost completely absorbed in the Earth's upper atmosphere. Absorption of this radiation results in the photodissociation of various atmospheric gases and is thereby a prime component of the complicated upper atmosphere photochem­ical system, including all major ozone production and destruction pro­cesses. It is reasonable to expect that variations in the solar UV pro­duce a di rect and measu rab 1 e response in va ri at ions of upper atmosphere temperature, motions, and concentrations of trace gases. This due to the fact that the solar UV radiation provides the major energy input to the interactive radiative/photochemical/dynamic processes at these levels and because radiation and most photochemical relaxation times are relatively short in the upper stratosphere and mesosphere.

Four limb scanning instruments on the Solar Mesosphere Explorer make measurements of ozone and other minor atmospheric constituents, for ex­ample, H20' and N0 2• A prime objective of the SME is to study the distri­bution and changes in the distribution of mesospheric ozone. To aid in

Ozone Symposium - Greece 1984 -656 -

C. S. Zerefos et al. (eds.), Atmospheric Ozone© ECSC, EEC, EAEC, Brussels and Luxembourg 1985

the interpretation of observed changes in ozone, accurate daily measure­ments of the solar irradiance at Lyman-a and in the spectral interval 175 to 300 nm are required. For this reason a small two channel spectrometer was included in the SME observatory. This scanning spectrometer has 0.75 nm spectral resolution and takes a solar data sample once per rotation of the spacecraft (12 seconds) as the Sun passes through the plane perpendi­cular to a diffusive screen scattering radiation into the spectrometer. To cover the full spectral range the instrument uses two different scat­tering screens, also two separate optical channels each with its own exit slit and photomult i p 1 i er tube detector. The short wavelength channel uses a cesium-iodide photocathode covering the spectral range 115 to 250 nm and the long wavelength channel uses a cesi um-te 11 uri de photocathode for the range 175 to 300 nm. For each calendar day a solar spectrum is pieced together, calibrated and integrated in 1.0 nm intervals using data of the short wavelength channel from 115 to 185 nm and the data from the long wavelength channel above 185 nm. For the discussion that follows these data have been integrated in 5.0 nm intervals in order to improve the signal to noise ratio of the data. Figure 1 is the mean solar irrad­iance in 5.0 nm intervals for all of 1983.

In addition to the pri mary set of scatteri ng screens a separate set of calibration screens are used with a low duty cycle «1% of solar viewing time) and their comparison with the screens used for the dai ly observations provides an estimate of the scattering su rface degradation. The resulting correction to the SME data is moderately small and does not exceed 6% at any wavelength for the en­tire year of 1982 and is considerably less than 1% for 1983. After correcting the SME data for scatteri ng screen degradation there is still a net downward trend at all wavelengths during the 1982 and 1983 period. When the two years of SME

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SME IRRADIANCE (5nm)

100 120 140 160 180 200 220 240 260 280 300

WAVELENGTH (nm)

Figure 1. SME mean solar irradiance spectrum for the entire year 1983.

data are compared to data from three calibration rockets flown on May 17th, 1982, January 13th, 1983 and July 25th, 1983 (2,3,4), the agreement is within one to two percent and well within the error bars of the comparison. Therefore no adjustment to the SME calibration has been made.

2. INTERMEDIATE TERM SOLAR VARIATIONS

Ti me seri es of the SME data at each 5.0 nm i nterva 1 show a 27 -day solar variation superposed on a general downward trend. This downward trend represents an upper limit to the long term solar variability during

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this two year period. The trend is -30% at Lyman-a, -3.7% for 175 to 190 nm, -0.7% for 240 to 260 nm, and -0.2% for 260 to 300 nm. These latter two values are small in comparison with the probable error of the rela­tive measurement (probably one to two percent) and therefore would permit a "constant" or slightly increasing Sun during this two year period.

In order to study the i ntermedi ate term va ri at ions evi dent in the time series, the long term linear trends were first removed from the data. Figure 2 shows three wavelength intervals with the long term trends removed. Throughout most of this two year period the strong 27-day modulation of the irradiance dominates. Near the end of i981 a strong act i vity center on the Sun produced a well defi ned 27 -day vari a­tion and the declining phase of this activity persisted for approximately four rotations into 1982 (5,6). For the following three months activity was more uniformly distributed on the solar disk and no strong 27-day signal is evident. In June 1982 a single solar longitude again became active and a remarkable 27-day signal continued throughout 1982 and into 1983.

The root mean squares (RMS) of the SME solar data with the long term trends removed are given in Figure 3a as a function of wavelength. In general the RMS of these data is somewhat smaller for 1983 than for 1982

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o and above 210 nm it falls to the noise level of the individual measurements (~0.5%). Although § the RMS provides a quantitative ....

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SOLAR IRRADIANCE , - , L YMAN ·n

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of atmospheric phenomenon (un- w less precise solar observations )i +6

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corresponding atmospheric obser- ~~~~ vations, for example, the data sets provided by SME). For comparison, Figure 3b gives the percent solar variation for a single large and well defined rotation period. The period chosen was in July 1982 and is designated by the letters 'A' and 'B I in the Lyman-a time series of Figure 1. This single solar rotation produced a modul­ation approximately five times the RMS devi at i on of the ent ire data set (note: the RMS of Figure 3a represents the plus

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1982 1983

Figure 2. Two year time series of SME data for three wavelength intervals with the long term trend removed.

and minus deviation about the mean and the variation of Figure 3b is the tota 1 peak-to-peak change). London et a 1. (1) analyzed the fi rst 20 solar rotations observed by SME including the 18 rotations from January 1982 throug.h May 1983. Thei r work cal cul ated the percent range (maximum

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to minimum) for each rotation and from these values determined the average percent range as a function of wavelength. The average percent range is also a peak-to-peak value and is closely approximated by three times the RMS variation given in Figure 3a.

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100 120 140 160 180 200 220 240 260 280300

WAVELENGTH (nm)

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WAVELENGTH (nm)

Figure 3a. RMS of SME solar data time series as a function of wavelength. 3b. Total peak-to-peak variation for a typical large 27-day rotation as designated "A" and "8" in Figure 2.

An autocorrelation analyses of the SME irradiance time series show strong peaks at 27 days, 54 days, and 81 days. The magnitude of the 27 day peak in the autocorrelation is approximately 0.5 and fairly constant for wavelengths between 120 and 220 nm. Longward of 250 nm the magnitude of the 27 day peak is lost in the noise of the autocorrelation techni­que. The magni tude of the 54 day peak follows the 27 day peak but is weaker by approximately a factor of two. The anti-correlation at 13.5 day is weak, but nonetheless present from 120 nm to the Al umi num I ab­sorption edge near 208 nm.

3. CONCLUSIONS

Time series of the SME ultraviolet irradiance data for 1982 and 1983 show a strong intermediate term variation, characterized by the 27-day rotation period of the Sun, superposed on a long term downward trend of the data. This downward trend is considered evidence of a solar cycle variation and ranges from a 30% decrease (over the two year period) at Lyman-a, to 1 to _ 2% decrease near 200 nm, and to 0.2% decrease near 300nm. This latteF value is well below the relative accuracy of the measurements but provides an upper limit to the solar variations. After removing the long term trends from the data the intermediate term varia­tions remain. Although the magnitudes of these shorter term variations differ throughout this two year period, general statistical features have be~ deduced. The root mean square deviation of the data sets decreases with increasing wavelength from ±8% at Lyman-a to a value of less than ±I% above 200 nm. The average percent range in these 27 -day rotations (1), exceeds the RMS value by approximately a factor of three.

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ACKNOWLEDGMENTS

The SME project is managed by the Jet Propuslion Laboratory for the National Aeronautics and Space Administration. C.A. Barth is Principal Investigator for SME. The project is supported by JPL Contract 955357. The sounding rocket experiments are supported by NASA Grants NSG 5178 and NAG 5212.

REFERENCES

1. London, J., G.G. Bjarnason, G.J. Rottman, (1984). 18 Months of UV Irradiance Observations from the Solar Mesosphere Explorer. Geophys. Res. Lett., .!!..' 54.

2. Mount, G.H., G.J. Rottman, (1983). The Solar Absolute Spectral Ir-radiance 1150-3173 A: May 17, 1982. ~. Geophys. Res., 88, C9, 5403.

3. Mount, G.K., G.J. Rottman, (1983). The Solar Absolute Spectral Ir­radiance at 1216 A and 1800-3173 A: January 12, 1983. ~. Geophys. Res., ~, C11, 6807.

4. Mount, G.H., G.J. Rottman, (1984). The Solar Absolute Spectral Ir-radi ance 118-300 nm: July 25, 1983. ~. Geophys. Res., in press.

5~ qottman, G.J., C.A. Barth, R.J. Thomas, G.H. Mount, G.M. Lawrence, ·D.W. Rusch, R.W. Sanders, G.E. Thomas and J. London, (1982). Solar Spectral Irradiance 120 to 190 nm, October 13, 1981 - Janu­ary 3, 1982. Geophys. Res. Lett • .2..0 587.

6. Rottman, G.J., (1983). 27-Day Variations Observed in Solar Ultravio­let (120-300 nm) Irradiance. Planet. Space Sci ., 00, 1.

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