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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 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
Total solar irradiance observations UARS SOHO space era solar activity is historicallyhigh 20 Modern Maximum cycle 0 10 Maunder Minimum Sunspot Number
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/
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
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
Can magnetic fields explain irradiance variability directy? TSI correlates poorly with global magnetic field
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
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
Sunspots cannot account for the solar irradiance cycle varibility sunspots cause net irradiance decrease of 1 Wm2 during the solar cycle
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
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 ± 0.5ICH
Faculae Magnetic sources of irradiance brightening 1. Empirical Relation with Chromospheric Index: FF= a + bICH 2. Bolometric Facular Brightening: PF= FF/FQ = 5AFAC[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
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 + bPs(t) + cICHst(t) + dICHlt(t) 2. F(t) = FQ(1+ Ps(t)) + [a + bICH (t)] 3. F(t) = FQ (1 + Ps(t) + PF (t)) Fröhlich & Lean, GRL, 1998 Foukal & Lean, ApJ, 1988 Lean et al., ApJ, 1998
Models of total irradiance variability based on PSI and MgII
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.
Model accounts for observed total irradiance rotation and cycle
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)
Temperature record of northern hemisphere Maunder minimum
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
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)
Solar-stellar connection and reconstruction of solar irradiance
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
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.