Pergamon Adv. Space Rex. Vol. 29. No. 12, pp. 1957-1962.2002

0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain

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SOLAR UV IRRADIANCE VARIATION DURING CYCLES 22 AND 23

L.E. Floyd’, D.K. Prinz’, P. C. Crane’, and L. C. Herring’

‘Interferometrics Inc., 14120 Parke Long Ct., Chantilly, VA 20151 USA 2E. 0. Hulburt Centerfor Space Research, Naval Research Laboratory, Washington, D.C. 20375 USA

ABSTRACT

The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) aboard the Upper Atmosphere Research Satellite (UARS) has been measuring solar W irradiances since October 1991, a period which includes the decline of solar cycle 22 followed by the rise of cycle 23. Daily solar measurements include scans over the wavelength range 115-410 nm at 1.1 nm resolution. As expected, the measured time series of W irradiances exhibit strong periodicities in solar cycle and solar rotation. For all wavelengths, the UV irradiance time series are similar to that of the Mg II core-to-wing ratio. During solar cycle 22, the irradiance of the strong Ly-a line varied by more than a factor of two. The peak- to-peak irradiance variation declined with increasing wavelength, reaching -10% just below the Al edge at 208 nm. Between the Al edge and 250 nm the variation was -6-7%. Above 250 nm, the variation declines further until none is observed above -290 nm. Preliminary results for the first portion of cycle 23 indicate that the far W below the Al edge is rising at about the same rate as the Mg II index while the irradiances in the Lya! emission line and for wavelengths longer than the Al edge are rising more slowly - even after accounting for the lower level of activity of cycle 23. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

SUSIM UARS MEASUREMENTS OF SOLAR UV IRRADIANCE The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) (Brueckner et al., 1993, 1996; Floyd et al., 1998a)

aboard the Upper Atmosphere Research Satellite (UARS) is a dual-dispersion spectrometer instrument which carries four stable deutet-ium lamps (Prinz et al., 1996) and redundant optical elements which are used to maintain its cali- bration throughout its mission. Responsivity degradation of the optical elements (i.e. filters and gratings) has proven to be an undesirable, but not unwanted, source of instrumental trends in solar UV it-radiance measurements (Floyd et al., 1996, 1998b). SUSIM’s wavelength-dependent responsivity is determined through intercomparison of its working channel with infrequently used reference optical channels. These reference channels are in turn calibrated through measurements of the deuterium lamps whose W radiant outputs are monitored through intercomparisons of scans of the four lamps by the same optical channel.

On 14 March 1999, the optical elements which comprise the SUSIM UARS daily standard optical channel (SC) were changed for all subsequent days. One reason for this fundamental change was that the SC responsivity at important short wavelengths had become so low that spectral irradiances below 150 nm of sufficient precision could no longer be obtained with the original working channel. Another reason was that this change made it possible to begin using two additional reference channels with heretofore fresh (i.e. unused) primary gratings. One result of this change in the SUSIM measurement operations was that new procedures and methods were required to assess the changes in instrument responsivity which then determine the UV spectral it-radiances after that date.

SUSIM began making its daily solar UV irradiance measurements when solar activity was still near the maximum levels of solar cycle 22. After 1996, solar UV spectral irradiances have begun the rise associated with solar cycle 23. V20 is the latest released version of the SUSIM UV irradiance data. In this work, the V20 data set has been modified with updated responsivity calibrations affecting the data between 29 September 1996 and 8 August 1998 and supple- mented with additional data through 13 March 1999. This updated data set is provisional until the release of the next data version, V2 1.

1958 L. E. Floyd et al.

Composite Mg II Index

0.26Ot I . 1980

I 1990 Year

Fig. 1. Composite Mg II core-to-wing ratio index formed from NOAA SEC and SUSIM V19r3 indices.

Mg II CORE-TO-WING RATIO INDEX The Mg II core-to-wing ratio index has been shown to be an effective proxy for the variations in UV irradiance

over a wide range of wavelengths (Heath and Schlesinger, 1986). As implied by its name, the Mg II index is the ratio of the UV spectral irradiance at the center of the feature to those of its adjoining wings. The Mg II index is valuable because it is relatively insensitive to errors in instrument responsivity while remaining sensitive to emissions from that feature’s chromospheric core. Comparisons with Mg II provide a convenient way to characterize the variations in a given line or in a given wavelength range (DeLand and Cebula, 1998). For most wavelengths and time periods, an approximately linear relationship appears to exist between solar UV irradiance variations and the Mg II index.

Figure 1 displays a composite Mg II index formed from the NOAA SEC Mg II index (Viereck et al., 1999) and the SUSIM V19r3 Mg II index. The V19r3 index represents the most accurate and up-to-date Mg II index version derived from the SUSIM data and is freely available for downloading from the SUSIM ftp site (Floyd, 2001). Routinely, the SUSIM V19r3 Mg II index for a given day is made available after about 2-3 days. The index computation for the V19r3 version is therefore necessarily based on a projected responsivity - since calibration scans after the fact are needed to make a responsivity determination. The algorithm and formula used to generate the index were specifically designed to be insensitive to instrumental responsivity errors. Before the change in the SC, the Mg II wavelength region (275-287 nm) was scanned at 1.1 nm resolution twice daily, once as a part of the daily a full spectral scan (108-412 nm) and again for the Mg II region only. The daily Mg II index value reported for the V19r3 index was based on the full spectral scan. After the new SC began daily full spectral scans, the Mg II region was separately and redundantly scanned using the old SC to ensure that the measurements of one could be easily related to the other. These two scans result in two separate Mg II indices. Although each is calculated identically the absolute levels of the two resulting indices need not be the same and are, in fact, different. However, as is the case with Mg II derived from different instruments having different resolutions and other characteristics, their respective time series are linearly related to one another unless instrumental trends are still present.

This composite index is formed from the type 1 NOAA index (itself a composite of indices derived from the SBUV and SBUV2 instruments aboard the Nimbus-7 and NOAA-9 spacecraft, respectively) for times before 1994 and an adjusted SUSIM V19r3 index thereafter. The adjustment factor is determined through linear regression of the SUSIM index after 1991 and the type 1 NOAA index. The SUSIM 1991 data are not used to relate the NOAA and SUSIM Mg II indices because of an instrumental trend in the index in the first few months of data (e.g. Cebula and DeLand, 1998). A Mg I (185 nm) core-to-wing ratio index time series shows that the probable cause of this trend is stray light in the SUSIM instrument that is a changing with time. It is expected that this trend will be removed in a later version

Solar UV Irradiance Variation 1959

of the SUSIM index. However, such a change should not affect the values of the composite index in any significant way.

15ot . V20 Measured lrradiance Variation ______________ .._..............

loo+- 01 Hi Lyman a c II

Cycle 22 Estimate

50 E _ _ . I si IV c IV

‘_ _ _ _ _’ - 0=

120 130 140 150 160

140 160 180 200 220

200 220 240 260 280 300 320

Fig. 2. Peak-to-peak solar cycle 22 UV irradiance variation. Dashed line is measured variation between 2 February 1992 and 29 May 1996. Solid line is fitted Mg II index estimate for the entire cycle based on a daily maximum recorded on 30 January 1991.

The result is a 20+ year time series which can be used as a proxy for UV irradiance variations generally. During cycles 21 and 22 which were of approximately equal magnitude, the long-term trends of sunspot number, Fia.7, and Mg II were similar. Indications are that solar cycle 23 will have somewhat lower levels of activity than the previous two cycles for which Fla.7 and Mg II are both available. As the cycle 23 maximum is reached, it will be interesting to observe if the relative long-term parity is maintained as peak solar activity changes from cycle to cycle.

SOLAR UV IRRADIANCE TIME SERIES The solid lines in Figures 3a, 3b, and 4 are the augmented V20 200-205 nm, 235-240 nm, and Ly-ar solar UV time

series. In this version of the UV spectral irradiance data, we have supplemented V20 irradiances through 8 August 1996 with new data to 13 March 1999 that are based on the latest version of the SUSIM wavelength- and time- dependent responsivity calibrations. Level 2 UV spectral irradiances at their measured resolution and wavelengths were integrated over the desired wavelength ranges to form the displayed time series. These interim data remain provisional as the responsivity calibrations for that period are still being evaluated and refined. The 200-205 nm integrated W irradiance is similar in behavior to that of continuum far UV wavelengths. Similarly, the variations of the 235-240 nm integrated irradiance exhibit behavior typical of the 208-250 nm wavelength range important for its effects on stratospheric ozone.

Estimated Solar Cycle 22 Variability On 2 February 1992, about six months after SUSIM W measurements began, a secondary peak of solar cycle 22

was reached. According to the composite Mg II index, this peak was only slightly below the earlier peak on 30 January 1991. Because the UV irradiance was not measured at the highest peak of activity of solar cycle 22 (as measured by the Mg II index) the solar cycle 22 variations of the W irradiance at various wavelengths cannot be directly computed. However, given the general correspondence between the Mg II index and the wavelength-dependent W irradiance, one can estimate the unmeasured solar cycle 22 peak it-radiance. Comparison with the fitted UV irradiance at the measured subsequent minimum gives the the solar cycle variation. Figure 2 displays an estimate of the solar cycle 22 variation based on the publicly available V20 SUSIM W irradiances. As outlined above, the integrated UV irradiance

1960 L. E. Floyd et al.

48

47

46

45

-.... Linear Mg II fit

i- = 0.965763

-----. Linear Mg II fit

r = 0.894860

43

1 .o

0.5

0.0

-0.5

-1.0 1992 1994 1996 1998 1992 1994 1996 1998

Fig. 3. SUSIM V20 UV irradiance time series for 200-205 nm and 235-240 nm and corresponding linear least squares fits to the Mg II index. Residuals of the fits are displayed below.

for each wavelength region is fitted to the composite Mg II index. The peak-to-peak variation of the fitted function is the solar cycle variation. An further advantage of this method is that because the Mg II time series is less noisy, measurement noise has less of an effect on the solar cycle variation estimates. The results are similar to results based the previous (~19) version of the SUSIM W irradiance data (Floyd et al., 1999). Generally, the variation is larger for shorter wavelengths and for strong emission lines. The strong Ly-a emission line varies by over a factor of two. The variations decline with increasing wavelength up to the Al edge (-208 nm) where the variation is about 10%. Between the Al edge and -252 nm (Mg edge), the peak-to-peak variation is 6-7%. At longer wavelengths excepting for the Mg II and Mg I absorption features, the solar cycle variation monotonically declines to less than the estimated long-term error (-2% l-a) near 290 nm. No significant long-term variation above this wavelength is measured.

Correspondence of Tie Series with the Mg II Index Proxy Previous work has found a relatively good correspondence between the Mg II index proxy and W spectral irradi-

antes (e.g. DeLand and Cebula, 1998). A proxy is valuable if it can effectively substitute for direct measurements of an important quantity and is easier to measure. Although Mg II (just as the UV irradiance) must be measured from space, the way in which it is defined makes it resistant to instrumental errors and thus far easier to measure accurately. Thus, if a reliable connection can be made between Mg II and W irradiance, fewer expensive measurements of W it-radiance from space might be needed. The level of correspondence of the various solar UV irradiance time series with the Mg II index is still being studied. It has been established that short-term variations in W irradiance and Mg II can be quite different as secured during the November 1994 through April 1995 period. (Floyd et al., 1998a, Crane et al., 2001) During this period, the time series of the integrated 200-205 nm h-radiance (and for that matter neighboring wavelength bins) shows predominant 13.5-day oscillations whereas the Mg II index shows significant 27- day components as well. The dissimilar behavior stems from differing limb darkening (Donnelly and Puga, 1990) of solar radiant emissions for the two wavelengths. The stronger limb darkening near 200 nm makes for more narrowly directed facular radiance.

The longer-term correspondence of variations in Mg II and W irradiances at various wavelengths is less well understood. It is widely supposed that UV irradiance variations are similar to that of the Mg II index for the long- term. However, the very same uncertainties in UV irradiance measurements which make the Mg II index attractive as a substitute render the connection correspondingly uncertain. For example, Dessler et al. (1998) in a review of the UARS W measurements, noted that the UARS measured Ly-cr irradiance bears a different relationship to the Mg II index than did Ly-(Y earlier measurements where the Ly-cr irradiance and the Mg II index were found to be linearly

Solar UV Irradiance Variation 1961

----.. Mg II linear fit Mg II short- and long-term

r = 0.916153 linear regression fit

- 10 r = 0.925568

,’ ! 1’1: .., ..$I _ ,.,b)

1992 1994 1996 1998 1992 1994 1996 1998

Fig. 4. SUSIM Ly-cr irradiance time series. On the left is displayed a linear fit to the Mg II index. On the right is a three parameter fit in which solar rotation (short-term) and solar cycle (long-term) components of the Mg II index are fitted separately. The residuals of each displayed below.

related. Figure 3 displays two representative integrated UV irradiance time series, least squares fits to the Mg II index, and their corresponding residuals for the 200-205 and 235-240 nm wavelength ranges. The variation of the first of these is about twice that of the latter. That the measured variations follow more closely that of Mg II in the former case than the latter is because the instrumental systematic error is a larger proportion of the observed variation. For both cases, the RMS of the fit residuals is within the estimated experimental error of 2% for this version of the data. Thus, we conclude that the SUSIM measurements show that the UV irradiance variations in these wavelength ranges are consistent with a long-term linear relationship with the Mg II core-to-wing ratio index. If this conjecture is strictly true, the residuals would be identical to the systematic errors in the instrumental calibration.

A somewhat different relationship exists for the SUSIM Ly-a h-radiance. Previously, Floyd et al., (1997) using SUSIM data from 1991-1996 showed that separating the Mg II index into its short- and long-term components and fitting the Ly-cr irradiance time series to a linear function with separate coefficients for each of the two components significantly improved the quality of the fits over fits of the Mg II index only. Figure 4 shows the two fits updated with additional data through 13 March 1999 along with the corresponding residuals. As before, the residuals continue to show that the solar rotation (-27 day) variations are better accounted for by the three-parameter fit, through solar minimum. However, this correspondence breaks down during the rise associated with solar cycle 23. There are at least two possible explanations for this anomalous behavior. One possibility is that the long-term (i.e. solar cycle) trend in the Ly-a! irradiance does not always follow that of the Mg II index. This is plausible since Mg II and Ly- a! originate different layers in the solar atmosphere as suggested by Woods et al. (2000). Alternatively, there may systematic errors in the determination of instrumental responsivity for Ly-cr wavelengths. However, the abrupt rise in the residual time series completely coincident with a rise in solar activity during August 1997 is not characteristic of instrumental responsivity errors. Systematic errors in responsivity are almost always slowly varying making it unlikely that a responsivity error would precisely counteract a sudden rise in solar activity (as given by the Mg II index). Alternatively, significant responsivity errors in SUSIM might have occurred in cycle 22 and thus altered the true solar cycle trend Obviously, SUSIM’s responsivity to Ly-a! as well as the resulting irradiance time series will be a focus of future study.

SUMMARY and CONCLUSIONS Selected preliminary UV irradiances measured by SUSIM UARS between 12 October 1991 and 13 March 1999

are presented. This time period covers the last secondary peak of solar cycle 22, solar minimum, and approximately

1962 L. E. Floyd et al.

half of the solar cycle 23 ascent. With the exception of the Ly-a, irradiance and to the present limits of experimental measurement accuracy, the long-term time series variations are the same as those of the Mg II index. The Ly-o irradiance measurements show considerably lower irradiance during the rise of solar cycle 23 than is implied by cycle 22 h-radiances and the Mg II index proxy. The responsivity calibrations which were used to derive these UV spectral irradiances are still in development.

ACKNOWLEDGEMENTS This work was supported by NASA-Defense Purchase Requests S14798D and S10108X.

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Brueckner, G.E., L.E. Floyd, P.A. Lund, D.K. Prinz, and M.E. VanHoosier, Solar ultraviolet spectral-irradiance ob- servations from the SUSIM-UARS experiment, Metrolog& 32,661-665, 1996.

Cebula, RI?, and M.T. DeLand, Comparisons of the NOAA-11 SBUV/2, UARS SOLSTICE, and UARS SUSIM Mg II Solar Activity Proxy Indexes, Solar Physics, 177, 117, 1998.

Crane, Patrick C., Linton E. Floyd, Lynn C. Herring, J. W. Cook, and Dianne K. Prinz, The Excess Center-to-Limb Function for Solar Active Regions at Ultraviolet Wavelengths Determined from UARS/SUSIM Ultraviolet Irradi- ante Measurements, submitted to Ap. J., 2001.

DeLand, M.T. and R.P. Cebula, NOAA 11 Solar Backscatter Ultraviolet, model 2 (SBUV/2) instrument solar spectral irradiance measurements in 1989-1994. 2. Results, validation, and comparisons, JGR, 103, 16251-73, 1998.

Dessler, A.E., M.D. Burrage, J.-U. Grooss, J.R. Holton, J.L. Lean, et al., Selected Science Highlights from the First 5 Years of the Upper Atmosphere Research Satellite (UARS) Program, Rev. Geophys., 36, 183, 1998.

Donnelly, R.F. and L.C. Puga, Thirteen-day periodicity and the center-to-limb dependence of UV, EUV, and X-ray emission of solar activity, Solar Physics, 130,369, 1990.

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Floyd, L.E., P.A. Reiser, PC. Crane, L.C. Herring, D.K. Prinz, and G.E. Brueckner, Solar Cycle 22 UV Spectral Irradiance Variability: Current Measurements by SUSIM UARS, SoEar Physics, 177,79-87,1998a.

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