Solar UV irradiance measurements: The UARS and EOS SOLSTICE

Phys.Chem. Earth (C).Vol. 25,No. 5-6,pp. 401-404,2000 0 2000 Elsevier Science Ltd.

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Solar UV. Irradiance Measurements: The UARS and EOS SOLSTICE G. Rottman Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80304, USA Received 31 October 1999; accepted 15 March 2000

There is every expectation that the UN&!? and SORCE

Abstract Solar radiation at wavelengths below 300 nm is almost completely absorbed by the Earth’s atmosphere, becoming the dominant direct energy source and playing a major role in atmospheric chemistry and dynamics. Even small changes in this incoming radiation field will have both direct and indirect influences on atmospheric processes, and perhaps will affect climate as well. The SOLSTICE on NASA’s UYRS has made daily measurements since October 1991 and continues operation today. A second generation SOLSTICE is being developed, and it will be launched on NASA’s SORCE mission in July 2002.

observation programs will overlap, providing uninterrupted data series for almost twenty years.Q 2000 Elsevier Science Ltd. All rights reserved

1 Introduction Some of the very earliest space missions included devices to measure solar ultraviolet irradiance, but for the most part these measurement programs lacked the necessary precision and accumcy to record true solar variability over long time periods. The measurement technology has continued to improve, and today reliable measurements extend over time scales up to, and including, the 1l-year solar cycle.

Table 1 Instrument SME SBUV Nimbus-7 NOM-9 NOM-11 NOM-14 UABS SOLSTICE UABS SUSIM EBS-2 GOME ATLAS: S-SBUV SUSIM SOLSPEC Other Shuttle S-SBW Correspondence

to: G.

an

Wavelength Range 115 to 300 mn

Time Interval Oct. 1981 to April 1989

160 to 400 nm 160 to 405 nm 160 to 405 mu 160 to 405 run 119 to 420 mu 115 to 410 nm 240 to 790 run

Nov. 1978 to Feb. 1987 March 1985 to present Feb. 1989 to Oct. 1994 Feb. 1995 to Oct. 1995 Oct. 1991 to present Oct. 1991 to present Apr. 1995 to present Mar. 1992, April 1993, and Nov. 1994

160 to 405 run 115 to 410 nm 200 to 3200 mn

five calibration flights Oct. 1989 - Jan 1996

160 to 405 nm

Rottman 401

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G. Rottman: Solar UV Irradiance Measurements

Table I identifies many of the major missions that have made solar irradiance measurements between 120 and 300 nm during the two most recent solar cycles. The table includes the spectml coverage and dates of operation of the various programs. Typically these instruments are grating spectrometers with varying degrees of preflight and in-flight calibrations. The Upper Atmosphere Research Satellite, UARS, was launched in September 1991 and includes two ultraviolet irradiance instruments - the Solar Stellar Irradiance Comparison Experiment, SOLSTICE, and the Solar Ultraviolet Spectral Irradiance Monitor, SUSIM (Brueckner et al., 1993). Although this report will concentrate on the observations of SOLSTICE, the estimates of short-term and long-term solar variations derived from the data of the two instruments are in close agreement (Floyd et al., 1998).

2 The upper atmosphereresearch satellite UARS operates in a near circular orbit with an altitude of 600 km and an orbit inclination of 57”. The spacecraft carries ten scientific instruments studying the Earth’s atmosphere with special emphasis on stratospheric ozone (Reber, 1990). The instruments measure atmospheric composition, dynamic and energetics. The UARS spacecraft is three-axes stabilized and earth oriented, and therefore the solar instruments are mounted to a sun-pointed platform (SSPP) and they are pointed to the sun with a precision of about one arc minute. During each of the sixteen orbits per calendar day the instruments collect solar data for periods of approximately 35 minutes. Although UARS had a design lifetime of only eighteen months, it now continues to operate after more than eight years. Likewise the solar instruments continue to operate in a nominal fashion. Hopefully the UARS observational program may extend for at least another four to five years.

3 The solar stellar comparisonexperiment SOLSTICEcovers the spectral interval 120 to 420 nm with three separate optical channels, each providing a spectml resolution of between 0.15 and 0.3 mn. Details of the instrument design and calibration are provided in papers by Rottman et al. (1993) and Woods et al. (1993). Basically, the three spectrometers are pointed at the Sun with a small entrance slit (< lOAcm2), followed by a difkxtion grating, and an exit slit establishing a resolving power of about 1000. Behind the exit slits are photomultiplier tubes, each with a cathode tailored to the specific spectml range.

As with all devices of this type, the challenge arises in establishing the on-orbit performance. How does the sensitivity of the spectrometer change during the mission? The UARS SOLSTICE has the unique capability of addressing this issue by observing stars using the very same set of optics and detectors employed for the solar observations. Only apertures and observing time are changed in order to accommodate the nearly eight orders magnitude difference between the solar and stellar irradiance levels - the entrance slit is changed to 2 cm2and the resolving power is decreased to about 30. The stars therefore serve as reference standards, and allow an independent evaluation of instrument degradation. Based on the theory of stellar evolution, the early, blue stars selected for the calibration program are thought to vary by only small fractions of one percent over thousands of years. Moreover the use of many stars (typically 15 to 20 stars) provides a means of identifying and removing pathological (varying) stars. In a broader sense, the SOLSTICE technique of directly comparing the Sun to a set of stars serves as a mechanism of relating today’s measurements from UARS to future solar measurements - assuming that the same solar/stellar technique is used with the same set of reference stars.

4 Solstice observations The SOLSTICE has several modes of operation and a number of full or partial scans of the irradiance spectrum are acquired during a single calendar day. These are accumulated into a single ‘daily” spectrum, and Figure 1 shows a typical spectrum (binned in 1 nm spectml samples). Basically the W spectnun can be approximated as a continuum with superposed absorption features (Fraunhofer lines) at the longer wavelengths and emission lines at shorter wavelengths. In addition to these features a number of distinctive absorption edges, for example the ahnninum edge near 208 nm, also distinguish the solar spectrum. The various portions of the specuum originate at different heights in the solar atmosphere - the longer wavelengths originate in the photosphere, and progressing to shorter wavelengths the emission originates at higher and higher levels in the Sun. Most of the short wavelength emission lines originate in the solar chromosphere.

403

G. Rottman: Solar UV h-radiance Measurements

to the 11-year solar cycle - cycle 22 peaking in 1990-91 andcycle23expectedtopeakin2000-01withaminimum falling between the two. Three examples of the 27day variation are pointed out as A,B,andC. Theratioofpeaktovalleyforeachofthese provides the wavelength dependence of the 27day variation. Although quite similar from one rotation to the next, the wavelength dependence is not always and precisely the same. A typical rotational variation (rotation A) as a function of wavelength is shown in Figure 3, but both the magnitude and the detail shape will vary from one rotation to the next. 200

3do

Wavelength (nm)

L.

.

.

( . . . . , , . . . ( . ., . , . . . . , . .

. , . ,3%

Figure I. Solar Inzdiance: llSto42Onm

5

Solstice time series

If an individual wavelength in Figure 1 is selected, time series during the UARS mission can be constructed. Figure 2 illustrates these data at lsvo wavelengths, one near 160 nm and the second for the feature identified in Figure 1 as Mg II ( see discussion ofthe Mg II index in deToma et al., 1997).

Wavelength (nm) .I8 .$ .I7

Mg II279.6 nm (inde

Figure 3. The wavelength dependence of a “t+aI” solar rotation. For purposes of iktration this ratio at tbe longer wavelengths is ampIiied by a factor of 10 and corresponds to the ordinate scale at the right.

.I6 .I5

i

2.3 2.2 2.1 2.0 1.9 1.8 1994 1992 F&we 2. Time series of solar tiiance;

1996

1998

October 1991 through early 1999.

6 Solar variations Two types of variation are immediately obvious - a higherfrequency variation with a period of about 27days related to the solar rotation, and a much longer term variation decreasing from early in 1992 to late 1996, and then beginning to rise again. ,This long-term variation is reIate4i

The longer term solar cycle variations seen in Figure 2 are still being evaluated. SOLSTICE data reprocessing will be complete late in 1999, and definitive values of the longterm variations will be forthcoming after that time. There is now general agreement that for solar cycle 22 the variation is as large as a factor of two at the shortest wavelengths, decreasing to roughly 10% near 200 nm. Proceeding to wavelengths above 200 mu the solar variability continues to decrease, and at about 300 nm it becomes smaller than the SOLSTICEmeasurement capability of about 1%. 7 The EOS SORCE mission The two UARS instruments will continue to collect reliable data in the spectral region 120 to 200 nm, and with solar variations exceeding several percent at these wavelengths, the UARS instruments are capable ofproviding a definitive measurement of the rise of solar cycle 23. These two

404

G. Rottman: Solar UV Irradiance Measurements

instruments, together with the SBW-2 and GGME instruments, will also provide measurements from 200 to 300 nm. Here the solar variations am only a few percent at the shorter wavelengths, decreasing to only about 1 % near 300 mn. For these observations, the instrument precision and relative accuracies are marginal, and, in the final analysis, ambiguity may remain concerning true solar variability near 300 mn. Considering the wavelength interval 300 to 400 nm, although all four instruments also cover this spectral range and may provide limited information on the intermediate term solar variations, they will not be able to establish the long-term variability over the solar cycle. At wavelengths longward of 300 nm, the solar variations probably do not exceed a few tenths of a percent and present instrument capabilities do not yet achieve the required levels of precision and accuracy to make these measurements. Certainly, models will provide guidance and estimates of solar variability at longer wavelengths, but the actual observations to confirm and constrain the models await the next generation of solar irradiance programs. The next major NASA mission to provide ultraviolet irradiance measurements will be the Solar Radiation and Climate Experiment (SORCE) to be launched in July of 2002. This mission will include a second generation SOLSTICE - an improved instrument similar to the UARS SOLSTICE. In addition, a Spectral Irradiance Monitor (S&l) will extend the spectral range to the visible and near infia-red (-200 nm to 2000 run).

This new long-wavelength channel is a single-element prism spectrometer with an electrical substitution radiometer (ESR) as a detector. The SIM science goal is to realize an absolute accuracy of better that 1% and a relative accuracy (over a five year period) of better than 0.01%. Acknowledgments The UARS SOLSTICE project is supported at the University of Colorado by NASA Contract NAS5-97145and NASA Grant NAG5-6850. The EOS SORCE Mission is supported by NASA Contract NAS5-97045. References Bruecktser, G.E., K.L. Edlow, LE. Floyd, J.L. Lean, and M.E. VanHoosier, The Solar Ultraviolet Spectral Irradiance Monitor (SUSlM) Experiment on Board the Upper Atmospheric Research Satellite &JARS), J. Geophys. Res., 98,10695-10711,1993. De Toma, G., O.R. White, B.G. Knapp, G.J. Rottman, and T.N. Woods, Solar Mg II core&wing index: compa&m of SBW2 and SOSLTICE time series, J. GeophyJ. Res., 102,2597-2610,1997. Floyd, LE., PA Reiser, P.C. Crane, LX. Herring, D.K. Prinz, and G.E. Brueckner, SUSIM UARS measurementp ofsolar cycle 22 W indance variability, SoZarPhysics, 177,79-87.1998. Reber, jC., The Upper Atmosphere R-h Satellite &JARS), Geqhy. Res. Letters, 20,1215-1218.1990 Rottman, G.J., T. N. Woods, and T.P. Spam, Solar Stellar Irradiance Comparison Experiment I: 1 Instrument Design and Gperation, J. Geophys. Res.. 98,10,667-10,677,1993.

Woods, T.N., G.J. Rottman, and G. Ucker, Solar stellar Irradiance Comparison Experiment I: 2 Instrument Calibration, J. Geophys. Res., 98, 10,679-10,694. 1993.