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Adv. Space Res. Vol. 21, No. 12, 1927-1932.2001 pp. 8 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved

Printed in Great Britain 0273- 1177101 $20.00 + 0.00

PII: SO273-1177(01)00272-l

THE SOLAR CYCLE VARIATION IN ULTRAVIOLET IRRADIANCE

G. Rottman, T. Woods, M. Snow, and G. DeToma

Lahoratot;l. /or .1 tnzospheric and Spwe Physics, Universit~~ of’ Colorado. Boulder, CO, 803094590, USA

ABSTRACT

The Solar Stellar Irradiance Comparison Experiment (SOLSTICE) is one of the ten science instruments on the Upper Atmosphere Research Satellite (UARS), launched in 1991 and now successfully operated for more than nine years. The SOLSTICE makes daily observations of solar spectral irradiance in the interval 120 to 320 nm - radiation important to ozone in the Earth’s middle atmosphere. Nine years of SOLSTICE observations now provide a reliable estimate of solar-cycle variations, extending from early in 1992 near the peak of solar cycle 22, through solar minimum in late 1996, and now back to the high levels of solar cycle 23. These observations indicate almost a factor of two variation near Lyman-a (121.6 run), decreasing to less than 10% near 200 nm, and to less than 1% near 300 nm. 0 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

TNTRODUCTION

NASA’s Upper Atmosphere Research Satellite (UARS) is a mission dedicated to improving our understanding of ozone in the Earth’s middle atmosphere (Reber, 1990). The spacecraft carries ten instruments to measure the composition and structure of the middle atmosphere with additional instruments to collect information on atmospheric dynamics and energy input. Special emphasis has been given to the measurement of ozone and other key gases that influence ozone concentration and distribution.

The Sun is the dominant direct energy input to the middle atmosphere, and changes in the solar radiation, primarily ultraviolet radiation with wavelengths shorter than 300 nm, will lead to changes in atmospheric composition, temperature, and dynamics. In order to study atmospheric processes, especiaIly those involving photochemistry, the UARS observations must include precise and reliable measurements of solar ultraviolet radiation. Moreover, in order to fully understand trends in atmospheric data, whether trends during the UARS epoch or longer-term trends relating UARS data to past and future observations, today’s solar measurements must be amenable to intercomparison. The UARS solar observations must be of sufficient quality and accuracy to allow valid comparisons to past and, especially, to future observations.

THE UPPER ATMOSPHERE RESEARCH MISSION The UARS Spacecraft

The UARS was launched in September of 1991 with a nominal mission life of eighteen months. The spacecraft and eight of the ten instruments now continue to operate after more than nine years, and there is every expectation that successful operation may continue for another three to four years. The spacecraft orbit is nearly circular with an altitude near 600 km and an inclination of 57”. The spacecraft is three-axis, stabilized and maintains an orientation with one side facing nadir, another side facing the Sun. As the orbit precesses approximately every 40 days, the Sun “moves” through the orbit plane and the spacecraft must be turned 180” (YAW around) to keep the Sun side properly oriented.

In order to understand the energy input to the atmosphere UARS carries instruments to measure both solar radiation and energetic particles and tields. Three instruments measure solar radiation: one measuring the total solar irradiance (radiant flux density) and the other two measuring the ultraviolet spectral irradiance. All three of these solar instruments are mounted and co-aligned on a pointed platform on the Sun-side of the spacecraft, and they are pointed along the Sun vector with a precision and accuracy of about one arc minute.

1928 G. Rottman et al.

I‘hc (ot;11 solar irradiance’(TS1) instrument is the ACRIM II (Willson. 1994) and the two UV spectral instrumcu(s ;IIY the Solar Stellar Irradiance Comparison Experiment, SOLSTlCE; (Rottman et al., 1993), and the Solar Ultra\rolct Spectral Irradiance Monitor, SUSIM, (Brueckncr et al., 1993). Both of the spectral irradiance instruments co\.er’ about thesame spectral interval, namely 120 to 420 nm, with about the same spectral resolution of 0.1 to 0.3 nut. Detailed comparisons of the data from the two experiments are in close agreement (Woods et al., 1996) and additIonal comparisons and validations are on going.

Key to the reliability of long-term data sets is the ability to identify and remove drift and change in the instrument response. The two solar UV instruments approach this challenge in quite different ways. SUSIM uses standard lamps (DA lamps) and fully redundant detectors and optical channels employed with varying duty cycles. SOLSTIC’I;. on the other hand, uses a quite different technique of observing bright, blue stars with the same set of optics anti clo~ccto~~ used for the solar observations. These stars then become the standard “candles” to which the Sun is comparccl.

The SOLSTICE Instrument Solar Obser\ ;ttrons

The SOLSTICE has a requirement of measuring solar ultraviolet irradiance from 120 nm to 420 nm, achieving a spectral resolution of at least Inm, and correcting for changes in instrument response with an accuracy on the order of 1% throughout the UARS mission (nominal lifetime of 18 months). The intensity of solar irradiance increases by :I factor of five orders of magnitude over the requested spectral range, and moreover optical coatings and dctcct~r response change dramatically over these ultraviolet wavelengths. To accommodate these observational challenges, SOLSTICE has three completely separate spectral channels - each with its own detector, gratings and optics, and shaping filters (Rottman et al., 1993 and,Woods et al., 1993). The channels are designated as the G-channel covering wavelengths between 115 and 185 nm, the F-channel between 170 and 320 nm and finally the N-channel between 280 and 420 nm. Figure 1 illustrates the general shape of the solar spectrum and indicates where the three spectral channels are pieced together. The slight bit of wavelength overlap between the channel pairs provides some limited redundancy. but more important it serves as verification and validation of solar data derived i&m the two independent channels;

The solar spectrum of Figure 1 has the following general features and characteristics. It appears as a pseudo continuum superposed with both absorption and emission features. The longest wavelengths are overlaid with absorption lines and ionization edges and the shorter wavelengths are more dominated by emission lines. This spectrum originates at varying levels of the solar atmosphere - the longest wavelengths are emitted from the photosphere and the shorter wavelengths emitted at the higher layers of the chrornosphere and transition region. Likewise the various spectral features originate at different levels (and temperatures) of the solar atmosphere. The ir-ratlrancc of Figure 1 represents roughly 10% of the total solar irradiance (TSI), and the fact that TSI varies by only = 0.1% must constrain the combined ultraviolet ( 100 < h < 400 nm) to vary less than 1 %. Nevertheless, since the shortest wavelengths contribute only a very small fraction of TSI (e.g., Lyman-cc at I2 I .6 nm is less than 10e5 of TSI), and moreover. since they originate in the more active and disturbed levels of the higher solar atmosphere, they may vary by quite large factors and still be consistent with a 0. loo Lariation of TSI. The SOLSTICE observations and estimates of ultraviolet variation are presented in hollowing sections.

1 100

$ lo

El 1.0

0.1

0.01

Fig. 1. SOLSTICE solar irradiance spectrum at the instrument resolution. Positions of the transitions between the three spectral channels, G, F, and N, are indicated. Dominant absorption and emission features in the spectrum are also identified

Solar Cycle in UV Irradiance 1929

Stellar Observations SOLSTICE detectors have roughly five orders of

magnitude dynamic range. With only a shght optical reconfiguration the spectrometer sensitivity increases by roughly five additional orders of magnitude, and thereby SOLSTICE achieves the unique ability of observing both the Sun and bright, blue stars. Only apertures are changed - the optics and detectors are the same for the both the solar and stellar observations. This SOLSTICE capability serves two essential purposes. First, the stars provide a reliable and direct in-flight method of identifying and correcting the SOLSTICE sensitivity. In a straightforward manner the SOLSTICE sensitivity is adjusted at each wavelength so that separately, and as an ensemble, the stellar flux levels do not vary with time. Figure 2 illustrates the SOLSTICE technique and shows the observations of eight different stars at a wavelength of 171 nm. The dashed line passing through the points is a “best” estimate of the change in the response of the instrument at this single wavelength, and indicates a sensitivity degradation of about 4% per year. This calibration procedure is carried out at 54 separate wavelengths spanning the full spectral range of

Fig. 2. Eight different stars observed by the SOLSTICE G-channel at 171 nm. The dashed line is a least squares. multivariate fit to these data and represents the degradation of the SOLSTICE at this wavelength.

SOLSTICE. The present uncertainty estimate for the SOLSTICE degradation analysis is wavelength dependent with a conservative estimate of about f 2%. Further refinements to the stellar data processing may ultimately improve this uncertainty to near * 1% for UARS SOLSTICE, but only at the shorter wavelengths below 200 nm.

The second important aspect of the SOLSTICE technique is that it establishes the ratio of the solar ultraviolet irradiance to the ultraviolet flux from a number of stars. If the stellar flux remains constant over arbitrarily long time periods, future comparisons of the Sun to the same set of stars will provide a direct method of intercomparing today’s UARS solar observations to those future ones. All is of course dependent on the assumption that the stars remain very stable and emit a nearly constant ultraviolet brightness - a reasonable assumption since the stars selected are young, blue stars (spectral classification 0, B, and A). The theory of stellar evolution (Mihalas and Binney, 1981) prescribes only exceptionally small changes in brightness (one part in lo4 in time periods of 10” years) for these types of stars. The SOLSTICE technique is of course enhanced by the fact that many stars are used, and data of the type shown in Figure 2 can provide an accurate mean value against which each individual star can be examined. Stars that vary with respect to this mean value or show other type of pathological behavior are removed from the analysis.

SOLAR IRRADIANCE MEASUREMENTS Spectral Scan Data

There are a number of observational modes that the SOLSTICE employs - each accomplished using a different configuration of the instrument. For the majority of the solar observing opportunities, the spectrometer is in a spectral scan mode where the instrument records a sample and then increments the grating drive by one step. The instrument is quite versatile and can scan in either direction (increasing or decreasing wavelength), from any starting point, and with varying integration times (full spectral scan periods as short as 5 minutes to longer than one orbit). The instrument operates autonomously from most spacecraft functions, and during a single calendar day ten to fifteen independent solar observations are made at each wavelength setting.

The ground data processing takes each individual solar observation and converts it to an irradiance value (Woods et al, 1993) by applying the instrument sensitivity, including all known effects of pointing, temperature, spacecraft voltage levels, and instrument degradation. The observed irradiance is then corrected to a mean distance of 1 astronomical unit (1 AU). The processing then combines all irradiance values obtained during the calendar day (0:OO to 24:00 UT) and averages them to obtain the mean daily spectrum - the SOLSTICE mean daily spectrum therefore represents the value for 12:00 UT + 12 hours. The algorithms to process the SOLSTICE data continue to be improved, and the results provided in this report are referenced as the Version 13 data product.

1930 G. Rottman er al.

Figure I is a typical daily spectrum with a few important spectral features identified. This spectrum is at instrument resolution of about 0.15 nm for the G-channel. 0.25 nm resolution for F-channel, and 0.3 nm for the N- channel. Jn general, inaccuracies and slight shifts in the grating (wavelength) drive are problematic at this resolution, and the data are therefore binned to a spectrum with 1 nm samples (centered on the half nm). This daily “level 3” SOLSTICE product is available to the public through the EOS Distributive Active Archive Center, DAAC,

(l~ttp:~~tl:~ac.~s~c‘cttla~a.~ov/data/dataset~UARS/O 1 InstrumcntsiSOLSTICE)

Time Series The UARS SOLSTICE data span the period from October 3, 1991 through the present. Data that have been

calibrated, corrected for instrument degradation, and forwarded to the Goddard DAAC now include the beginning of the mission through December 1999. From this data base time series can be constructed for any single wavelength, or for any range of wavelengths. Figure 3 shows examples of time series at two different wavelengths, with time range extending from October of 1991 through the mid 1999. The top panel is the Lyman-a line (121.6 nm) identified in Figure I, and the bottom panel is a time series of irradiance integrated between 200 and 205 nm.

200to205nm

.042 ' I

1992 1994 1996 1998 2000

Fig. 3. Time series for two different wavelengths of SOLSTICE irradiance data. The higher frequency is modulation due to the 27-day rotation period of the Sun, and the longer term variation, from high levels in early 1992 through a minimum in 1996 and back to the higher levels, is the solar cycle variation. The data indicated as maximum and minimum are values used to determine the solar cycle variation of Figure 4.

RESULTS Intermediate Term Variations

The time series of Figure 3 are quite typical of the UARS era. The solar irradiance was at high levels early in the mission, decreased to low values in 1996, and has now regained the higher levels seen early in the mission. In addition to this longer term variability, the data also display a striking higher frequency variation with a period of about 27 days. Both features of the solar time series are well known, one representing the 1 l-year solar cycle variation and the other the 27-day solar rotation variation. The variations presented in both time scales are related to the storage and release of magnetic energy in the Sun (Spruit, 1994). This activity causes enhanced emission From all layers of the solar atmosphere - only a small percentage change for the photosphere, but much larger for the chromosphere, transition region and corona. The appearance and location of active areas o!l the solar disk is

Solar Cycle in UV Irradiance 1931

non-uniform, and therefore as the Sun rotates, ‘,“T approximately every 27-days, the irradiance is modulated by the passage of the bright regions resulting in the strong 27-day variation. These intermediate term variations are well studied and are discussed elsewhere . (see for example, Rottman and Woods. 1994 and * Rottman, 2000).

3

Solar Cycle Variations The longer-term variations evident in the time

series of Figure 3, first from high levels in early 1992 to low levels in 1996 and then back to higher levels in 1999, are indicative of solar cycle variations. This ultraviolet variability follows the shape and phase of other indices of solar activitv (Harvev and White. 1999). The actual peak of solar cycle 2.2 was earlier in 1991 and proceeded the launch of UARS; however, for this discussion the high levels in early 1992 are considered representative of solar maximum conditions. Likewise, the high levels seen in 1999 proceed the actual peak of solar cycle 23 that is anticipated to occur in 2001. Awaiting final analysis of additional SOLSTICE data only the decline of cycle 22 is considered here, and the estimate of solar cycle change is obtained by taking a ratio of average values in early 1992 to similar values in 1996. These two periods are identified in Figure 3, and an 81-day average value has been used at both the maximum and the minimum in order to remove the effect of the 27-day variation. Proceeding wavelength by wavelength the ratio of maximum to minimum has been

Wavelength (nm)

Fig. 4. Solar cycle variation as a function of wavelength obtained by taking the ratio of irradiance values at the time indicated “maximum” to those at the time “minimum” in Figure 3. The general nature of the variability shows high values of tens of percent at the shortest wavelengths originating in the solar chromosphere, and decreasing to a few percent to even less than 1% between 200 and 300 nm where the emission originates in the upper regions of the solar photosphere.

. ” t

lIII..(II.IJ.II...I..ll 120 140 160 180 200 220 200 220 240 260 260 300 320

calculated and the result is provided in Figure 4. Notice that at the shortest wavelengths the variability is greatest, especially for the strong chromospheric emission lines (e.g., Lyman-a at 121.6 nm, N V at 124 nm, Si IV at 139 nm and C IV at 150 nm). The variability steadily decreases to a value of only 6% near 200 nm, and then precipitously drops by another factor of 2 moving across the aluminum ionization edge at 208 nm. Between 200 and 300 run the solar cycle (cycle 22 only) diminishes to less than 1% near 300 run, a value that may be compromised by the uncertainty in the instrument degradation analysis of about + 2% (see instrument discussion above).

One aspect of Figure 4 has been highlighted (dashed oval) near 180 run, and at least a portion of the offset seen here is likely due to an anomaly still present in the instrument degradation functions. Referring to Figure 1, the transition from the G-channel data to the F-channel data occurs at 180 nm, and the offset requires close inspection of the degradation curves in this overlap region. However, a portion of the feature may also be solar in origin due the Si II emission lines at 180.8 and 18 1.7 nm. Ongoing SOLSTICE data analysis will hopefully resolve these questions in upcoming releases of the data. As mentioned earlier, the analysis presented here is from SOLSTICE version 13 data, and a refined version 15 data, including new stellar degradation analyses, is presently being evaluated. These version 15 data should be released through the Goddard DAAC early in 2001.

ACKNOWLEDGEMENTS The UARS SOLSTICE Project is supported at the University of Colorado by NASA contract NAM-97145

and by NASA grant NAG5-6850. B. Knapp, C. Russell, and B. Boyle form the core of the SOLSTICE operations and data processing team, and contribute to the success of this experiment. C. Jackman is the NASA UARS Project Scientist, and together with the entire UARS project team, they insure the continued success of the UARS mission.

1932 G. Rottman er al.

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Harvey, K. L., a~tcl Il’hite, 0. R., “What IS Solar Cycle Minimum?“, J. Geophys. Res., 104, 19759-19764. 1999. Mihalas, D., and t+illl\ey, J., Gcrkctic I.strwwn~~ Strrrcttrre and Kinematics, p. 135-139, W. H. Freeman, New York,

1981. Reber, C., “The Upper Atmosphere Research Satellite (UARS)“, Geoph_~~. Rex Letters, 20, 121% 12 18, 1990. Rottman, G. J., Woods, T. N., and Sparn, T.P., “Solar Stellar Irradiance Comparison Experiment 1: 1. Instrument

design and operation”, J. Geophys. Res., 98, 10667-I 0678, 1993. Rottman, G. J.. and Woods, T. N., “UARS SOLSTICE”, SPIE Proceedings, 2266,317-327, 1994. Rottman, G.. “Solar UV h-radiance Measurements: The UARS and EOS SOLSTICE”, Phys. Chem. Earth,, 25, 491-

404,200o. Spruit, H.C., “Theoretical Interpretation of Solar and Stellar Irradiance Variations”, in The Sun as a Variable Star,

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Willson, R. C., “lrradiance observations of SMM, Spacelab 1, UARS, and ATLAS experiments”, in The Sun us u Vuriable Stur, ed. J. M Pap, C. Friihlich, H. S. Hudson, and S. K. Solanki, Cambridge Univ. Press, Cambridge. p. 54-62, 1994.

Woods, T. N., Rottman, G.J., and Ucker, G.J., “Solar Stellar Irradiance Comparison Experiment 1: 2. Instrument calibration”, J. Geophys. Res., 98, 10679- 10694, 1993.

Woods, T. N, Prinz, D.K., Rottman, G.J., London, J., Crane, et al., “Validation of the UARS Solar Ultraviolet Irradiances: Comparison with the ATLAS - I, - 2 Measurements”, J. Geophys. Res., 101,9541-9569, 1996.