Solar Irradiance Variability and Solar Irradiance Variability and ClimateClimate
Solar Irradiance Variability and Solar Irradiance Variability and ClimateClimate
ObservationsObservations total irradiance since 1978
Empirical ModelsEmpirical Models sources and proxies of variability modeled variations: present, past, future
Solar-Terrestrial InfluenceSolar-Terrestrial Influence Past Climate: Maunder Minimum How much influence comes from the Sun
Claus Fröhlich1 and Judith Lean21) PMOD/WRC, Davos, Switzerland
2) Naval Research Laboratory, Washington DC
Total solar irradiance observationsTotal solar irradiance observations
space era solar activity is historically high
Maunder Maunder MinimumMinimum
ModernModernMaximumMaximum
Suns
pot N
umbe
r
UARSUARS
20cycle 0 10
SOHOSOHO
Total solar irradiance databaseTotal solar irradiance database
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/
Version: 24.00composite_d24_00.asc
Total solar irradiance database:Total solar irradiance database:Differences from compositeDifferences 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 variabilityTotal solar irradiance variability
composite total solar irradiance record:
Fröhlich & Lean, GRL,
1998
0.2-0.3% 27-day solar rotation
0.1% (1000 ppm) 11-year solar cycle longer-term variations not yet
reliably detected
solar irradiance increases when solar activity is high
Can magnetic fields explain irradiance Can magnetic fields explain irradiance variability directy?variability directy?
TSI correlates poorly with global
magnetic field
Sunspots:Sunspots:Magnetic sources of Magnetic sources of irradiance dimmingirradiance dimming
Bolometric Sunspot Blocking: Bolometric Sunspot Blocking:
PS= FS/FQ
= ASPOT[CS-1](3+2)/2
MDI 29 Mar 2001
FS .. irradiance change from spotFQ .. quite Sun irradiance .. wavelengthASPOT .. fractional disk area of spot .. heliocentric locationCS .. 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 Rotation of sunspots causes large dips in total solar irradiancedips in total solar irradiance
sunspots do not account for all variability during solar rotation:
• PS uncertainties• other variability sources
RO
ME
PS
PT
IM
AG
ES
RO
ME
PS
PT
IM
AG
ES
Sunspots cannot account for the solar Sunspots cannot account for the solar irradiance cycle varibilityirradiance cycle varibility
sunspots cause net irradiance decrease of 1 Wm2 during the solar cycle
1996
-06-
1619
96-0
6-16
1998
-06-
0419
98-0
6-04
2000
-02-
2520
00-0
2-25
Composite chromospheric Composite chromospheric irradiance indexirradiance index
BBSO Ca KBBSO Ca K
Lean et al., JGR, 106, 10645, 2001
MgII index:ratio of core-to-
wing emission in Fraunhofer line
near 280 nm
core
wing wing
Total solar irradiance brightness Total solar irradiance brightness residuals track chromospheric indexresiduals track chromospheric index
Residual = F –FQ-FQxPs
• highly correlated r=0.95
Resid = - 13.53 ± 0.06 + 106.2 ± 0.5ICH
• similar power distribution
1. Empirical Relation with 1. Empirical Relation with Chromospheric Index:Chromospheric Index:
FF= a + bICH
2. Bolometric Facular 2. Bolometric Facular Brightening:Brightening:
PF= FF/FQ
= 5AFAC[CF-1]R(, )/2FF .. irradiance change from faculaeFQ .. quite Sun irradiance .. wavelengthAFAC .. fractional disk area .. heliocentric locationCF .. facular contrastR .. center-to-limb function
P
SP
T
29 M
ar
200
1
FaculaeFaculaeMagnetic sources of irradiance brighteningMagnetic sources of irradiance brightening
Total solar irradiance variability Total solar irradiance variability model formulation model formulation
F(t) = FF(t) = FQ Q + + FFSS(t)(t) + + FFF F (t) (t)
Approaches:
Quiet Sun Irradiance
Irradiance = Sunspot Blocking
Facular Brightening
+ +
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 Models of total irradiance variability based on PSI and MgIIbased on PSI and MgII
Empirical models of total irradiance Empirical models of total irradiance variability account for >85% of variancevariability account for >85% of variance
Trend corresponds to -3.3 ppm/a. Compared to the 2 uncertainty of the composite of 3 ppm/a this is barely significant.
Model accounts for observed total Model accounts for observed total irradiance rotation and cycleirradiance rotation and cycle
Sources of irradiance variability are Sources of irradiance variability are wavelength dependentwavelength dependent
sunspotsfaculae
Solar Active Region: BBSO Image
(Y. Unruh)
(Y. Unruh)
Band Contribution to TSIUV ~ 8%VIS~44% IR ~48%
EUV <0.0004%
Solar irradiance and the Earth’ climateSolar irradiance and the Earth’ climate
Temperature record of northern Temperature record of northern hemispherehemisphere
Maunderminimum
Solar activity proxies -- 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.
Long-term solar activityLong-term solar activity
DA
TA
SO
UR
CE
S:
Bal
iuna
s &
Jas
trow
, 19
90
Stu
iver
& B
razi
una
s, 1
993
Bee
r e
t al
., 1
988
Ca Brightness of Sun-like StarsCa Brightness of Sun-like Stars
Num
ber
Num
ber
Solar Activity Solar Activity ProxiesProxies
Solar twins and sun-like stars in Solar twins and sun-like stars in cluster M67cluster 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 Solar-stellar connection and reconstruction of solar irradiancereconstruction of solar irradiance
Climate models forced by TSI Climate models forced by TSI variability variability
Future total solar irradiance and Future total solar irradiance and climate forcingclimate forcing
Sun’s role in future climate change depends on irradiance cycles and trends relative to anthropogenic scenarios
Anthropogenic Scenarios
• IS92aIPCC, 1995
• AlternativeHansen et al, 2000
•11-year cycles based onSchatten et al., 1996 Hathaway et al., 1999 Thompson, 1993
• background is ±0.04Wm-2/year
Lean, GRL, 2001
Long-term trend during last 23 years:Long-term trend during last 23 years:•approx. 0.7 3 ppm/a.Variations are related to magnetic Variations are related to magnetic features: features: •sunspot darkening and faculae brightening•empirical models account for a large part (>90%) of the observed variations.Long-term changes of TSI influence climate: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.
Summary: TSI variability, solar-Summary: TSI variability, solar-stellar connection and Earth’ climatestellar connection and Earth’ climate