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SCIENCE DIRECT-

doi: lO.l016/SO273-1177(03)00148-O

SUSIM’S ll-YEAR OBSERVATIONAL RECORD OF THE SOLAR UV IRRADIANCE

L.E. Floyd’, J.W. Cook2, L.C. Herring’, and P.C. Crane”

‘Interjerometrics Inc., 14120 Parke Long Court, Suite 103, Chantilly, VA 20151, USA 2E 0 Hulburt Center for Space Research, Naval Research Laboratory, Washington, DC 20375, USA . .

3 Remote Sensing Division, Naval Research Laboratory, Washington, DC 20375, USA

ABSTRACT

The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM), a wavelength-scanning, dual-dispersion, dual- spectrometer instrument aboard the Upper Atmosphere Research Satellite (UARS), has measured the solar ultraviolet (UV) spectral irradiance (115-410 nm) since October 1991. This 11-year period, the duration of a solar activity cycle, extends from a late secondary maximum of solar cycle 22 through the intervening solar minimum and the maximum of solar cycle 23. Accordingly, SUSIM observed nearly the entire maximum-to- minimum variation of the solar UV irradiance of both solar cycles. The UV irradiance variations during the two solar cycles are compared. Apart from solar rotation effects and to within experimental accuracy, they show similar variation in the UV spectral irradiance. Solar cycle amplitudes calculated after removing solar rotation effects were -50% for the strong 0 I, C II, and Si IV emission features below 145 nm, -S-18% between the Al edge and 145 nm, respectively, and ~4% between the Al edge and 263 nm. The amplitude of the solar cycle periodicity was not detected above -300 nm. 0 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.

BACKGROUND

Accurate observations of solar UV spectral irradiance must inevitably be made from above the Earth’s absorbing atmosphere. Systematic measurements of the solar UV spectrum began roughly three decades ago; Floyd et al. (2002a) and Rottman et al. (2003) provide recent reviews of this subject. It is common for uncertainties resulting from incompletely understood changes in the measuring instruments’ responsivities to severely affect those measurements. Exposure to the solar UV and EUV radiation are understood to cause these responsivity changes. Meanwhile, questions about the fate of Earth’s ozone layer led NASA in 1991 to launch the UARS, an experimental mission devoted to the study of stratospheric and mesospheric processes affecting ozone (Reber et al., 1993). The solar UV irradiance is of central importance to these middle atmospheric processes because ozone is both created and destroyed by its various spectral components. To enhance the accuracy and reliability the solar UV irradiance measurements made by UARS over the long term, instrumental calibration, two UARS solar UV experiments of differing design were selected. Both the Solar-STellar InterComparison Experiment (SOLSTICE) and SUSIM carry their own means of calibration (Rottman et al., 1993; Brueckner et al., 1993). SUSIM carries four stable deuterium lamps and makes solar and lamp measurements using redundant optical channels made up of elements having differing exposures.

As of October 2002, both UARS solar UV instruments are still operating, having just completed an unprecedented 11 years of measurements. Brueckner et al. (1996) and Floyd et al. (1998a,1999,2002b) have described earlier versions of the SUSIM data. In this paper, we present and analyze the SUSIM V21 irradiance data set, which extends from 12 October 1991 through 27 April 2002.

Adv. Space Res. Vol. 31, No. 9, pp. 211 l-2120.2003 Q 2003 COSPAR. Published by Elsevier Ltd. All rights reserved Printed in Great Britain 0273-l 177/03 $30.00 + 0.00

2112 L. E. Floyd et al.

02 lamp spectrum

wavelength (nm)

Fig. 1. Measured UV spectral irradiance of the sun (solid line) on 2 February 1992 at 1.1 nm resolution and of a deuterium lamp (dashed line) at 5 nm resolution. Prominent solar spectral features are identified.

SUSIM EXPERIMENT

Figure 1 displays a solar UV irradiance spectrum at 1.1 nm resolution measured by SUSIM. To the lowest order of appromation, it is that of a blackbody of about 5770 K. Descriptions and explanations of the origin of the solar UV irradiance spectrum are found in Avrett (1998) and Foukal (1990). Also shown is the spectrum of one of the deuterium lamps at 5 nm resolution. A full account of SUSIM instrument and initial operations was given in Brueckner et al. (1993). The following brief summary updates that description.

Events Daily solar scans at mid (1.1 nm) resolution and weekly solar scans at high (0.15 nm) resolution were

conducted whenever possible. An example when no scans were taken was during the investigation into problems with the UARS solar array drive clutch. This caused a 38-day continuous outage in June and July of 1992. The solar array drive was permanently “parked” in a single position beginning in March 1995. Afterward, SUSIM was usually turned off for periods of 3-7 days on an average of once every 35 days when the solar array is poorly illuminated in order to conserve spacecraft power. Through October 2002, SUSIM performed mid and high resolution UV spectral scans (115-410 nm) for 89% and 10% of all days, respectively.

Hardware problems ended the continuous telemetry data storage provided by the two UARS telemetry tape recorders in October 1999. Because SUSIM itself has no capability for storage of irradiance data, after this time, irradiance measurements were scheduled only when telemetry contacts to the UARS were available through NASA’s Tracking and Data Relay Satellite System (TDRSS). Telemetry contacts via the TDRSS are not continuous, but rather occur in discrete time segments. Solar measurements must be acquired during spacecraft day and when the Sun is not obscured by the UARS solar array. Also, to reduce noise, measurements are not taken while UARS itself passes through the South Atlantic Anomaly. As a result of these practical constraints, SUSIM’s daily scans must be gathered in more numerous and shorter wavelength segments. These constraints also occasionally limit the types of daily scans that can be done and have nearly eliminated the weekly high resolution scans. On 14 March 1999, the grating pair and short wavelength MgFs entrance filter used in obtaining the daily irradiances were changed because of low signal levels below 160 nm. Before 27 November 1991, solar pointing errors were typically much greater than 2 arcmin. After July 2000, the UARS Earth sensors and star trackers could no longer be reliably used to maintain spacecraft attitude; this severely degraded the solar pointing which was until then done open loop. Starting in January 2002, UARS solar pointing used the more effective closed-loop method which has improved the pointing error performance to much better than 1 arcmin.

SUSIM’s 1 l-Year Solar UV Record 2113

Responsivity and its degradation The correct determination of the responsivity throughout the duration of the SUSIM experiment is cen-

tral to the task of accurately measuring UV spectral irradiances. A summary of the evolution of various wavelength-dependent responsivities in the SUSIM instrument is given by Floyd et al. (1998b). Prinz et al. (1996) describe the corresponding in-flight evolution of the output from SUSIM’s deuterium lamps. Floyd (1999) describes the loss of responsivity in the working channel. DeLand et al. (2003) show the lamp measurements of the changing responsivity of the biannual reference. channel.

After launch, it was soon apparent that measurements by the working channel of the deuterium lamps do not accurately measure for its responsivity degradation. The reason for this is understood to be that the footprint of UV exposure on the surfaces of the gratings is not the same for solar and lamp illumination. This nonuniform exposure of the gratings also increases the sensitivity to solar pointing errors. Because this effect is related to solar exposure, it is smaller for the infrequently used reference channels. Accordingly, the scans of the lamps are used to calibrate the reference channels, whose calibration is transferred to the working channel (Floyd et al., 1996). S everal reference channels are now scanned on monthly, half-yearly, yearly, and longer cadences.

Data The standard SUSIM data product (called L3BS) consists of UV solar indices (e.g., Mg II) and 1-nm

integrated irradiances (centered on the half nm) for each day. This can be obtained from NASA’s GSFC Distributed Active Archive Center (DAAC) or from SUSIM’s FTP site (Floyd and Esfandiari, 2003). Al- ternatively, measured irradiances at both 1.1 nm and 0.15 nm resolutions on instrumental wavelengths (i.e., level 2 data) can be obtained from the latter location.

The latest and sixth release (V21) of the SUSIM UV spectral irradiance data extends from 11 October 1991 to 27 April 2002. By contrast, the Mg II index (version V21r2) currently extends beyond this, to 12 November 2002. New values of the SUSIM Mg II index time series are made available usually a few days after the underlying measurements are gathered. In order to keep the index current, the calculation is made based on extrapolated responsivity projections. This can accurately be done for the Mg II index and not the underlying spectral irradiances because of its relative insensitivity to responsivity errors as is explained below. Version 21 is considered to be provisional version since the full analysis of lamp and solar calibration data is not yet complete. At the conclusion of this analysis and after production and validation, we expect that a V22 version will be released sometime in mid-2003. The level of change between versions may well be substantial, especially for Ly-a.

Comparisons Intercomparisons among the UARS and shuttle-based experiments over limited time periods have been

performed (Woods et al., 1996; Cebula et. al, 1996) and solar UV reference spectra based on these data have been generated (Thuillier et al., 2003). DeLand and Cebula (1998) previously compared UV irradiance time series from SUSIM and SOLSTICE for an earlier version of these data. DeLand et al. (2003) compare version V21 of the SUSIM data with the latest (V18) from SOLSTICE.

Several recent experiments have begun or will begin soon. Most important of these follow-on UV irradiance experiments are the Spectral Irradiance Monitor (SIM) and an improved version of the SOLSTICE both aboard the SORCE (Solar Radiation and Climate Experiment) satellite that is now expected to launch in January 2003 (Harder et al., 2000; McClintock et al., 2000). Earlier in 2002, the TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics) SEE (Solar EUV Experiment) began making observations of the far UV UP to 200 nm (Woods et al., 1998). Likewise, SCIAMACHY aboard Envisat-1 began solar UV measurements above 240 nm (No61 et al., 1998). Measurement comparisons with these experiments may help to validate and diagnose both the SUSIM and the other experimentss. In particular, 1994 was the year of the last irradiance comparison of a well-calibrated experiment with the UARS solar UV instruments,

UV SPECTRAL IRRADIANCE TIME SERIES

Displayed in Figure 2 are representative SUSIM solar UV irradiance time series for Ly-a, the Mg II core-to- wing ratio index, and irradiances integrated over the 170-175 nm, 200-205 nm, and 230-235 nm wavelength

2114 L. E. Floyd et al.

ranges. The integrated time series are derived from the level 2 1.1 nm resolution spectral irradiances. For wavelength regions where continuum radiation dominates, we compare UV irradiances integrated over several nm in wavelength. Below 142 nm, the integration regions are organized around the strong emission line features, Ly-cw, 0 I, C II, and Si IV. The long-term l-a errors for the V21 spectral irradiances are roughly estimated to be 2% above about 170 nm, 5% for 142-170 nm, 12% for the latter three emission features, and 30% for Ly-a. Woods et al. (1996) presents more comprehensive error analyses for an earlier version of the SUSIM data. The Mg II core-to-wing ratio extends beyond the last major calibration since it can be accurately calculated without accurate responsivities.

The time series begin during the latter stages of the solar activity maximum in 1991, the descent to solar minimum in 1996, and extending through the following solar maximum in 2000-2001. They show similar behavior dominated by two periodic variations: that of the solar rotation (-27 day) and that of the solar activity cycle (~11 year). The period from November 1994 through April 1995 provides a good example of short-term time series behavior that differs significantly for different wavelength regions. Variations in the solar UV spectral irradiance are understood to be caused by bright regions which are created, decay, and rotate across the solar disk. Differences from time to time in the number, size, contrast, and distribution active regions cause the observed solar UV irradiance variations.

“’ I” ” ‘I” “’ I”” ‘I ” I” I’ ““l’l “‘I”’ ” I’ “‘11”“‘1”“~ Lyman a lrradiance (mW/m’)

200-205 nm Integrated lrradiance (mW/m’)

250 230-235 nm Integrated lrradiance (mW/m*)

SUSIM Mg II Core-to-Wing Ratio (V21r2)

0.26 -

0.25 ~~~IIIIIIIII~IIIIIIIIJIIIIIIIIIIIIIIIIIIIII~IIIII~IIIII~~IIIIIIIIII_ 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

Year

of

Fig. 2. Representative UV integrated irradiance time series as measured by SUSIM for Ly-a, 170-175 nm, 200-205 nm, and 230-235 nm, and the Mg II core-to-wing ratio index.

SUSIM’s 1 l-Year Solar UV Record 2115

Mg II Core-to-Wing-Ratio Index The Mg II absorption feature near 280 nm is a result of a complex blend of a pair of overlapped pho-

tospheric absorption lines and a pair of highly variable chromospheric emission lines. The Mg II index is loosely defined to be the ratio of irradiance at the center of the Mg II feature to that of its adjoining wings (Heath and Schlesinger, 1986). Of course, this definition of the Mg II index is flexible allowing for countless possible algorithms. Because the solar variation found in the photospheric wings is relatively small, the index is very sensitive to changes in solar activity. Use of the irradiance ratio effectively addresses the uncertainty in responsivity because for typical instruments the responsivities errors cancel out in the ratio.

The Mg II index serves as an effective proxy for the variations in UV irradiance over a. wide range of wavelengths (e.g., Cebula and DeLand, 1998). For most wavelengths and especially for time periods longer than a few solar rotations, an approximately linear relationship is has been found to exist between solar UV irradiance and the Mg II index time series. The level of the correspondence between the Mg II index and its value as a spectral irradiance proxy remains an active research area. Comparisons with the Mg II index also provide a convenient way to characterize the variations in a given line or in a given wavelength range as is demonstrated elsewhere in this volume (DeLand and Cebula, 1998).

The bottom panel of Figure 2 displays the SUSIM V21r2 Mg II core-to-wing-ratio index time series, which extends from 11 October 1991 to 12 November 2002. Inaccuracies in this time series is caused by several factors and can be divided into absolute, long-term, and short-term components. The primary cause of uncertainty in the absolute level of the index is the instrument function and its wavelength resolution. Because the index is used only as a relative measure, the absolute uncertainty is neither relevant nor estimated. Errors in instrumental responsivity and stray light can cause long-term errors in the index. The algorithm by which the Mg II index is calculated is designed to eliminate responsivity errors of up to first order in wavelength. Error analysis studies show very little sensitivity to errors in responsivity and an uncertainty of about 2% (1-o) of the maximum-to-minimum solar cycle variation due to errors in stray light.

It is interesting to note that while sunspot number peaked in 2000, the peak in the Mg II index occurred in very early 2002. The sunspot number is a derived quantity based on human counting of dark solar surface features while the Mg II index is a ratio based on direct spectral irradiance measurements. Other than an observed overall long-term correlation, there is no physical description that explicitly links sunspot number with the Mg II index. Accordingly, it is not unexpected that the sunspot number and the Mg II index will significantly deviate from one another from time to time. Differences among solar indices and their proxy relationships to solar total and spectral irradiance are currently actively studied (Pap, et a,l. 2002; Sprigg and Pap: 2003).

Not surprisingly, Mg II indices derived from different UV irradiance experiments have also been found to be linearly related to one another. For example, separate fits of the NOAA-9 Mg II index fcJr 1991-1994 or of the GOME Mg II index (Weber et al., 1998) for 199552002 to the SUSIM V21r2 index give correlation coefficients in excess of 0.99. This characteristic has led to the construction of composite Mg II indices that now extend 24 years, more than two solar activity cycles in length from data from several experiments. In this paper, we use a SUSIM composite time series formed using the Nimbus-7/NOAA-9 (type 1 from Viereck and Puga, 1999) and the SUSIM V21r2 Mg II indices. The SUSIM V21r2 composite index is displayed in Figure 3. Details on its construction are given in Floyd et al. (2002b). Note that the highest daily peak of Mg II of either solar cycle occurs in solar cycle 22. Since the Mg II index time series is dominated by solar cycle and solar rotation periodicities, we can send it through a ‘low pass” filter to remove the latter periodicity. An 81-day FWHM Gaussian filter was chosen for this purpose, In contrast to the daily result. the solar cycle amplitude as revealed in the 81-day Gaussian filtered index reached a slightly higher level in solar cycle 23 than it did in solar cycle 22.

Ly-a! irradiance During the descent from the peak of solar cycle 22, the Ly-o irradiance declined by more than 56%.

The ascent during solar cycle 23 is not nearly as large. DeLand et al. (2003) compare the SUSIM Ly-o irradiance with that of SOLSTICE. They find that the two time series are in approximate agreement during solar cycle 22, but that the SUSIM Ly-cr irradiance trends lower during solar cycle 23. Close examination

2116 L. E. Floyd et al.

0.290 0.285

0.280

0.275

0.270

0.265

1980 1985 1990 1995 2000

Fig. 3. Daily values of SUSIM V21r2 Composite Mg II core-to-wing ratio index. The thick line denotes the same filtered with a centered 81-day FWHM Gaussian function.

of the top and bottom panels of Figure 2 shows that while the short-term variations in the Ly-o irradiance correspond to that of the Mg II core-to-wing ratio index, the longer term variation is quite different. While the peaks of the Mg II index during solar cycles 22 and 23 are about the same height, for Ly-cw, the cycle 23 peak is much lower than that of cycle 22. The responsivity degradation for wavelengths below 130 nm is more intense than in any other wavelength region. Accordingly, the uncertainty in the Ly-CY irradiance measurements is also large. Currently, time- and wavelength-dependent determinations of the responsivity degradation both in the lamps and in the reference channels are still being analyzed. Preliminary results indicate that the degradation in the lamp output at wavelength of Ly-cu was overestimated and that the Ly-cu iriadiance reached similar peak levels during solar cycles 22 and 23. Such a result would agree well with the Ly-a results of Woods et al. (2000).

SOLAR CYCLE UV IRRADIANCE VARIATION

An important goal of SUSIM was to measure the variation of the solar UV irradiance over a solar cycle time frame. Fortuitous timing of the start of the experiment allowed SUSIM (and SOLSTICE) to measure nearly all of the decline of one solar cycle followed by the entire rise of another in only an 11-year period. In early February of 1992, approximately six months after the start of SUSIM UV measurements, a secondary peak of the solar cycle 22 maximum was reached. According to the Mg II index metric, this 1992 solar activity peak was only slightly lower than the levels achieved earlier in solar cycle 22 (see Figure 3). Although the entire variation of solar cycle 22 was not measured, the composite Mg II index can be employed to extend the measurements backward in time to estimate this variation. Now that SUSIM has now also taken measurements the solar cycle 23 maximum, a comparison of the variation over the two solar cycles can be made. The results of these analyses are displayed in Figure 4.

We estimate the actual solar cycle 22 variation by linear least-squares fitting the irradiance in each wavelength range with the SUSIM V21r2 composite Mg II index. The lone exception was the Ly-a irradiance which was fit using separate parameters for the long- and short-term components of Mg II as described in Floyd et al. (1997). The Mg II index fits are then used to estimate the UV spectral irradiance variation in solar cycle 22. Only Mg II and UV irradiance data from cycle 22 were included in these fits that are used to generate the cycle 22 variation, thus preventing influence by the behavior during solar cycle 23. Two measures of solar cycle variation are computed. The peak-to-peak variation is calculated by comparing the peak of the fitted irradiance of solar cycle 22 (early 1991) with the minimum of the corresponding fitted irradiance in 1996. The other measure of solar cycle variability that we consider is the amplitude of the solar cycle periodicity. To estimate this, the fitted integrated irradiance is passed through a Gaussian filter having a 81-day FWHM of with the purpose of eliminating the effect of solar rotation and any other higher frequency periodicities. A filter width of 81 days (three times the duration of a solar rotation as observed from the Earth) was chosen to ensure that virtually all solar rotation effects are removed without significantly altering the solar cycle dependence. A comparison of the the solar cycle 22 maximum and

SUSIM’s 1 l-Year Solar UV Record 2117

- Cycle 22 Peak-to-Peak Cycle 23 Peak-to-Peak Cycle 22 Amplitude Cycle 23 Amplitude

+ Si Edges + 30

ti 8 5 20 a

10

140 160 180 200

Fig. 4. The wavelength-dependent solar UV irradiance variation for solar cycles 22 and 23 based on version V21 of the SUSIM UARS measurements given as the percentage increase from solar minimum. The peak-to-peak variation is estimated using the fits of the composite Mg II index to the time series of the integrated irradiance for each wavelength bin. The solar cycle amplitudes are based on 81-day Gaussian smoothing where short-term effects such as solar rotation are effectively removed.

2118 L. E. Floyd et al.

minimum of this smoothed function generates the amplitude of the solar cycle 22 modulation. For solar cycle 23, the measured irradiances are available for the minimum between solar cycles 22 and

23 and the maximum of cycle 23. So, in addition to the two calculations for solar cycle 23 made using the same method as was used for solar cycle 22, we calculate the amplitude of the solar cycle periodicity using the measured integrated UV irradiances themselves. For each wavelength region, the integrated irradiance

150 200 250 300 wavelength (nm)

Fig. 5. The percentage difference between the two estimates of the amplitude of the solar cycle 23 variation with short-term effects removed. One estimate is 81-day Gaussian of the integrated (measured) irradiances in each wavelength bin. The other is obtained by finding the maximum and the minimum of the 81-day Gaussian smoothed linear least squares fit of the Mg II index to the integrated irradiances in each wavelength region.

time series is passed through the 81-day FWHM Gaussian filter. As before, comparison of the minimum (in 1996) and maximum (at the end of 2002) of the smoothed time series generates the amplitude of the solar cycle modulation. The peak-to-peak variation based on the measured irradiances was not calculated to avoid corruption of the results stemming from “noise” in the daily spectral irradiance data.

Figure 4 shows the results for wavelengths from Ly-cx to 320 nm. Although SUSIM makes daily measure- ments up to 410 nm, no solar cycle variation above instrumental uncertainty has yet been directly detected from the measurements for 300-410 nm. The percentage difference between the solar cycle 23 amplitude calculated from the Mg II fitted irradiances and from the measurements themselves is displayed in Figure 5. Note that the two methods are in agreement to less than 1% for 160-310 nm and to less than 10% for shorter wavelengths. This validates the use of the Mg II model irradiances for wavelengths above 160 nm for use in variability estimates for solar cycle 22.

The strongest variations are found at the shortest wavelengths and in emission lines. For example, the 0 I (triplet), C II (doublet), and S IV (doublet) emission features have solar cycle amplitudes of about 50%. For obvious reasons, the peak-to-peak variations are larger than the solar cycle amplitude alone. In solar cycle 22, the solar cycle amplitude represents a smaller portion of the the peak-to-peak variations than it does in solar cycle 23, indicating a stronger effect of solar rotation at the solar cycle peak of the former. The abrupt changes in solar cycle variation at the Al and perhaps the Mg edges reveal the corresponding abrupt changes in the emission height in the solar atmosphere. Given the long-term l-a errors, the solar cycle amplitudes of the UV irradiance variations of solar cycles 22 and 23 are seen to be of comparable strength for most wavelengths. From the Al-edge (~208 nm) to about 263 nm, the solar cycle amplitude represents roughly 4% of the solar minimum irradiance. This amplitude is about 8% at 205 nm shortward of the Al-edge and rises to about 18% at -145 nm. For Ly-cr, the solar cycle 22 variation is substantially larger than in solar cycle 23, but a recent analysis indicates that this is an instrumental effect.

SUMMARY and FUTURE PLANS

SUSIM UARS UV spectral irradiance time series for selected wavelength ranges from October 1991 to April 2002 are presented. These reported UV irradiances are provisional as the responsivity calibrations on

SUSIM’s 1 l-Year Solar UV Record 2119

which these were based are still being developed. The solar cycle variation as a function of wavelength is estimated for solar cycles 22 and 23. To within instrumental accuracy and after solar rotation effects are removed, the amplitudes of the solar cycle UV irradiance variation for solar cycles 22 and 23 are comparable at most wavelengths. The actual maximum UV irradiance is somewhat higher for solar cycle 22 except for 140-180 nm wavelengths.

SUSIM UARS still makes its daily measurements at nearly its original level of capability. The experiment will continue into 2003 as a part of the UARS Traceability Mission. Its goal is to extend the UARS datasets until follow-on irradiance and constituent measuring missions have begun. Long-term, multi-decadal datasets such as for the solar UV irradiance are believed to be crucial in our understanding of the terrestrial atmosphere and climate. Long-term datasets not only provide required input for terrestrial models, but also provide the basis for an understanding of solar mechanisms. Only through better understanding the underlying mechanisms of solar variability will we be able to anticipate the ranges of possible future solar behavior.

ACKNOWLEDGEMENTS

With sincere and profound regret, the authors acknowledge the passing of Dr. Dianne K. Prinz, our dear colleague, mentor, and SUSIM Principal Investigator 1998-2000, after an extended illness. She contributed in large measure to the success of SUSIM and is deeply missed. We thank NOAA/SEC for making their Mg II index data available. We also thank the two referees for thorough and thoughtful suggestions. This work was supported by NASA-Defense Purchase Request S-44722-G.

REFERENCES

Avrett, Eugene H., Modelling Solar Variability-Synthetic Models, Solar Physics, 177! 449.-469, 1998. Brueckner, G.E., K.L. Edlow, L.E. Floyd IV, J.L. Lean, and M.E. VanHoosier, The Solar Ultraviolet Spectral

Irradiance Monitor (SUSIM) experiment on board the Upper Atmosphere Research Satellite (UARS), J. Geophys. Res., 98, 10695-10711, 1993.

Brueckner, G.E., L.E. Floyd, P.A. Lund, D.K. Prinz, and M.E. VanHoosier, Solar ultraviolet spectral- irradiance observations from the SUSIM-UARS experiment, Metrologia, 32, 661-665, 1996.

Cebula, R.P., G.O. Thuillier, M.E. VanHoosier, and E. Hilsenrath, Observations of the solar irradiance in the 200-350 nm interval during the ATLAS-l mission: a comparison among three sets of measurements - SSBUV, SOLSPEC, and SUSIM, Geophys. Res. Lett., 23, 2289-2292, 1996.

Cebula, R.P., and M.T. DeLand, Comparisons of the NOAA-11 SBUV/2, UARS SOLSTICE, and UARS SUSIM Mg II Solar Activity Proxy Indexes, Solar Physics, 177, 117-132, 1998.

DeLand, M.T. and R.P. Cebula, NOAA 11 Solar Backscatter Ultraviolet, model 2 (SBUV/2) instrument solar spectral irradiance measurements in 1989-1994. 2. results, validation, and comparisons. .1. Geophys. Res., 103, 16251-73, 1998.

DeLand, M.T., L.E. Floyd, G.J. Rottman, and J.M. Pap. St,atus of Solar UV Irradiance Data, Adv. Space Res., this issue, 2003.

Floyd, L.E., L.C. Herring, D.K. Prinz, and G.E. Brueckner, Maintaining calibration during the long term space flight of the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM), SPIE Proceedings. 2831, 36-47, 1996.

Floyd, L., G. Brueckner, P. Crane, D. Prinz, and L. Herring, Correlations of Solar Cycle 22 UV Irradiance, Proc. 31st ESLAB Symp., Correlated Phenomena at the Sun, in the Heliosphere and in Geospace, ESTEC, Noordwijk, The Netherlands, ESA SP-415, 235-242, 1997.

Floyd, L.E., P.A. Reiser, P.C. Crane, L.C. Herring, D.K. Prinz, and G.E. Brueckner. Solar Cycle 22 UV spectral irradiance variability: current measurements by SUSIM UARS, Solar Physics, 177, 79%87, 1998a.

Floyd, L.E., L.C. Herring, D.K. Prinz, and P.C. Crane, Instrument responsivity evolution of SUSIM UARS, SPIE Proceedings, 3427, 445-456, 199813.

Floyd, Linton, Filter responsivity degradation caused by solar UV exposure. Adv. Space Res., 23. 1459-1462, 1999.

Floyd, L.E., D.K. Prinz, P.C. Crane, L.C. Herring, and G.E. Brueckner, Measurements of solar ITV irradiance

2120 L. E. Floyd et al.

variation, Adv. Space Res., 24, 225-228, 1999. Floyd, Linton, W. Kent Tobiska, and Richard P. Cebula, Solar UV irradiance, its variation, and its relevance

to the Earth, Adv. Space Res., 29, 1427-1440, 2002a. Floyd, LE., D.K. Prinz, P.C. Crane, L.C. Herring, Solar UV irradiance variation during cycles 22 and 23,

Adv. Space Res., 29, 1957-1962, 2002b. Floyd, L.E., and A.E. Esfandiari, f tp : //susim. nrl .navy .mil/pub/uars, 2003. Foukal, Peter, Solar Astrophysics, John Wiley & Sons, New York, 1990. Harder, J., G. Lawrence, G. Rottman, and T. Woods, The spectral Irradiance Monitor (SIM) for the SCRCE

mission, SPIE Proceedings, 4135, 204-214, 2000. Heath, D.F., and B.M. Schlesinger, The Mg 280-nm doublet as a monitor of changes in solar ultraviolet

irradiance, J. Geophys. Res., 91, 8672, 1986. McClintock, W.E., G.J. Rottman, and T.N. Woods, SOLar Stellar Irradiance Comparison Experiment

II (SOLSTICE II) for the NASA Earth Observing System’s Solar Radiation and Climate Experiment Mission, SPIE Proceedings, 4135, 225-234, 2000.

Noel, S., H. Bovensmann, J. P. Burrows, et al., The SCIAMACHY instrument on ENVISAT-1, SPIE Proceedings, 3498 94-104, 1998.

Pap, J.M., M. Tmmon, L. Floyd, C. Frohlich, and Ch. Wehrli, Total Solar and Spectral Irradiance Variations from Solar Cycles 21 to 23, Adv. Spa. Res., 29, 1923-1932, 2002.

Prinz, D.K., L.E. Floyd, L.C. Herring, and G.E. Brueckner, On-orbit performance of deuterium calibration lamps during four years of SUSIM operations on UARS, SPIE Proceedings, 2831, 25-35, 1996.

Reber, Carl A., Charles E. Trevathian, Robert J. McNeal, and Michael R. Luther, The Upper Atmosphere Research Satellite (UARS) M ission, J. Geophys. Res., 98, 10643310647, 1993.

Rottman, Gary J., Thomas N. Woods, and Thomas P. Sparn, Solar-Stellar Intercomparison Experiment 1 1. Instrument Design and Operation, J. Geophys. Res., 98, 10667-10677, 1993.

Rottman, Gary, Linton Floyd, and Rodney Viereck, Measurement of the solar ultraviolet irradiance, in Solar Variability and its Effect on the Earth’s Atmosphere and Climate System, edited by J. M. Pap, P. Fox, C. Frohlich, H.S. Hudson, J. Kuhn, J. McCormack, G. North, W. Sprigg, S.T. Wu, American Geophysical Union, Washington, DC, in press, 2003.

Sprigg, William, and Judit M. Pap, Science Requirements and Required Future Measurements, op. cit., 2003.

Thuillier, Gerard, Linton Floyd, Thomas N. Woods, et al., Solar Irradiance Reference Spectra, op. cit., 2003.

Viereck, R. and L.C. Puga, The NOAA Mg II core-to-wing solar index: construction of a 20-year time series of chromospheric variability from multiple satellites, J. Geophys. Res., 104, 9995-10005, 1999.

Weber, Mark, John P. Burrows, and Richard P. Cebula, GOME solar UV/VIS irradiance measurements between 1995 and 1997 - first results on proxy solar activity studies, Solar Physics, 17'7, 63-77, 1998.

Woods, T.N., D.K. Prinz, G.J. Rottman, et al., Validation of the UARS solar ultraviolet irradiances: com- parison with the ATLAS 1 and 2 measurements. J. Geophys. Res., 101, 9541-9569, 1996.

Woods, T., F. Eparvier, S. Bailey, et al., TIMED Solar EUV Experiment, SPIE Proceedings, 3442, 180, 1998.

Woods, T.N., W.K. Tobiska, G.J. Rottman and J.R. Worden, Improved solar Lyman alpha irradiance modeling from 1947 through 1999 based on UARS observations, J. Geophys. Res., 105, 27195, 2000.

Email address of L.E. Floyd linton.floyd@nrl.navy.mil Manuscript received 19 October 2002; revised 3 February 2003; accepted 5 Feburary 2003