Long-Term Variations in UV and EUV Solar Spectral Irradiance

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1 Long-Term Variations in UV and EUV Solar Spectral Irradiance Linton Floyd 1 Don McMullin 2 1 Interferometrics Inc. / Naval Research Laboratory 2 Space Systems Research Corporation / Naval Research Laboratory International Workshop on Solar Variability, Earth s Climate and the Space Environment Bozeman MT June 1-6, 2008

2 Relevance of the Study of Solar EUV/UV Irradiance Knowledge of the Solar UV irradiance helps us better understand: Solar mechanisms Earth s atmosphere Earth s climate

3 Solar EUV/UV and Solar Atmospheric Structure Solar EUV/UV originates in: corona, transition region, chromosphere, and upper photosphere where much of the solar irradiance variation occurs. Above the temperature minimum, surface structures limit the usefulness of this 1-D model. From: Fox (2004).

4 Relevance to Earth s Atmosphere Absorption of Solar EUV/UV Solar UV absorption drives atmospheric: constituent densities, thermal structure, and dynamics. Solar UV is absorbed by: ozone ( nm) molecular oxygen ( nm) Haigh, 2004

5 Solar UV and Earth s Climate Climate and weather data shows connections to solar activity, e.g. QBO, NAO, and SST. Models show possible solar UV connections to dynamical changes descending from the stratosphere to the troposphere. Cosmogenic isotopes show correlations to climate over the past two millennia, independent of Milankovich (orbital and terrestrial attitude) changes. Solar causal connections to climate are poorly understood. Solar UV variation is a leading candidate.

6 Solar UV/EUV Irradiance Spectrum 4 Solar Ultraviolet Irradiance Spectrum Log 10 Spectral Irradiance (mw/m 2/nm) HeII EUV FUV Ly α CII CIV OI SiIV CIII Ly β HeI SiII 5778 K Blackbody Al edge Mg edge Mg II K H Ca II MUV NUV VIS -2-3 Ly edge Thuillier et al. (2005) wavelength (nm)

7 Solar EUV/UV Irradiance Spectrum extends from 30 nm in the EUV to the visible (400 nm) spans roughly 5 orders of magnitude contains about 8.7% of the total solar flux shows exponential increase in FUV to Al-edge (208 nm) for increasing λ, the spectrum is characterized by: strong emission lines ( nm) absorption lines ( nm) line-blanketed continuum continuum at 160 nm from solar temperature minimum

8 Measurements of the Solar UV/EUV Irradiance Instrumental Responsivity Calibration Typical source of largest uncertainty is changing instrumental responsivity. End-to-end calibration methods utilize measurements which are: stellar (e.g. SOLSTICE, SORCE), of lamps (e.g. SUSIM, SOLSPEC/ISS), vicarious (e.g./ NOAA-11/SBUV2, SEM), or redundant (e.g. SORCE, TIMED, SUSIM)

9 Measurements of the Solar UV Irradiance Coverage of Past Experiments Temporal Coverage of Solar UV Irradiance Experiments 400 Key: Nimbus-7/NOAA 9,11 onboard cal underflights GOME 300 no in-flight cal wavelength (nm) 200 AE-C OSO3 OSO5 SME UARS SUSIM,SOLSTICE 100 AE-E OSO4 AEROS-A AEROS-B 0 SM5 SNOE year SEM

10 Measurements of the Solar UV Irradiance Coverage of Present and Future Experiments GOME GOME-2 SCIAMACHY OMI AURA SOLSPEC NPOES (?) NOAA-N (?) PICARD/PREMOS wavelength (nm) 200 UARS SUSIM,SOLSTICE NOAA-16,17 SORCE SIM,SOLSTICE II PICARD/PREMOS SOL-ACES 100 SEE/ TIMED GOES (N-P) SEM SORCE XPS SDO EVE? 0 SNOE year

11 Solar UV Irradiance Experiments: Data Comparisons Data Comparisons: Ly-α UARS Solar Lyman α Irradiance 7 SOLSTICE V18 Upper panel shows the Ly-α irradiances from SUSIM and SOLSTICE. Lower panel displays their differences. x10 11 ph/sec/cm SUSIM V21r3 The two experiments show better relative than absolute agreement (mostly). SOLSTICE - SUSIM Average = STD = Year

12 Solar UV Irradiance Experiments: Data Comparisons Data Comparisons: nm Integrated Irradiance 50 UARS Solar Irradiance nm SOLSTICE V18 Upper panel are the nm integrated irradiances from SUSIM and SOLSTICE. mw/m SUSIM V21 Lower panel displays their differences The two experiments show better relative than absolute agreement (mostly). SOLSTICE - SUSIM 4 Average = STD = Year

13 Solar UV Irradiance Variations Sources and spatial distribution of UV radiance Variations in received solar UV irradiance are caused by the emergence and decay of active regions as they transit the solar disk. Active regions contain enhanced: UV brightness (faculae and plages) localized enhanced magnetic fields Upper right: BBSO Ca II k line brightness Lower right: GONG Magnetogram (Sources: BBSO)

14 Solar UV Irradiance Variations Time series characteristics UV irradiance time series periodicities dominated by: solar rotation ( 27 day) solar cycle ( 11 yr) SUSIM Irradiance Time Series Ly α (mw/m 2 ) nm (mw/m 2 ) nm (mw/m 2 ) Variation at different wavelengths are in phase. Occasionally, the short-term behavior can be quite different among various wavelength ranges (see figure) nm (mw/m 2 ) Mg II Core-to-Wing Ratio JUN 94 JUL AUG SEP OCT NOV DEC JAN 95 FEB MAR APR MAY

15 EUV Irradiance Variation over the Solar Cycle wavelength dependence in the EUV ( nm) 300 Estimates derived from TIMED SEE V9 200 SC variation (%) 100 HeII HeI CIII Ly β CIII wavelength (nm)

16 Solar UV Irradiance Variation over the Solar Cycle wavelength dependence in the FUV ( nm) Variations shown are derived from UARS SOLSTICE; similar results have been obtained from SUSIM and NOAA-11 SBUV2. Relative FUV irradiance variations are larger: for shorter wavelengths, and in emission lines (e.g. Ly-α, Cii, Siiv, Civ, and Siii). Adapted from: Rottman, Floyd, and Viereck (2004).

17 Solar UV Irradiance Variation over the Solar Cycle wavelength dependence in the MUV ( nm) Relative variations are roughly constant up to about 263 nm with larger variations in absorption line cores (e.g. Mg II). Above about 290 nm, the variation is below experimental uncertainties. Adapted from: Rottman, Floyd, and Viereck (2004).

18 Solar UV Irradiance Variation over the Solar Cycle wavelength dependence in the NUV ( nm) Relative variations are: much stronger in absorption lines uncertain, but less than 1% overall estimates obtained through signal detection methods Solar Cycle Variation from Synthetic Solar Model From: Fox (2004).

19 Contribution of UV Irradiance Variation to total solar irradiance (TSI) variation UV energetic variation dominated by longer wavelengths larger relative variations below 200 nm are insignificant variation for nm highly uncertain contribution of UV to TSI variation (0.1%) range from 17% to 60%

20 Solar Mg II Core-to-Wing Ratio Index irradiance ratio of the core of the MgII absorption feature (280 nm) and its nearby wings 400 Solar Mg II Absorption Feature 1.10 nm sensitive to the large solar variation in the core while effectively removing instrumental effects which vary weakly with λ derived from the measurements of many experiments having unique instrumental properties (e.g. resolution) Irradiance (mw/m 2 /nm) nm 0.01 nm (Hall & Anderson) k h various Mg II index series are linearly related (r > 0.98) wavelength (nm)

21 Composite Solar Mg II Core-to-Wing Ratio Index components of 2004 version NOAA SEC Composite Mg II Core-to-Wing Ratio Index Viereck et al. (2004) SC 21 SC 22 SC Nimbus-7 NOAA-9 NOAA-11 Solar UV Experiments SOLSTICE which produce SUSIM or will produce GOME a MgII Index NOAA-16 SCIAMACHY NOAA-17 SORCE OMI GOME-2 NOAA-N?

22 Composite Solar Mg II Core-to-Wing Ratio Index current version (2008) NOAA SWPC MgII Composite, May MgII Index (dimensionless) year

23 Comparison of Mg II Index with UV Irradiances SUSIM Ly-α, nm, and nm mw/m Ly-α r = Data Mg ΙΙ Fit 6 Relative long-term variations of the UV irradiance ( nm) are well described by the Mg II index (within experimental uncertainties). Above 290 nm, Lean et al. (1997) report that sunspots also contribute significantly. Uncertainty as a fraction of the solar variation grows for increasing λ. residuals mw/m 2 mw/m 2 residuals residuals nm r = nm r = Data Mg ΙΙ Fit Data Mg ΙΙ Fit Year % % %

24 Solar Ultraviolet Irradiance Research Theoretical and modeling research Composite Mg II Index (Viereck, Weber, and others) Composite solar UV irradiances (Snow, DeLand, and others) Solar cycle dependence of solar UV irradiance (Floyd, Pagaran, and others) Empirical past and predictive models of solar UV irradiance (Tobiska and others) Semi-empirical models of solar UV irradiance (Solanki, Krivova and others; Morrill and others; Ermolli and others; Unruh and others) Synthetic solar irradiance model (Fontenla, Kurucz and others)

25 Solar Ultraviolet (UV & EUV) Irradiance Interesting Questions for Further Research What are the detailed mechanisms of solar UV irradiance variation? What is the connection between magnetic activity and UV irradiance variations? What is the contribution of UV variation to that of the TSI? How much does the solar UV vary over time periods longer than the solar activity cycle? What was the solar UV irradiance during the Maunder Minimum? How well does the Mg II index describe relative irradiance variations from the EUV to the visible?

26 Suggested and Planned Future Directions for solar EUV/UV irradiance research Continued UV spectral irradiance measurements especially those by instruments with in-flight end-to-end calibrations Improvements in long-term calibration of UV instruments (e.g. SORCE) Imaging in the UV (e.g. Picard) perhaps from different directions with simultaneous UV irradiance measurements Continued and improved measurements of solar activity indices (e.g., F 10.7, Mg II, He 1083 EW, and even SSN)

27 Acknowledgements many thanks to organizers and sponsors I thank the organizers, especially Judit Pap, Dibyendu Nandi, and Dean Pesnell as well and the conference sponsors: NASA LWS, Montana State University, SCOSTEP/CAWSES, UMBC/GEST, and IHY. This work was supported by a grant from the NASA Living with the Star Program (contract NNH05CD10C).

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