Solar Radiative Output and its Variability

Preethi GanapathyNovember 22, 2005

Claus Frölich and Judith Lean

Solar Irradiance Variability

• Historical Investigations• Contemporary Investigations• Limitations of Contemporary Observations• Solar Radiation Properties• Sources of Variability• Global Effects• Summary

Introduction

• What is Solar Irradiance• Irradiance is the amount of solar

radiation that a radiometer observes at the mean Sun-Earth distance.

Historical Investigations

• Sunspots observed in 1610 by Galileo, Goldsmid, Harriet and Scheiner [1].

• 1645-1715: Maunder Minimum, Little Ice Age in Europe.

• In mid 1800’s, Pouillet and Herschel created the pyrheliometer to measure irradiance [1].

• In 1843, Samuel Heinrich Schwabe established the 11 year solar cycle.

Historical Investigations• In 1881, Samuel Langley used his bolometer to gather data and

calculate solar constant [1].

• Until 1980, solar radiative output was considered a constant. Absorption and reflection by Earth’s atmosphere interfered with measurements.

• Solar activity levels were determined by tree rings and ice core drilling [3,4].

Eddy, (1976)

Contemporary Investigations

• In 1980, the NIMBUS 7 satellite carried the Earth Radiation Budget Experiment. Electrically Calibrated Radiometers (ECR) monitored the Sun.

• Other missions include:• Upper Atmosphere Research Satellite (UARS)• Solar and Heliospheric Observatory (SOHO)• ACRIMSAT• Solar Radiation and Climate Experiment (SORCE)

Contemporary Investigations

• ACRIM data shows short term fluctuations in TSI due to 11 year solar cycle.

• During 11 year solar cycle, TSI varies yearly by 0.1%-1.4 W/m2.

• Spectral Irradiance causes this change-wavelength dependent

• UV, X-ray, and radio wavelengths of spectrum vary most.

Contemporary Investigations

Frolich and Lean (2004)

Limitations of Recent Observations

• Instruments today are subject to uncertainty.

• Absorption by the Earth’s atmosphere, oceans and land have not been measured accurately [2].• Atmosphere absorbs 20-25% of radiation.

• Land absorbs 45-50% of radiation.

• Remaining nearly 30% is reflected by clouds.

• Amount of data available spans only 25 years.

• Most existing data measures TSI so there is a lack of information on Spectral Irradiance.

Properties of Solar Radiation

• Spectral Distribution• Spectral features produced by

absorption and emission.Frolich and Lean (2004)

Properties of Solar Radiation

• Spatial Distribution• Inhomogeneous

distribution.• Temperature and

density of atmosphere is altered.

• Changes in specific features relates better to irradiance.

Frolich and Lean (2004)

Sources of Variability

Solar Activity originates in a cycle of magnetic flux caused by a dynamo near bottom of convection zone at 0.7RSun.

• Sunspots deplete local solar irradiance• Faculae enhance local solar irradiance• Comparison of strengths of sunspots

and faculae determine irradiance variability

Global Effects

• Earth’s equilibrium temperature is maintained by incoming and outgoing radiation.

• Solar activity may be responsible for half of 0.6 degrees of global warming in the past 110 years (IPCC).

• Cloud Coverage may change.• Hole in ozone layer can be affected by

oscillating UV levels.

Summary

• Observational record of TSI through space based observations show that the Sun is 0.1% brighter during maxima than during the minima of the 11 year cycle.

• Long term trends are still under discussion.• 80% of observed variance of TSI comes from

combination of sunspot darkening and faculae brightening.

• Current space based instruments still have some limitations.

• Understanding Solar Irradiance may help in understanding the greenhouse effect and other terrestrial climate processes.

References

[1] High Altitude Observatory, History of Solar Physics. http://www.hao.ucar.edu/Public/education/Timeline.D.html#1800

[2] Lamont-Doherty Earth Observatory, Solar Radiation and Climate Experiment. http://eobglossary.gsfc.nasa.gov//Library/SORCE/printall.php[3] Bruderheim Rea, Global Warming Explained. http://www.fathersforlife.org/REA/warming7.htm[4] Secrets of the Ice, An Arctic Expedition. http://www.secretsoftheice.org/icecore/studies.html[5] Frölich, Claus and Judith Lean. Solar Radiative Output and its Variability. A&A Rev., 12, 273 (2004).