Mem. S.A.It. Vol. 76, 850c© SAIt 2005 Memorie della

Calculating solar UV spectral irradianceusing observed spectral radiance

and full disk Ca II K images

J. Morrill

E. O. Hulburt Center for Space Research, Naval Research Laboratory, Washington DC20375-5352, USAe-mail: morrill@shogun.nrl.navy.mil

Abstract. An empirical model of solar UV spectral irradiance has been developed at theNaval Research Laboratory. This model uses observed spectral radiance measurements fromthe HRTS-9 rocket and the UV spectrograph on SKYLAB in conjunction with full disk CaII K images to identify various solar surface features on order to estimate the full diskirradiance. The initial wavelength coverage was a narrow range (180 Å) near the Mg IIdoublet at 2800 and was derived from HRTS-9 spectral observations. This range is beingexpanded to cover from roughly 2000 to 4000 using SKYLAB data and future plans includethe extension of this range down below 1000 Å.Currently the model identifies three surface features: quiet sun, active sun, and sunspots.The model requires detailed knowledge of the center-to-limb variation (CLV) of these threefeatures. Originally, the CLV was only available for the quiet sun and so this is now usedfor all three features. However, we will be examining other data and theoretical sourcesof CLV for active regions and sunspots. Since the spectral radiance observations are fromfilm data, calibration required the use of irradiance spectra on a set of extremely quiet daysfrom the SUSIM and SOLSTICE instruments on the UARS satellite. Currently, the modelhas concentrated on the time period between 1990 through 1995 but this is being expandedtoward the present in order to extend the comparison between model and observed (UARS)spectra.This paper is a review of the model that will include the latest understanding of the CLVof the quiet sun, the methods of calibration, and the identification of the above surfacefeatures. Future plans to utilize the recently digitized film archives of Ca II K images, suchas Mt Wilson, and the Observatories of Rome and Paris, will also be discussed in additionto the planned methods to model the spectrum into the EUV.

Key words. Sun: irradiance – Sun: atmospheres

1. Introduction

Recent efforts to produce estimates of total andspectral solar irradiance that occurred duringthe last several centuries have relied on sun

spot records, the aa geomagnetic index, andtime profiles of cosmogenic isotopes from treerings (14C)and ice cores (10Be). Although thesemethods have varying levels of reliability theystill suffer from uncertainty especially with re-

Morrill: Solar UV irradiance from Ca II K images 851

spect to spectral irradiance (Frohlich and Lean2004). This paper discusses the development

of a model to estimate the solar spectral ir-radiance in the 2000 to 4000 Å range basedon Ca II K images and observed spectra ofquiet sun, active sun, and sunspots. The spec-tra were measured by the HRTS-9 rocket andthe SO82A spectrograph on SKYLAB and weare currently examining several sets of Ca II Kimages observed with electronic detectors.

Once completed, this model will be used toestimate solar spectral irradiance and the Mg IIindex (Vierick et al. 1995) back to at least theearly 1900’s by using Ca II K images that wereoriginally recorded on film and are currentlybeing converted to digital form. Several dataarchives exist such as the images at Mt WilsonObservatory, the Observatory of Rome, and theObservatory of Paris. This paper will discussthe details of the model and preliminary resultsthat have used full disk Ca II K images fromBig Bear Solar Observatory (BBSO) (Lean etal. 1998) and spectra from the SUSIM-UARSinstrument (VanHoosier 1996).

2. Details of the Irradiance Model

The current full disk irradiance model is a threecomponent model that uses observed spectrafrom quiet sun, active regions and sunspots aswell as center-to-limb variation (CLV) all athigh resolution (0.015 Å). The mathematicalformulation of this model is shown by the fol-lowing formula,

FFD(λ) = (Σi(QS )BQS (λ)CLV(λ, r(i)) +

Σ j(AR)BAR(λ)CLV(λ, r( j)) +

Σk(S S )BS S (λ)CLV(λ, r(k)))dA (1)

where FFD (λ) is the full disk solar irradiance,BQS (λ) is the disk center quiet sun brightness,BAR(λ) is the disk center active region bright-ness, BS S (λ) is the disk center sunspot bright-ness, CLV(λ,r) is the quiet sun limb CLV, anddA is the angular size of the Ca II K pixels(generally 1 arc-second). The wavelength cov-erage is 2760 to 2880 Å for the HRTS-9 spec-tra and about 2000 to about 4000 Å for theSKYLAB spectra.

Fig. 1. Spectra derived from HRTS-9 and used forthe irradiance model, (a) active sun spectrum, (b)quiet sun spectrum, and (c) sunspot spectrum.

The model uses the full disk Ca II K im-age of the sun that has been corrected for flatfielding and for the center-to-limb variation ofCa II K. A histogram is determined for eachimage. The central quiet sun portion is fit witha gaussian and the upper and lower thresholdsare determined as the points where the gaus-sian drops to 10−5 of a solar disk.

852 Morrill: Solar UV irradiance from Ca II K images

Center−to−Limb Varation Near Mg II

2780 2800 2820 2840 2860Vacuum Wavelength [Å]

0

200

400

600

800

Arc

seco

nds

Fro

m D

isk

Cen

ter

to L

imb

Fig. 2. Center-to-limb variation (CLV) for the quietsun derived from HRTS-9.

3. Input UV Spectra, CLV, and Ca II KImages

The spectra of the three solar surface featuresconsidered in this model were derived frommeasurements made by the HRTS-9 spectro-graph that flew on April, 18, 1995. The deriva-tion of these spectra was presented elsewhere(Morrill et al. 2001) and the three are shownin Fig. 1. Spectrograms from this flight werealso used to derive average spectra and CLVin the wavelength region around the Mg II h& k doublet at 2800 Å. Here a spectrogramaligned to observe the quiet sun center-to-limbvariability was used to produce polynomial fitsto the spectral variations at nearly full reso-lution (about 0.03 Å). The quiet sun CLV isshown in Fig. 2. The above spectra were cal-ibrated using the CLV curves and spectra ob-served by the SUSIM and SOLTICE instru-ments on UARS on several days where the sunis extremely quiet.

The current model results use a set of CaII K images from BBSO which have been cor-rected for flat field variations and CLV for CaII K. The model calculates a histogram dis-tribution of the Ca II K intensity which hasa Gaussian central peak that is produced bythe quiet sun, a shoulder at higher intensi-ties is produced by the active region plage,and a lower level shoulder that is produced bysunspots. Examples are shown below.

4. Model Results: Spectra and the MgII Index

Two examples of the output of our model arepresented Fig. 3 and Fig. 4. The top-left panelsshows the corrected Ca II K image, the top-center shows the histogram distribution of theCa II K intensity plotted on a log scale, andthe top-right panels show the resulting esti-mated spectra. The bottom row of Fig.s 3 and 4show a closeup of the estimated Mg II h and klines at full resolution, a comparison betweenthe SUSIM full disk irradiance spectrum andthe estimated spectrum with the resolution de-graded to that of the SUSIM spectrum (1.5 Å),and the difference between the SUSIM and es-timated spectra.

Fig. 3 shows one of the days used for cali-bration and the histogram of Ca II K intensityshows that the emission is almost entirely quietsun. The difference between the two spectra(lower right panel) shows mostly noise whichis likely due to slight difference in the wave-length scales of the two spectra. Fig. 4 shows aday of increased activity. Here the active regionportion of the histogram has increased signifi-cantly and the presence of a few sunspots canbe seen as well. The difference shows similarnoise but the presence of a negative offset inthe region of the Mg II h and k lines is signif-icant and implies a correction to the active re-gion spectra may be needed. We are currentlyconsidering ways to improve the agreement inthis region.

Once these estimated spectra are calculateda Mg II index can be derived and compared tothe NOAA Mg II index. Since these spectra areat higher resolution than those used to calcu-late the NOAA composite Mg II index (Viericket al. 1995) a linear fit is performed betweenthe values derived from the estimated spectraand the NOAA index. This fit is then appliedto the estimated values and the resulting com-parison appears in Fig. 5. Fig. 6 show the twoindices over plotted and indicate the estimatedvalues do a reasonable good job reproducingthe NOAA index.

Morrill: Solar UV irradiance from Ca II K images 853

Fig. 3. Model results for extremely quiet day. Panel from left to right, top to bottom are (a) Ca II K image,(b) Ca II K intensity histogram, (c) estimated spectrum at full resolution, (d) blowup of the region near MgII h and k lines, (e) comparison of estimated spectrum and SISUM-UARS spectrum for this day, and (f)difference between spectra in (e).

Fig. 4. Same as Figure 3 except for day of increased activity.

854 Morrill: Solar UV irradiance from Ca II K images

Fig. 6. Time dependent NOAA composite with es-timated Mg II index over plotted.

Fig. 7. Estimated quiet sun full disk irradiance com-pared to SUSIM-UARS spectrum on same day asFigure 3.

Fig. 5. Correlation between NOAA composite MgII index and estimated Mg II index with linear fittingapplied.

5. SKYLAB Results and FutureEfforts

The most recent efforts have involved the anal-ysis of spectral data from the SKYLAB S082Aspectrograph. With this data we will expand thespectral range of the model from 2000 to 4000Å. At present, spectra and CLV for SKYLABquiet sun observations have been derived from2100 to 3200 Å and efforts are underway toextend this range to the full 2000 to 4000 Årange and to add spectra from active regionsand sunspots. The disk center radiance fromSKYLAB is presented in Fig. 7 which com-pares the estimated spectrum to the radiancederived from SUSIM and the SKYLAB CLV.

Other future efforts include the expansionof the spectrum to wavelengths below 1000Å. This will involve studying correlations be-tween the Mg II index and observed EUVemissions as well as additional SKYLAB ob-servations at wavelengths below 1000 Å. Also,we plan to utilize Ca II K images from recentlydigitized film archives such as Mt Wilson, theRome Observatory, and the Paris Observatory.

6. Conclusions

In this paper we have presented the details of arelatively simple irradiance model and the ini-tial results. These results include estimated fulldisk irradiance spectra for a number of sam-ple days and the time series of the Mg II indexderived from estimated spectra. Comparisonsbetween estimated spectra and observationsfrom the SUSIM-UARS instrument show goodagreement over most of the wavelength range.Differences in the region near the Mg II h andk line cores will be examined and may indi-cate the nead to use a slightly modified activeregion spectrum. Comparison of the estimatedMg II index with the NOAA composite MgII index showed a good correlation and thatmost of the detailed variation of the NOAAindex were recovered by the estimated index.Spectra from SKYLAB will soon increase themodel wavelength range from 2000 to 4000 Åalthough current SKYLAB quiet sun spectrarange from 2100 to 3200 Å. Future plans in-clude the expansion of the of the wavelengthrange below 1000 and the use of recently digi-tized film archives of Ca II K images that will

Morrill: Solar UV irradiance from Ca II K images 855

extend our estimated spectra back to the early1900’s and possibily the late 1800’s.

Acknowledgements. This research was sponsoredby a NASA LWS grant and the Office of NavalResearch. The author acknolwedges the efforts of S.Weaver in the developement of the irradiance codeand helpful conversations with J. Lean, C. Frohlich,L. Floyd, J. Pap., and P. Fox.

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Lean, J., J. Cook, W. Marquette, & A.Johannesson, 1998, Astrophys. J., 492, 390

Morrill, J. K., Dere, & Korendyke, C. 2001,Astrophys. J., 557, 854

VanHoosier, M. 1996, SPIE, 2831, 57Viereck, R., & Puga, L. 1995, J. Geophys.

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