Solar Radiative Output and its Variability

1
Solar Radiative Output and its Variability
Claus Frölich and Judith Lean

• Preethi Ganapathy
• November 22, 2005

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.
• 1645-1715 Maunder Minimum, Little Ice Age in
  Europe.
• In mid 1800s, Pouillet and Herschel created the
  pyrheliometer to measure irradiance 1.
• In 1843, Samuel Heinrich Schwabe established the
  11 year solar cycle.

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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 Earths
  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)

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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.

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Contemporary Investigations
Frolich and Lean (2004)
9
Limitations of Recent Observations

• Instruments today are subject to uncertainty.
• Absorption by the Earths 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.

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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 0.7RSun.
• Sunspots deplete local solar irradiance
• Faculae enhance local solar irradiance
• Comparison of strengths of sunspots and faculae
  determine irradiance variability

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Global Effects

• Earths 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.

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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.

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References
1 High Altitude Observatory, History of Solar
Physics. http//www.hao.ucar.edu/Public/educat
ion/Timeline.D.html1800 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.fathersfor
life.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. AA Rev., 12, 273 (2004).