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Solar Irradiance Variability and Climate

Solar Irradiance Variability and Climate. Claus Fröhlich 1 and Judith Lean 2 1 ) PMOD/WRC, Davos, Switzerland 2 ) Naval Research Laboratory, Washington DC. Observations total irradiance since 1978 Empirical Models sources and proxies of variability

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Solar Irradiance Variability and Climate

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  1. Solar Irradiance Variability and Climate Claus Fröhlich1 and Judith Lean2 1) PMOD/WRC, Davos, Switzerland 2) Naval Research Laboratory, Washington DC Observations total irradiance since 1978 Empirical Models sources and proxies of variability modeled variations: present, past, future Solar-Terrestrial Influence Past Climate: Maunder Minimum How much influence comes from the Sun

  2. Total solar irradiance observations UARS SOHO space era solar activity is historicallyhigh 20 Modern Maximum cycle 0 10 Maunder Minimum Sunspot Number

  3. Total solar irradiance database Version: 24.00 composite_d24_00.asc The dispersion of the original data is more than 7 times the solar cycle amplitude. The trend of the composite (difference between minima) is +7 ppm. Data and plots at: http://www.pmodwrc.ch/data/irradiance/composite/

  4. Total solar irradiance database: Differences from composite • drifts in radiometer stability can reach fractions of the solar cycle amplitude • largest drifts tend to occur at start of mission • the most controversial changes in HF are the two glitches in late 1989

  5. Total solar irradiance variability • 0.2-0.3% 27-day solar rotation • 0.1% (1000 ppm) 11-year solar cycle • longer-term variations not yet reliably detected composite total solar irradiance record: Fröhlich & Lean, GRL, 1998 solar irradiance increases when solar activity is high

  6. Can magnetic fields explain irradiance variability directy? TSI correlates poorly with global magnetic field

  7. Sunspots: Magnetic sources of irradiance dimming Bolometric Sunspot Blocking: PS= FS/FQ = ASPOT[CS-1](3+2)/2 MDI 29 Mar 2001 FS .. irradiance change from spot FQ .. quite Sun irradiance  .. wavelength ASPOT .. fractional disk area of spot  .. heliocentric location CS .. contrast (area-dependent) of spot (3+2)/2 .. center-to-limb function Hudson et al., 1982; Fröhlich et al.,1994; Brandt et al., 1994; Chapman et al., 1996

  8. Rotation of sunspots causes large dips in total solar irradiance ROME PSPT IMAGES • sunspots do not account for all variability during solar rotation: • PS uncertainties • other variability sources

  9. Sunspots cannot account for the solar irradiance cycle varibility sunspots cause net irradiance decrease of  1 Wm2 during the solar cycle

  10. Composite chromospheric irradiance index BBSO Ca K MgII index: ratio of core-to-wing emission in Fraunhofer line near 280 nm wing wing core 1996-06-16 1998-06-04 2000-02-25 Lean et al., JGR, 106, 10645, 2001

  11. Total solar irradiance brightness residuals track chromospheric index Residual = F –FQ-FQxPs • highly correlated r=0.95 • similar power distribution Resid = - 13.53 ± 0.06 + 106.2 ± 0.5ICH

  12. Faculae Magnetic sources of irradiance brightening 1. Empirical Relation with Chromospheric Index: FF= a + bICH 2. Bolometric Facular Brightening: PF= FF/FQ = 5AFAC[CF-1]R(, )/2 FF .. irradiance change from faculae FQ .. quite Sun irradiance  .. wavelength AFAC .. fractional disk area  .. heliocentric location CF .. facular contrast R .. center-to-limb function PSPT 29 Mar 2001

  13. Total solar irradiance variability model formulation Sunspot Blocking Quiet Sun Irradiance Facular Brightening Irradiance = + + F(t) = FQ + FS(t)+ FF (t) Approaches: 1. F(t) = a + bPs(t) + cICHst(t) + dICHlt(t) 2. F(t) = FQ(1+ Ps(t)) + [a + bICH (t)] 3. F(t) = FQ (1 + Ps(t) + PF (t)) Fröhlich & Lean, GRL, 1998 Foukal & Lean, ApJ, 1988 Lean et al., ApJ, 1998

  14. Models of total irradiance variability based on PSI and MgII

  15. Empirical models of total irradiance variability account for >85% of variance Trend corresponds to -3.3 ppm/a. Compared to the 2suncertainty of the composite of ±3 ppm/a this is barely significant.

  16. Model accounts for observed total irradiance rotation and cycle

  17. Sources of irradiance variability are wavelength dependent Solar Active Region: BBSO Image (Y. Unruh) faculae sunspots • Band Contribution to TSI • UV ~ 8% • VIS~44% •  IR ~48% •  EUV <0.0004% (Y. Unruh)

  18. Solar irradiance and the Earth’ climate

  19. Temperature record of northern hemisphere Maunder minimum

  20. Long-term solar activity Solar activityproxies -- cosmogenic isotopes in tree-rings and ice-cores (below), geomagnetic activity, and the range of variability in Sun-like stars (right) -- suggest that long-term fluctuations in solar activity exceed the range of contemporary cycles. Number Solar Activity Proxies Ca Brightness of Sun-like Stars DATA SOURCES: Baliunas & Jastrow, 1990 Stuiver & Braziunas, 1993 Beer et al., 1988

  21. Solar twins and sun-like stars in cluster M67 The solar-type stars in the open cluster M67 (constellation Cancer) have solar-age and solar-metallicity: 76 ‘solar-type’ stars (with unreddened colors in the range +0.60 <= B-V <= +0.76) and 21 ‘solar-twins’ (+0.63 <= B-V <= +0.67) have been observed (Giampapa et al. 2000)

  22. Solar-stellar connection and reconstruction of solar irradiance

  23. Climate models forced by TSI variability

  24. Future total solar irradiance and climate forcing • 11-year cycles based on • Schatten et al., 1996 Hathaway et al., 1999 Thompson, 1993 • background is ±0.04Wm-2/year Lean, GRL, 2001 • Anthropogenic Scenarios • IS92a • IPCC, 1995 • Alternative • Hansen et al, 2000 Sun’s role in future climate change depends on irradiance cycles and trends relative to anthropogenic scenarios

  25. Summary: TSI variability, solar-stellar connection and Earth’ climate Long-term trend during last 23 years: approx. 0.7 ± 3 ppm/a. Variations are related to magnetic features: sunspot darkening and faculae brightening empiricalmodels account for a large part (>90%) of the observed variations. Long-term changes of TSI influence climate: extrapolation to past still quite uncertain; the sun has probably not influenced our climate during the past 20-30 years. Before, at most ½ of the climate change could be due to the sun. changes of spectral distribution may be more important for sun-climate connection than just (energetic) changes of TSI.

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