The Shuttle SBUV (SSBUV) and NOAA-11 SBUV/2 instruments measured solar spectral UV irradiance during the maximum and declining phase of solar cycle 22. The SSB UV data accurately represent the absolute solar UV irradiance between 200-405 nm, and also show the long-term variations during eight flights between October 1989 and January 1996. These data have been used to correct long-term sensitivity changes in the NOAA-11 SBUV/2 data, which provide a near-daily record of solar UV variations over the 170-400 nm region between December 1988 and October 1994. The NOAA-11 data demonstrate the evolution of short-term solar UV activity during solar cycle 22

Cebula, Richard P.

DeLand, Matthew T.

Hilsenrath, Ernest

English

NASA Technical Reports Server

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SSBUV AND NOAA-I 1 SBUV/2 SOLAR VARIABILITY MEASUREMENTS
Raytheon STX Corporation, Lanham, USA *
ERNESTHII.SF.JV,RATIq
NASA Goddard Space Flight Center, Code 916, Greenbelt, USA
Summary: The Shuttle SBUV (SSBUV) and NOAA-11 SBUV/2 instruments measured solar
spectral UV irradiance during the maximum and declining phase of solar cycle 22. The SSBUV data
accurately represent the absolute solar UV irradiance between 200--,405 nm, and also show the
long-term variations during eight flights between October 1989 and January 1996. These data have
been used to correct long-term sensitivity changes in the NOAA-11 SBUV/2 data, which provide a
near-daily record of solar UV variations over the 170--400 nm region between December 1988 and
October 1994. The NOAA-11 data demonstrate the evolution of short-term solar UV activity during
solar cycle 22.
Keywords:
1. INTRODUCTION
In order to fully understand the origins of long-term trends in stratospheric ozone, spectral and
temporal variations in solar irradiance must be characterized. The integrated (or total) solar
irradiance is known to be constant to better than 0.3% over the 1l-year solar cycle (Lean, 1991).
Long'ward of approximately 290 nm, spectral solar irradiance can be observed from the ground, but
instrumental limitations and atmospheric effects preclude measurement of predicted sub-l%
variations over long time scales. The solar irradiance shortward of 290 nm in the ultraviolet (UV)
region must be observed from space, using either short-term observations from recoverable
instruments such as rockets and Space Shuttle flights, or long-term observations from satellite
instruments. Rocket measurements have historically had significant uncertainties in absolute
calibration (typically 10-20%) which have prevented the determination of long-term (i.e. solar
cycle) changes from widely separated observations. Satellite data sets of solar UV irradiance
typically suffer from rapid deterioration in instrument sensitivity with time, such that long-term
trends are difficult to determine accurately.
In this paper, we describe the use of coincident Shuttle SBUV (SSBUV) data to create an
accurate long-term solar UV irradiance data set from the NOAA-I1 SBUV/2 instrument. The
excellent absolute and relative accuracy of the SSBUV irradiances allows the NOAA-I 1 data to
have a long-term uncertainty of less than ±1% at 400nm over 5.5 years, and better than .-_.2% for all
data longward of 200 rim. The combined SSBUV and NOAA-11 data indicate a long-term decrease
of approximately 7% at 205 nm between the maximum of solar cycle 22 in 1989-1991 and the near-
minimum conditions of October 1994, when NOAA-I 1 ceased solar measurements. NOA.A-11 data
also show the evolution of solar rotational activity in Cycle 22. Additional description of the long-
term calibration of the NOAA- 11 data can be found in Cebula et al. (I998b).
i
Address: Raytheon STX Corporation, 4400 Forbes Blvd., l.,anham, MD 20706 USA
e-mail: mdeland@stx.com
Studia geoph, et geed. 42 (1998), i
© 1998 StudiaGeo s.r.o., Prague
https://ntrs.nasa.gov/search.jsp?R=19980162995 2020-06-15T23:44:49+00:00Z
M.T. DeLand et al.
2. SSBUV DATA
The SBUV/2 instrument continuously measures backscattered terrestrial radiance at
12 discrete wavelengths between 252-340 rim, then measures solar irradiance once per
week at the same wavelengths to produce spectral albedos which can be inverted to
produce stratospheric ozone profiles (Frederick et al., 1986). Three SBUV/2 instruments
have flown on NOAA TIROS spacecraft between 1985 and 1997 (NOAA-9, NOAA-I 1,
NOAA-14), and three more launches are planned during the next decade. These
instruments also make dafly'_spectral scan measurements over the wavelength range
160--405 nm with a resolution of -1.I nm. The SSBU'V instrument is the engineering
model of the SBUV/2 instrument, which was modified to fly on the Space Shuttle to
provide validation and long-term calibration (Frederick et al., 1990). SSBUV made eight
flights between October 1989 and January 1996 (Cebula et al., 1998a). Because SSBUV
was calibrated before, during, and after each mission, its solar irradiance data have
significantly better accuracy than previous satellite instruments. Comparisons with
coincident measurements in March 1992 from the UARS SUSIM and SOLSTICE satellite
instruments (Woods et al., 1996) and the ATLAS SUSIM and SOLSPEC Shuttle
instruments (Cebula et al., 1996) found that between 200-400 nm, the SSBUV data
represented the mean solar UV irradiance me_ured by five independent instruments to
approximately -+2..5% (2 c) accuracy, with no spectral dependence. This is a considerable
improvement over the _+10% inter-instrument differences shown by Cebula and
Hilsenrath (1992).
As noted previously, no measurable solar irradiance changes are expected at
wavelengths longward of 300 nm on solar cycle timescales (Lean, 1991). The long-term
accuracy of the SSBUV data is thus indicated graphically by Figure la, where the
irradiance data from each flight, averaged over 380-390 nm and normalized to SSBUV-2,
vary by < I% over the first 7 flights. At shorter wavelengths, where long-term solar
variations become significant, the SSBUV data show changes of 3-4% at 240-250 nm
(Figure lb) and 6--7% at 200-208 nm (Figure lc) relative to the SSBUV-2 flight. The
SSBUV-I data were omitted from these figures because of calibration problems. The
magnitude of solar variability at 205 nm is important in determining the external forcing
of stratospheric photochemistry. Because the SSBUV data span only 3-8 days during
each flight, a flight-averaged irradiance value can be affected by the phase of solar
rotational modulation (i.e. peak or valley). Thus, the SSBUV data in Figure 1 incorporate
both solar rotational variations and long-term solar change. We can use the daily
NOAA- I 1 irradiance data to fully separate these two effects.
3. NOAA-I 1 DATA
NOAA-I1 solar spectral irradiance measurements processed with fulI internal
corrections exhibit significant long-term drifts at 3.< 300 nm (up to 20% at 205 nm),
because the SBUV/2 instrument is not capable of determining end-to-end calibration
changes. Comparisons with coincident SSBUV observations provide a suitable reference
source for evaluating and correcting these drifts (Cebula et al., 1994). Figure 2 shows the
coincident spectral ratios between NOAA-11 and SSBUV data for the first seven SSBUV
ii $tudia geoph, et geod. 42 (1998)
Y.
SSBUV and NOAA- 11 SBUV/2 Solar...
flights, averaged over the dates of each flight. All ratios have been normalized to remove
the absolute spectral bias between NOAA-1 1 and SSBUV. Each ratio was fit spectrally
with a smoothing spline function to remove residual effects of small wavelength
registration errors. The degradation fit values from all flights at each wavelength were
then fit with a quadratic function to determine the temporal dependence of the NOAA-1 1
correction. Examples of these data and the corresponding fits are shown in Figure 3.
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Fig. 1. Time series of SSBUV flight-averaged irradiance data: (a) 380-390 nm; (b) 240-250 nm;
(c) 200-208 nm. All data have been normalized to the SSBUV-2 results.
Studia geoph, et geod. 42 (1998) 111
M, T. DeLand et al.
NOAA-1 1 Response Change using SSBUV Data
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Fig. 2. Ratios of coincident NOAA-I 1 SBUVI2 and SSBUV spectra for the first seven SSBUV
flights, normalized to remove absolute bias. Dates of SSBUV solar irradiance observations during
each flight are listed.
After removing the SSBUV-based correction for sensitivity change, the NOAA-11
irradiance data give a good picture of solar UV variability during the maximum and
declining phases of Cycle 22. Because coincident SSBUV data were used to correct
NOAA-11 instrument changes, the derived long-term NOAA-11 solar variations will be
approximately equal to the SSBUV results. The time series plot of the 380-390 nm band
average in Figure 4a shows that the drift in the NOAA-I 1 data is less than +1% over
5.5 years. At shorter wavelengths, such as the 240-250 nm band (Figure 4b), the corrected
NOAA-I I data clearly show the extended activity maximum of solar cycle 22 during
1989-1991 and the sharp decline beginning in April 1992. Short-term variations due to
rotational modulation are generally 2-3% peak-to-peak in this wavelength region. The
heavy line in Figure 4b is a 27-day running average of the irradiance data, and indicates a
long-term decrease in solar output of -3% from late 1989 through October 1994. The
200-208 nm data, shown in Figure 4c, show similar temporal variations on both short and
long time scales, with an increased amplitude because these data are taken shortward of
the AI ionization edge. The long-term decrease implied by the 27-day average curve in
this panel, which is effectively free of rotational modulation effects, is approximately 6%.
This is consistent with the result found previously by SSBUV, showing that the SSBUV
flight averages are not significantly impacted by rotational modulation effects. Recent
calculations with 2-D atmospheric models using similar solar forcing find that the
corresponding variation in globally averaged total ozone is approximately 1.5(:t'0.5)%
iv Studia geoph, et geod. 42 (I 998)
SSBUV and NOAA-I1 SBUV/2 Solar ...
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DATZ
Fig. 3. NOAA-1 ] instrument sensitivity change at 205, 300, and 390 nm. Data values (symbols)
are taken from Figure 2, and the curves are quadratic fits to each data set.
(Fleming et aL, 1995; Jackman et aL, 19963. The persistent solar rotational modulation in
the NOAA- 11 data, with a peak-to-peak amplitude of 4-6% during solar maximum and
2-3% during the declining phase of Cycle 22, can be studied further using statistical
techniques.
4. PERIODOGRAM ANALYSIS
Time series such as the NOAA-11 h-radiance data in Figure 4c are difficult to analyze
with traditional power spectral analysis techniques, because they contain sizeable gaps
which must be filled, thus possibly altering the statistical properties of the data set. The
periodogram formulation presented by Home and Batiunas (1986) successfully handles
such data sets without filling or interpolation. Applying the periodogram technique to the
2(X)-208 nm irradiance data produces the power spectrum shown in Figure 5, using 150
equally spaced frequencies corresponding to periods between 10 and 50 days. The dashed
line indicates the power level at which the result is 99.9% statistically significant. We find
two significant rotational peaks at approximately 26 and 29 days, and a peak at - 13.5 days
which has less significance.
In order to understand the presence of multiple rotational periods, we can adapt the
periodogram technique to characterize the evolution of solar activity. Following the
method of Bouwer (1992), we apply the periodogram to a 256-day window of data, then
step the window through the data set at intervals of 64 days. The combined "dynamic
power spectrum" results from all such periodograms are shown in Figure 6, where the
Studia geoph, et geod. 42 (1998) V
M, T. DeLand et aL
lowest contour level represents 99.9% significance for these data. The strong rotational
modulation signal (relative power > 40) during solar maximum has a 29-day period during
early 1989, which shifts to a 26-day period by mid-1990, then drifts to a 27-28 day period
by mid-1993. This evolution is the source of the double rotational peaks in Figure 5. The
dynamic power spectrum also shows significant power with a period of- 13.5 days during
late 1991 and early 1993. This represents true solar variability rather than aliasing of the
27-day rotational modulation, as shown by the time series in Figure 4c. An approximate
13-day period indicates the presence of active regions in opposing hemispheres
simultaneously (DeLand and Cebula, 1998).
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Fig. 4. Time series of NOAA-I 1 corrected irradiance data: (a) 380-390 nm; (b) 240-250 nm;
(c) 200-208 rim. The heavy solid line in panels (b) and (c) is a 27-day running average of each data
set,
vi Studia geoph, et g¢od. 42 (1998)
SSBUV and NOAA-I 1 SBUV/2 Solar ...
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NOAA-t I SBUV/2 PERIODOGRAM for 200-208 nTr_ Data
Time Interval = t988/340 to 1994/289
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Fig. 5. Periodogram of NOAA-I 1 200-208 nm data for December 1988--October 1994. The
dashed line denotes 99.9% statistical significance.
1995
1994
1993
1992
Jggl
1990
NOA.A-1 t SBUF/2 Dyr_arnicPower Spectrum
Wavelength Band = 200-208 nTn
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Fig. 6. Dynamic power spectrum of NOAA-1I 200-208 nm irradiance data. Dashed contour
represents 99.9% statistical significance; additional contours are in steps of I0, using the same scale
as Figure 5.
Studia g¢oph, et geod.42 (1998) vii
M,T.DeLand etal.
5. CONCLUSION
Solar spectral irradiance data covering the wavelength range 170--405 nm from the
SSBUV and NOAA-11 SBUVI2 instruments are now available for the period December
1988 to January 1996. The SSBUV data provide accurate absolute irradiances at 9-12
month intervals from the maximum of solar cycle 22 to near minimum. These data have
also been used to correct long-term instrument sensitivity changes in the NOAA-I 1 data,
which provide a near-daily record of solar UV irradiance from December 1988 to October
1994. The SSBUV-corrected NOAA-I1 SBUV/2 data show a decrease of 6--7% in
205 nm irradiance during this period, which demonstrates the magnitude of solar forcing
for stratospheric photochemistry. Model calculations with similar long-term solar
variations give corresponding changes of approximately 1.5% in total ozone. The
NOAA-11 data also show the evolution of solar rotational variations about their nominal
27-day period during solar cycle 22.
Acknowledgements: This research was supported by NASA contract NA55-31755 and NASA
grant NASW-4864.
Manuscript received." 19. November 1997; Revisions accepted: 25 February 1998
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Studia geoph, et geod. 42 (1998) ix


SSBUV and NOAA-11 SBUV/2 Solar Variability Measurements

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