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