1. Solar Radiative Output and its Variability Preethi Ganapathy November 22, 2005 Claus Frölich and Judith Lean

2. Solar Irradiance Variability Historical Investigations Contemporary Investigations Limitations of Contemporary Observations Solar Radiation Properties Sources of Variability Global Effects Summary

3. Introduction What is Solar Irradiance Irradiance is the amount of solar radiation that a radiometer observes at the mean Sun-Earth distance.

4. Historical Investigations Sunspots observed in 1610 by Galileo, Goldsmid, Harriet and Scheiner [1].[1]. 1645-1715: Maunder Minimum, Little Ice Age in Europe. In mid 1800’s, Pouillet and Herschel created the pyrheliometer to measure irradiance [1].[1]. In 1843, Samuel Heinrich Schwabe established the 11 year solar cycle.

5. Historical Investigations In 1881, Samuel Langley used his bolometer to gather data and calculate solar constant [1].[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)

6. 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)

7. 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/m 2. Spectral Irradiance causes this change- wavelength dependent UV, X-ray, and radio wavelengths of spectrum vary most.

8. Contemporary Investigations Frolich and Lean (2004)

9. 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].[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.

10. Properties of Solar Radiation Spectral Distribution Spectral features produced by absorption and emission. Frolich and Lean (2004)

11. 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)

12. Sources of Variability Solar Activity originates in a cycle of magnetic flux caused by a dynamo near bottom of convection zone at 0.7R Sun. Sunspots deplete local solar irradiance Faculae enhance local solar irradiance Comparison of strengths of sunspots and faculae determine irradiance variability

13. 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.

14. 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.

15. 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 http://eobglossary.gsfc.nasa.gov//Library/SORCE/printall.php [3] Bruderheim Rea, Global Warming Explained. http://www.fathersforlife.org/REA/warming7.htm http://www.fathersforlife.org/REA/warming7.htm [4] Secrets of the Ice, An Arctic Expedition. http://www.secretsoftheice.org/icecore/studies.html 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).