Comparison of Solar UV Spectral Irradiance from SUSIM and SORCE

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  • 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: jeff.morrill@nrl.navy.mil

    L. FloydFM Technologies, Chantilly, VA 20151, USAe-mail: linton4@gmail.com

    D. McMullinSpace Systems Research Corp., Alexandria, VA 22314, USAe-mail: donald.mcmullin@nrl.navy.mil

    mailto:jeff.morrill@nrl.navy.milmailto:linton4@gmail.commailto:donald.mcmullin@nrl.navy.mil

  • J.S. Morrill et al.

    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 SunEarthsystem, 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

  • J.S. Morrill et al.

    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

    http://wwwsolar.nrl.navy.mil/uars/

  • 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_produ...

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