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 UV varies the most when variability is given as a ratio (percentage) However Variability given in energy units is more appropriate for climate studies 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)
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 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 Out of Phase Wavelengths 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) Versus Var. = Max/Min - 1 1.4% 1.6% 1.9% (raw data) 8.0% -4.1% Technique Uncertainty is ~20%
This technique can be tested with missions that have measured over cycle minimum SC 21 22 23 24 SME UARS SORCE 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 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) 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 400-1600 nm SSI / TSI 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 CCD Assy Rotational Prism Carrier Shutter/Photodiode Assy Fery Prism Assy CCD Aperture Ch. A Aperture Ch. B Aperture Ch. C Aperture External Flex ESR Detector Assy Vacuum Door Mechanism Fine Sun Sensor Focal Plane Module