Comparison of Solar Minima
Using Solar Ultraviolet
Irradiance Data
Matthew DeLand and Richard Cebula
(with thanks to Barry Schlesinger, Ernie Hilsenrath, Don Heath,
Gary Rottman, Tom Woods, Linton Floyd, Gerard Thuillier,
Guenther Brueckner, Dianne Prinz)
Science Systems and Applications, Inc.
(SSAI)
SORCE Science Meeting, Keystone, CO
19 May 2010
Atmospheric Impact of Solar UV
Solar UV Measurement History
CHALLENGE: Instrument
response changes are
typically large and rapid
relative to solar variations.
Different techniques to
characterize these changes
have been used.
25%
SOLSTICE: Stellar Observations
SUSIM: Onboard Calibration
25%
25%
SBUV/2: Reference Flights
Solar Minima Comparison –
Single Instrument
• NOAA-9 SBUV/2
data cover two
minima (1985-1986,
1995-1996).
• Ratio of average
spectra shows ~1%
difference for most
wavelengths.
• Observed changes
below 260 nm less
than prediction based
on ACRIM TSI trend.
DeLand et al. [2004]
Public Solar UV Data Sets
DeLand and Cebula [2008] (updated with SORCE data)
Creation of Composite Data Set
• Use 1 nm binned products (public release) for consistency.
• Normalize data to reference spectrum [Thuillier et al., 2004] to
remove large-scale biases.
• Intercomparison of instruments during overlap periods to
identify discrepancies.
• Compare each instrument’s data to solar activity proxy index to
help identify possible sensor issues.
• Generate synthetic values to fill data gaps.
• Create single product for full spectral/temporal range.
• M. T. DeLand and R. P. Cebula (2008), J. Geophys. Res., 113,
A11103, doi:10.1029/2008JA013401.
Normalize Irradiance Data
• Use Thuillier et al.
spectrum as reference.
• Use coincident data
for each instrument
where available.
• ATLAS-1 mission (29
Mar 1992) overlaps
NOAA-9, NOAA-11,
SUSIM, SOLSTICE
• Use smoothing fit to
minimize wavelength
scale difference
effects.
Intercomparison (1 of 387)
Comparison to Solar Proxy
Selection of Data
(Thanks to Frank Eparvier for the design concept)
Content of Product Data Set
• Daily 1 nm spectra covering 120-400 nm from
November 1978 to June 2005.
• Source flag (e.g. instrument or proxy; also note if
spectral and/or temporal interpolation performed).
• Absolute uncertainty (spectral values taken from
individual instrument analysis).
• Relative uncertainty (time-dependent values taken
from individual instrument analysis).
Composite Product: 150-154 nm
Data vs. Proxy – 150-154 nm
synthetic
SME
SUSIM
SOLSTICE
synthetic
Validation Comparisons
Solar Minima Comparison –
Composite Product
• Use same dates as
NOAA-9 comparison
(average 27 days at
minimum).
• Three instruments
used in 1986, two
different instruments
used in 1996.
• Same Y-axis range,
but endpoints shifted
and 120-170 nm data
added.
Extension of Composite
Product Using SORCE Data
• SORCE data coverage begins May 2003
(SOLSTICE), April 2004 (SIM).
• All data (so far) at lower activity levels than
ATLAS-1 reference spectrum Æ normalization
based on ATLAS-3 reference spectrum.
• Normalization dates with matching solar activity
level not available until late 2005 (2 years after
launch) Æ accuracy of degradation correction
comes into play.
• Variable SIM resolution introduces other issues
for NUV data that are not treated at this time.
Extended Data Set Structure
Normalization (no overlap)
• Select date with
comparable solar
activity to reference
spectrum (ΔMgII =
±0.3%, Δ<MgII>81
= ±0.1%).
• Accuracy of
normalization
depends on spectral
consistency of solar
variation, long-term
instrument
characterization.
Normalization Ratio vs. ATLAS-3
SOLSTICE
SIM
Solar Minima Comparison –
Extended Composite
• Choose most recent
minimum in April
2009.
• Results look
consistent with
previous plots.
• SORCE uncertainty
values estimated
from on-line notes
and discussion.
• However, this is
only a snapshot.
Extended Composite Data #1
Extended Composite Data #2
Extended Composite Data #3
Conclusion and Status
• Comparison of solar UV irradiance between minima requires
excellent long-term calibration accuracy.
• NOAA-9 SBUV/2 data show no change between Cycles 21/22
and 22/23 within long-term uncertainty.
• Composite UV irradiance data set covers 1978-2005 (available
on-line at LISIRD web site,
lasp.colorado.edu/lisird/cssi/cssi.html) Æ can be
extended to latest minimum using SORCE data.
• Comparison of minima from different cycles is complicated by
absolute calibration differences, normalization of data to create
composite product, residual instrument response changes.
• No clear evidence for multi-decade trends in UV spectral
irradiance data.