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Investigating the variability of solar UV Radiation

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07/11/06 Solar UV radiation - ET -701-1237-00 1

Investigating the variability of solar UV Radiation

•Sun-Earth Distance

•Extra-terrestrial radiation (Top of the atmosphere)

•Variability due to the atmosphere:

– Atmospheric composition

• Ozone

• Sulphure dioxide

• Aerosols

• Clouds

• Molecules

•Solar Zenith Angle

•Ground reflection

•Ground topography

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Solar UV irradiance Variability The sun-earth distance

The Earth rotates around the sun in an ellipse, with the Perihelion (shortest distance) during

the winter solstice and the Aphelion (farthest distance) at the Summer Solstice.

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07/11/06 Solar UV radiation - ET -701-1237-00 3

Solar UV irradiance Variability The sun-earth distance

The solar radiation reaching the Earth varies according to the change in distance between the Earth and the sun, (following the square law decrease of radiation from a point source).

The variation between winter and summer is about 6.8%, and can be determined by the following formula where f represents the solar irradiance variation at the Earth relative to the standard distance of 1 astronomical Unit (AU)

g=2*pi*(dayofyear-1)/365;

f=1.000110+0.034221*cos(g)+0.001280*

sin(g)+0.000719*cos(2*g)+

0.000077*sin(2*g);

winter solstice

summer solstice

winter

solstice

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Solar Radiation at the top of the atmosphere

Most of the energy reaching the Earth’s atmosphere originates from the photosphere and is considered the

“surface” of the sun.

Its effective temperature is about 6000 K.

The emission of the photosphere is a continuum,

superimposed with absorption and emission lines called

Fraunhofer lines.

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07/11/06 Solar UV radiation - ET -701-1237-00 5

Fraunhofer lines in the solar spectrum

Ca Ca & Fe H Fe Na H O

2

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07/11/06 Solar UV radiation - ET -701-1237-00 7

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Solar variability and Climate

0 50 100 150 200

1600 1650 1700 1750 1800 1850 1900 1950 2000

N U M B E R

YEAR

Maunder Dalton

su n sp ot s

Year

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07/11/06 Solar UV radiation - ET -701-1237-00 9

Total Solar Irradiance (TSI)

Cycle amplitude 1 W/m

2

or

~0.1%

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Solar UV variability

The Sun attained its sunspot maximum of its 11-years solar cycle in the year 2000.

These images are captured using Fe XII 195 Å emissions showing the solar corona at a temperature of about 1 million K. Many more sunspots, solar flares, and coronal mass ejections occur during the solar maximum. The progression towards more

active regions and the number/size of magnetic loops is unmistakable.

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07/11/06 Solar UV radiation - ET -701-1237-00 11

Influence of sun spots and faculae

0.2%

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The Solar Ultraviolet Spectral Irradiance Monitor – SUSIM

The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) is a dual dispersion spectrometer

instrument which measured from near-Earth orbit the absolute irradiance of the sun in the

ultraviolet (UV) wavelength range of 115 nm to 410 nm.

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07/11/06 Solar UV radiation - ET -701-1237-00 13

The Solar Ultraviolet Spectral Irradiance

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UV Spectral Irradiance Time series from SUSIM

1992 2003

12%

8%

3%

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07/11/06 Solar UV radiation - ET -701-1237-00 15

1992 2003

Solar cycle variation

Floyd et al., 2004

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Solar UV variability

http://ssbuv.gsfc.nasa.gov/solar.html

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07/11/06 Solar UV radiation - ET -701-1237-00 17

Solar UV irradiance Variability

Summary:

• The solar UV irradiance below 280 nm varies with the 11 year solar cycle.

• The variability increases with decreasing wavelength:

– at 140 nm, the variability between minimum and maximum of the solar cycle is about 50%

– at 200 nm, the variability is about 8%

– above 290 nm the variability of the solar UV irradiance is less than 2% and cannot be resolved with current instruments.

Therefore:

• As we will see next, the Solar UV variability is large at wavelengths below about 250 nm which are responsible for the ozone production in the Earth atmosphere. Studies indicate possible Climate variability due to variations in the solar UV irradiance

• At the surface, where only wavelengths longer than 290 nm can penetrate, the solar UV

variability from the sun itself is negligible.

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