Solar Extreme Ultraviolet Irradiance Measurements During Solar Cycle 22

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  • 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 803073000, U.S.A.

    (Received 25 October 1996; accepted 4 December 1996)

    Abstract. The solar extreme ultraviolet (EUV) irradiance, the dominant global energy source forEarths 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 3050% 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 AEE spectra by as much as a factor of 2 at some wavelengths. Our sounding-rocket flightsduring the past several years (19881994) 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 1721, 1996.

    Solar Physics 177: 133146, 1998.c

    1998 Kluwer Academic Publishers. Printed in Belgium.

  • 134 THOMAS N. WOODS ET AL.Table I

    Launch times and solar conditions for the 19921994 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 30103 nm 168.9 132.5 3136.107 Oct. 4, 1993 17:45 UT 26 nm 121.5 93.2 6

    17103 nm36.124 Nov. 3, 1994 18:45 UT 2120 nm 85.9 82.4 13

    While there have been many recent measurements of the solar far ultravi-olet (FUV: 115200 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. Woods et al.(1996) present solar irradiance spectra in 1 nm intervals for the full 119410 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 Woods et al. (1994a) describe the optical properties of theserocket instruments. The EGS is a 14 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. Bailey et 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 610%, 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 (Woods et 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. Bailey et al. (1998) describe in more detailthese XPS calibration results. The uncertainty of the XPS calibrations is about 1020%, 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 51.960 nm (at 103.8120 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

  • 136 THOMAS N. WOODS ET AL.

    scattered light as described by Woods et 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

    C

    m

    = C

    r

    e

    C

    r

    ; (1)where C

    m

    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 forC

    r

    ,

    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 onlasp.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

  • SOLAR EXTREME ULTRAVIOLET IRRADIANCE MEASUREMENTS DURING CYCLE 22 137

    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. Bailey et 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,

  • 138 THOMAS N. WOODS ET AL.

    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 310%.Woods et 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 C III 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 C III line is more saturated than expected.

    The 1994 measurement, being the only complete spectrum and the one closestto solar minimum conditio...