NRLEUV 2: A new model of solar EUV irradiance variability

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    solar observations, full-disk solar images, and a database of atomic physics parameters to calculate the solar EUV irradiance. Recent

    ray (SXR) wavelengths (150 A) to many physical process-es in the Earths upper atmosphere and ionosphere, the

    ments have been taken by the Solar EUV Experiment(SEE) on the TIMED spacecraft (e.g., Woods et al.,1998a). In the absence of routine monitoring several empir-

    et al., 1994; Tobiska and Eparvier, 1998).To complement existing solar irradiance measurements

    observations of the Sun. In our model emission measuredistributions for the quiet Sun, coronal holes, and activeregions are used to compute synthetic spectra for thesesolar features. The synthetic spectra are then combinedwith limb-brightening curves and areas derived from thefull-disk solar images to compute the irradiance for a givenE-mail address: hwarren@nrl.navy.mil

    Advances in Space Research 3magnitude and variability of the solar irradiance at thesewavelengths is not well understood. During the past severalsolar cycles routine irradiance monitoring in the EUV hasbeen carried out very rarely. Most of our knowledge onEUV irradiance variability has been derived from theAtmospheric Explorer E (AE-E) observations takenbetween 1976 and 1980 (e.g., Hinteregger et al., 1981).Since February 2002 routine EUV irradiance measure-

    and the empirical models derived from them, we havedeveloped a new approach to modeling solar EUV irradi-ance variability. Our model, which we refer to asNRLEUV, is based on intensities derived from emissionmeasure distributions and full-disk solar images. The emis-sion measure distributions, which attempt to describe thetemperature and density structure of the solar atmosphere,are determined from spatially and spectrally resolvedupdates to the model include the calculation of a new quiet Sun dierential emission measure distribution using data from the CDSand SUMER spectrometers on SOHO and the use of a more extensive database of atomic physics parameters. Here, we present com-parisons between the NRLEUV quiet Sun reference spectrum and solar minimum irradiance observations. Although there are manyareas of agreement between the modeled spectrum and the observations, there are some major disagreements. The computed spectracannot reproduce the observed irradiances at wavelengths below about 160 A. The observed irradiances appear to overstate the magni-tude of the EUV continua. We also present some initial comparisons between the NRLEUV irradiance variability model and TIMED/SEE data. We nd that the NRLEUV model tends to overpredict the absolute magnitude of the irradiance at many wavelengths. Themodel also appears to underpredict the magnitude of the solar-cycle and solar rotational variation in transition region emission lines. 2006 Published by Elsevier Ltd on behalf of COSPAR.

    Keywords: Solar cycles; EUV irradiance

    1. Introduction

    Despite the importance of solar radiation at extremeultraviolet (EUV) wavelengths (501200 A) and soft X-

    ical models have been developed which use the existingirradiance observations and proxies for solar activity topredict the solar irradiance on days when observationsare not available (e.g., Hinteregger et al., 1981; RichardsNRLEUV 2: A new model of

    Harry P

    E.O. Hulburt Center for Space Research, Code 7670, N

    Received 15 February 2005; received in revised f

    Abstract

    NRLEUV represents an independent approach to modeling theing irradiance observations, our model utilizes dierential emission0273-1177/$30 2006 Published by Elsevier Ltd on behalf of COSPAR.doi:10.1016/j.asr.2005.10.028lar EUV irradiance variability

    arren

    l Research Laboratory, Washington, DC 20375, USA

    27 September 2005; accepted 19 October 2005

    ns EUV irradiance and its variability. Instead of relying on exist-easure distributions derived from spatially and spectrally resolved

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  • peaked function of temperature and observations of manydierent lines are required to determine the dierentialemission measure distribution.

    In the earlier version of the irradiance model we usedobservations of EUV emission lines from the Harvardspectrometer on Skylab (Vernazza and Reeves, 1978).For our new survey of the quiet Sun we have used 20CDS spectral atlas observations taken between Marchand June 1996. Each CDS spectral atlas covers an area2000 24000. Approximately 20,000 individual spectra wereaveraged together to construct a composite spectrum fromwhich the line intensities were derived. The CDS observa-tions cover the upper transition region and corona. Toextend the emission measure to lower temperatures weuse SUMER observations taken February 9 and 22 1997.During these observations the SUMER slit was movedalong the central meridian from south pole to north pole.As many as 9500 individual spectra were used to computeintensities for each spectral line. For both the CDS and

    ace Research 37 (2006) 359365day. A detailed description of the NRLEUV model is givenby Warren et al. (1998a) and Warren et al. (2001). Compar-isons between our modeled irradiances and observationsand empirical models are given by Warren et al. (1996),Warren et al. (1998b), and Lean et al. (2003).

    Recently, Warren (2005) updated the NRLEUV quietSun irradiance spectrum to consider solar spectra takenwith the Coronal Diagnostic Spectrometer (CDS) and theSolar Ultraviolet Measurements of Emitted Radiation(SUMER) spectrometers own on the Solar and Helio-spheric Observatory (SOHO). These spectrometers havehigh spectral resolution, which allows us to better observemany more emission lines than were available with earlierinstruments. Furthermore, the SOHO spectrometers havetaken extensive observations, which allows us to constructcomposite spectra that are more representative of the quietSun. Finally, in computing the latest NRLEUV quiet Sunirradiance spectrum we have utilized the newest version(4.2) of the CHIANTI atomic database Young et al.(2003). The new version of CHIANTI contains emissivitiesfor about 50,000 lines, compared with only about 2000 forthe database used in computing the earlier version of themodel.

    In this paper, we present a brief overview of the modeland a review of the similarities and dierences between cal-culations from NRLEUV and irradiance observations ofthe quiet Sun. We have found that the CDS and SUMERquiet Sun observations are generally consistent with theearlier Harvard Skylab measurements. One area of dis-agreement is at the highest temperatures. The dierentialemission measure distribution derived from the SOHOobservations is signicantly lower than the Harvard SkylabDEM at the highest temperatures. We also present someinitial comparisons between the NRLEUV irradiance var-iability model and the TIMED/SEE data. We nd somesignicant dierences between our calculated irradiancetime series and the observed irradiances.

    2. The NRLEUV model

    2.1. Emission measure formalism

    In our calculations the observed radiance for an optical-ly thin emission line depends both on the atomic transitionsinvolved and on the conditions in the solar atmosphere,

    Ikul ZTGulT nT dT . 1

    The quantity nT n2e ds=dT is the dierential emissionmeasure, ne is the electron density, ds is the dierential dis-tance along the line of sight, and T is the electron temper-ature. The quantity Gul (T) is the radiant power density,which is often referred to as the contribution function,and contains all of the factors which are known (or as-sumed to be known) about the transition, such as the rate

    360 H.P. Warren / Advances in Spof collisional excitation and the elemental abundance of theemitting atom. The power density is typically a stronglySUMER observations we have only used data within40000 of disk center so that limb-brightening does not eectthe observed intensities.

    The intensities derived from our new survey of quiet Sunintensities observed with the CDS and SUMER spectrom-eters are generally in good agreement with those derivedfrom the Harvard instrument. For example, the intensitiesof the three emission lines that were observed by all threeinstruments, He I 584 A, Mg X 625 A, and O V 630 A, allagree to within 25% of each other. Furthermore, we alsond generally good agreement when we compare theobserved and theoretical ratios for many emission lines.For example, the various ratios for all 10 of the O III emis-sion lines observed with SUMER and CDS lie within25% of theoretical calculations.

    The dierential emission measure derived from our newsurvey of quiet Sun intensities is shown in Fig. 1. For this

    4.5 5.0 5.5 6.0Log T (K)

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    Fig. 1. The quiet Sun dierential emission measure. Distributions derived

    from both Harvard Skylab (dotted line) and SOHO (solid line) observa-tions are shown. From Warren (2005).

  • work the emission measure is represented as a series ofspline knots. The values of the emission measure at thespline knots are varied to minimize the dierences betweenthe observed and computed intensities. The most signi-cant change in the new emission measure is at temperaturesabove about 1 MK. This dierence is driven by the smallradiance of the Si XII 520.68 A line in the CDS spectra rel-ative to previous observations. Vernazza and Reeves (1978)report 25.44 erg cm2 s1 sr1 for this line while we mea-sure 4.98 0.06. Emission lines from other ions formedat high temperatures such as Fe XII, Si X, Si X, and FeXIV, are consistent with the lower values for the emissionmeasure.

    The dierential emission measure distribution allows usto compute the disk center intensities of optically thin emis-sion lines at any wavelength. The disk-integrated ux (theirradiance) is related to the disk center radiance by therelation

    F k pR2R2

    hIki; 2

    where Rx is the solar radius, R is the EarthSun distance,and Ik is the disk-averaged radiance:

    hIki 2IkZ 10

    Rkll dl. 3

    In this expression Ik is the specic radiance at disk-center,Rk (l) Ik (l)/Ik (l = 1) parameterizes the center-to-limbvariation of the radiance, and l = cosh, where h is theheliographic latitude. For optically thin emission the radi-ance at the limb increases with the path length through thesolar atmosphere, which indicates Rk (l) = 1/l. The inten-sities of optically thick emission lines are generally ob-served to be independent of l (see Warren et al., 1998a),corresponding to Rk (l) = 1.

    2.2. Calculated irradiances

    Model irradiances are computed by convolving theDEM with the emissivity for each line computed with CHI-ANTI and applying the simple limb-brightening curve (Eq.(3)). The contribution of optically thin freefree thermalbremsstrahlung is also included. The irradiances for theoptically thick EUV emission lines and continua are deter-mined from the observed values. The intensities for theoptically thick lines and continua determined from thepresent CDS and SUMER observations are also very closeto the intensities used in our previous model. The irradi-ances for these lines are computed assuming no limbbrightening.

    The computed solar EUV irradiance spectrum for thequiet Sun is shown in Fig. 2. For comparison, irradiances

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    NCAR/NRLEUV v2Fig. 2. Quiet Sun irradiances from NRLEUV and Woods et al. (1998b). Thefunction of wavelength. From Warren (2005).

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    bottom panel shows the ratio of the new model to the observations as a

  • from the 1994 Woods et al. (1998b) rocket ight are alsoshown. There are two signicant areas of disagreementbetween the NRLEUV irradiance spectrum and the obser-vations. First, it is clear that the continuum irradiancesmeasured during the rocket ight are systematically higherthan those in the NRLEUV model. The dierences are par-ticularly large in regions where the irradiances are relativelysmall, such as the regions near 600 and 900 A. The intensi-ties observed with SUMER in these wavelength ranges gen-erally agree with those from the Harvard spectrometer,although it appears that the NRLEUV model is missingsome relatively weak optically thick emission linesKretzschmar et al. (2004). The inclusion of these lines isnot enough to account for this discrepancy, suggesting thatinstrumental eects in the rocket data lead to an overesti-mate of the irradiance at these wavelengths.

    The second obvious problem is the uxes at wavelengthsbelow about 160 A. The observed irradiances at thesewavelengths are about a factor of 4 greater than thoseinferred from the model. The SC21REFW uxes Hintereg-ger et al. (1981) at these wavelengths, however, are general-ly close to those we have calculated. Terrestrialphotoelectron observations have suggested that the solarirradiances at these wavelengths given in SC21REFW aretoo small by factors of 23 (e.g., Richards et al., 1994).

    To investigate this wavelength range in more detail wecompare the NRLEUV and SC21REFW reference spectrawith the solar minimum irradiance observations of Manson(1972) and recent observations of the Sun-like star a Cent-auri A taken with the Low Energy Transmission Grating(LETG) on Chandra (Raassen et al., 2003). As is shownin Fig. 3, these comparisons indicate that the solar spec-trum at these wavelengths is not well understood. Manyemission lines in the observed spectra are not representedin our calculated spectrum or in the SC21REFW referencespectrum. The CHIANTI database clearly needs to beupdated further to include the atomic data relevant to theseemission lines. The Woods et al. (1998b) rocket data utiliz-es model spectra based in part on SC21REFW to convertthe uxes measured with broad band silicon photodiodesto spectrally resolved irradiances. Thus, the dierencesbetween NRLEUV and the rocket data are also likely tobe inuenced by the absence of some emission lines inthe SC21REFW spectrum.

    3. Solar variability

    We have not yet integrated SOHO CDS and SUMERobservations into the NRLEUV solar variability model.Like the previous version of the quiet Sun spectrum, the

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    Fig. 3. Solar and stellar irradiances at wavelengths between 60 and 120 Apanels. Solar irradiances from NRLEUV and SC21REFW are shown in theof a Centauri A are shown in the bottom panel (Raassen et al., 2003). The N

    Gaussian smoothing kernel that matches the spectral resolution of the Mansonfeatures that are missing from the NRLEUV and SC21REFW spectra. From

    100 120elength ()

    en et al. 2003)

    he solar minimum observations of Manson (1972) are shown in all threetwo panels. Stellar irradiances at 1 AU from Chandra LETG observationsEUV, SC21REFW, and a Centauri irradiances have been convolved with a

    (1972) observations. The Manson (1972) observations show many emissionWarren (2005).

  • rst version of the NRLEUV solar variability model wasbased on active region observations from the Harvard Sky-lab spectrometer published by Vernazza and Reeves (1978).The reliance on a single active region spectra is likely to bea signicant limitation of the current model. Our expecta-tion is that intensities averaged over an active region arelikely to dier greatly from region to region and any oneregion is unlikely to be representative. We are currentlyanalyzing a large number of CDS active region spectra toevaluate the range of active region intensities.

    Comparisons between the NRLEUV model and exist-ing EUV irradiance models have been carried out byLean et al. (2003). These comparisons showed that theNRLEUV model tended to predict both lower absoluteirradiances and small solar cycle-variability than theother models. For example, between 50 and 1050 ANRLEUV predicts an average total EUV energy of2.9 mW m2. This is smaller than the total EUV energiesof 3.75.6 mW m2 predicted by the empirical models.Similarly, the NRLEUV predicts a solar cycle variationof 1.9 in the total EUV energy, compared with variationsof 2.32.9 predicted by the empirical models. Lean et al.(2003) concluded that the NRLEUV reproduced theobserved solar rotational modulations as well or betterthan other models.

    We have made some preliminary comparisons betweenthe latest release of the SEE observations (data version7) and calculated NRLEUV irradiances. In theNRLEUV model we have used image data from the peri-od between 1991 and 1996 to calculate irradiance timeseries. These time series have been correlated with prox-ies for solar activity (the F10.7 cm radio ux and the MgII core-to-wing ratio) to extend the irradiance model totimes outside of this period. The correlation coecientfor the irradiances computed from the images and theproxies for solar activity are generally very high(0.95), suggesting that the use of the proxies does notintroduce signicant errors into time series computedfrom the model. This assumption, however, has yet tobe thoroughly tested.

    For these comparisons we convolve the NRLEUV spec-tra calculated for each day with a triangular window 1.67 AFWHM. We then select a narrow wavelength range aroundeach line from both the SEE observations and the modelcalculations. Spectra and irradiance time series for 4 wave-length ranges are shown in Figs. 4 and 5. These compari-sons show both areas of agreement and signicantdierences. The absolute irradiances are generally within30% of each other. If we scale the NRLEUV irradiancesto match the average values observed with SEE, both the

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    Fig. 4. Solar irradiances from SEE and the NRLEUV model. (Top paneldotted vertical lines indicate the wavelength range used to compute the ir

    spectra. (Bottom panels) Observed and modeled irradiance time series for thesehave been scaled to match the average value of the SEE observations. The sca610 620 630 640 650Wavelength ()

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    ectra for the wavelength ranges near 304 A (He II) and 630 A (O V). Theiances. No scalings have been applied to either the observed or modeled

    wavelength ranges. To facilitate comparisons the NRLEUV model spectraling factor is indicated in the gure.

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    364 H.P. Warren / Advances in Sprotational modulation and solar cycle variability are alsoclose for some lines. The irradiances for the O V line arean obvious exception to this. The NRLEUV model clearlyunderestimates both the rotational modulation and thesolar-cycle variability by a signicant amount. Compari-sons for O II/O III 835 A, O IV 554 A, and O VI 1032 A alsoshow similar discrepancies. The low levels of solar cycleand rotational modulation result from the relatively lowquiet Sun to active region contrasts indicated in the Vern-azza and Reeves (1978) spectra. The observed contrast forO V 630 A, for example, is only a factor of 4. Interestingly,the observed contrasts for the optically thick emission linesare somewhat greater, about 13 for He I 584, 522, and517 A, and about 10 for He II 304 A. The observed varia-tions for these lines are more closely reproduced by themodel, suggesting that the typical active region intensitiesfor the optically thin transition region emission lines muchlarger than what is suggested by the Vernazza and Reeves(1978) data.

    The comparisons between SEE and NRLEUV alsoshow signicant dierences between the modeled andobserved values for the underlying continua. The irradianc-es near the peaks of the strong emission lines are generallyclose. In the regions where the uxes are weak, however,there are dierences of an order of magnitude or more.These dierences are also evident in many of the otherwavelength ranges we have examined.

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    Fig. 5. Observed and modeled solar irradiances for the wav340 350 360 370 380 390Wavelength ()

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    Research 37 (2006) 3593654. Summary

    We have presented an overview of recent updates to theNRLEUV irradiance model. The comparison of the SOHOCDS and SUMER quiet Sun data with similar observa-tions from the Harvard Skylab instrument Vernazza andReeves (1978) and the results from an irradiance rocketight Woods et al. (1998b) indicate that the solar EUVspectrum in the quiet Sun is reasonably well understoodat wavelengths above 160 A. These comparisons also indi-cate that there are some processes that the dierential emis-sion measure formalism cannot account for.

    The comparisons between SEE observations andNRLEUV irradiance model indicate some signicant dif-ferences, particularly for emission formed in the transitionregion.

    Future work will focus on using observations from CDSand SUMER to model active region emission. The primarylimitation of our previous work is the reliance on a singleactive region spectrum to dene the active region dieren-tial emission measure. Also note that the intensities fromthe active region DEM were mapped to Ca II k-line inten-sities by assuming a linear relationship between the two(see Lean et al., 1998). Since Ca II images are not availablefor the specic active region in the Vernazza and Reeves(1978) active region spectrum, this linear relationship isnot well constrained by the observations. As is illustrated

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    elength ranges near 780 A (Ne VIII) and 368 A (Mg IX).

  • nd Cdie

    aceby the PSPT and CDS observations of an active regionshown in Fig. 6, there are signicant dierences betweenthe Ca II k-line and transition region emission.

    Despite these dierences it still may be possible to usethe Ca II k-line images to accurately infer the distributionof intensities in the transition region. Studies have shownthat the total intensity in an EUV emission line from anactive region is well correlated with the total unsigned mag-netic ux from that active region (e.g., Fludra and Ireland,2003; Schrijver, 1987). Unfortunately, the magnetic ux isdicult to measure near the limb, making it dicult touse magnetograms as the basis for irradiance modeling.The Ca II k-line intensity and the unsigned magnetic uxare well correlated Schrijver et al. (1985), thus it shouldbe possible to correlate k-line intensities with EUV lineintensities. There is a wealth of high quality Ca imagesand CDS active region spectrum available for the descend-ing phase of the current solar cycle.

    Acknowledgments

    PSPT Ca II k 27Jun2001 17:23:19

    Fig. 6. A precision Solar photometric telescope (PSPT) Ca II k-line image ain common, the variations in intensity across the active region are clearlyH.P. Warren / Advances in SpThe SEE experiment on the TIMED mission was spon-sored by NASAs Oce of Space Science. The PSPT Caimage is courtesy of HAO PSPT project team. HAO is adivision of the National Center for Atmospheric Researchwhich is supported by the National Science Foundation.

    References

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    NRLEUV 2: A new model of solar EUV irradiance variabilityIntroductionThe NRLEUV modelEmission measure formalismCalculated irradiances

    Solar variabilitySummaryAcknowledgmentsReferences