Solar PhysDOI 10.1007/s11207-014-0535-5
Comparison of Solar UV Spectral Irradiancefrom SUSIM and SORCE
J.S. Morrill · L. Floyd · D. McMullin
Received: 29 August 2011 / Accepted: 12 April 2014© Springer Science+Business Media Dordrecht 2014
Abstract Knowledge of solar spectral irradiance (SSI) is important in determining the im-pact of solar variability on climate. Observations of UV SSI have been made by the So-lar Ultraviolet Spectral Irradiance Monitor (SUSIM) on the Upper Atmosphere ResearchSatellite (UARS), the Solar-Stellar Irradiance Comparison Experiment (SOLSTICE), andthe Solar Irradiance Monitor (SIM), both on the Solar Radiation and Climate Experiment(SORCE) satellite. Measurements by SUSIM and SORCE overlapped from 2003 to 2005.
SUSIM and SORCE observations represent ∼20 years of absolute UV SSI. Unfortu-nately, significant differences exist between these two data sets. In particular, changes inSORCE UV SSI measurements, gathered at moderate and minimum solar activity, are afactor of two greater than the changes in SUSIM observations over the entire solar cycle.In addition, SORCE UV SSI have a substantially different relationship with the Mg II in-dex than did earlier UV SSI observations. Acceptance of these new SORCE results imposesignificant changes on our understanding of UV SSI variation. Alternatively, these differ-ences in UV SSI observations indicate that some or all of these instruments have changes ininstrument responsivity that are not fully accounted for by the current calibration.
In this study, we compare UV SSI changes from SUSIM with those from SIM andSOLSTICE. The primary results are that (1) long-term observations by SUSIM and SORCEgenerally do not agree during the overlap period (2003 – 2005), (2) SUSIM observations dur-ing this overlap period are consistent with an SSI model based on Mg II and early SUSIMSSI, and (3) when comparing the spectral irradiance for times of similar solar activity on ei-ther side of solar minimum, SUSIM observations show slight differences while the SORCE
J.S. Morrill (B)Space Sciences Division, Naval Research Laboratory, Washington, DC, USAe-mail: [email protected]
L. FloydFM Technologies, Chantilly, VA 20151, USAe-mail: [email protected]
D. McMullinSpace Systems Research Corp., Alexandria, VA 22314, USAe-mail: [email protected]
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observations show variations that increase with time between spectra. Based on this work,we conclude that the instrument responsivity for SOLSTICE and SIM need to be reevaluatedbefore these results can be used for climate-modeling studies.
Keywords Solar UV spectrum · Solar spectral irradiance · Space-based observations
1. Introduction
Understanding the impact of solar variability on terrestrial climate requires detailed knowl-edge of both solar spectral irradiance (SSI) and total solar irradiance (TSI). The importanceof SSI is due to the wavelength-dependent nature of both solar variability (Krivova, Solanki,and Floyd, 2006; Morrill, Floyd, and McMullin, 2011) and the absorption of solar radiationby the terrestrial atmosphere (Chou and Suarez, 2002). Observations of SSI in the ultraviolet(UV) have been made by various space-based missions since 1978. Two of these missionsare (1) the Upper Atmosphere Research Satellite (UARS), which included the Solar Ul-traviolet Spectral Irradiance Monitor (SUSIM) and Solar-Stellar Irradiance ComparisonExperiment (SOLSTICE) instruments, and (2) the Solar Radiation and Climate Experiment(SORCE) satellite, which includes the SOLSTICE and Solar Irradiance Monitor (SIM) in-struments. UARS/SUSIM observations extended from 1991 into 2005 (DeLand and Cebula,2008) and the SORCE observations began in 2003 and continued until recently (Harderet al., 2010; Unruh, Ball, and Krivova, 2011).
The SUSIM observations have produced the longest absolute solar UV irradiance dataset and have been used in studies of the Sun and its impact on Earth (e.g. Rozanovet al., 2006). The SORCE observations have also been used in studies of the Sun–Earthsystem, and in a recent study these were used to investigate the impact of SSI vari-ability on climate (e.g. Cahalan et al., 2010). SORCE measurements indicate that re-cent changes in UV irradiance are quite different from those measured by SUSIM andother instruments in earlier time periods (DeLand and Cebula, 2008, 2012; Lean and De-Land, 2012). In particular, differences between UV SSI measurements made by SORCEat moderate and minimum solar activity (Haigh et al., 2010) are a factor of two greaterthan changes over the entire solar activity cycles measured previously by other instru-ments including SUSIM (Krivova, Solanki, and Floyd, 2006; DeLand and Cebula, 2012;Lean and DeLand, 2012).
Acceptance of these new SORCE results requires significant changes in our understand-ing of how UV SSI varies during the solar cycle. Furthermore, if earlier measurements arealso to be accepted, the behavior of the Sun during the most recent declining phase of solaractivity must be quite different than the behavior during previous solar cycles. An alternateconclusion is that the measurements of some or all of these instruments have errors largerthan claimed. For instruments that observe solar irradiance, one of the main causes of suchsizable errors is due to changes in instrumental responsivity, or degradation, that is not fullyaccounted for by the instrument calibration. Whether or not the SUSIM or SORCE data setsare adversely affected by uncorrected instrumental degradation is one of the main topicsaddressed in this article.
To examine the differences between the SORCE and SUSIM observations, we presentcomparisons of UV spectral irradiance observations from SUSIM on UARS with obser-vations from the two SORCE SSI instruments, SOLSTICE and SIM. Results from theSOLSTICE instrument on UARS did not overlap with the SORCE mission and thereforeare not discussed here. The primary results of this study are based on two comparisons.
Comparison of Solar UV Irradiance: SUSIM and SORCE
First, we compare irradiance time-series observed by SUSIM and SORCE during the over-lap time period, from mid-2003 to mid-2005, along with results from a SUSIM-based UVirradiance model (Morrill, Floyd, and McMullin, 2011) through most of the SORCE mis-sion. Second, we compare UV spectra observed at times of similar solar activity on eitherside of the last two solar minima. Specifically, we compare observations by SUSIM aroundthe previous solar minimum and observations by SORCE around the most recent solar min-imum. It is not the intent of this study to examine the sources of instrumental degradationbut rather to determine, as best possible, which of these data sets is most accurate, based oninternal consistency and in relation to solar proxies, specifically the Mg II index.
There have been several recent efforts that have used the SORCE observations in at-mospheric modeling studies (Cahalan et al., 2010; Haigh et al., 2010; Merkel et al., 2011;Swartz et al., 2012; Wen et al., 2013; Ermolli et al., 2013). One main point of these articleshas been to further our understanding of the impact of solar variability on climate variabilityby using SORCE solar irradiance observations as the solar input. These studies have exam-ined the modeled stratospheric temperature response or compared recent ozone observationswith model results. The work by Haigh et al. (2010) provides an extremely good demonstra-tion of the spectral irradiance variation observed by SORCE. Our present effort includes acomparison of spectral irradiance variation derived from SUSIM observations with similarobservations by SOLSTICE and SIM using the format presented by Haigh et al. (2010).
In Haigh et al. (2010), solar spectra from SORCE and spectra from a solar irradiancemodel by Lean (2000) were averaged over two ten-day periods about 3.5 years apart. Theseperiods were centered on 21 April 2004 and 7 November 2007. These two time periodsoccurred during the most recent declining phase of solar activity, with the latter time periodbeing very near solar minimum. Differences between these averaged spectra were shownfrom 150 nm to 730 nm in Figure 1 of Haigh et al. (2010). That figure showed two mainresults over the 3.5 year time period examined in that study. While the SORCE results didshow the expected decrease in UV irradiance with decreasing solar activity, they also showeda much greater decrease in UV spectral irradiance than was indicated by the Lean-modelresults. Contrary to the decrease in the UV, the SORCE spectral irradiance observations inthe visible and near-IR show an increase with decreasing solar activity, which is opposite tothe results of the Lean model.
From previous efforts in the study of solar spectral irradiance (Lean, 1984; Lean et al.,1997, 1998; Morrill, Dere, and Korendyke, 2001; Morrill, 2005; Floyd et al., 2005; Fontenlaet al., 2009; Krivova, Solanki, and Floyd, 2006; Krivova, Solanki, and Unruh, 2011;Krivova et al., 2009; Pagaran, Weber, and Burrows, 2009; Morrill and Korendyke, 2008;Morrill, Floyd, and McMullin, 2011; Ball et al., 2011; Thuillier et al., 1997; Unruh,Ball, and Krivova, 2011; Unruh, Solanki, and Fligge, 2000; Unruh et al., 2008; etc.), in-creases in UV irradiance with increasing solar activity are neither unexpected nor sur-prising. The magnitude of the change in solar UV irradiance that was reported by Haighet al. (2010), however, is much larger than expected and these results were characterizedas a “solar surprise” (Garcia, 2010; Voiland, 2010). We have estimated the total changein UV irradiance, from a period of moderate solar activity to a period near solar mini-mum, observed by SORCE, to be about 1.14 W/m2 from Figure 1 of Haigh et al. (2010)with a similar estimate from Harder et al. (2009). This is a factor of two larger thanthe change in the UV irradiance for an entire solar cycle (∼0.5 W/m2) that has been re-ported elsewhere (Krivova, Solanki, and Floyd, 2006; Pagaran, Weber, and Burrows, 2009;Morrill, Floyd, and McMullin, 2011).
Haigh et al. (2010) noted that the wavelength dependent decrease in UV irradiance rangefrom four to six times larger than is expected by our previous knowledge, with the largest
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difference between the SORCE observations and the Lean model results occurring between200 nm and 370 nm (see also Garcia, 2010). They also stated that “currently there is insuf-ficient observational evidence to validate the spectral variations observed by SIM or fullycharacterize other solar cycles, but our findings raise the possibility that the effects of solarvariability on temperature throughout the atmosphere may be contrary to current expecta-tions.” While this is a reasonable caveat, the work by Haigh et al. (2010) is one of severalrecent studies mentioned above that have used the SORCE observations as SSI inputs whilethese results are still controversial (Ball et al., 2011; Unruh, Ball, and Krivova, 2011; De-Land and Cebula, 2012; Lean and DeLand, 2012). By comparing SUSIM and SORCE UVobservations we examine in detail the differences in SSI from these instruments and howthese differences vary in time.
There are several basic conclusions that are derived from our present analysis. First, thecomparisons of observations during the 2003 – 2005 overlap period show that both the valueand slope of the SUSIM time series disagree with SORCE time series from both SOLSTICEand SIM. Second, the time series of SUSIM observations from 2003 to 2005 generally agreewith results of the SUSIM-SSI solar irradiance model (Morrill, Floyd, and McMullin, 2011),which is based on SUSIM observations prior to 2000. This indicates that the SUSIM SSIobservations are internally consistent. Third, comparisons between averaged SUSIM spectraon either side of the previous solar minimum (1996) have similar spectral irradiance valueson days with the same average Mg II index, as expected. On the other hand, when thesecomparisons are performed with SORCE spectra on either side of the most recent solarminimum (2009), we find differences in the UV spectra that are larger than those observedby SUSIM and these differences in SORCE observations increase with increasing separationin time between spectra. This kind of behavior of the SORCE observations suggests thatchanges in instrument responsivity have not been adequately corrected, especially during theearly portion of the SORCE mission when the rate of degradation would be greatest (Floydet al., 1998). SUSIM observations extend to ∼410 nm so the present analysis focuses ondifferences between SUSIM and SORCE at UV wavelengths. We will, however, discuss theimplications of the present analysis on SORCE/SIM observations at longer wavelengths.
2. Sources of Solar UV Irradiance Data
As mentioned above, the focus of the present article is on SUSIM and SORCE UV observa-tions in addition to a model based on the Mg II index and SUSIM observations prior to April1999 (Morrill, Floyd, and McMullin, 2011). This section describes the sources of irradianceobservations from SUSIM on UARS and from SOLSTICE and SIM on SORCE.
The UARS spacecraft was launched from the Space Shuttle on 14 September 1991and observations were made by SUSIM from 12 October 1991 until 1 August 2005, justprior to the de-commissioning of UARS. This data set covers the wavelength range of110 nm to 412 nm (Brueckner et al., 1993; Woods et al., 1996). The SORCE satellitewas launched 25 January 2003 and has been making solar spectral irradiance observationssince March 2003 (Harder et al., 2009). The combined spectral irradiance observations fromthe SOLSTICE and SIM instruments covers the solar spectrum with a maximum range of115 nm to 2700 nm (Harder et al., 2005b; Rottman et al., 2005). The last portion of the14-year SUSIM data set overlaps with the beginning of the SORCE observations, allowinga direct comparison between data sets from these two missions.
The SUSIM data set that overlaps the SORCE observations is from the V22 SUSIMdata product, which is available online (http://wwwsolar.nrl.navy.mil/uars/). The SUSIM-SSI
Comparison of Solar UV Irradiance: SUSIM and SORCE
model results were derived from linear fits of the high-resolution SUSIM spectral irradianceto the NOAA Mg II index (Morrill, Floyd, and McMullin, 2011). In addition to overlappingobservations, part of our comparison will include spectra that were estimated using ourSUSIM-SSI model. The SUSIM observations used for this model were observed during the1990s (1991 – 1999). This model uses the Mg II index as the only input to generate a solarirradiance spectrum from 1 nm to 410 nm where the portion above 150 nm is based onSUSIM observations and provides an extremely good estimate of SUSIM irradiance.
The SORCE data sets we examined were downloaded from the SORCE web site (http://lasp.colorado.edu/sorce/data/data_product_summary.htm) and were used without correc-tion. Here we discuss results from several different versions of the SORCE observations.The most recent data sets used here (SOLSTICE Ver. 12 and SIM Ver. 19) were downloadedin February and December 2013, respectively. The earlier data sets (SOLSTICE Ver. 10 andSIM Ver. 17) and (SOLSTICE Ver. 9 and SIM Ver. 15) were downloaded in March 2011and July 2008, respectively. The most recent spectra from SOLSTICE (Ver. 12) are fromMay 2003 to July 2013 and cover the wavelength range from 118 nm to 310 nm. The mostrecent spectra from SIM (Ver. 19) cover the time period from April 2003 to May 2011 andcover the wavelength range from 240 nm to 2413 nm. In this study we focus primarily onthe portion of SORCE observations that overlap with SUSIM between 150 nm to 410 nm.
3. SUSIM and SORCE Calibration Methods
A detailed description of the UARS/SUSIM instrument was presented by Brueckner et al.(1993) and includes both the properties of the instrument and the basic calibration proce-dure. The SUSIM instrument calibration was determined before flight by measurements atthe Synchrotron Ultraviolet Radiation Facility (SURF) at the National Institutes of Standardand Technology (NIST). During normal operations, SUSIM used duplicate channels withmultiple gratings, detectors, and filters in addition to onboard calibration lamps to determinethe calibration and long-term degradation. Calibration runs were scheduled throughout theUARS mission, and these calibration data were used to determine the calibration and degra-dation corrections. The validation of the SUSIM calibration was discussed by Woods et al.(1996) and further details of the SUSIM calibration have been the subject of several publi-cations. The topics addressed in these papers included correction for instrumental stray light(Reiser et al., 1994), performance of the calibration lamps (Prinz et al., 1996), degradationof the optical filters (Floyd, 1999), and the overall responsivity degradation (Floyd, Prinz,and Brueckner, 1994; Floyd et al., 1994, 1996, 1998, 1999). The status of the SUSIM ir-radiance was addressed by DeLand et al. (2004) and the SUSIM observations have beenincluded in a recent composite UV SSI data set (DeLand and Cebula, 2008).
The details of the SORCE spectral observations were discussed by Harder and co-workers (Harder et al., 2009, 2010). Detailed descriptions of these instruments and calibra-tion procedures were discussed in several papers. The absolute calibration of the SOLSTICEinstrument was also determined before flight by measurements at SURF/NIST. During nor-mal operations, SOLSTICE made stellar observations of several A and B type stars, whichwere used for calibration (McClintock, Rottman, and Woods, 2005; McClintock, Snow, andWoods, 2005; Snow et al., 2005a). The SIM instrument calibration involved a componentlevel calibration in conjunction with a measurement equation to determine the instrumentsensitivity and long-term degradation (Harder et al., 2005a, 2005b, 2009 – SupplementaryMaterial, 2009; Harder et al., 2010). An end-to-end pre-flight calibration was made withan FEL quartz halogen lamp, although this source only achieved 2.5 % of the solar flux.
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Figure 1 The solar UVspectrum for comparisonwith the irradiance differencesshown below.
Additional post-flight measurements of the SIM engineering unit at NIST were performedin support of the pre-flight calibration (Harder et al., 2010). Both SORCE instruments havetwo separate channels that are used by the SORCE team to help understand the long-termdegradation.
4. Irradiance Differences Between Moderate and Minimum Solar Activity
An example of the UV portion of the solar spectrum examined in this study is shown inFigure 1. Some of the important features are the Al edge at 210 nm, the Mg II doublet,which is the single deep feature at 280 nm, and the Ca II K and H lines, which are the twofeatures near 393 nm and 397 nm, respectively. The Mg II doublet and the nearby continuumare used to generate the well-known Mg II index and the short-wavelength Ca II line is usedto produce images of Ca II K that show the presence of numerous surface features includingquiet Sun, active regions, network, and sunspots. We present this spectrum to provide areference scale for the plots of irradiance differences described below.
An effective means of displaying changes in solar irradiance is to examine the differencesbetween spectra at various times; these are referred to as difference curves. Differences insolar irradiance from various sources are presented in Haigh et al. (2010). The differencecurves in their Figure 1 were derived from UV, visible, and near-IR spectra that were aver-aged over two ten-day periods centered on 21 April 2004 and 7 November 2007. The UVportions of these difference curves are shown in our Figure 2. In this study we used threeversions of the publicly available SORCE data sets that were downloaded from the SORCEweb site, whereas the study by Haigh et al. (2010) used a data set that was provided by theSORCE team (SOLSTICE Ver. 10 and SIM Ver. 17). The main difference between the pub-licly available SIM (Ver. 17) data set and the one used by Haigh et al. (2010) is that the SIMdata set used by Haigh extends below 310 nm. To confirm the similarity between the data setwe downloaded and the one used by Haigh et al. (2010); we digitized and smoothed the irra-diance difference curves from the SORCE observations presented in Figure 1 of Haigh et al.(2010). The digitized difference curves for SIM (blue curves), SOLSTICE (green curves),and the Lean model (light blue curves) are presented in our Figure 2.
Also shown in Figures 2(a) and (b) is the irradiance difference curve from the SUSIM-SSImodel (Morrill, Floyd, and McMullin, 2011). The SORCE Mg II index (Snow et al., 2013)was the only input to the SUSIM-SSI model to generate irradiance spectra for the twotime periods in Haigh et al. (2010). The ten-day average Mg II index values are 0.2695for 21 April 2004 and 0.2635 for 7 November 2007. The resulting difference curve fromthe SUSIM-SSI model is the red curve, which basically follows the Lean curve (light blue)throughout the 200 nm to 400 nm range.
Comparison of Solar UV Irradiance: SUSIM and SORCE
Figure 2 The irradiance difference curves from observations by SORCE and model results by Lean (2000)and Morrill, Floyd, and McMullin (2011). These curves are the differences between spectra averaged overtwo ten-day periods around 21 April 2004 and 7 November 2007. Figure 2 is similar to a figure presented inHaigh et al. (2010) and their curves for SIM (blue), SOLSTICE (green), and Lean (light blue) are included.The heavy black curves are from the current analysis and use the two most recent publicly available SORCEdata sets. The SORCE data used in the present study were (a) SOLSTICE Ver. 10 and SIM Ver. 17 and(b) SOLSTICE Ver. 12 and SIM Ver. 19. Haigh et al. (2010) used SOLSTICE Ver. 10 and SIM Ver. 17. Notethat the SIM data set used by Haigh et al. extends below 310 nm. The four horizontal bars labeled (a) through(d) indicate the passbands for the time series in the next two figures. The vertical dashed line at 310 nm isthe dividing line between SOLSTICE and SIM observations used in this study and the vertical dashed line at393 nm is the upper limit of the SUSIM-SSI model results in which we have confidence.
The black curves in Figures 2(a) and (b) are the difference curves we generated from thetwo most recent SORCE data sets. The black curves in Figure 2(a) show results from theprevious SORCE data sets (SOLSTICE Ver. 10 and SIM Ver. 17), which were the versionsused by Haigh et al. (2010). The black curves in Figure 2(b) are the results from the mostrecent SORCE data sets (SOLSTICE Ver. 12 and SIM Ver. 19). The most recent publiclyavailable SIM data set (Ver. 19) does extend below 310 nm, but we focused on the SIMobservations above this wavelength. As can be seen in Figure 2(a), the overlap is nearlyidentical between the difference curves from the present study and those from Haigh et al.(2010). The results for the most recent SORCE data sets, however, show significant changes.Figure 2(b) shows that the most recent SOLSTICE data set above 250 nm yields a differencecurve that is closer to the curves produced by both the Lean and SUSIM models. The resultsfrom the most recent SIM data set are more complex and agree better with these models inthe 310 nm to 340 nm and 380 nm to 400 nm ranges, but less so in the 350 nm to 380 nmrange.
The difference curves in these figures are snapshots that compare two specific timesduring the most recent decline in solar activity. While they do show variations betweendifferent SORCE data versions and the two models, an alternative comparison is to examineirradiance time series in various passbands. The four passbands we examine are shown bythe horizontal bars in Figure 2 labeled “a” through “d”, which indicate the passbands overwhich irradiances are summed to generate time series that are presented in the next section(see Figures 3 and 4).
5. Irradiance Time Series – Long Term Variability
In this section we describe the irradiance time series in the four passbands identified inFigure 2. The four passbands are (a) 220 – 250 nm, (b) 260 – 280 nm, (c) 315 – 335 nm,
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Figure 3 Long-term irradiance time series from several sources. The passbands are (a) 220 – 250 nm,(b) 260 – 280 nm, (c) 315 – 335 nm, and (d) 360 – 390 nm. The SORCE curves are those nearest the SOL-STICE or SIM version labels and these figures show the most recent SOLSTICE (Ver. 12) and SIM (Ver. 19)data sets (purple lines) along with several earlier versions (green lines: SOLSTICE Ver. 9, SIM Ver. 15;blue lines: SOLSTICE Ver. 10, SIM Ver. 17). The SUSIM-SSI model estimate is the red curve nearest theSUSIM label and the SUSIM observations are the heavy green triangles overlaying this curve. All curveswere smoothed with a single 27-day boxcar. Days of major SUSIM calibrations where the lowest cadencereference channels and lamps were scanned are indicated by the heavy black tick marks above the SUSIMobservations. The two sets of vertical dashed lines identify the dates from the study by Haigh et al. (2010).The open diamonds are on dates with the same Mg II index (0.2668) on either side of the most recent solarminimum and the dot-dashed lines are at the level of the irradiance of earlier date. The open triangles are themeasured SORCE irradiance on the date after solar minimum and the error bars indicate the uncertainty basedon the time between the two points. The tick marks and Mg II index values along the bottom are addressed inthe discussions of Figures 5 and 6 and the tick mark labels (a – h) refer to the difference curves in Figure 6.The SIM time series plots (c, d) have fewer Mg II index values and tick marks since the SIM Ver. 19 data setdoes not extend as far as the SOLSTICE Ver. 12 data set.
and (d) 360 – 390 nm. Time series were generated by summing over the above passbandsin the daily spectra. The resulting time series are presented in Figure 3 and show curvesfrom the SORCE data sets mentioned in Section 3 above; three for SOLSTICE (Ver. 9, 10,and 12), and three for SIM (Ver. 15, 17, and 19). The time series from these three versionsare the green, blue, and purple curves, respectively. Also present in Figure 3 are time seriesof SUSIM observations (green triangles) that overlapped the first two years of the SORCEmission as well as irradiance estimates derived from the SUSIM-SSI model (red curves).These model results used the SORCE Mg II index as the only input. This index has justrecently been corrected (Snow et al., 2013) to account for changes in one of the SOLSTICEchannels from 200 nm onward. In each of these figures (Figure 3(a) through Figure 3(d))vertical dashed lines indicate the dates from Haigh et al. (2010) over which the spectra wereaveraged to produce the difference curves in Figure 2. Both the SUSIM-SSI model estimates
Comparison of Solar UV Irradiance: SUSIM and SORCE
Figure 4 Short-term irradiance time series for a one-year period. Here the irradiance time series has beensmoothed with a double 27-day boxcar function and then subtracted from the original irradiance time series.The resulting difference was then smoothed for plotting purposes. These plots show that the short-term timeseries is nearly identical for the SUSIM observations, the SUSIM model, and the SORCE/SOLSTICE ob-servations below 300 nm. At the longer wavelengths the results for the SUSIM model and observations areof similar magnitude, but are poorly correlated. The SORCE/SIM results show significantly higher residuals.Note the scale factor for the SORCE/SIM short-term time series in 5c (10×) and 5d (2×). The correlationcoefficients between the SUSIM observations and SUSIM-SSI model are (a) 0.73, (b) 0.53, (c) 0.01, and(d) 0.10. The correlation coefficients between the SUSIM and SORCE observations are (a) 0.69, (b) 0.48,(c) −0.04, and (d) 0.05.
and SUSIM observations were smoothed by twice convolving with a 27-day boxcar func-tion, as were the SORCE observations, thus smoothing these irradiance values roughly ontoa solar rotation time scale.
As part of the standard SUSIM observing program, major instrument calibrations wereperformed during this two-year overlap period with SORCE, and these calibrations wereused to correct the SUSIM observations during this time period. Major calibrations were per-formed on the following days; 08 July 2003, 29 December 2003, 19 June 2004, 27 February2005, and 30 July 2005. These calibration days are indicated in Figure 3 as the vertical tickmarks above the SUSIM observations. The impact of these calibrations on the long-term be-havior of irradiance can be seen in all four passbands by the overlap of the SUSIM observa-tions and the SUSIM-SSI model, with the best fit occurring at the shorter wavelengths. Thetwo points to note are that (1) the scatter in the observations is relatively uniform around themodel results, and that (2) the difference between SUSIM observations and the SUSIM-SSImodel results range from 0.1 % to 0.5 %. These two points reflect the long-term stability ofthe SUSIM data set.
As mentioned, the overlap between the SUSIM observations and SUSIM-SSI model re-sults is an indication of the internal consistency of the SUSIM results and the validity andstability of the SUSIM calibration- and degradation-correction methods. This is primarily
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Figure 5 The Mg II index time series for the two most recent solar minimum periods. The plots of the mostrecent minimum uses the SORCE Mg II index (Figure 5(a)) and the plots of the previous minimum uses theNOAA Mg II index (Figure 5(b)). The dates and Mg II index values used in Figure 6 are plotted in thesefigures. The differences between the average spectra for these dates are shown in Figure 6.
because the model is based only on the Mg II index and SUSIM observations prior to April,1999. As a result, the effect of the calibrations during the last six years of the SUSIM mis-sion did not impact the model results. If the SUSIM degradation correction was not accurate,then the model would most likely have predicted higher values, and the long-term overlapbetween the observations and model would not have been as good as is shown in thesefigures.
The time series for SOLSTICE in Figures 3(a) and (b) shows no significant change be-tween the three data sets in the 220 – 250 nm passband (Figure 3(a)). This lack of changeis also seen in the overlap of SOLSTICE difference curves in Figures 2(a) and (b) 250 nm.There is, however, a noticeable change in the 260 – 280 nm passband (Figure 3(b)) for themost recent data set (Ver. 12) compared with the earlier versions (Ver. 9 and 10). This isthe reason why the SOLSTICE difference (current analysis and Haigh et al. (2010)) curvesdo not overlap above 250 nm in Figure 2(b). In addition, there are weak periodic featuresin the SOLSTICE curves in Figures 3(a) and (b) that appear to follow the timing of theEarth’s perihelion. Measured irradiances that are corrected to 1 AU almost certainly shouldnot exhibit this variation as there is no known solar cause. Instead, its presence could bethe result of an erroneous 1 AU correction, albeit this is unlikely. A more probable causemight be the result of subtle instrumental temperature effects, which result indirectly fromthe changing Sun–Earth distance. Another consequence of the change that occurred in the260 – 280 nm time series between SOLSTICE Ver. 10 and Ver. 12 is that the Ver. 12 timeseries now shows a minimum in irradiance near the observed solar-cycle minimum (see alsoFigure 5(a)). The earlier SOLSTICE versions (Ver. 9 or 10) did not show a minimum inthis passband, although a minimum is present in both SOLSTICE Ver. 10 and Ver. 12 in the220 – 250 nm passband.
For the SIM time series in Figures 3(c) and (d), significant differences exist between thethree SIM data sets (Ver. 15, 17, and 19). The most obvious difference is the dramatic changein shape and slope between these three versions in the 315 – 335 nm and 360 – 390 nm pass-bands, respectively (Figures 3(c) and (d)). Another issue is that the SIM data prior to 21 April2004 were removed from the SIM Ver. 17 data set, apparently because of difficulties with theprism control system during the first ten months of operations (Harder et al., 2005a). Thisearly portion of the SIM observations has been included in the most recent data set (Ver. 19)and may show negative degradation (or sensitivity increase) during the earliest portion of theSORCE mission. Furthermore, the annual variation seen in the SOLSTICE observations was
Comparison of Solar UV Irradiance: SUSIM and SORCE
much more pronounced in the SIM Ver. 17 315 – 335 nm passband than in the SOLSTICEdata sets. This annual behavior is much less obvious in the Ver. 17 360 – 390 nm passband.This variation is greatly improved in the most recent SIM data set (Ver. 19) in both the315 – 335 nm and 360 – 390 nm passbands. A notable change in the Ver. 19 time series isthat neither SIM passband shows a minimum in irradiance near the minimum in solar ac-tivity. We mention these points since the 315 – 335 nm passband is where the disagreementbetween SORCE irradiance observations and SUSIM-SSI model results is greatest.
The final topic to be addressed concerns the most recent data versions of the long-termtime series displayed in Figure 3. Because the most recent SORCE data sets (SOLSTICEVer. 12 and SIM Ver. 19) extend beyond the latest solar minimum, irradiances on eitherside of the minimum can be compared in the following way: in this approach, days withthe same smoothed Mg II index value can be selected before and after the most recentsolar minimum, and the average irradiance values for these two days can be compared. InFigures 3(a) – (d), two days on either side of the recent minimum with the same index value(0.2668) were selected from the smoothed SORCE Mg II index. These days are indicated bydiamonds on either side of the most recent solar minimum connected by dot-dashed lines.The smoothed irradiance value before the solar minimum determines the level of the dot-dashed line in each figure. The triangles with error bars on the dates after solar minimumindicate the irradiance level from the SORCE curve on this day. The error bars indicatethe uncertainty estimates based on the time between these points and the reported stabilityof these instruments. The uncertainty values we used are 0.3 %/year (0.2 % – 0.5 %/year)for SOLSTICE and 0.03 %/year for SIM, both from the SORCE data header. The SUSIMuncertainty values are about 0.3 %/year for observations below 300 nm and 0.03 %/yearobservations above 300 nm, and these values were derived from Floyd (1999).
For SOLSTICE, the irradiance value after solar minimum in the 220 – 250 nm passband(Figure 3(a)) falls just outside the uncertainty range, but the irradiance value in the 260 –280 nm passband (Figure 3(b)) is within the relative uncertainty. For SIM, the irradiancevalue in the 315 – 335 nm and 360 – 390 nm passbands (Figures 3(c) and (d)) is outside theuncertainty in both cases. This type of analysis will be extended in Section 7, where spectrabefore and after the last two solar minima are compared. As part of the comparison of spectraon days with the same smoothed Mg II index on either side of the solar minimum, Figure 3contains a series of five pairs of tick marks, below the SORCE time series in each figure.These tick marks indicate days with the same value of the smoothed Mg II index. The Mg II
index for each pair of tick marks is listed on the right side of each figure. As we discuss inmore detail in Section 7, the difference between spectra on these days shows the variationin SUSIM and SORCE calibration as a function of time on either side of solar minimum.
6. Irradiance Time Series – Short-Term Variability
To examine short-term irradiance variability, the time series in Figure 3, which have beensmoothed with a double 27-day box car, were subtracted from the original unsmoothed timeseries. These residual time series were examined during several one-year periods, but inFigure 4 we confine our attention to a single time interval. Differences in the short-termvariability curves from the various SORCE data sets were found to be minimal, so only themost recent versions for SOLSTICE and SIM are presented. These short-term time serieswere generated for the same four passbands as in Figure 3. The starting and ending dates ofthis time period were chosen so that this one-year period falls entirely within the SORCEand SUSIM measurement overlap. For the short-wavelength UV passbands in Figures 4(a),
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(b) (220 – 250 nm and 260 – 280 nm), the residuals from SOLSTICE and SUSIM observa-tions, and SUSIM-SSI model results, all agree extremely well (see the caption of Figure 4 forthe correlation coefficients). This indicates that the SUSIM and SOLSTICE data sets bothhave nearly identical short-term variability, and both are driven in the same manner as theMg II index, by magnetically active surface features and solar rotation (e.g. see Snow, Mc-Clintock, and Woods, 2010). Similar behavior was seen by Ball et al. (2011), who comparedthe SATIRE model with SIM observations in the 201 – 300 nm range and by Unruh, Ball,and Krivova (2011), who examined SSI results from instruments on UARS and SORCE.
For the long-wavelength UV passbands in Figures 4(c) and (d) (315 – 335 nm and 360 –390 nm) the situation is very different. Here the SUSIM and SIM observations do not showthe same temporal variability as was seen at the shorter wavelengths and the correlation be-tween the observed and estimated SUSIM residuals appears to be very weak (see the captionof Figure 4 for the correlation coefficients). The range of values in the SUSIM residuals fromthe observed and estimated irradiances is of a similar magnitude, but the range of values inthe SIM residuals is significantly larger (see SIM scaling factors in Figures 4(c) and (d)). TheSIM residuals are roughly a factor of ten larger than the SUSIM values in the 315 – 335 nmpassband and roughly a factor of two larger in the 360 – 390 nm passband. The source ofthis relative increase in scatter in the SIM residuals is not clear at present, although it maybe due to limitations in the SIM analog-to-digital converter (ADC) (Harder et al., 2005a). Itis interesting to note, however, that the scale factors between the SIM and SUSIM residualsare roughly the same as the factors between the irradiance differences for SIM and SUSIMshown in the two long wavelength UV passbands in Figure 2.
Several other one-year time intervals were examined, and the results were approximatelythe same for both SUSIM and SORCE. It is important to note, however, that any similaritiesor differences in the short-term time series are less significant than the differences in thelong-term time series, and the short-term time series are included primarily for complete-ness. The main focus of this article involves the differences in the long-term behavior, whichare related to the mischaracterization of the degradation of instrumental sensitivity.
7. Comparison of Spectra Across the Solar Minimum
To examine instrumentally driven changes in spectra over a multiyear time period, we com-pared 27-day average spectra on either side of solar minimum on days with the samesmoothed Mg II index. For this comparison, we used two time series of the Mg II indexthat are displayed in Figure 5. For the SORCE data the SORCE Mg II index was used andfor the SUSIM data the NOAA Mg II index was used. Figure 5(a) shows the SORCE Mg II
index (Snow and McClintock, 2005; Snow et al., 2005b) around the most recent solar mini-mum, which has been recently corrected and is on the same scale as the NOAA Mg II index(Snow et al., 2013). Figure 5(b) shows the NOAA Mg II index (Viereck and Puga, 1999;Viereck et al., 2001a, 2001b; Morrill et al., 2011) during the previous solar minimum. Inthese figures, the indices were smoothed in two ways: once with a three-day boxcar func-tion, and then twice with an 81-day boxcar function. The three-day smooth was done fordisplay purposes only.
In the following analysis, a series of Mg II index values were selected from the moreheavily smoothed SORCE index (Figure 5(a)) on a series of dates, 0.2 years apart, afterthe most recent solar minimum starting at about 2009.9 and extending to about 2011.7.A set of corresponding dates prior to the recent solar minimum, which had the same Mg II
index values, were then selected from the heavily smoothed SORCE Mg II index curve in
Comparison of Solar UV Irradiance: SUSIM and SORCE
Figure 5(a). After selecting these dates for the SORCE observations, a set of dates wereidentified for the SUSIM observations on either side of the previous solar minimum, whichhad the same set of Mg II index values, which were selected from the heavily smoothedNOAA Mg II index in Figure 5(b). Several of these dates and index values are shown inFigures 5(a) and (b) by diamonds connected by dashed lines. Results from this set of datesare presented in Figure 6. After selecting these dates, spectra were averaged over a 27-dayperiod around these dates to average over solar irradiance changes that would occur during afull solar rotation. This procedure is similar to the calibration procedure used by DeLand andCebula (2008) to normalize the Nimbus 7 and SME SSI observations with the SOLSPECreference spectrum (Thuillier et al., 2004a, 2004b).
The differences between averaged spectra for two dates with the same Mg II index arenot visible when plotted on the same scale as Figure 1. The small irradiance differencesbetween the pairs of spectra for the dates given in Figure 5 become more apparent in theplots of Figure 6. The difference curves in Figure 6 are the differences between the averagespectra before solar minimum minus the average spectra after solar minimum (before minusafter). Each plot in Figure 6 contains the difference between pairs of average spectra fromSORCE (blue curves) and SUSIM (green curves). The light dotted and dashed curves are0.1 % and 0.2 % of the solar spectrum, respectively, to place these differences on a relativescale.
The Mg II index values for Figure 6 are (a, b) 0.2642, (c, d) 0.2652, (e, f) 0.2672,(g) 0.2686, and (h) 0.2704. The SORCE spectra in Figures 6(a), (c), and (e) are from the pre-vious SORCE data set (SOLSTICE Ver. 10 and SIM Ver. 17) and so represent the irradianceobservations used by the studies mentioned earlier (Cahalan et al., 2010; Haigh et al., 2010;Merkel et al., 2011; Swartz et al., 2012). The SORCE spectra used in Figures 6 (b, d, f, g,and h) are from the most recent SORCE data sets (SOLSTICE Ver. 12 and SIM Ver. 19). Thefirst three pairs of Figures 6 (a, b; c, d; and e, f) compare the two most recent SORCE datasets at the same Mg II index value. Since the current SOLSTICE data set (Ver. 12) extendsbeyond the previous SOLSTICE data set (Ver. 10) and both recent SIM data sets, Figure6(g) and (h) only shows the SOLSTICE portion of the difference curves from Ver. 12 sincea comparison with the earlier version is not possible. The SUSIM difference curves are fromspectra taken on the dates indicated in Figure 5(b) and listed in Figure 6. Since we examinedonly the final SUSIM data set (Ver. 22), the SUSIM difference curves are the same in thefirst three pairs of panels in Figures 6 (a, b; c, d; and e, f).
The SOLSTICE data extend over the entire range of Mg II index values used, and thecurves in Figure 6 show that the differences between spectra before and after minimumincrease with increasing Mg II index and increasing time between spectra. As the Mg II
index increases, the time difference between spectra increases, and the comparison is madebetween observations earlier in the SORCE mission with observations later in the mission.Figure 6 shows that for days with the same Mg II index, the average SOLSTICE spectralirradiance measured early in the SORCE mission is higher than the average spectral irradi-ance measured late in the mission, and this difference increases with increasing differencein time between spectra.
For SIM Ver. 17, the differences are negative and it is difficult to determine a clear trendthat includes observations earlier in the SORCE mission since the SIM (Ver. 17) data setdoes not extend as far back as SOLSTICE. The magnitude of the SIM difference appears tobe increasing with increasing time between spectra and as earlier observations are comparedwith later ones. The oscillations at the end of the SIM Ver. 17 time series (Figures 3(c)and (d)) appear to affect these difference curves. For SIM Ver. 19, the differences betweenspectra are positive and show a dramatic increase with increasing time between spectra. The
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Figure 6 The difference curves for the pairs of spectra that correspond to the dates indicated in Figures 3and 5. Figures (a, c, e) are from the previous SORCE data version (SOLSTICE Ver. 10 and SIM Ver. 17)and (b, d, f, g, h) are from the most recent SORCE data versions (SOLSTICE Ver. 12 and SIM Ver. 19). Theblue curves in each figure are the difference curves between the two 27-day average spectra (SOLSTICE andSIM) around the indicated dates for SORCE, and the green curve is the difference curve for SUSIM. Theblack dotted and dashed curves are 0.1 % and 0.2 % of the solar spectrum, respectively, for reference. TheMg II index values are (a, b) 0.2642, (c, d) 0.2652, (e, f) 0.2672, (g) 0.2686, and (h) 0.2704. The SOLSTICEdata cover the entire range of Mg II index values (a – h), but the SIM data only covers values through 0.2672(a – f). Note that the SOLSTICE difference is less in Ver. 12 than in Ver. 10 and the SIM difference haschanged sign between Ver. 17 and Ver. 19. The magnitude of the differences for both SOLSTICE and SIMtend to increase with increasing separation in time. The SUSIM differences tend to vary around zero andremain within 0.2 % except for the last two plots below 300 nm.
Comparison of Solar UV Irradiance: SUSIM and SORCE
difference between these two SIM versions (Ver. 17 and 19) can be clearly seen in the timeseries in Figures 3(c) and (d). As mentioned, the most recent SIM data set (Ver. 19) doesnot show a minimum in irradiance near the minimum in solar activity in either passband(Figures 3(c), (d), and 5(a)). Including both earlier and more recent SIM observations wouldbe helpful in this analysis so that spectra near the beginning of the SORCE mission couldbe compared with the most recent observations.
The SUSIM difference indicates that the SUSIM spectral irradiance has some variationwith increasing time between spectra, but this difference is generally within 0.2 % of thesolar spectrum and tends to scatter around zero. The exception is in Figures 6(g) and (h),where the SUSIM difference has exceeded 0.2 % below 300 nm.
The basic assumption in this comparison is that the Mg II index is a good measure ofUV SSI, and the variability in this proxy tracks the variability of UV irradiance (Cebula,DeLand, and Schlesinger, 1992; DeLand and Cebula, 1993; Viereck et al., 2001b; Morrill,Floyd, and McMullin, 2011). Figure 6 shows differences between spectra measured on daysthat are separated in time by several years but have the same average Mg II index values.Based on the above assumption, these average spectra should be the same or nearly so.The results presented in Figure 6 show that the SORCE data vary significantly across thesolar minimum and that the magnitude of differences increase with increasing separationin time between observations. The SUSIM data, on the other hand, are shown to have asmaller difference that tends to scatter around zero and remains relatively constant in time(within 0.2 % above 250 nm and throughout most of these plots). One explanation is thatthe SOLSTICE and SIM calibration and degradation corrections are not maintained over the5- and 7.5-year time periods, respectively, shown in Figure 6. On the other hand, the resultsfor SUSIM appear to show that the SUSIM calibration is better maintained over the 6-yeartime period shown in Figure 6.
Because the differences in the SORCE data across the solar minimum shown in Fig-ure 6 are similar to the irradiance differences presented by Haigh et al. (2010) and thesedifferences tend to increase with increasing time between spectra, it is difficult to accept thevalidity of the SORCE UV irradiance results presented in Harder et al. (2009) and Haighet al. (2010).
8. Discussion
We have performed a detailed examination of SUSIM and SORCE solar UV spectral irra-diance observations made over several time periods during the last two solar cycles. Thiswas done by comparing overlapping observations during the most recent declining phase ofSolar Cycle 23 and by examining observations around the two most recent solar minima.
The first comparison, displayed in Figure 2, shows irradiance differences from the twomost recent SORCE data sets and our SUSIM-SSI model. These figures are similar to thework by Haigh et al. (2010) and the curves from that article for SOLSTICE, SIM, and theLean NRLSSI model are included in both Figures 2(a) and (b). The difference curves fromthe current work shown in Figure 2 are between average irradiance spectra from two ten-dayperiods about 3.5 years apart during the declining phase of the most recent solar cycle. TheSUSIM-SSI model results generally agree with the results from the NRLSSI model by Lean(2000). By using the two most recent SORCE data versions (SOLSTICE Ver. 10 and 12; SIMVer. 17 and 19), Figure 2 shows the recent changes to the SORCE data sets. Figure 2(a) usesthe same data sets as were used by Haigh et al. (2010), as indicated by the overlap betweentheir results (green and blue curves) with the present results (black curves). Figure 2(b)
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shows that the results from the most recent SOLSTICE data set (Ver. 12) are closer to bothmodel results above 250 nm. The SIM results are more complicated and agree better withboth model results for wavelengths in the range of 310 nm to 340 nm and 380 nm to 400 nm,but agree less well in the range from 350 nm to 380 nm. While these recent versions showimproved agreement with both model results, significant differences still remain.
Figure 3 shows irradiance time series from SUSIM and SORCE data sets. The SUSIMtime series includes observations from the final two years of UARS operations, which over-lapped the first two years of the SORCE mission. Results from our SUSIM-SSI model overa majority of the SORCE time period are also shown. The observations and model estimatesof SUSIM irradiance agree very well, especially at shorter wavelengths. The SUSIM modelwas derived from observations earlier in the 14-year SUSIM mission (prior to April, 1999).Consequently, the close and detailed agreement between the SUSIM model results and themost recent SUSIM observations is an excellent indication of the internal consistency of theSUSIM results and tends to validate the stability of the SUSIM calibration and degradationcorrection.
During the overlap period of SORCE and UARS (2003 – 2005) shown in Figure 3, fiveSUSIM calibrations were conducted that included the use of onboard calibration lamps.These calibrations were used to correct the SUSIM observations. As noted above, if theSUSIM calibration method was inaccurate, then it is very likely that the SUSIM-SSI modelresults would have predicted higher values during this overlap period because of contin-ued sensitivity degradation. Because of the design of SORCE, no in-flight calibration withonboard light sources has been made for the SIM instrument, although the SOLSTICE in-strument did perform periodic stellar observations for calibration purposes. The SIM cor-rections involve the use of two spectrograph channels with interconnected optical systems(Harder et al., 2005b, 2009, including the supplementary material). An important point tonote from Figures 3(a) – (d) is that there is both an offset in irradiance and a difference inslope between the SUSIM and SORCE results. Both instruments were observing the sameobject during this overlap period, so offsets are related to differences in absolute calibration,and differences in the slope of the curves in Figure 3 are directly related to differences in theprocedures that correct for the degradation in instrumental sensitivity. One main concern isthat Figure 3 shows that the most recent SIM data set (Ver. 19) no longer has a minimum inirradiance that corresponds to the minimum in solar activity (2009).
In Figure 4, the short-term time series showed similar behavior for SORCE and SUSIMin all four passbands we examined. At the two shorter wavelength bands, the SOLSTICE andSUSIM observations and SUSIM-SSI model results all showed nearly identical variability.At the longer wavelengths, the variability of the SUSIM observations and model estimatewas of the same magnitude, but showed little temporal similarity. The SIM results showedsimilar variability except that it was larger than either of the SUSIM results; 10 times largerin the 315 – 335 nm passband and twice as large in the 360 – 390 nm passband. This short-term variability is not surprising in any of these passbands. Even if these results indicatethat the short-term variability is similar for both SUSIM and SORCE, the difference inthe long-term variability between these instruments is most important for understandingthe degradation in instrumental sensitivity. It is worth noting that the scale differences inshort-term variability for SIM relative to SUSIM in Figure 4(c) (315 – 335 nm) is similar tothe scale factor between the SIM irradiance differences and irradiance differences from theSUSIM-SSI or Lean (2000) model results shown in Figure 2.
To check the internal consistency of the SUSIM and SORCE observations over time, weexamined differences in 27-day average spectra on either side of the two most recent solarminima for days with the same Mg II index. Figure 5 shows the Mg II index plots for the last
Comparison of Solar UV Irradiance: SUSIM and SORCE
two solar minima and several dates with the same smoothed Mg II index values on eitherside of solar minimum. The assumption here is that days with the same Mg II index shouldhave nearly identical solar spectra. This comparison shows that the SUSIM differences tendto be very small below 300 nm and generally within 0.2 % of the solar spectrum above300 nm. The SORCE spectra, on the other hand, show much larger differences across thesolar minimum. Considering the results from the most recent SORCE data sets, these differ-ences in Figure 6, compared with Figure 2(b), are roughly 140 % at 290 nm for SOLSTICEand 100 % at 320 nm for SIM, and these differences tend to increase with increasing timebetween spectra. The significant differences between pairs of average SORCE spectra thatshould be nearly identical calls into question the SORCE long-term degradation correctionprocedure. Consequently, it is difficult to justify the validity of the solar UV irradiance dif-ference presented by Harder et al. (2009), Haigh et al. (2010), or even the results shownhere from the most recent SORCE data versions. If the apparent problems with the SORCEcalibration- and degradation-correction procedures in the UV are verified, then this will raisequestions about the procedures in the visible and near-IR, thus also calling into question theSORCE results at wavelengths above 400 nm.
If, on the other hand, we accept the SORCE irradiance spectra as accurate, we are pre-sented with alternatives that are difficult to reconcile. Either the Mg II index has never beenlinearly related (i.e., does not correlate well) with solar UV irradiances or it did so only dur-ing earlier solar cycles, but not during the time around the most recent solar minimum. If theindex has never been linearly related to irradiance, then the absolutely calibrated solar UVirradiance measurements, performed by experiments such as SBUV2 onboard NOAA-11,have reported inaccurate irradiances. A further consequence would be that the Mg II index,which had been understood to be an accurate proxy of solar activity, is neither an accuratenor an adequate proxy. If instead the Mg II index were correlated with solar UV irradiance,then both the Mg II index proxy and the SUSIM UV spectral irradiance measurements wereaccurate during earlier periods, but not now.
Acceptance of this latter case is especially difficult since the past interpretations of solarimages both in the Mg II and Ca II regions would need to be modified (Donnelly, White,and Livingston, 1994; Morrill, Floyd, and McMullin, 2011). We instead conclude that theSORCE degradation correction is inadequate in its present form or should at least be reeval-uated. The main conclusion here is that the different calibration methods for these two mis-sions have not produced a unified set of SSI observations. While we tend to favor the SUSIMcalibration for the various reasons we have mentioned, a reevaluation of the properties andcalibration procedures of both instruments will probably be required to resolve the differ-ences between the SUSIM and SORCE results.
9. Conclusion
We have compared observations from the UARS/SUSIM instrument with observations fromthe SORCE/SOLSTICE and SORCE/SIM instruments. This was done, in part, by examiningdifferences between spectra on either side of solar minimum at times of similar solar activityas well as solar spectral irradiance time series during the overlap period of SORCE andUARS. Several conclusions have been drawn from this analysis. First, neither the magnitudenor the slopes of the SUSIM and SORCE observations agree with one another during theoverlap period (2003 – 2005). Here the differences in slope are most important and indicatedifferences in the procedures that correct for degradation of instrumental sensitivity. Second,the SUSIM irradiance time series during this overlap period agree with model results based
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on SUSIM observations during the 1990s. This is significant since it shows that the SUSIMresults are internally consistent and tends to validate the SUSIM calibration procedures andstability of the SUSIM irradiance values over the course of the UARS mission. Lastly, whencomparing the irradiance for times of similar solar activity on either side of solar minimum,SUSIM observations show relatively smaller differences while the SORCE observationsshow greater variation that increases with increasing time between spectra. Based on theseresults, it is difficult to accept the validity of the current solar irradiance values from SORCE(Harder et al., 2009; Haigh et al., 2010) and this, in turn, calls into question the atmosphericmodel results that are based on the current SORCE observations (Cahalan et al., 2010; Haighet al., 2010; Merkel et al., 2011; Swartz et al., 2012; Wen et al., 2013; Ermolli et al., 2013).
The issues outlined by the present analysis suggest that further evaluation of the SORCEcalibration and degradation correction is called for. Similar conclusions have been made byseveral other studies (Ball et al., 2011; Unruh, Ball, and Krivova, 2011; DeLand and Cebula,2012; Lean and DeLand, 2012). While we consider the SUSIM SSI irradiance observationsto be more accurate than the SORCE results, especially with respect to solar variability,a review of both instruments and calibration procedures needs to be performed to confirm thestatus of both data sets. The result of such an effort would provide a two-decade long UV SSIdata set from both missions and validate the visible and NIR observations from SORCE. Toidentify the impact of solar spectral irradiance variability on climate, efforts of this kind mustbe made to ensure the quality of these and other historical data sets, such as SBUV, for use inclimate models (e.g. the European program SOLID; http://projects.pmodwrc.ch/solid/. Untilsuch an activity is completed, our understanding of the impact of solar spectral irradiancevariability on terrestrial climate will remain uncertain.
Finally, while a reevaluation of the SIM and SOLSTICE calibration and degradationcorrection procedures would apply to the SORCE mission, this reevaluation could also affectother versions of the SIM instrument (e.g. the Total and Spectral Irradiance Sensor (TSIS)).The SORCE/SIM instrument is a two-channel instrument, while newer versions (TSIS/SIM)will be three-channel instruments. The addition of a third channel may well provide a meansto avoid the apparent difficulties associated with a two-channel instrument with no on-boardcalibration light source capability. The proper use of these three channels, however, is likelyto require careful thought in the planning of mission operations to ensure the acquisition ofthe high-quality data set needed to understand the impact of solar variability on the changingclimate.
Acknowledgements This effort was supported by NASA grants NNH–10CC79C, NNH–05AB56I,W-10136. The SORCE data were downloaded from the SORCE web site at (http://lasp.colorado.edu/sorce/data/data_product_summary.htm) and the SUSIM data were downloaded from (http://wwwsolar.nrl.navy.mil/uars/). All data sets were used without correction. One of us (L. Floyd) acknowledges support of the In-ternational Space Science Institute (Bern) while working with the international team on interpretation andmodeling of SSI measurements. The authors gratefully acknowledge the efforts of L. Hutting in the prepara-tion of various aspects of this paper.
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