Status of UARS solar UV irradiance data

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<ul><li><p>ar</p><p>b,</p><p>Green</p><p>e Lon</p><p>sics/U</p><p>ht Ce</p><p>ed for</p><p>Abstract</p><p>mospheric photochemistry and heating. Direct mea-</p><p>surement of these variations can only be made from</p><p>space. Solar UV activity variations on rotational time</p><p>scales (approximately 27 days) have been observed since</p><p>directly measure solar UV irradiance variations over a</p><p>The Upper Atmosphere Research Satellite (UARS)</p><p>was launched in September 1991 to simultaneously</p><p>monitor many aspects of the Earths atmosphere and its</p><p>energetic inputs. Two instruments on UARS are dedi-</p><p>cated to measure solar UV irradiance: the Solar Ultra-</p><p>violet Spectral Irradiance Monitor (SUSIM) (Brueckner</p><p>et al., 1993) and the Solar Stellar Intercomparison Ex-</p><p>ch 34* Corresponding author. Tel.: +1-301-867-2164; fax: +1-301-867-The existence of variations in solar activity at visible</p><p>wavelengths through changes in sunspot number hasbeen known for centuries. More recently, solar vari-</p><p>ability on time scales from months to years has been</p><p>observed in radio ux measurements (Tapping, 1987)</p><p>and near-IR absorption line width measurements</p><p>(Harvey and Livingston, 1994). Knowledge of solar</p><p>variations at ultraviolet (UV) wavelengths is critically</p><p>important for understanding the Earths atmosphere</p><p>and climate system because of its dominant role in at-</p><p>complete solar cycle with a single instrument have cov-</p><p>ered only a portion of the activity cycle (e.g. Nimbus-7Solar Backscatter Ultraviolet spectrometer (SBUV)</p><p>(DeLand and Cebula, 2001); Solar Mesosphere Explorer</p><p>(SME) (Rottman, 1988); NOAA-11 SBUV/2 (DeLand</p><p>and Cebula, 1998a)). The NOAA-9 SBUV/2 instrument</p><p>observed the entire range of solar cycle 22 from March</p><p>1985 to February 1997, but does not currently have an</p><p>adequate long-term calibration (DeLand and Cebula,</p><p>1998b).Accurate measurement of solar ultraviolet (UV) irradiance variations is important for understanding both solar processes and</p><p>energetic input to the Earths atmosphere. Satellite instruments are capable of providing such data, but must correct for signicant</p><p>spectral and temporal response changes during the observing lifetime of the instrument. The Upper Atmospheric Research Satellite</p><p>(UARS) carries two instruments dedicated to monitoring long-term solar UV irradiance: the Solar Ultraviolet Spectral Irradiance</p><p>Monitor (SUSIM) and the Solar Stellar Intercomparison Experiment (SOLSTICE). Both instruments include comprehensive on-</p><p>board calibration systems designed to characterize and remove response changes from the irradiance data. This paper presents the</p><p>status of the SUSIM and SOLSTICE data sets. We nd that both instruments produce meaningful direct estimates of solar cycle UV</p><p>irradiance variations in the wavelength range 120250 nm. Between 250 and 300 nm, the reduced magnitude of solar variability</p><p>becomes comparable to the long-term calibration uncertainty. Longward of 300 nm, solar cycle irradiance variations cannot be</p><p>detected. The SUSIM and SOLSTICE irradiance data sets represent the rst fully calibrated solar UV irradiance data sets to cover a</p><p>complete solar cycle.</p><p> 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.</p><p>Keywords: Solar UV irradiance; UARS solar UV irradiance data; SUSIM and SOLSTICE instruments</p><p>1. Introduction the late 1960s (Heath, 1973). Most previous attempts toStatus of UARS sol</p><p>M.T. DeLand a,*, L.E. Floyd</p><p>a Science Systems and Applications, Inc., 10210b Interferometrics Inc., 14120 Park</p><p>c Laboratory for Atmospheric and Space Phyd GEST/UMBC, NASA Goddard Space Flig</p><p>Received 29 November 2002; received in revis</p><p>Advances in Space Resear2151.</p><p>E-mail address: matt_deland@sesda.com (M.T. DeLand).</p><p>0273-1177/$30 2004 COSPAR. Published by Elsevier Ltd. All rights reserdoi:10.1016/j.asr.2003.03.043UV irradiance data</p><p>G.J. Rottman c, J.M. Pap d</p><p>belt Rd., Suite 400, Lanham, MD 20706, USA</p><p>g Ct., Chantilly, VA 20151, USA</p><p>. Colorado, Boulder, CO 80309-0590, USA</p><p>nter, Code 680, Greenbelt, MD 20771, USA</p><p>m 23 January 2003; accepted 14 March 2003</p><p>(2004) 243250</p><p>www.elsevier.com/locate/asrperiment (SOLSTICE) (Rottman et al., 1993). Each</p><p>ved.</p></li><li><p>instrument performs complete end-to-end calibration</p><p>on-orbit, using dierent techniques to characterize</p><p>changes in instrument response. SUSIM and SOL-</p><p>STICE can thus provide independent, fully calibrated</p><p>data sets to directly estimate solar cycle irradiance</p><p>variations.Previous studies using SUSIM and SOLSTICE data</p><p>have demonstrated absolute calibration agreement to</p><p>3% over 150410 nm (Woods et al., 1996), and long-term agreement with NOAA-11 SBUV/2 data to within</p><p>1% during 19911994 (DeLand and Cebula, 1998a).UARS has now operated from the end of the solar cycle</p><p>22 maximum in late 1991 through the maximum of solar</p><p>cycle 23 in 20002002. The SUSIM and SOLSTICEdata sets are the rst solar UV irradiance measurements</p><p>to cover the full dynamic range of a solar cycle with self-</p><p>consistent calibrations. This paper presents the current</p><p>status of these irradiance data sets, using the most recent</p><p>products from both instrument groups.</p><p>use of a working channel, which experiences more than</p><p>99% of the cumulative solar exposure, and a reference</p><p>channel, which is only operated at approximate 6 month</p><p>intervals. Fig. 1 shows the spectral dependence of</p><p>changes in SUSIM instrument response as measured by</p><p>the reference channel from December 1991 throughAugust 1998. The rate of response change is approxi-</p><p>mately linear with exposure time, and increases rapidly</p><p>shortward of 170 nm. By 1998, SUSIM was experiencing</p><p>severe degradation eects in the working channel at</p><p>k &lt; 140 nm that compromised the ability to produceirradiance data. In early 1999, the reference channel was</p><p>converted to use as the working channel, and a previ-</p><p>ously unused set of gratings became the new referencechannel (Floyd et al., 2002). The current SUSIM irra-</p><p>diance data set, designated V21, uses calibration data</p><p>from this revised conguration to extend coverage</p><p>through April 2002 (Floyd et al., 2003). The SUSIM</p><p>calibration goal is 15% relative accuracy at the end of</p><p>the data record, with larger uncertainties corresponding</p><p>244 M.T. DeLand et al. / Advances in Space Research 34 (2004) 2432502. Calibration</p><p>The SUSIM instrument is a dual-dispersion spec-</p><p>trometer, using two overlapping wavelength segments</p><p>(110265 nm, 235410 nm) to cover the spectral range</p><p>between 115 and 410 nm (Brueckner et al., 1993). Daily</p><p>irradiance measurements are performed with the mod-</p><p>erate resolution mode (Dk 1:1 nm) and low resolutionmode (Dk 5 nm). SUSIM uses multiple gratings, l-ters, and detectors in dierent combinations with on-</p><p>board deuterium lamps to determine instrument</p><p>response changes. Further details are given in Floyd</p><p>et al. (1998, 1999). A key element of this method is theFig. 1. SUSIM reference channel measurements from March 1992to shorter wavelengths. The largest component of this</p><p>uncertainty is the determination of wavelength-depen-dent and time-dependent sensitivity changes. These re-</p><p>sults, in turn, rely on the assumption that calibration</p><p>lamps with dierent operating cadences can be described</p><p>by the same model of irradiance degradation as a</p><p>function of working time. Other error sources include</p><p>photodiode gain and stray light correction. An extensive</p><p>discussion of error sources for both SUSIM and</p><p>SOLSTICE is given in Woods et al. (1996).The SOLSTICE instrument is a modied Monk</p><p>Gillieson spectrometer (Rottman et al., 1993). Data</p><p>from three overlapping channels (G 119190 nm,F 170320 nm, N 280420 nm) are used to con-to March 1999, relative to instrument sensitivity at launch.</p></li><li><p>struct a complete solar spectrum covering 119420 nm.</p><p>Each daily average spectrum combines data from all</p><p>available orbits. SOLSTICE uses young bluewhite</p><p>stars (spectral types A and B), which are believed to be</p><p>stable to one part in 104 over thousands of years, as</p><p>calibration sources to monitor instrument responsechanges (Woods et al., 1993). By changing the aperture</p><p>size (decrease by 104), integration time (increase by 103),</p><p>and spectral bandpass (increase by 101), stellar mea-</p><p>surements can be made with comparable photon uxes</p><p>to solar measurements. Changes in observed stellar</p><p>signals should therefore represent changes in instrument</p><p>response. SOLSTICE currently uses an ensemble of 18</p><p>stars to track instrument calibration. Time series arecreated at 18 separate wavelengths throughout the</p><p>spectral range of each channel to characterize instru-</p><p>ment response. Fig. 2 shows an example of the stellar</p><p>data for a single wavelength (153.5 nm) in the G channel.Spline ts are used to determine the continuous time-</p><p>dependent change in instrument response at the wave-</p><p>3. Irradiance data</p><p>The standard irradiance product for both SUSIM</p><p>and SOLSTICE is a daily average spectrum, binned at</p><p>1.0 nm on 0.5 nm centers (k 119:5, 120.5,. . . nm). Weconstructed time series in 5 nm bands over the wave-</p><p>length range 120300 nm. The nominal boundaries were</p><p>shifted to create a 3 nm band for the Lyman a emissionline (121.6 nm), and a pair of slightly wider bands (200</p><p>207, 208214 nm) to minimize the impact of the Al</p><p>absorption edge at 207.6 nm. For longer wavelengths</p><p>(300410 nm), 10 nm band averages were used. Signi-</p><p>cant outliers were identied by processing the time serieswith singular spectrum analysis (e.g. Pap and Frohlich,1999) and comparing the reconstructed time series with</p><p>the original data. Retaining the rst 20 terms produced a</p><p>reconstructed time series with </p></li><li><p>for 1</p><p>246 M.T. DeLand et al. / Advances in Space Research 34 (2004) 243250+25% by mid-2000 (Fig. 3(c)). Most of this dierence is</p><p>believed to be uncorrected degradation in the SUSIM</p><p>data during the later part of the record. Recent analysis</p><p>has produced an improved characterization of deute-rium calibration lamp changes that will signicantly</p><p>reduce the error in the SUSIM Lyman a data (Floydet al., 2003). The solar cycle variation in SOLSTICE</p><p>Lyman a irradiance is approximately 70% based on a27-day smoothed time series (to remove rotational</p><p>modulation eects). This range increases to 90100% if</p><p>unsmoothed daily values are evaluated. The relative</p><p>Fig. 3. Irradiance data at 120122 nm, normalized to the average value</p><p>in percent.drift between SUSIM and SOLSTICE through 1996 isless than 0.2 of the full solar cycle variation in irradiance</p><p>(0:2 DFcycle). For most of the spectral bands in the farUV spectral region (k 120200 nm), the agreementbetween SOLSTICE and SUSIM is very good. Typical</p><p>dierences in irradiance time series are 2% or less.Some spectral bands in this wavelength region, such as</p><p>165169 nm (not shown here), do show slightly higher</p><p>irradiance values during the Cycle 23 maximum than atthe end of Cycle 22. The dierence in amplitude is po-</p><p>tentially signicant (0:1 0:2 DFcycle), but depends onthe accuracy to which long-term calibration uncertain-</p><p>ties can be assigned.</p><p>Knowledge of solar irradiance variations in the</p><p>spectral region near the Al absorption edge at 207.6 nm</p><p>is important because of its dominant role in strato-</p><p>spheric photochemistry (Brasseur and Solomon, 1986).Fig. 4 shows that SOLSTICE and SUSIM agree on the</p><p>amplitude of the solar cycle signal in the 200207 nm</p><p>wavelength band to within 1% over 9 years. Fluctua-tions of 12% are observed throughout the dierence</p><p>time series, most notably during the solar minimum</p><p>period of 19951997. The SOLSTICE data reach aminimum value in April 1995 and July 1997, while the</p><p>SUSIM data have a well-dened minimum value in</p><p>August 1996. This behavior is consistently observed in</p><p>both data sets between approximately 180240 nm. Theamplitude of the observed solar cycle irradiance varia-</p><p>tion is thus approximately 68%, depending on the</p><p>choice of solar minimum. This result is consistent with</p><p>NOAA-11 observed changes of 7.0(1.8)% (Cebulaet al., 1998). Accurate knowledge of the 200207 nm</p><p>irradiance variation is important for understanding the</p><p>behavior of upper stratospheric ozone, where solar cycle</p><p>3 December 1991: (a) SOLSTICE data; (b) SUSIM data; (c) dierencevariations of 6% are observed at 12 mbar (McCormackand Hood, 1996). The relative drift between the SUSIM</p><p>and SOLSTICE data sets is less than 0:2 DFcycle.The magnitude of solar activity drops by a factor of</p><p>two longward of the Al edge, and remains roughly con-</p><p>stant between 210255 nm. This complicates the deter-</p><p>mination of long-term irradiance variations, because</p><p>calibration errors leading to time-dependent drifts tend to</p><p>have a weak spectral dependence and thus become rela-tivelymore important at longer wavelengths. Fig. 5 shows</p><p>data from this spectral region, using the wavelength band</p><p>245249 nm. A time-dependent drift is observed in the</p><p>dierence plot immediately (Fig. 5(c)), reaching 5% byfall 2000. This drift appears to be largely due to uncor-</p><p>rected degradation in the SOLSTICEdata, which fall well</p><p>short of reaching the maximum Cycle 22 values. For</p><p>comparison, estimated solar cycle variations at thesewavelengths are 34% (Lean, 1991). Similar behavior is</p><p>observed over the spectral region 230270 nm, with the</p><p>largest drift at 250 nm. The SOLSTICE instrumentteam is evaluating techniques to reduce this error. The</p><p>solar cycle variation of approximately 4% at 245249 nm</p><p>observed by SUSIM is consistent with the solar cycle 22</p></li><li><p>M.T. DeLand et al. / Advances in Space Research 34 (2004) 243250 247decrease of 3.5(1.8)% observed by NOAA-11 SBUV/2(DeLand and Cebula, 1998a).</p><p>Solar activity levels drop by an additional factor of</p><p>two longward of the Mg edge at 255 nm, further com-</p><p>plicating the task of detecting long-term variations.</p><p>Fig. 6 shows that the SOLSTICE and SUSIM data at</p><p>270274 nm generally agree to within 1%, with a largerdip during 19971998 that appears to be in the SOL-</p><p>STICE data set. Predicted solar cycle irradiance varia-tions in this wavelength region are only 12% (Lean,</p><p>1991; DeLand and Cebula, 1993). Thus, even if each</p><p>UARS instrument meets its desired goal for long-term</p><p>accuracy, the combined uncertainty is approximately</p><p>equal to the signal for which detection is intended.</p><p>Fig. 4. Irradiance data at 200207 nm</p><p>Fig. 5. Irradiance data at 245249 nmThe observation of long-term variations in TSI at the0.1% level implies that near-UV (k &gt; 300 nm) irradiancechanges must be as large or larger to compensate for the</p><p>darkening eect of visible sunspots. However, main-</p><p>taining satellite instrument calibration to sub-1% accu-</p><p>racy over 10 years to allow direct observations of these</p><p>variations is dicult. The stars used by SOLSTICE do</p><p>not have sucient output to serve as adequate long-term</p><p>calibration sources for N -channel data (300420 nm).SOLSTICE irradiance data in this wavelength region</p><p>were therefore forced to have no long-term change using</p><p>a spline t. The SUSIM calibration derived from on-</p><p>board lamps produces irradiance data that are at to</p><p>better than 1% longward of 300 nm, with short-term</p><p>. Identications are as in Fig. 3.</p><p>. Identications are as in Fig. 3.</p></li><li><p>porate active network variations.</p><p>4 nm</p><p>248 M.T. DeLand et al. / Advances in Space Research 34 (2004) 243250The diculty of maintaining accurate satellite in-</p><p>strument calibration for...</p></li></ul>

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