Solar Ultraviolet Irradiance Variations over Four Solar Cycles

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1 Solar Ultraviolet Irradiance Variations over Four Solar Cycles Tom Woods Laboratory for Atmospheric and Space Physics University of Colorado SSI Data Sets ( 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

2 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)

3 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

4 Understanding degradation is critical for understanding solar variations Measurement = SIM uncorrected data at 280 nm

5 Technique: pick days of similar solar activity levels equal-distance from Min. SC Proxy composite: <TSI> <27 days> SSN (daily) <SSN> <Mg C/W> <Lyman- > F10.7 <F10.7> Range: as each scaled to 100 for its SC variability range Reference Level is somewhat arbitrary Picked 2004/255

6 Assumption: average of variability before and after minimum = real solar variability Average Variability 1.5% (corrected) Var. = Max/Min % 1.6%

7 Assumption: average of variability before and after minimum = real solar variability Average Variability 1.5% (corrected) Versus Var. = Max/Min % 1.6% 1.9% (raw data) 8.0% -4.1% Technique Uncertainty is ~20%

8 This technique can be tested with missions that have measured over cycle minimum SC SME UARS SORCE Proxy composite: <TSI> <27 days> SSN (daily) <SSN> <Mg C/W> <Lyman- > F10.7 <F10.7> Range: as each scaled to 100 for its SC variability range ASIDE: Proxy composite has lower minimum in 2008, so variability for SC can be relatively larger for this technique (e.g. PMOD TSI is x 2 larger) Reference Level is somewhat arbitrary Picked 2004/255

9 NRLSSI Comparison for Same Levels

10 SME over solar cycle min Ideal Comparison Best Wavelengths nm

11 UARS SUSIM SC Min Best Wavelengths nm

12 SORCE L3 SC Min SORCE L3 SOLSTICE: nm SIM: nm Best Wavelengths nm & nm

13 Combine the best of each to make Composite Solar Cycle Variability Note that full solar cycle variation is about a factor of 3 more

14 Combine the best of each to make Composite Solar Cycle Variability Note that full solar cycle variation is about a factor of 3 more

15 SSI / TSI from this composite SSI (1- uncertainty is about 30%) Wavelength (nm) nm is 1.2% of TSI nm is 28% nm is 116% So the nm range is most important for this comparison to TSI

16 DeLand SBUV-UARS SSI Comparison to this new Composite Measurement Best Wavelengths for SBUV nm

17 Solar Models compared to Composite nm

18 Solar Models compared to Composite nm

19 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 nm SSI = 84% TSI (expect ~88%)

20 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

21 Backup Slides

22 SORCE L3 SC minimum technique result TSI 0.032%

23 All Models compared to Composite nm SSI / TSI nm 96% 130% 104% N/A 84% TSI 0.032% Note that SIM nm SSI / TSI = 88% (expected for values above)

24 List of days of same level Solar Cycle Minimum 1986/ / /280

25 PMOD TSI suggests same levels are reasonably selected. 3- variation for detrended 27-day smoothed TSI = 313 ppm

26 SRPM Comparison for same levels

27 Woods & Rottman Model Comparison for same levels

28 UARS SOLSTICE SC Min Best Wavelengths nm

29 TIMED SEE SC Min Best Wavelengths nm

30 Example DeLand SSI time series (red diamonds are reference days)

31 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