Available online at www.sciencedirect.com

www.elsevier.com/locate/asr

Advances in Space Research 43 (2009) 349–354

Comparison of solar soft X-ray irradiance from broadbandphotometers to a high spectral resolution rocket observation

Thomas N. Woods *, Phillip C. Chamberlin

Laboratory for Atmospheric and Space Physics (LASP), University of Colorado, 1234 Innovation Drive, Boulder, CO 80303, USA

Received 6 August 2008; received in revised form 21 October 2008; accepted 22 October 2008

Abstract

The solar soft X-ray (XUV; 1–30 nm) radiation is highly variable on all time scales and strongly affects the ionosphere and upperatmosphere of Earth, Mars, as well as the atmospheres and surfaces of other planets and moons in the solar system; consequently,the solar XUV irradiance is important for atmospheric studies and for space weather applications. While there have been several recentmeasurements of the solar XUV irradiance, detailed understanding of the solar XUV irradiance, especially its variability during flares,has been hampered by the lack of high spectral resolution measurements in this wavelength range. The conversion of the XUV photom-eter signal into irradiance requires the use of a solar spectral model, but there has not been direct validation of these spectral models forthe XUV range. For example, the irradiance algorithm for the XUV Photometer System (XPS) measurements uses multiple CHIANTIspectral models, but validation has been limited to other solar broadband measurements or with comparisons of the atmosphericresponse to solar variations. A new rocket observation of the solar XUV irradiance with 0.1 nm resolution above 6 nm was obtainedon 14 April 2008, and these new results provide a first direct validation of the spectral models used in the XPS data processing. Therocket observation indicates very large differences for the spectral model for many individual emission features, but the differencesare significantly smaller at lower resolution, as expected since the spectral models are scaled to match the broadband measurements.While this rocket measurement can help improve a spectral model for quiet Sun conditions, many additional measurements over a widerange of solar activity are needed to fully address the spectral model variations. Such measurements are planned with a similar instru-ment included on NASA’s Solar Dynamics Observatory (SDO), whose launch is expected in 2009.� 2008 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Solar ultraviolet irradiance; Flares; Space weather

1. Introduction

The solar soft X-ray (XUV, defined here as 1–30 nm)radiation is highly variable on all time scales with varia-tions for both short-term (minutes, flares), mid-term(months, solar rotation), and long-term (years, solar cycle)ranging from factors of two to a hundred (wavelengthdependent). These solar XUV variations directly affectthe composition, density, and temperature in Earths’ andMars’ ionosphere and upper atmosphere. Furthermore,the solar driven atmospheric processes are wavelength

0273-1177/$34.00 � 2008 COSPAR. Published by Elsevier Ltd. All rights rese

doi:10.1016/j.asr.2008.10.027

* Corresponding author.E-mail address: tom.woods@lasp.colorado.edu (T.N. Woods).

dependent and thus dependent on the intrinsic solar vari-ability at the appropriate wavelengths. The large flaresare of major concern for space weather applications caus-ing detrimental effects on communication and navigationsystems due to ionospheric changes (e.g., Lanzerotti,2001; Kintner et al., 2007) and on satellite tracking asrelated to satellite drag changes due to solar forcing ofthe neutral density (e.g., Sutton et al., 2006).

Accurate measurements of the solar ultraviolet spectralirradiance, along with an understanding of its variabilityon all time scales, are required for atmospheric studiesand application for space weather operations. While therehave been several recent broadband measurements of thesolar XUV irradiance by the XUV Photometer System

rved.

350 T.N. Woods, P.C. Chamberlin / Advances in Space Research 43 (2009) 349–354

(XPS), detailed understanding of the solar XUV irradiance,especially its variability during flares, has been hamperedby the lack of high spectral resolution from these measure-ments. The flare variations are better understood in theextreme ultraviolet (EUV; 27–120 nm) and far ultraviolet(FUV; 120–200 nm) wavelengths where there are higherresolution spectral measurements of the solar variability(e.g., Woods et al., 2005a). However, the results with thebroadband measurements at shorter than 27 nm initiallyyielded conflicting results with atmospheric responses toflares (e.g., Strickland et al., 2007). The source for these ini-tial differences is the choice of the solar spectral model usedin converting the XUV broadband photometer data intoirradiance units. A new algorithm has been developed forprocessing the XPS data with dynamic solar spectra thatinclude a flare component, and the new XPS Level 4 prod-ucts are significantly improved for flare events (Woodset al., 2008).

The focus for this paper is the direct validation of theCHIANTI spectral models used in processing the XPSdata. Prior validation has been limited to other solarbroadband measurements or with comparisons of theatmospheric response to solar variations (Woods et al.,2008). This new validation is based on the results from arecent rocket observation of the solar XUV irradianceabove 6 nm with 0.1 nm resolution on 14 April 2008.

2. Improved irradiance algorithm for XPS

XPS is a set of filter photometers that measure the solarirradiance from 0.1 to 27 nm with an additional channel atthe important H I Lyman-a line at 121.6 nm. The XPS isone of four different solar irradiance instruments onNASA’s Solar Radiation and Climate Experiment (SOR-CE) satellite (Rottman, 2005; Woods et al., 2005b) and isalso part of the Solar EUV Experiment (SEE) on NASA’sThermosphere, Ionosphere, Mesosphere, Energetics, andDynamics (TIMED) satellite (Woods et al., 2005a). In eachXPS there are a total of twelve silicon photodiodes, eightwith metal films directly deposited on them, one with121 nm interference filters in front, and three with barephotodiodes. The filter material (either metal coating orinterference) establishes the wavelength sensitivity overbroadbands of about 7–10 nm and also blocks the longwavelength solar radiation that would overwhelm the rela-tively weak signal at these short X-ray wavelengths.

The primary data products from XPS are the solar XUVirradiance in its broadbands and also at higher spectral res-olution based on scaling model spectra to the XPS signallevels. The time cadences for the XPS results include thedaily averaged irradiances and higher cadence of a fewminutes. Because of the satellite orbit and instrument con-figuration, the TIMED XPS has about 3% duty cycle forsolar observations, and the SORCE XPS has about 70%duty cycle for solar observations. The TIMED daily mea-surements began in January 2002, and the SORCE dailymeasurements began in March 2003, and data from both

can be found at the LASP LISIRD database (http://lasp.colorado.edu/lisird).

The original XPS Level 4 algorithm scaled a daily refer-ence spectrum to match the signals (currents) of the XPSphotometers and is described in detail by Woods et al.(2005b). The scale factors for the reference spectrum aredetermined in three bands at 0–4, 4–14, and 14–27 nm. Thisapproach of having a single reference spectrum for eachday to generate a XUV spectrum in 1 nm intervals for eachXPS measurement works quite well for non-flare measure-ments but severely over estimates the flare irradiance by afactor of 2 or more as determined from measured atmo-spheric response to the flares (e.g., Strickland et al., 2007;Tsurutani et al., 2005). The reason for this algorithmbreakdown for flares is due to the flare spectrum changingsignificantly, namely to increase more at the shorter wave-lengths. Consequently, this algorithm is expected to overestimate the flare irradiance with a static reference spec-trum because the photometers are most responsive at theshorter wavelengths. That is, the photometer signal goesup dramatically, by a factor of about 50 for very largeflares, with the real spectral increase expected to be mostlynear 2 nm (Rodgers et al., 2006), but the irradiance calcu-lation uses the lower photometer responsivity at longerwavelengths, which in turn yields high estimates for theflare irradiance.

An improved algorithm, as described in detail by Woodset al. (2008), is to use a combination of reference spectrathat are representative of both flare and non-flare activity.The CHIANTI version 5.2 spectral model (Dere et al.,1997; Landi et al., 2006) is used to generate reference spec-tra representative for non-flare and flare measurements.The CHIANTI spectral model includes options with stan-dard differential emission measures (DEMs) and also iso-thermal spectra appropriate for the Sun.

Two non-flare spectra are estimated on a daily basisusing the minimum current from the XPS Ti-coated pho-tometer. A combination of the standard DEM options inCHIANTI are used for the non-flare components, beinga solar cycle minimum spectrum and an active region spec-trum. The active region reference spectrum is simply theactive region DEM in the CHIANTI spectral model;whereas, the solar cycle minimum reference spectrum is acombination of the quiet Sun DEM and coronal holeDEM. Although neither TIMED nor SORCE hadobserved solar cycle minimum conditions at the time whenthe algorithm was developed, the lowest XPS measure-ments fit well with the average of the CHIANTI quietSun DEM spectrum and coronal hole DEM spectrum;therefore, this average is chosen to be the solar cycle min-imum reference spectrum, which is the focus here for thecomparison to the rocket observation in April 2008.

The flare spectrum is estimated for every observationusing the difference from the measurement and the dailyminimum current (non-flare part). Isothermal spectra areused for the flare component. The temperature used for theisothermal spectra is derived from the GOES (Geostationary

T.N. Woods, P.C. Chamberlin / Advances in Space Research 43 (2009) 349–354 351

Operational Environmental Satellite) X-Ray Sensor (XRS)measurements of the solar X-rays. This technique is similarto that defined by Garcia (1994) in that the ratio of the longand short channel XRS measurements provide a monotonicfunction for temperature, but in the XPS Level 4 algorithmthe CHIANTI isothermal models are convolved with XRSresponsivities to determine the relationship of the XRS ratioto flare temperature. Flares are known to be multi-tempera-ture emissions (e.g., Aschwanden, 2007), but the single tem-perature approach is a simple, reliable approach for dataprocessing because the GOES solar X-ray data are availablenear realtime and because the GOES flare temperature rela-tionship is well characterized (e.g., Garcia, 1994). Fortu-itously, these flare temperature results are very similar tothe detailed derivation of the flare DEMs during differentflare phases (Chifor et al., 2007). The primary concerns withthe flare component in the new XPS algorithm are the lack ofcool plasma contributions and incomplete coverage for theimpulsive phase of the flares. The rocket observation in April2008 was during quiet Sun conditions, so there is no new val-idation result for the flare component used in the XPS dataprocessing. This will wait for future validation with high-res-olution measurements of flares provided by SDO EVE.

The new XPS Level 4 Version 9 data products that incor-porate the combination of daily components and a flarecomponent of the CHIANTI spectral models are significantimprovements over the previous Version 8 data. For one,the flare reference spectra are uniquely different than thedaily (non-flare) reference spectra, and the temperaturefor the flare spectra are based on the well characterizedGOES solar X-ray measurements. Whereas, the previousXPS version used a single reference spectrum for eachday, which did change from day-to-day to account for solaractivity but did not have a flare component. Because of themore realistic flare reference spectra, the new XPS flareresults are consistent for the first time with the atmosphericresponse to flares. While evaluating the XPS results, onedoes need to keep in mind that the XPS Level 4 data prod-ucts are actually a combination of high-resolution spectrafrom the CHIANTI model with the XPS direct broadbandmeasurements with only 7–10 nm resolution. In otherwords, the irradiance over broad ranges (�10 nm) is consid-ered much more accurate than the accuracy of the irradi-ance at higher resolution, but the broadbands are notpractical for some space weather applications.

There are many comparisons and validation for the newXPS Level 4 data products as shown in Woods et al.(2008), so only the key results are summarized here.

� The CHIANTI model is not adequate for the opticallythick He II 30.4 nm emission as reflected in the 27–31 nm comparison to the EUV Grating Spectrograph(EGS) on TIMED SEE. The XPS Level 4 product in thisband is 21% lower than the EGS measurements.� The offsets between the new Version 9 XPS data and the

previous version are due to revised responsivities in thenew version.

� The previous results in the 7–17 nm band had unrealis-tically large day-to-day variations.� The previous results showed more variability during

flares; the new version is considered the more accuratevariation for flares. The flare variations are now reducedby about a factor of 4 in the 7–17 nm range and a factorof 2 in the 0–7 nm range. The new flare variations areslightly larger by about 20% in the 17–27 nm range thanthe previous XPS version and are also consistent withthe spectral measurements by EGS in the 27–40 nmrange.� Comparisons of atmospheric responses to solar variabil-

ity are improved with the new XPS data product. Thesecomparisons include measurements of the total electroncontent (TEC) and dayglow observations from theTIMED Global Ultraviolet Imager (GUVI; Stricklandet al., 2007).

3. Rocket observations at higher spectral resolution

A new higher spectral resolution measurement of thesolar XUV irradiance was obtained from NASA’s sound-ing rocket 36.240 that was launched on 14 April 2008.One of the two instruments in this rocket payload is theEUV Variability Experiment (EVE), the prototype of asimilar EVE instrument that will launch on the SolarDynamics Observatory (SDO) (Woods et al., 2006). TheMultiple EUV Grating Spectrograph (MEGS) channelsof EVE measure the solar irradiance from 6 to 106.0 nmat better than 0.1 nm spectral resolution in three overlap-ping channels. While the main purpose of this rocket flightis to provide the fifth underflight calibration for TIMEDSEE, the key importance of this rocket observation is thesolar XUV and EUV irradiance at higher spectral resolu-tion of 0.1 nm and during solar cycle minimum conditions(Woods et al., in press). These rocket results, shown as thegreen lines in Fig. 1, are of critical significance here for pro-viding direct validation of the solar cycle minimum refer-ence spectrum used in processing the broadband datafrom XPS.

EVE consists of several channels to measure the spectralirradiance from 0.1 to 5 nm at 1 nm resolution, 5–105 nmwith a resolution of 0.1 nm, plus the hydrogen Lyman-aline at 121.5 nm with 1 nm resolution. EVE is designed tomeasure the full spectral range on a 10-s time cadence.The primary, high spectral resolution irradiance measure-ments are made by the Multiple EUV Grating Spectro-graphs (MEGS). The MEGS is composed of twospectrographs: MEGS A is a grazing incidence spectro-graph covering the 5–39 nm range, and MEGS B is atwo-grating, cross-dispersed spectrograph covering the35–106 nm range. Both MEGS A and B have a back-illu-minated 1024 � 2048 CCD camera from MIT LincolnLaboratory. MEGS A includes two slits, MEGS A1 andMEGS A2, which share the same grating and detectorbut are offset so the A1 and A2 spectra are on opposite

Fig. 1. Comparison of XPS Level 4 results (black) to rocket MEGS observation (green) on 14 April 2008. Both are shown in 0.1 nm intervals, but MEGShas slightly higher resolution.

352 T.N. Woods, P.C. Chamberlin / Advances in Space Research 43 (2009) 349–354

halves of the detectors. Thin film filters are used with theMEGS A slits to isolate 5–20 nm for A1 and 17–39 nmfor A2. An unused part of MEGS A detector is used fora pinhole camera as a Solar Aspect Monitor (SAM) to pro-vide solar position information and also an X-ray image/spectrum from 0.1 to 5 nm. Chamberlin et al. (2007a)and Crotser et al. (2007) provide more details on theMEGS channels. EVE also includes the University ofSouthern California EUV SpectroPhotometers (ESP) thathas five EUV bands to provide cross-calibration for MEGSA and also higher cadence (3 s) observations (Didkovskyet al., 2007). The prototype EVE and flight EVE channelsare almost identical, with the main difference being thatthe prototype EVE uses single, fixed filter for each channel(versus filter mechanism with redundant filters). TheMEGS A and B channel results are included in this com-parison to the XPS spectral results.

4. Direct validation of XPS spectral irradiance model

While the new algorithm for converting XPS data intoirradiance units is considered to be more accurate and isthought to be providing more realistic results of the solarXUV spectral variations (Woods et al., 2008), these XPSresults are based on spectral models scaled to match the

level of the XPS broadband measurements. This concernneeds higher spectral resolution observations, over a vari-ety of solar activity, to more directly validate the CHI-ANTI models used in the XPS processing. The plannedsolar EUV observations from SDO will surely provide suchdirect validation, but for now, the rocket observation on 14April 2008 with the prototype MEGS instrument providesa direct validation for solar cycle minimum conditions,being one of the three key spectral models used in XPS dataprocessing.

The XPS Level 4 result for 14 April 2008 and the rocketobservation are shown in Fig. 1. There are many differencesbetween them by factors of 10 or more for many of theindividual emission lines, as shown in Fig. 2. The meanand standard deviation of the ratio of rocket to XPS Level4 at 0.1 nm resolution are 1.6 and 4.5, respectively. At thishigher resolution, the XPS Level 4 irradiance spectrum isnot in good agreement with the rocket observation. Thisresult implies that the XPS solar cycle minimum spectrum,comprised of the CHIANTI quiet Sun and coronal holeDEM spectra, could be significantly improved by usingthe rocket spectrum that has about 10% accuracy.

The XPS Level 4 results are significantly improved ifthey are binned down to lower resolution, but much ofthe practicality and use of these spectral measurements

Fig. 2. Ratio of rocket MEGS observation to XPS Level 4 irradiance. Thecomparison at 5 nm resolution is shown as the green line.

T.N. Woods, P.C. Chamberlin / Advances in Space Research 43 (2009) 349–354 353

are then lost. The CHIANTI model spectra are scaled tothe XPS broadband measurement as part of the XPS Level4 algorithm, thus improvement is expected for comparisonsat 5–10 nm resolution. The green line in Fig. 2 shows theratio of rocket to XPS Level 4 at 5 nm resolution. The ratiois 1.12, 0.82, 0.65, and 1.11 for the bands at 0–10, 10–20,20–30, and 30–40 nm, respectively. These differences of11–35% are within the uncertainty (accuracy) of the XPSLevel 4 irradiances.

This rocket observation can be used to significantlyimprove the spectral distribution within the solar cycleminimum spectrum used in the XPS Level 4 algorithm.However, this single rocket observation cannot addressthe active region spectrum or flare spectra also used inthe XPS Level 4 algorithm. Such improvements must waitfor the future SDO EVE and also SDO AIA (AtmosphericImaging Assembly) observations. The higher resolutionirradiance spectra from EVE and solar images at severaldifferent emission temperatures from AIA will initiate anew validation effort of these XPS results and will likelylead to improvements for the CHIANTI and other spectralmodels of the Sun.

The XPS Level 4 data products are applicable for inclu-sion in solar irradiance spectral models and for a variety ofspace weather research and applications. For example, thelatest version of the Flare Irradiance Spectral Model(FISM) already includes the XPS Level 4 results (Cham-berlin et al., 2007b). A caveat is that the XPS Level 4 irra-diances are most accurate at low resolution (10 nm) but dohave significant uncertainties for individual emission lines.Future efforts to further improve the solar XUV irradi-ances, such as with new SDO results, are important forspace weather operations and forecasts of the ever-chang-ing ionosphere and thermosphere.

Acknowledgments

This research was supported by NASA contractNAS5-97045 to the University of Colorado. Thanks to

Vanessa George for her assistance with this manuscript.The CHIANTI spectral model is a collaborative projectinvolving the NRL (USA), RAL (UK), MSSL (UK),the Universities of Florence (Italy) and Cambridge(UK), and George Mason University (USA). The SOR-CE data are available from http://lasp.colorado.edu/sor-ce/. The TIMED SEE data are available from http://lasp.colorado.edu/see/.

References

Aschwanden, M.J. RHESSI timing studies: multithermal delays. Astro-phys. J. 661, 1242–1259, 2007.

Chamberlin, P.C., Hock, R.A., Crotser, D.A., Eparvier, F.G., Furst, M.,Triplett, M.A., Woodraska, D., Woods, T.N. EUV VariabilityExperiment (EVE) Multiple EUV Grating Spectrographs (MEGS)radiometric calibrations and results. Proc. SPIE 6689, 66890N–66890N-9, 2007a.

Chamberlin, P.C., Woods, T.N., Eparvier, F.G. Flare Irradiance SpectralModel (FISM): daily component algorithms and results. SpaceWeather 5, S07005, 2007b.

Chifor, C., Del Zanna, G., Mason, H.E., Sylwester, J., Sylwester, B.,Phillips, K.J.H. A benchmark study for CHIANTI based on RESIKsolar flare spectra. Astron. Astrophys. 462, 323–330, 2007.

Crotser, D.A., Woods, T.N., Eparvier, F.G., Triplett, M.A., Woodraska,D.L. SDO-EVE EUV spectrograph optical design and performance.Proc. SPIE 6689, 66890M–66890M-11, 2007.

Dere, K.P., Landi, E., Mason, H.E., Monsignori Fossi, B.C., Young, P.R.CHIANTI – an atomic database for emission lines. Astron. Astrophys.Suppl. 125, 149–173, 1997.

Didkovsky, L., Judge, D., Wieman, S., Woods, T., Chamberlin, P., Jones,A., Eparvier, F., Triplett, M., Woodraska, D., McMullin, D., Furst,M., Vest, R. SDO EVE ESP radiometric calibration and results. Proc.SPIE 6689, 66890P–66890P-12, 2007.

Garcia, H.A. Temperature and emission measure from GOES soft X-raymeasurements. Solar Phys. 154, 275–308, 1994.

Kintner, P.M., Ledvina, B.M., de Paula, E.R. GPS and ionosphericscintillations. Space Weather 5, S09003, 2007.

Landi, E., Del Zanna, G., Young, P.R., Dere, K.P., Mason, H.E.,Landini, M. CHIANTI – an atomic database for emission lines VII.New data for X-rays and other improvements. Astrophys. J. Suppl.162, 261–280, 2006.

Lanzerotti, L.J. Space weather effects on technologies, in: Song, P., Singer,H.J., Siscoe, G.L. (Eds.), Space Weather, AGU Monograph 125,Washington, DC, 2001.

Rodgers, E.M., Bailey, S.M., Warren, H.P., Woods, T.N., Eparvier, F.G.Soft X-ray irradiances during solar flares observed by TIMED-SEE. J.Geophys. Res. 111, A10S13, 2006.

Rottman, G. The SORCE mission. Solar Phys. 230, 7–25, 2005.Strickland, D.J., Lean, J.L., Daniell Jr., R.E., et al. Constraining and

validating the Oct/Nov 2003 X-class EUV flare enhancements withobservations of FUV dayglow and E-region electron densities. J.Geophys. Res. 112, A06313, 2007.

Sutton, E.K., Forbes, J.M., Nerem, R.S., Woods, T.N. Neutral densityresponse to the solar flares on October and November 2003. Geophys.Res. Lett. 33, L22101, 2006.

Tsurutani, B.T., Judge, D.L., Guarnieri, F.L., et al. The October 28, 2003extreme EUV solar flare and resultant extreme ionospheric effects:comparison to other Halloween events and the Bastille Day event.Geophys. Res. Lett. 32, L03S09, 2005.

Woods, T.N., Eparvier, F.G., Bailey, S.M., Chamberlin, P.C., Lean, J.,Rottman, G.J., Solomon, S.C., Tobiska, W.K., Woodraska, D.L. TheSolar EUV Experiment (SEE): mission overview and first results. J.Geophys. Res. 110, A01312, 2005a.

Woods, T.N., Rottman, G., Vest, R. XUV Photometer System (XPS):overview and calibration. Solar Phys. 230, 345–374, 2005b.

354 T.N. Woods, P.C. Chamberlin / Advances in Space Research 43 (2009) 349–354

Woods, T.N., Lean, J.L., Eparvier, F.G. The EUV Variability Experiment(EVE) on the Solar Dynamics Observatory (SDO): science plans andinstrument overview, in: Gopalswamy, N., Bhattacharyya, A. (Eds.),Proceedings of the International Living with a Star Workshop. QuestPublications, Mumbai, India, pp. 145–151, 2006.

Woods, T.N., Chamberlin, P.C., Peterson, W.K., Meier, R.R., Richards,P.G., Strickland, D.J., Lu, G., Qian, L., Solomon, S.C., Iijima, B.A.,

Mannucci, A.J., Tsurutani, B.T. XUV Photometer System (XPS):improved solar irradiance algorithm using CHIANTI spectral models.Solar Phys. 249, 235–267, 2008.

Woods, T.N., Chamberlin, P.C., Harder, J.W., Hock, R.A., Snow, M.,Eparvier, F.G., Fontenla, J., McClintock, W.E., Richard, E.C., SolarIrradiance Reference Spectra (SIRS) for the 2008 Whole HeliosphereInterval (WHI). Geophys. Res. Lett., in press.