Quantitative assessment of the impact of solar ultraviolet irradiance variations on stratospheric ozone abundances currently requires the use of proxy indicators. The Mg II core-to-wing index has been developed as an indicator of solar UV activity between 175-400 nm that is independent of most instrument artifacts, and measures solar variability on both rotational and solar cycle time scales. Linear regression fits have been used to merge the individual Mg II index data sets from the Nimbus-7, NOAA-9, and NOAA-11 instruments onto a single reference scale. The change in 27-dayrunning average of the composite Mg II index from solar maximum to solar minimum is approximately 8 percent for solar cycle 21, and approximately 9 percent for solar cycle 22 through January 1992. Scaling factors based on the short-term variations in the Mg II index and solar irradiance data sets have been developed to estimate solar variability at mid-UV and near-UV wavelengths. Near 205 nm, where solar irradiance variations are important for stratospheric photo-chemistry and dynamics, the estimated change in irradiance during solar cycle 22 is approximately 10 percent using the composite Mg II index and scale factors

Cebula, Richard P.

Deland, Matthew T.

English

NASA Technical Reports Server

N95- 11124
CHANGES IN PHOTOCHEMICALLY SIGNIFICANT SOLAR UV SPECTRAL IRRADIANCE AS ESTIMATED BY
THE COMPO6ITE MG H INDEX AND SCALE FACTORS
Matthew T. DeLand and Richard P. Cebula
Hughes STX Corporation, Lanham, MD 20706 USA
ABSTRACT: Quantitative assessment of the impact of solar
ultraviolet irradianee variations on stratospheric ozone
abundances currently requires the use of proxy indicators.
The Mg II core-to-wing index has been developed as an
indicator of solar UV activity between 175-400 nm that Is
independent of most instrument artifacts, and measures solar
variability on both rotational and solar cycle time scales.
Linear regression fits have been used to merge the individual
Mg II Index data sets from the Nimbus-7, NOAA-9, and NOAA-
11 instruments onto a single reference scale. The change in
27-day running average of the composite Mg II index from
solar maximum to solar minimum is approximately 8% for
solar cycle 2 l, and approximately 9% for solar cycle 22
through January 1992. Scaling factors based on the short-
term variations in the Mg II index and solar lrradtanee data
sets have been developed to estimate solar variability at
mld-UV and near-UV wavelengths. Near 205 nm, where solar
lrradianee variations are important for stratospheric photo-
chemistry and dynamics, the estimated change in irradlanee
during solar cycle 22 is approximately 10% using the compos-
ite Mg II Index and scale factors.
INTRODUCTION
Variations in solar ultraviolet (UV) Irradianee have
been correlated with changes in stratospheric ozone and
temperature on short timeaeales of days to weeks [e.g.
Keatlng et al., 1987; Hood and dirlkowtc, 1991] and long
timescales of years to decades [e.g. Keatlng et al., 1981].
Satellite instruments provide the only daily solar ultraviolet
observations with wide spectral coverage over long time
periods. The NOAA-9 and NOAA- 11 SBUV/2 instruments have
been making solar observations beginning in March 1985 and
December 1988 respectively, and continuing to the present
[Cebula and DeLand, 1992]. The variation in solar irrad-
lance at 205 nm during solar cycle 21 has been estimated to
be 5-8% from SBUV data [Schlestnger and Cebula, 1992] and
2-10% from SME data [Rottman, 1988]. The magnitude of the
uncorrected instrument change In the SBUV/2 data exceeds
the estimated solar irradlanee variation for long-term use
[Cebula and Hllsenrath, these proceedings]. The SBUV/2
instruments incorporate an on-board calibration system to
monitor changes in diffuser reflectance as a correction to the
derived ozone abundances [Weiss et al., 19911. Successful
927
results from this calibration system only provide information
on time-dependent changes in one component of the SBUV/2
optical system, with no provision for the end-to-end calibra-
tions required for accurate long-term solar irradlanee
observations.
The limitations of current absolute UV lrradlanee
measurements have forced the use of proxy indexes to
represent solar UV variability, for many purposes, such as
atmospheric modelling. The 10.7 em radio flux (FI0.7) is
frequently used as a proxy for both short-term and long-term
solar UV activity [Keatin9 et al., 1981; Ebel et al.. 1986;
Chandra, 1991 ], but it is not generated in the same layers of
the solar atmosphere as ultraviolet radiation in the 200-300
nm wavelength region, which drives stratospheric ozone
photochemistry. Fl0.7 is generated in the solar corona, and
exhibits different behavior on solar rotational timescales than
middle-UV and near-UV solar lrradlance generated in the
solar chromosphere [Donnelly, 1990, 1992]. The equivalent
width of the He I 1083 nm line, which does have a chromo-
spheric origin, has been used as an index of solar UV activity
since 1974 [Harvey, 1984]. However, the He I daily record Is
only complete at the 60-70% level and contains many data
gaps of between 3 and 10 days, which affects studies of
short-term variability. In order to identify the contribution of
solar activity to stratospheric ozone variations, a proxy Index
Is needed which has a good correlation with solar irradiance
variations in the 200-300 nm region, approximately daily
measurements to allow accurate characterization of short-
term variations, and a data record covering one or morc solar
cycles to address the magnitude and phase of long-term
variations.
The Mg II core-to-wing Index of solar variability was
first developed for the Nimbus-7 SBUV instrument by Heath
and Schlesinger [ 1986], and has been extended to the NOAA-9
and NOAA-I1 SBUV/2 instruments [Cebula et al., 1992;
DeLand and Cebula, 1992]. The Mg II index is defined as the
ratio of the irradlance in the core of the unresolved Mg II
doublet at 280 nm, which Is sensitive to solar activity
variations In the chromosphere, to the irradiance in the wings
of the Mg II line, which approximates the local photospheric
continuum. Previous work [Heath and Schleslnger, 1986;
Donnelly et al., 1987; DonneUy, 1988] has demonstrated that
the Mg II Index effectively represents short-term solar irrad-
https://ntrs.nasa.gov/search.jsp?R=19950004711 2020-06-16T10:05:50+00:00Z
lance variations between approximately 200 and 300 nm.
The use of an Irradlance ratio eliminates wavelength-indepen-
dent instrument sensitivity change effects in the Mg II index,
and the choice of equally spaced wing wavelengths eliminates
most wavelength-dependent effects as well. The absolute
value of the Mg II index is strongly dependent on the exact
wavelengths and the bandpass of the instrument being used
because of the large difference in lrradlance between the Mg
II line core and the line wings [Hall and Anderson, 1988].
MG IIINDEX MEASUREMENTS
Long-term data sets of the Mg II index derived from
continuous scan solar Irradlance measurements over the 160-
400 nm wavelength region are now available from three
separate SBUV-series Instruments. Daily values of the
Nimbus-7 SBUV Mg It index time series from November 1978
to March 1987 are shown in Figure HaL NOAA-9 SBUV/2 Mg
II index values between March 1985 and November 1991 are
shown in Figure 10b), and NOAA-I I SBUV/2 Mg II index
values from December 1988 through January 1992 are shown
in Figure l(c). The NOAA-9 Mg II index values are sensitive
to instrument noise during periods of low solar activity
[DeLand and Cebula, 1992]. The NOAA-9 and NOAA-I1 Mg
It index results presented by Donnelly [1988, 1990] are
derived from SBUV/2 step scan measurements at selected
individual wavelengths about the MgII line. This product has
less sensitivity to irradiance variations in the Mg It llne than
the continuous scan Mg It index presented here because of
the choice of local continuum wavelengths [DeLand and
Cebula, 1992]. The Mg II index data derived from NOAA-9
and NOAA-11 continuous scan measurements will be used
here for continuity with the Nimbus-7 data, which are only
available in this format.
COMPOSITE MG II INDEX I_ESULTS
The variations in nominal Mg II index value between
the SBUV and SBUV/2 instruments shown in Figure 1 and the
Mg N INDEX DATA SET_
t,_'e_r[' T 'H ] T_'LI' ' [ ' T T T ' T_ / r T T T 'q ' " ']j2_:-
errs
_ o me
o ass
0200 •
i T , ,.,.  l ['&kik'li'1
o_t, • ± ._A _Li , I i I , I I]'ttlli,lliilWtl _ I . I , I , I
oilf, T T' I ' I ' I ' I ' I ' I ' I I
o211o [ NO#_i- i I
o _
o_.6 __Ld.__ I i I i I , I , I I I , l _ I _ I
DATE
FIGURE I. Mg II Index time series: (a) Nlmbus-7, (b)
NOAA-9, (c) NOAA-11. The NO_u_,-9 and NOAA-1 1 MgII data
have been smoothed with a 5-day binomial-weighted average.
o 2_,o
o Z85
I o 2"_5
o _o
o z65
t'
o 2f_
COMPOSITE l_g II INDEX
_T-"T -_ _-1 -T-r-' r'l' I ' 1 '1'1 'l '
10_8 tt_o0 1oo_ 1904 I_ I_ I_O I_
DATg
I12
I10 !
II
Ir
FIGURE 2. The composite Mg It Index time series MglIc(t}
from November 1978 to January 1992, smoothed with a
27-day running average. The right-hand scale uses the
average of the September 1986 data as a reference value for
normalization. The plotted error bars represent -+2 o errors.
possible variations in Mg II index sensitivity due to Instru-
mental differences must be considered ifa single continuous
Mg It Index data set is desired. Linear regression analysis
allows the Incorporation of both absolute and relative
differences between Mg It index data sets. Relationships
between Nimbus-7 and NOAA-9 contemporaneous Mg II index
data derived by linear regression, as well as between NOAA-9
and NOAA- 11. have been used to construct a composite Mg II
index referenced to the NOAA-9 Mg II index scale. Further
details of the procedure can be found in DeLand and Cebuta
[ 1992]. Using a 27-day running average of the composite Mg
It index time series to remove the effects of rotational modula-
tion, the change between solar maximum and solar minimum
for solar cycle 21 is approximately 8% (Figure 2), where the
average of the September 1986 data represents solar mini-
mum values. Solar cycle 22 shows an Increase of approxi-
mately 9% in the composite 1_ II index from solar minimum
to solar maximum in ¢mrly 1989. However, changes in the
NOAA-9 wavelength scale may be responsible for approxi-
mately 1% of this increase [DeLand and Cebula, 1992]. The
2 o error estimates for each regrtmaion fit plotted In Figure 2
indicate that the difference between the magnitudes of cycles
21 and 22 is not statistically significant.
COMPOSITE SCALE FACTORS
In order to estimate solar vari_bility at other ultravio-
let wavelengths using the Mg II index, a set of scale factors
must be derived to relate changes in the Mg II index to
Irradiance changes at each wavelength. Scale factors were
developed and presented for the Nlmbus-7 SBUV instrument
by Heath and Schleslnger [ 1986], and for NOAA-9 SBUV/2 by
Cebula et al. [ 1992], based on the strength of 27-day solar
rotational modulations. The creation of a composite Mg It
index from the Nimbus-7, NOAA-9, and NOAA- I l Mg II index
data sets suggests the need for a corresponding composite
928
zo COMPOSITE SCALE FACTORS
-o2 ,,,I,,,I,,,I,,,I,,,I,,,I,,,I,,,l,,,I,,,I,,,I,,,-
1@+0 18o 20O ;z20 24O _ _ _ 32O 34O 3CO 38O lO0
WAV£LENGTH [nm I
FIGURE 3. Composite scale factors Fs{A) for solar ultraviolet
variability In the wavelength region 170-400 nm at 0.2 nm
resolution, expressed as percent change in Irradianee for a
I% change in the Mg II index.
scale factors data set. Details of the procedure used In
deriving the needed scale factors are given in DeLand and
Cebula [1992]. Figure 3 plots the composite scale factors
Fs(A) for the wavelength range 170-400 nm, with 2 a error
bars shown at selected wavelengths. Features exhibiting
significant solar variability in Figure 3 include the Al I
absorption edge at 208 nm, the Mg It line at 280 nm, and the
Ca I] K and H lines at 393 and 397 nm. Except for a few
identifiable features, the composite scale factors can be
interpreted as showing no solar variability beyond 290 nm
within the measurement noise. The close agreement in
magnitude between the scale factors at 205 nm and the Mg lI
line supports the suggestion that the similar brightness
temperatures of these features will lead to similar responses
to irradlance changes [Heath and Schlesinger, 1986].
Some question exists as to the validity of estimating
long-term irradiance variability using the Mg II index scale
factors, which are derived from short-term variations. The
relative strength of rotational modulations between different
wavelengths appears to be constant during the solar cycle
[Heath and Schlesinger, 1986]. Predictions of long-term solar
lrradlance changes using the Mg II index and scale factors will
only be in error if the relationship between long-term and
short-term variability at a given wavelength differs significant-
ly from the relationship for the Mg lI index. The estimated
variability of solar irradianee at 205 nm using the scale
factors in Figure 3 is up to 7% over a solar rotation, and
approximately 9.8_+0.7% (95% confidence level) during solar
cycle 22, based on the 27-day running average of the compos-
Ite Mg It Index shown in Figure 2. The estimated solar
irradiance change at 205, 250, and 300 nm from 1978-1992
using the composite Mg It Index and scale factors is shown in
Figure 4, with error bars representing the 95% confidence
limit In the derived scale factors. A similar analysis by Lean
[1991] using the He I 1083 nm equivalent width data as an
Index of solar UV activity yields comparable results.
CONCLUSIONS
Understandingofthe variabilityofsolar UV lrradiance
is recognized as a critical element in determining long-term
ozone changes, The composite Mg lI index provides a solar
UV proxy indicator that is directly related to wavelengths
affecting ozone, and avoids instrument change effects which
prevent the use of absolute irradtances. Scale factors derived
from rotational modulation amplitudes allow estimates of
trradiance changes at mtd-UV wavelengths from changes in
the Mg II index. The composite Mg Il index and scale factors
are available on CD-ROM from NSSDC [Larko and McPeters,
1992]. Additional SBUV/2 instruments planned for launch at
approximate 2-year intervals through the year 2000 offer the
opportunity to construct a valuable long-term record of solar
middle ultraviolet variability.
ACKNOWLEDGEMENTS: This work was supported by NASA
contracts NAS5-29386, NAS5-31380, and NAS5-31755. The
assistance of Drs. W. G. Planet and J. H. Liencsch and Mr. H.
D. Bowman of NOAA/NESDIS in providing the NOAA-9 and
NOAA- l i SBUV/2 data is greatly appreciated.
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929
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6
5
4
3
3
1
0
-1
-2
-3
-4
-5
-6
1978
ESTIMATED SOLAR IRRADIANCE CHANGES
T I I I I I
! _ :1
: i'. ;:"' : _ ' : ::
_': '_. ':,;,::_ : '.I ..,, ;J_ _,:
."i,_ _!,,:'i _:: !::: : , -' ''_
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• _..-,-_.... .............. _ .... -_ -
;_,_, _-/ 300 nm _ -_.
', : , _l _ "
1980 198_ 1990 199_
• Ij _'
', _ ,.
_. '._:.:..,./,,_
L_ nm
I I I I I I
1904 1986 1988
DATE
FIGURE 4. Estimated solar irradlance change at 205, 250, and 300 nm during 1978-1992, computed from the
composite Mg II index and scale factors.
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930


Changes in photochemically significant solar UV spectral irradiance as estimated by the composite Mg II index and scale factors

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