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Advances in Space Research 46 (2010) 296–302

Solar spectral irradiance variability in the ultraviolet from SORCEand UARS SOLSTICE

M. Snow *, W.E. McClintock, T.N. Woods

Laboratory for Atmospheric and Space Physics, University of Colorado, 1234 Innovation Dr, Boulder, CO 80303, USA

Received 23 November 2009; received in revised form 26 March 2010; accepted 30 March 2010

Abstract

The SOLar-STellar Irradiance Comparison Experiment (SOLSTICE) on the SOlar Radiation and Climate Experiment (SORCE) hasbeen measuring the solar spectral irradiance on a daily basis since early 2003. This time period includes near-solar maximum conditions,the Halloween storms of 2003, and solar minimum conditions. These results can be compared to observations from the SOLSTICE Iexperiment that flew on the Upper Atmosphere Research Satellite (UARS) during the decline of the previous solar cycle as well as withcurrently operating missions. We will discuss similarities and differences between the two solar cycles in the long-term ultraviolet irra-diance record.� 2010 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Solar ultraviolet irradiance; Solar cycle variability

1. Introduction

Solar spectral irradiance (SSI) in the far ultraviolet(FUV: 120–200 nm) and middle ultraviolet (MUV:200–300 nm) is the primary driver for changes in the heat-ing, composition, and dynamics of the middle atmosphere.Variations in stratospheric ozone are largely produced bysolar UV irradiance changes, and the long term behaviorof UV SSI may even play a role in global climate change(Lean et al., 2005). UV irradiance accounts for only a smallfraction of the total solar irradiance (TSI), but since it isformed in the solar chromosphere, it is much more variablethan the visible irradiance produced by the photosphere.Therefore it contributes significantly to the total TSI vari-ability. Understanding the long-term variation of this spec-tral region is an important part of understanding thenatural drivers of global climate change.

The SOLar-STellar Irradiance Comparison Experiment(SOLSTICE) concept has flown on both the Upper Atmo-

0273-1177/$36.00 � 2010 COSPAR. Published by Elsevier Ltd. All rights rese

doi:10.1016/j.asr.2010.03.027

* Corresponding author.E-mail addresses: snow@lasp.colorado.edu (M. Snow), mcclintock@

lasp.colorado.edu (W.E. McClintock), woods@lasp.colorado.edu (T.N.Woods).

sphere Research Satellite (UARS) and SOlar Radiationand Climate Experiment (SORCE) platforms (Rottmanet al., 1993; McClintock et al., 2005). The former waslaunched in 1991 and operated from the peak of solar cycle22 through the peak of cycle 23. The latter instrument waslaunched in 2003, slightly after the peak of cycle 23, andcontinues to operate, measuring the solar spectral irradi-ance (SSI) from 115 to 320 nm. Although the two instru-ments are not identical, they share many common designelements, particularly in their FUV channels. Both aregrating spectrometers with 0.1 nm spectral resolution and115–180 nm FUV passbands. They also both rely on stellarirradiance observations for tracking instrumental changes,thus the long-term accuracy for each SOLSTICE is on theorder of 0.5% for most wavelengths (Snow et al., 2005a).The SOLSTICE MUV channel is similar but covers the170–320 nm range. Only the solar FUV results throughmid-2008 are presented in this paper.

2. FUV variability

In order to make a comparison of the solar variabilityobserved during the UARS and SORCE epochs, we must

rved.

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first choose equivalent time intervals. Our strategy was tofind the time of minimum rotational activity in each datasetand then consider the 5 year period leading up to it, plus anadditional 6 months following minimum. Fig. 1 shows theMg II index for the two time periods. The following sec-tions will describe the overall behavior of the FUV irradi-ance during these two time periods in an attempt todetermine differences and similarities in the two datarecords.

2.1. Rotational variation

Because active regions on the Sun have lifetimes of severalmonths on average and active network can take even longerto decay (Worden et al., 1998), the rotation of the Sun intro-duces a strong 27-day periodicity to the solar irradiance timeseries. This variability on rotational time scales is larger dur-ing times when there are more (and larger) active regions onthe sun. During times of low activity, the rotational modula-tion of the irradiance can become very small.

The top panel of Fig. 2 shows the time series of the com-posite Mg II index (Viereck et al., 2004; Viereck and Puga,1999) for the two time periods. The index measurementsfrom the two time periods have been overplotted as a func-tion of elapsed time in days since the start of the interval.The bottom panel shows the statistical variance,Pðxi � �xÞ2, of the Mg II index over an 81-day moving win-

dow. Time periods with a large rotational amplitude willhave a relatively larger variance in the measurements. Assolar activity decreases, the variance in the index will alsodecrease.

The published Mg II time series (Viereck et al., 2004)needed to be extended to the current epoch (R. Viereck,

Fig. 1. Equivalent time periods in solar cycle 22 and 23 determined by matchingyears leading up to minimum plus 6 months afterwards. The period from 1992 tcycle 23 from 2003 to 2008 was measured by SORCE SOLSTICE.

private communication) for use in this study. This exten-sion was done using the same methodology as for the pub-lished result. Note that this composite is not the same asthe dataset available on the NOAA ftp site.

2.2. Solar cycle variation

The variation in SSI shown in Fig. 2 is for just one wave-length (280 nm). The next logical step in the comparison ofthe two solar cycles is to look at the long-term variability asa function of wavelength. Fig. 3 shows the ratio of the max-imum yearly average irradiance to the minimum yearlyaverage irradiance for both SORCE and UARS time-frames. In both datasets, the irradiance has been binnedto 1-nm intervals for comparison.

The solar cycle variability observed during the SORCEmission is similar to the results described in Rottmanet al. (2001) for UARS SOLSTICE during cycle 22. In bothsolar cycles, the magnitude of change of the continuumincreases smoothly from about 5% at 180 nm to about20% near 120 nm. The emission lines have a greater vari-ability, on the order of 30–50%. The magnitude of thelong-term variability observed by SORCE SOLSTICE isgenerally smaller than was measured by UARS SOL-STICE. The following section will use a proxy model totry to determine if these differences are to be expected ornot.

2.3. Variation predicted by proxy

Although the UARS is no longer operational, there wasa 2-year period of overlap between it and SORCE. Fig. 4shows the time series of irradiance at one wavelength for

minimum rotational activity levels. The time periods highlighted are the 5o 1997 was measured by UARS SOLSTICE, and the corresponding part of

Fig. 2. Rotational variability based on Mg II index. (top) Mg II index for the time ranges shown in Fig. 1 overplotted as a function of days elapsed sincethe start of the time period for both datasets. (bottom) Variance in the index over a moving 81-day window.

Fig. 3. Solar cycle variability as a function of wavelength for cycles 22 and23 in the FUV. The variability of the continuum rises smoothly withdecreasing wavelength. Sharp features in the variability are due tochromospheric emission lines, which have a significantly larger variabilitythan the continuum.

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both datasets. Unfortunately, only a small subset of theoverlapping UARS data has been processed so far. Thatdata are shown as plus symbols in Fig. 4.

As the UARS spacecraft aged, some of the subsystemssuffered failures. The tape recorder ceased operations inlate 1999, and from that point forward, only data takenduring real-time contacts was transmitted to the ground.Since these contacts were intermittently scheduled, the datarecord had many gaps after 1999. The computer codes thatprocessed the data at the UARS Central Data Handling

Facility (CDHF) could not handle these gaps properly.New processing code running on SORCE computers havebeen able to process that data, and the new version 19 isshown in Fig. 4. Some data from early in the UARS mis-sion were processed with this new system as well as selecteddays in the later stages of the mission. When this codebecomes fully operational, we will be able to examine theoverlap from 2003 to 2005 in much greater detail. Thenet effect is that the comparison of the two solar cycles can-not be made with overlapping measurements at the currenttime, so a proxy model based on the UARS SOLSTICEmeasurements and then extended to the SORCE era mustbe used.

In this case, we will use a proxy irradiance based on theMg II index (Heath and Schlesinger, 1986; Viereck andPuga, 1999; Viereck et al., 2004) to connect the UARSand SORCE epochs. The Mg II index is the ratio of thechromospheric emission near 280 nm to the emission fromthe photospheric pseudo-continuum on either side of it.Since it is a ratio of irradiances, it is less sensitive to instru-mental artifacts than a single irradiance measurement.

To create this proxy model, the Mg II index is scaled tomatch the measured irradiance over a specified time inter-val. Since the formation region in the solar atmosphere forthe core of the Mg II doublet is about the same as that forFUV lines, changes in the Mg II index are highly correlatedwith changes in the FUV irradiance. We can make use ofthis fact to determine scaling factors (also known as con-trast ratios) for each wavelength. This method of determin-ing the linear regression coefficients between the index andirradiance has been used by many authors such as DeLand

Fig. 4. Time series irradiance for both UARS and SORCE SOLSTICEs. The plus symbols are UARS SOLSTICE data that have been processed onSORCE computer systems (see text for details).

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and Cebula (1998) and Floyd et al. (2002) to create anempirical model irradiance. Fig. 5 shows the correlationbetween the irradiance at a typical FUV wavelength(155 nm) and the Mg II index. The scaled index measure-ment can then be used as a proxy for the irradiance atany epoch where the index is available. The NOAA com-posite Mg II index time series is continuously available

Fig. 5. Mg II proxy scaling factor. (top panel) The 1 nm binned irradiance meatime range as the irradiance measurement. (bottom panel) Correlation betweenshown in the lower right.

from 1978 to the present, so it is well-suited to comparisonsbetween the UARS and SORCE time frames.

This simple proxy model can be used to predict the FUVirradiance with a precision of about 1%. The left panel ofFig. 6 shows the correlation coefficient for the Mg II indexand 1-nm binned irradiance as a function of wavelength forthe entire FUV range. The correlation is extremely good

sured by UARS SOLSTICE. (middle panel) The Mg II index over the samethe Mg II index and the observed irradiance. The correlation coefficient is

Fig. 6. (left) Correlation coefficient between the scaled Mg II index and the measured irradiance as a function of wavelength. The correlation is extremelygood throughout the FUV range, only decreasing above 175 nm. (right) Precision of proxy model. This plot shows the ratio of the proxy model to themeasurements for the data shown in Fig. 5. The mean is centered at 1 with a Gaussian width of less than 1%.

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because the Mg II core at 280 nm is formed in the samepart of the solar atmosphere as the FUV. The correlationdoes decrease at the long-wavelength end of the FUVrange, but is still above 90% at all wavelengths consideredin this study.

The right panel of Fig. 6 shows the precision of theproxy model. The ratio of the predicted to observed irradi-ances is a distribution that is centered at 1 with a Gaussianwidth of 0.7%. Therefore one could conclude that the dif-ference between the proxy model prediction and theobserved shown in Fig. 7 is not likely due to any statisticalerror in the model.

Fig. 7 shows the size of the solar cycle predicted for theUV during the SORCE era. The proxy model agrees verywell with the SORCE measurements above about155 nm. The 140–155 nm region predicts a smaller cyclethan observed by about 5%. The observations and modelcome back into agreeement from 140 to about 130 nm,

Fig. 7. Solar cycle variability predicted by proxy scaling factors. There isgood agreement between the model prediction and the SORCE measure-ments at some wavelengths, but about a 5% difference at others.

and then the model predicts a larger cycle than observedfor the shortest wavelengths. Fig. 11 from Snow et al.(2005a) indicates that the degradation at 150 nm for SOR-CE SOLSTICE is larger than at surrounding wavelengths,and therefore might be less well known. The proxy modeluses a chromospheric index, so one might expect that itdoes not reproduce transition region behaviors (such asLyman a) very well. That may explain the lack of agree-ment at the shortest wavelengths.

3. Minimum reference spectrum

An additional useful comparison that can be madebetween the two solar cycles is the minimum irradiancespectrum. Fig. 8 (left) shows the solar irradiance binnedto 1 nm for the dates of minimum activity. The SORCEspectrum at minimum shows a lower absolute irradiancein 2007 than UARS SOLSTICE observed in 1996.

The right hand panel of Fig. 8 shows the ratio of the twospectra from the left hand side, but also divided by theratio of the calibration difference between the two instru-ments. Fig. 13 of McClintock et al. (2005) shows the ratioof the two irradiances at the start of the SORCE mission.Any systematic errors in the calibration between the twoinstruments will be captured in this ratio, so it can be usedto normalize the ratio of the solar minimum spectra. Thequantity plotted in the right panel of Fig. 8 is thus

R ¼ ESORCE2007=EUARS1996

ESORCE2003=EUARS2003

: ð1Þ

This ratio is a combination of a real change in solar min-imum in the current cycle relative to the previous cycleand uncorrected degradation in UARS and/or SORCESOLSTICE. From the shape of this ratio, it would seemthat the region from 140 to 155 nm is anomalously low.As discussed above, this is likely due to an underestimateof the degradation in the current version of the SORCE

Fig. 8. (left) FUV solar spectrum at minimum activity for both solar cycle 22 and 23 in W m�2 nm�1. (right) Ratio of spectra for the two solar cycles afternormalizing for calibration differences between the UARS and SORCE instruments. The current solar minimum is about 5% lower than the previousminimum.

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SOLSTICE data. Otherwise, the ratio is a fairly constant6% lower irradiance in 2007 than at the previous solarminimum in 1996. The TSI record also shows a 0.2% low-er value for 2007 compared to 1996 using the Frolich andLean composite (Frohlich and Lean, 1998) including up-dates on the PMO ftp site. Therefore a lower value forthe UV SSI at minimum in cycle 23 relative to cycle 22is quite possible.

4. Summary and conclusions

We have compared the behavior of the SSI during thedeclining phases of cycles 22 and 23 as observed by theSOLSTICE instruments on the UARS and SORCE space-craft. The irradiance variations measured by SORCE SOL-STICE are generally similar to those reported for UARSSOLSTICE (Rottman et al., 2001) and SUSIM (Floydet al., 2002), as one might expect.

The observed solar cycle variability is larger than wouldbe predicted from the composite Mg II index proxy modelat some wavelenths. In the 140–155 nm region, this couldbe due to a poorly understood degradation correction. Atshorter wavelenths, it could be due to differences in chro-mospheric and transition region variability.

The absolute irradiance at in November, 2007 is approx-imately 6% lower than the cycle 22 minimum seen in 1996.However, this finding is only significant at about the 1 � rlevel. The major sources of uncertainty in this result arefrom the degradation functions of both UARS and SOR-CE SOLSTICE. The uncertainty in tracking the degrada-tion correction for both SOLSTICE instruments is on theorder of 0.5% per year. The uncertainty for this correctionin each SOLSTICE at the time of solar minimum (1996 or2007) is approximately 2.5%, thus the ratio is uncertain toabout 4%. The next largest source of uncertainty is theabsolute calibration of the two instruments. When the

two effects are added in quadrature, the total uncertaintyof the solar minimum ratio is approximately 5%. So it islikely in a statistical sense that the irradiance in 2007 waslower than it was in 1996, the uncertainties in the measure-ments make it difficult to accurately say how much lower itwas.

Future refinements to the SORCE degradation functionmay reduce this uncertainty somewhat, and using the SOL-STICE stellar measurements may also improve the accu-racy of the calibration correction. Initial results usingstellar spectra indicate that the minimum irradiance ratioat the long-wavelength end of the FUV range is closer to1 than is shown in Fig. 8. Work on improving our under-standing of the difference in ultraviolet irradiance levelbetween the two minima is ongoing.

All of the SOLSTICE irradiance data used in this studyare available for download from the LASP InteractiveSolar Irradiance Datacenter (LISIRD) http://lasp.colo-rado.edu/lisird/ (Snow et al., 2005b).

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