Solar Ultraviolet Irradiance
Variations over Four Solar Cycles
Tom Woods
Laboratory for Atmospheric and Space Physics
University of Colorado
email: tom.woods@lasp.colorado.edu
SSI Data Sets (http://lasp.colorado.edu/LISIRD)
SME: G. Rottman
UARS SOLSTICE: G. Rottman & T. Woods
UARS SUSIM: L. Floyd
TIMED SEE: T. Woods & F. Eparvier
SORCE SOLSTICE: M. Snow & W. McClintock
SORCE SIM: J. Harder & J. Fontenla
Composite SSI (SBUV): M. DeLand
SSI Models
NRLSSI: J. Lean
SATIRE: W. Ball
SRPM: J. Fontenla
W&R: T. Woods & G. Rottman
Example solar cycle (SC) variability

UV
Note that red lines
are variations that
are out of phase with
the solar cycle.
Solar Cycle (SC)
variability from the
NRLSSI model
(J. Lean)
UV varies the most when
variability is given as a
ratio (percentage)
However…
Variability given in
energy units is
more appropriate
for climate studies

new SSI variability results
from SORCE
In addition to the infrared,
some visible wavelengths
are out of phase with the
solar cycle
 Also, there is more UV
variability than expected


Out of Phase
Wavelengths
Are the SIM and model
differences possibly related to
unresolved instrument trends?
◦ Checking these results is challenging
◦ Other validation techniques are
needed than direct comparisons
Figures from J. Harder et al., GRL, 2009
Understanding degradation is critical
for understanding solar variations
Measurement = SIM uncorrected data at 280 nm
Technique: pick days of similar solar
activity levels equal-distance from Min.
SC
21
22
23
24

Proxy composite:
◦
◦
◦
◦
◦
◦
◦
◦

<TSI> <27 days>
SSN (daily)
<SSN>
<Mg C/W>
<Lyman- >
F10.7
<F10.7>
Range: 0-700 as each
scaled to 100 for its SC
variability range
Reference Level is
somewhat arbitrary
◦ Picked 2004/255
Assumption: average of variability before
and after minimum = real solar variability
Average
Variability
1.5%
(corrected)
Var. = Max/Min - 1
1.4%
1.6%
Assumption: average of variability before
and after minimum = real solar variability
Average
Variability
1.5%
(corrected)
Var. = Max/Min - 1
Versus
1.4%
1.9%
(raw data)
Technique
Uncertainty
is ~20%
1.6%
8.0%
-4.1%
This technique can be tested with missions
that have measured over cycle minimum
SC
21
22
SME
23
UARS
24

◦
◦
◦
◦
◦
◦
◦
◦
SORCE
ASIDE: Proxy composite has lower minimum in 2008,
so variability for SC 23-24 can be relatively larger for
this technique (e.g. PMOD TSI is x 2 larger)
Proxy composite:

<TSI> <27 days>
SSN (daily)
<SSN>
<Mg C/W>
<Lyman- >
F10.7
<F10.7>
Range: 0-700 as each
scaled to 100 for its SC
variability range
Reference Level is
somewhat arbitrary
◦ Picked 2004/255
NRLSSI Comparison for “Same” Levels
SME over solar cycle min 21-22
Ideal
Comparison
Best Wavelengths
140-210 nm
UARS SUSIM – SC Min 22-23
Best Wavelengths
140-260 nm
SORCE L3 – SC Min 23-24
SORCE L3
SOLSTICE: 115-308 nm
SIM: 308-1600 nm
Best Wavelengths
115-270 nm & 310-400 nm
Combine the best of each to make
Composite Solar Cycle Variability
Note that full solar cycle variation
is about a factor of 3 more
Combine the best of each to make
Composite Solar Cycle Variability
Note that full solar cycle variation
is about a factor of 3 more
SSI / TSI from this composite SSI
(1- uncertainty is about 30%)
Wavelength (nm)
0-200 nm is 1.2% of TSI
0-300 nm is 28%
0-400 nm is 116%
So the 300-400 nm range is most important for this comparison to TSI
DeLand SBUV-UARS SSI Comparison
to this new Composite Measurement
Best Wavelengths for SBUV
140-200 nm
Solar Models compared to Composite
0-220 nm
Solar Models compared to Composite
180-400 nm
Conclusions
Observations over a solar cycle minimum
permit independent check on degradation
trend (at ~30% of the variability)
 Solar ultraviolet variability observations and
models agree best at wavelengths shorter
than 210 nm
 This SORCE re-analysis suggest lower
variability than Harder et al. (2009) but still
higher than some models for above 260 nm

◦ More consistent with SUSIM, SATIRE, SRPM
◦ 0-1600 nm SSI = 84% TSI (expect ~88%)
Future SSI Measurements

Can new instruments have little or no
degradation for the UV-Vis-NIR ?

NOAA/NASA TSIS SIM
◦ 3 channels: daily, monthly, annual
◦ Improved lower-noise version of SORCE SIM
Féry prism spectrometer covering the full wavelength range from the UV to IR
using only one optical element for spectral dispersion and image quality
Backup Slides
SORCE L3
SC minimum technique result
 TSI
0.032%
All Models compared to Composite
SSI / TSI
400-1600 nm
0-1600 nm
96%
130%
104%
N/A
84%
 TSI
0.032%
Note that SIM 0-1600 nm SSI / TSI = 88% (expected for values above)
List of days of “same” level
Solar Cycle Minimum
1986/195
1996/140
2008/280
PMOD TSI suggests “same” levels are
reasonably selected.
3- variation for detrended 27-day smoothed TSI = 313 ppm
SRPM Comparison for “same” levels
Woods & Rottman Model
Comparison for “same” levels
UARS SOLSTICE – SC Min 22-23
Best Wavelengths
120-170 nm
TIMED SEE – SC Min 23-24
Best Wavelengths
0-140 nm
Example DeLand SSI time series
(red diamonds are reference days)
Overview of TSIS SIM
Rotational Prism
Carrier
CCD Assy
Shutter/Photodiode
Assy
Fery Prism Assy
CCD Aperture
External Flex
Ch. A Aperture
ESR Detector Assy
Ch. B Aperture
Ch. C Aperture
Fine Sun
Sensor
Vacuum Door
Mechanism
Focal Plane
Module