Download - Solar Extreme Ultraviolet Irradiance Measurements During Solar Cycle 22

SOLAR EXTREME ULTRAVIOLET IRRADIANCE MEASUREMENTSDURING SOLAR CYCLE 22 �

THOMAS N. WOODS, GARY J. ROTTMAN, SCOTT M. BAILEY andSTANLEY C. SOLOMON

Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309-0590,U.S.A.

JOHN R. WORDENHigh Altitude Observatory, National Center for Atmospheric Research, Boulder,

CO 80307–3000, U.S.A.

(Received 25 October 1996; accepted 4 December 1996)

Abstract. The solar extreme ultraviolet (EUV) irradiance, the dominant global energy source forEarth’s atmosphere above 100 km, is not known accurately enough for many studies of the upperatmosphere. During the absence of direct solar EUV irradiance measurements from satellites, the solarEUV irradiance is often estimated at the 30–50% uncertainty level using both proxies of the solarirradiance and earlier solar EUV irradiance measurements, primarily from the Air Force GeophysicsLaboratory (now Phillips Laboratory) rockets and Atmospheric Explorer (AE) instruments. Oursounding rocket measurements during solar cycle 22 include solar EUV irradiances below 120 nmwith 0.2 nm spectral resolution, far ultraviolet (FUV) airglow spectra below 160 nm, and solar soft X-ray (XUV) images at 17.5 nm. Compared to the earlier observations, these rocket experiments providea more accurate absolute measurement of the solar EUV irradiance, because these instruments arecalibrated at the National Institute of Standards and Technology (NIST) with a radiometric uncertaintyof about 8%. These more accurate sounding-rocket measurements suggest revisions of the previousreference AE–E spectra by as much as a factor of 2 at some wavelengths. Our sounding-rocket flightsduring the past several years (1988–1994) also provide information about solar EUV variabilityduring solar cycle 22.

1. Introduction

The ionosphere and thermosphere are established by the absorption of the solarvacuum ultraviolet (VUV: below 200 nm) radiation by the major species in theatmosphere: O2, N2, and O. The thermal structure and electric fields created in theupper atmosphere lead to winds in the thermosphere with velocities as much as100 km per hour. The solar VUV radiation is also a catalyst for many chemicalcycles in the upper atmosphere including water, ozone, and odd nitrogen cyclesin the mesosphere. These atmospheric processes related to photoionization, pho-todissociation, and photoexcitation are expected to be as variable as the intrinsicsolar variability at the appropriate wavelengths. Precise specification of the largestglobal energy source, the solar VUV spectral irradiance and its variability, is thusimportant for those performing detailed studies of the many upper atmosphericprocesses.

� Paper presented at the SOLERS22 International Workshop, held at the National Solar Observat-ory, Sacramento Peak, Sunspot, New Mexico, U.S.A., June 17–21, 1996.

Solar Physics177: 133–146, 1998.c 1998Kluwer Academic Publishers. Printed in Belgium.

134 THOMAS N. WOODS ET AL.

Table ILaunch times and solar conditions for the 1992–1994 solar EUV measurements

NASA Date Time Spectral Solar 10.7 cm flux Geomagneticrocket range 81-day avg index

F10:7 hF10:7i Ap

36.098 Oct. 27, 1992 18:30 UT 30–103 nm 168.9 132.5 3136.107 Oct. 4, 1993 17:45 UT 2–6 nm 121.5 93.2 6

17–103 nm36.124 Nov. 3, 1994 18:45 UT 2–120 nm 85.9 82.4 13

While there have been many recent measurements of the solar far ultravi-olet (FUV: 115–200 nm) irradiance by the Upper Atmosphere Research Satellite(UARS) and the Space Shuttle ATLAS missions, there have only been a few sound-ing rocket measurements of the solar extreme ultraviolet (EUV: below 120 nm)irradiance. Atmospheric modelers often need reference solar spectra to characterizethe solar influence, perhaps using a few solar spectra as part of their input. However,there are no daily measurements of the solar EUV irradiance, and moreover theexisting solar proxy models of the solar EUV irradiance have inaccuracies. Forthese reasons, the solar EUV measurements taken on October 27, 1992, October 4,1993, and November 3, 1994 by our rocket experiments are presented for consider-ation as reference spectra for moderate to low solar activity. Some recent reviews ofthe solar EUV irradiance are given by Tobiska (1993), Simon and Tobiska (1991),and Lean (1987).

2. Instrumentation

The solar VUV spectra described in this paper are a combination of measurementsfrom three separate instruments. The UARS SOLSTICE measurements, whichhave been made daily since October 3, 1991, provide the data above 119 nm.Rottman, Woods, and Sparn (1993) and Woods, Veker, and Rottman (1993) describedetails about the SOLSTICE instrument, operation, and calibrations. The primarycomponents of the SOLSTICE are three grating spectrometers that are integratedinto a single housing and have a spectral resolution of about 0.2 nm. Although thespectral range for the SOLSTICE instrument is 119 to 420 nm, only the data below200 nm from SOLSTICE are presented here because this part of the solar spectrumis most important for studies of the mesosphere and thermosphere. Woodset al.(1996) present solar irradiance spectra in 1 nm intervals for the full 119–410 nmrange as well as solar variability information during the early UARS mission.

The two solar EUV instruments are flown on a sounding rocket payload and thusonly provide a measurement about once per year. Table I lists the launch times andthe solar activity levels for these flights. The two rocket instruments are the EUV

SOLAR EXTREME ULTRAVIOLET IRRADIANCE MEASUREMENTS DURING CYCLE 22 135

Grating Spectrograph (EGS) and the XUV Photometer System (XPS). Woods andRottman (1990) and Woodset al. (1994a) describe the optical properties of theserocket instruments. The EGS is a1

4 m Rowland circle grating spectrograph witha spectral range of 30 to 120 nm with 0.2 nm resolution. The XPS are a set ofsilicon XUV photodiodes to measure the integrated flux from 0 to 35 nm with aspectral resolution of about 5 to 10 nm as determined by the thin film filters thatare deposited directly on the photodiodes. Baileyet al. (1998) describe the XPSand their results in more detail.

3. Calibrations

The radiometric calibration of the UARS SOLSTICE is based on pre-flight calib-rations at the Synchrotron Ultraviolet Radiation Facility (SURF-II) at the NationalInstitute of Standards and Technology (NIST) in Gaithersburg, Maryland and in-flight calibrations using bright, early-type stars. The EGS calibrations, both pre-flight and post-flight, are also based on SURF calibrations. Woods, Ucker, andRottman (1993) and Woods and Rottman (1990) describe the calibration proced-ures for these two instruments. The 1� uncertainties of the SURF calibrations forSOLSTICE and EGS are about 3% and 6–10%, respectively. With the uncertaintyin the SURF radiance being only about 1%, the uncertainties in the counting statist-ics, wavelength scale, linearity correction, and field of view calibration contributethe most to the instrument calibration uncertainties (Woodset al., 1996). TheEGS calibration uncertainty is larger than that of SOLSTICE because its countingstatistics and linearity corrections are larger.

The radiometric calibration of the XPS is based on NIST calibrations by RandyCanfield for wavelengths above 5 nm and on a Fe-55 radioactive source at 0.2 nm.In addition, the electronics linearity is calibrated so that nonlinear effects at highcurrents can be properly corrected. Baileyet al. (1998) describe in more detailthese XPS calibration results. The uncertainty of the XPS calibrations is about 10–20%, which is even higher than SOLSTICE and EGS calibration uncertaintiesbecause the XPS calibrations are based on a secondary NIST standard (calibratedsilicon photodiodes) instead of a primary NIST standard (synchrotron radiation atSURF).

There are three enhancements to the EGS calibration than what is documentedby Woods and Rottman (1990). One is the improvement of the calibration of second-order efficiency from the grating by using a tin (Sn) filter at SURF. With the Snfilter transmission limited between 51.9 and 85 nm, second-order efficiency at 51.9–60 nm (at 103.8–120 nm first order) can be measured unambiguously.Because theEGS grating has no measurable higher-order efficiency below 90 nm, as confirmedby using SURF multiple beam energies and solar measurements, the use of theSn filter yields a more accurate measurement of the EGS second-order efficiency.A second calibration enhancement is the use of the more rigorous correction for


scattered light as described by Woodset al. (1994b). The third enhancement is animproved nonlinearity correction derived from counting probabilities presented byMelissinos (1966). This correction, which accurately fits the behavior of the EGSmicrochannel plate (MCP) detector, is

Cm = Cre�Cr� ; (1)

whereCm is the measured count rate,Cr is the real count rate, and� is the linearitytime constant. Because this equation does not have a simple inverse solution forCr,this linearity correction is applied using interpolation of a lookup table. A flat-fieldspectrum is first folded into each solar spectrum before the correction is appliedand then unfolded after the correction. By doing this flat-field correction, the�

parameter is found to be essentially the same value (1.9 ms) across the entire arraydetector. The flat field is derived by dividing a SURF spectrum by its spectrumsmoothed by 75 anodes.

4. Solar Measurements

The format chosen for reporting the solar irradiance is 1 nm intervals on 0.5 nmcenters and at 1 AU. In addition, the irradiances of 53 bright emission featuresare reported separately. These extracted emission lines are not removed from theirradiance spectra in 1 nm intervals. This format is equivalent to the format ofthe standard UARS Level 3 solar irradiance product above 115 nm. The emissionline irradiances are derived by first subtracting the local background signal (con-tinuum or nearby weak lines) and then integrating over the isolated emission line,typically by�0.2 nm. This line extraction is performed at instrument resolution,about 0.15 nm for both the rocket EUV and SOLSTICE FUV measurements. The1994 measurement is shown in Figure 1 at instrument resolution and with theextracted lines labeled. The spectra in 1 nm intervals and the extracted emissionlines can be obtained from the anonymous File-Transfer-Protocol (FTP) site on‘lasp.colorado.edu’ (see the Data Archive section). These spectra below 105 nmcan be converted easily to the solar irradiance format of Torr and Torr (1985)which includes selected emission features and 5 nm intervals without the separ-ated emission features. We however recommend the higher spectral resolution foratmospheric modeling because the Torr and Torr format for the solar irradianceintroduces systematic errors in calculating the solar energy deposited in the atmo-sphere. These errors are more sensitive to the absorption cross sections at coarserresolution and to the wavelength dependence of solar variability within each 5 nminterval.

There are two gaps in the 1992 and 1993 solar EUV spectra where measurementswere not made. In the 1992 data, the gaps are from 0 to 30 nm and from 103.5to 119 nm. In the 1993 data, the gaps are from 6 to 17 nm and from 103.5 to119 nm. The gaps below 30 nm are due to lack of instrumentation for that spectral


Figure 1. Solar VUV irradiance spectrum for November 3, 1994. The spectrum is at a spectralresolution of 0.3 nm. The 53 bright lines that are extracted are labeled. The ions in parentheses areweaker emissions blended with the brighter lines at our instrument resolution.

range during those flights, and the gaps above 103.5 nm are due to photoelectronsscattered onto this region of the detector via the impingement of the intense solarL� radiation on the stainless steel detector housing. These instrument deficiencieswere corrected for the 1994 rocket measurement which yielded a complete solarEUV spectrum from 0 to 120 nm, which together with the SOLSTICE measurementyields a complete spectrum from 0 to 400 nm on the same day.

For the XUV broadband measurements below 30 nm, the results from theHinteregger, Fukui, and Gilson (1981) proxy model of the solar irradiance arenormalized to match our XUV measurements in order to compile the spectrum in1 nm intervals. The daily 10.7-cm radio flux (F10:7) and the 81-day average of theF10:7 are the inputs for the Hinteregger proxy model. Adjustment factors for theHinteregger proxy model irradiances are derived over broad bands by comparingthe measured photometer current to the current predicted by convolving the XUVphotometer sensitivity with the model irradiances at 0.1 nm resolution. Baileyet al.(1998) describe in more detail how we derive the 1 to 30 nm region adjustmentfactors. The adjustment factors for the 1993 rocket measurements are 1.35 for 1 to6 nm and 2.06 for 17 to 30 nm. An additional XUV photometer was added for the1994 flight to measure the solar flux from 6 to 17 nm. The resulting adjustmentfactors for the 1994 measurements are 2.24 for 1 to 6 nm, 2.26 for 6 to 17 nm,


Figure 2. Comparison to proxy models. The comparisons of our 1994 measurement to the Hinteregger,Fukui, and Gilson (1981), Tobiska and Eparvier (1997), and Richards, Fennelly, and Torr (1994) proxymodels are shown in panels A, B, and C, respectively. Differences are discussed in the text.

and 1.57 for 17 to 30 nm. The ‘adjustment factors’ are the amounts by which theHinteregger model results should be multiplied for the given wavelength regions.

The UARS SOLSTICE measurements have been validated to other solar irradi-ance measurements made by UARS SUSIM, ATLAS SUSIM, and SSBUV (Woodset al., 1996). The typical differences between the FUV irradiances are 3–10%.Woodset al. (1996) give complete details of the validation comparisons and res-ults, as well as solar UV spectral irradiances which could be used as referencespectra from 119 to 410 nm. The rocket EUV measurements have had much lessvalidation due to the lack of other measurements. The calibration techniques anduncertainty analyses proven for SOLSTICE are applied to the rocket instrument-ation, so we expect the calculated uncertainties for the rocket measurements, thatrange from 10 to 15%, to be a realistic result. A first check in validating thesesolar measurements is to directly intercompare them. This comparison shows goodagreement with the values decreasing from 1992 to 1994 as expected and with-in the 10% uncertainty level. The only exception to this good agreement is thatthe CIII 97.7 nm emission in the 1994 measurement appears anomalously low.Although all other emissions appear to be corrected properly for nonlinear affectsusing Equation (1), it is possible that this CIII line is more saturated than expected.

The 1994 measurement, being the only complete spectrum and the one closestto solar minimum conditions, is compared to proxy model predictions to study theabsolute values being used in the proxy models. All proxy model comparisons use


the model predictions for the particular date of the measurement. Comparison ofthe Hinteregger, Fukui, and Gilson (1981) model to our 1994 measurement in the1 nm intervals, as shown in Figure 2(A), shows differences at many wavelengthsby as much as a factor of two. The Hinteregger model FUV results are lowerthan the SOLSTICE irradiances by a factor of about 1.5 except at L� where theyagree better (Woods and Rottman, 1997). For the EUV chromospheric emissions,the Hinteregger model results are, in general, higher than our rocket observationsby as much as a factor of 2. On the other hand, the Hinteregger model resultsare lower than our rocket measurements for the EUV continua (C, H, He). Theintegrated total of the Hinteregger model irradiance below 200 nm is 40% lowerthan our measurements, and the model total below 120 nm is only 10% less thanour measurement.

Another model comparison is to the Tobiska EUV97 model (Tobiska and Epar-vier, 1997) which uses the wavelength bin format defined by Torr and Torr (1985).The result of comparing the EUV97 model to our 1994 measurement is similarto the Hinteregger model comparison and is expected because both models relyheavily on the AE–E measurements. The comparison of the 5 nm bins is shownin Figure 2(B) as the line, and the comparison of the brighter emission lines isshown as the diamond symbols. These differences are more obvious by plotting thebrighter lines separately, and the differences apply to both the Hinteregger modeland the Tobiska model. In this format, one also notices that the coronal emissionsare predicted to be about 30% lower than the actual measurement. The integratedtotal of the EUV97 model below 105 nm is 10% higher than our measurement; thisgood agreement is offset by some predicted emissions being a factor of 2 brighterthan measured values and some emissions being a factor of 2 lower.

A third model comparison is to the Richards, Fennelly, and Torr (1994) EUVACsolar proxy model as shown in Figure 2(C). This comparison is essentially acomparison to their modified ‘F74113’ rocket measurement in which they increasedthe original irradiances below 25 nm. The comparison indicates that their increasebelow 15 nm by a factor of 3 is in excellent agreement with our measurement butthat their increase between 15 and 25 nm by a factor of 2 is too much of an increase.Most of the EUVAC emission lines are in good agreement with our measurement,but the EUVAC 5 nm bins above 30 nm are all smaller than our measurement bya factor of 2 to 3. The EUVAC model was also partially derived using measuredphotoelectron fluxes; however, the gap between 0 to 5 nm in the EUVAC modelis an important contributor to highly energetic photoelectrons. The addition ofsolar irradiances below 5 nm to the EUVAC model has the potential to require areduction in the other XUV bins in the EUVAC model in order to be consistentwith the photoelectron measurements. The integrated total of the EUVAC modelbelow 105 nm is 22% smaller than our measurement.

Comparisons of the measurements are also made with other reference solarspectra for moderate activity. The Donnelly and Pope (1973) solar spectrum isappropriate for anF10:7 of 150 and thus is compared to our 1992 measurement in


Figure 3. Comparison to other measurements. Our 1992 measurement is compared to other meas-urements also made at a moderate solar activity level. The comparisons are to the Donnelly and Pope(1973) reference spectrum, the Ogawa (1996, private communication) composite spectrum, and theWoods and Rottman (1990) spectrum in panels A, B, and C, respectively. Differences are discussedin the text.

Figure 3(A). The Donnelly and Pope irradiance is about a factor of 2 lower at mostwavelengths than our 1992 measurement. Notably exceptions where the agreementis quite good are for the 33.5 and 36.5 nm bins and the 60 to 64 nm region. The33.5 and 36.5 nm bins are dominated by coronal emissions which agree well in thiscomparison. On closer examination, we note that the 60 to 64 nm region is affectedby the placement of the bright MgX 61.0 nm and OV 63.0 nm emissions within the1 nm intervals. For the Donnelly and Pope spectrum, these lines are located entirelyin the 61.5 and 63.5 nm bins, respectively. Whereas in our rocket measurement,these lines are not extracted first and thus appear in adjacent bins, e.g., the 61.0 nmemission contributes to both the 60.5 and 61.5 nm bins. The integrated total of theDonnelly and Pope spectrum below 103 nm is 45% lower than our measurement.A part of this difference is expected to be due to different levels of solar activity,but only at the 10–20% level.

We also compare the 1992 measurement with another moderate solar activ-ity spectrum compiled by H. Ogawa (1996, private communication). The Ogawaspectrum is composed of measurements from 0 to 1 nm from Kreplin and Horan(1992), 1 to 5 nm from Freeman and Jones (1970), and above 5 nm from Van Tas-sel, McMahon, and Heroux (1981). The Van Tassel, McMahon, and Heroux (1981)spectrum above 5 nm was taken when theF10:7 was 158, so our 1992 measure-


ment is used in this comparison. The format for the Ogawa composite spectrum is1 nm intervals. This spectrum is presented in the SOLERS-22 WorkshopProceed-ings(Woodset al., 1997). The integrated total of the Ogawa spectrum between 30and 80 nm is 53% higher than our 1992 measurement. This comparison, shownin Figure 3(B), indicates that the Ogawa spectrum may be more appropriate forhigher solar activity or that there may be absolute calibration differences as largeas a factor of 3. The method of placing the bright lines into the 1 nm intervals mayalso affect this comparison. In addition, this Ogawa spectrum has a much lowerirradiance level between the bright lines than any of our measurements. This resultsuggests that the corrections for background noise and scattered light may be toolarge in the Ogawa spectrum or too small in our measurements. The scattered lightcorrection for our EUV measurements (Woodset al., 1994b) is validated usingthe solar spectrum heavily attenuated at lower altitudes during the rocket flight;therefore, additional investigation of the Ogawa composite spectrum is warranted.

Finally, the 1992 measurement during solar cycle decline is compared to our1988 observation during solar cycle rise (Woods and Rottman, 1990). The 1988measurement corresponds to a similar level of solar activity (F10:7 = 148), andits integrated spectrum below 103 nm is 16% less than the 1992 measurement.This difference is well within the expected range of solar variability and calibra-tion uncertainties. Better agreement for this comparison is expected because thesame spectrograph is used in the 1988 and 1992–1994 measurements with someimprovements made between each flight. The largest differences in this comparisonshown in Figure 3(C) is due to the placement of lines within the 1 nm intervals. Asexplained in the Donnelly and Pope analysis above, the bright emission lines nearthe 1 nm boundaries are placed in adjacent bins for the 1992–1994 measurements.However, the Woods and Rottman spectrum was generated differently in that theemission lines were first extracted and then 1 nm intervals of remaining irradiancesformed. The extracted lines were subsequently placed entirely into a single bin.The two different analysis schemes cause the 50.5, 61.5, 63.5, and 79.5 nm binsto have a higher ratio and the 49.5, 60.5, 62.5, and 78.5 nm bins to have a lowerratio due to the placement of the SiXII 50 nm, MgX 61 nm, OV 63 nm, and OIV

79 nm lines, respectively. In comparing the irradiances of the extracted lines, wecaution that the extracted lines reported in the Woods and Rottman (1990) have nobackground continuum removed, whereas the 1992–1994 measurements do. Forexample, the SiXII 50 nm emission is on the HeI continuum, and the OIV 78.7 and79 nm emissions are on the HI continuum.

5. Solar Variability

These 1992–1994 measurements occur during the declining phase of solar cycle 22with the October 1992 period being already half way down the cycle for most solarindices and the November 1994 period being close to solar minimum conditions.


Figure 4. Solar VUV variability at 1 nm and 5 nm resolution. The solar variability during thedeclining phase of solar cycle 22 is given as the ratio between the 1992 and 1993 data in 1 nm and5 nm intervals in panels A and B, respectively. The average 1� uncertainty for this ratio is shown asthe dashed line in panel A. The ratios of the 1992–1993 predictions from the Hinteregger, Fukui, andGilson (1981) proxy model (2), the EUV 97 proxy model (Tobiska and Eparvier, 1997) (�), and theEUVAC proxy model (Richards, Fennelly, and Torr, 1994) (4) are shown in panel B.

Solar irradiance variability is estimated by comparing the 1993 measurement to the1992 measurement as shown in Figure 4. Because the FUV measurements fromSOLSTICE are made with the identical instrument and because there exists in-flightcalibrations for SOLSTICE, the solar variability in the FUV has a reasonably smalluncertainty of about 3%. The 1992 and 1993 rocket measurements above 30 nmwere made with identical instrumentation although illuminated at slightly differentincidence angles, but the bafffle and detector filter in the rocket spectrograph wereimproved for the 1994 measurement. We therefore consider the comparison of the1992 to 1993 solar EUV irradiances to be better for estimating the solar EUVvariability from the rocket measurements. With no XUV measurements in 1992,this comparison is restricted to wavelengths above 30 nm. The comparisons of therocket measurements rely entirely on the absolute calibration of the instrumentswhich yield an average uncertainty of about 15% for the solar EUV variabilityratios.

The amount of variability from the Hinteregger, EUV97, and EUVAC proxymodels are compared to the measured 1992–1993 ratio at 5 nm resolution, asshown in Figure 4(B). All models predict higher variability at most wavelengths


Figure 5. Variability of selected lines compared to the UARS SOLSTICE MgII C / W index. Theamount of variability between 1992 and 1994 is given in the top left corner of each plot. The variabilityof the HI L� at 121.6 nm is about 30% less than the other HI emissions (the HI 95 nm emissionis typical). The HeI emissions (such as the 58.4 nm line shown) vary similarly as most of the HIemissions. The HeII emissions (such as the 164 nm line shown) vary more than the HeI emissions.Somewhat unexpectedly, several of the transition-region ions, such as the OIV, OV, N IV, and NeVIIemissions, show much less variability than the H and He emissions.

with the Hinteregger model slightly higher than the other models. It is encouragingthat the wavelength dependence of the model variability is similar to the measuredvariability. However, there are differences as large as a factor of 2 in the amount ofvariability (i.e., ratio – 1) between the models and the measurement.

Besides taking ratios of the spectra, another way to estimate solar variability isto plot the irradiances at a single wavelength versus a solar proxy. The SOLSTICEMg II core-to-wing (C / W) index is chosen as the proxy for chromospheric emis-sions, and the solar 10.7- cm radio flux (F10:7) is chosen as the proxy for coronalemissions. Examples of 6 emissions plotted versus the Mg C / W index is givenin Figure 5. While the different emissions from the same ion are not expected tovary exactly together due to different solar atmospheric temperatures and radiativetransfer conditions, most emissions from the same ion do track each other withina few percent. Examination of all 53 extracted lines reveals that the variabilityfor the same ions are within the expected uncertainty for the measurements; thusproviding a first-order validation of the uncertainties for these measurements.


While much of the solar EUV irradiance varies similar to the more preciselymeasured solar FUV variability, there are several differences worth noting. Thecoronal emissions, such as the MgX 61 nm and SiXII 50 nm emissions, show highersolar variability than chromospheric emissions as expected (Hinteregger, Fukui,and Gilson, 1981). The HI, HeI, and HeII emissions show similar variability,except that the HI L� emission shows about 30% less variability. The AE–Emeasurements showed similar results with the HI L� emission being about 20%less variable than the other H emissions (Hinteregger, Fukui, and Gilson, 1981).Somewhat unexpectedly, the OV, N IV, and NeVII emissions, all Be-like ions,show very little variability. Sorting variability by isoelectronic sequences, it isclear that the extracted emissions with isoelectronic sequences of Be, B, C, N, andO have much less variability than other emissions of the H, He, Li, Na, Mg, and Alsequences. These interesting results are likely related to the breakdown of emissionmeasure analysis found in our data by Judgeet al.(1995). Further work is needed,both with the irradiance data presented here and the data from SOHO, to examinethe causes of such differences. Such work is in progress (P. Judge, 1996, privatecommunication) .

Worden (1996) indicates that the solar FUV irradiance is driven primarily by thechanges in the area of the plage and active network on the solar disk. These changesinclude 27-day variability driven largely by the solar rotation of the plages andlong-term variability related to the evolution of plage erupting and then decayinginto active network during the solar 22-year magnetic cycle. The solar variabilityestimates presented here using the three measurements spaced about a year apartrepresents better the long-term variability. The solar FUV variability is betterpresented by Woodset al.(1996), Chandraet al.(1995), Londonet al.(1993), andWorden (1996), where the time series of the UARS solar irradiances have beenanalyzed in far more depth.

6. Conclusions

The solar VUV measurements taken on October 27, 1992, October 4, 1993, andNovember 3, 1994 by rocket experiments and the UARS SOLSTICE instrumentare presented for consideration as reference spectra for moderate to low solaractivity. The amount of solar variability estimated by intercomparing these threespectra indicate that these measurements are consistent with each other within theircalibration uncertainties. Comparison of the 1994 measurement to the Hinteregger,Fukui, and Gilson (1981), Tobiska and Eparvier (1997), and Richards, Fennelly,and Torr (1994) models revealed differences as large as a factor of 2 at manywavelengths. The source for these differences may well be the AE–E data setwhich is used in developing these models. While the proxy models predict betterthe amount of relative variability as compared to the measured variability, thereare significant and interesting differences in solar variability at some wavelengths


that deserve additional attention. Comparisons of these rocket data to the Donnellyand Pope (1973) and Ogawa (1996, private communication) reference spectra alsoreveal factors of 2 differences when compared at similar levels of solar activity.While solar variability can explain some of these differences, instrument calibrationerrors for earlier experiments may be the primary contribution to these differences.Finally, the comparison of the Woods and Rottman (1990) spectrum to the 1992measurement showed good agreement within the range of solar variability.

Improved estimates of the solar EUV variability is best addressed with a well-calibrated satellite instrument. The ESA/NASA SOlar and Heliospheric Observat-ory (SOHO), which was launched in December 1995, provides important inform-ation about the solar EUV variability. The SOHO CDS and SUMER instrumentsare making daily measurements of the solar VUV intensity (partial solar images,not full-disk irradiance), and the SOHO EIT and SEM instruments are making full-disk images and irradiance measurements at a few XUV wavelengths, respectively.Because these SOHO measurements do not cover the full VUV spectral rangebelow 200 nm for the full-disk irradiance, additional studies of the solar VUVirradiance are warranted. Our recent attempt to address the solar EUV irradiancevariability with our Solar EUV Experiment (SEE, PI: T. Woods) on the METEORsatellite was unsuccessful when the Conestoga launch vehicle failed during launchin October 1995. Our next SEE instrument is being prepared for a launch on theNASA TIMED satellite in the year 1999 and will provide daily measurements ofthe solar VUV irradiance from 0.1 to 200 nm with 0.4 nm resolution. The TIMEDmission will study the energetics and dynamics of the thermosphere, ionosphere,and mesosphere.

Acknowledgements

This research was supported by NASA/NSF Interagency Agreements S-09936-Fand S-87289-E. We are grateful to Chris Pankratz and Richard ‘Hipook’ Brownfor their support in improving processing algorithms for the EGS instrument. Wealso acknowledge useful discussions with Philip Judge concerning solar variabilitysources.

Data Archive

The solar irradiance spectra in 1 nm intervals and the extracted emission lines canbe obtained from the anonymous ftp site on ‘lasp.colorado.edu’ in the ‘pub/rocket/’directory. The ‘Index.txt’ file gives a listing and a short description of the files there.


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