The Constant Sun

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The Constant Sun

• Some important definitions and terminology
• Solar terms
• Solar wind
• heliosphere
• Suns structure
• The solar constant
• The temperature of the planets
• Importance of the atmosphere

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Before proceeding we need to define some basic
Solar terminology

• Suns interior everything below the photosphere
• Photosphere lowest layer of the Suns atmosphere
  (the visible surface of the Sun)
• Chromosphere the layer of the atmosphere just
  above the photosphere
• Corona the extremely hot outer layer of the
  Suns atmosphere
• Solar Wind particles emanating from the outer
  layers of the solar atmosphere that stream into
  space (even the Suns gravity cannot hold them
  in)
• The Heliosphere the region of space carved out
  of the interstellar medium by the solar wind

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The Structure of the Sun
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The Solar Wind

• The corona continues off into space and creates
  the Solar Wind
• Suggested to exist based on comet-tail
  observations

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Planetary Magnetospheres

• When the Solar Wind collides with planets that
  have a magnetic field, a magnetosphere is created
• Earth, Mercury, and the gas-giant planets all
  have magnetospheres
• Why do some planets have magnetic fields and
  others dont?

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The Sun is said to be like a blackbody what
does this mean?
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The Solar Constant

• Solar Luminosity
• Total energy emitted by the Sun per second
• L 3.9 x 1026 W
• Solar Flux
• Luminosity divided by Suns surface area (the
  amount radiated per area)
• Solar Constant
• The amount of Solar Flux arriving at Earth
• 1370 W/m2

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The solar constant
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The Suns energy reaching that is not reflected
or absorbed by the surface or atmosphere, is
absorbed by the Earths surface, heating it.The
heated Earth, radiates energy alsoIf you
balance heat input by the Sun and heat radiated
by a planet, you can determine the mean
temperature of a planet

• Requires knowing the albedo which is the
  fraction of incident energy that is reflected
  into space
• Difficult to measure the Earths albedo
• Can use measurements of Earthshine

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Table of the planets mean temperatures and their
respective predictions
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Earths Atmosphere and Global Temperature

• Earths temperature is substantially higher than
  predicted by the simple energy-balance argument
• The Earth should be below freezing
• Why is this?

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Atmospheric Transparency

• Much of the Suns visible light comes through the
  atmosphere
• Earths radiated infrared radiation cannot escape
  because elements in Earths atmosphere absorb it
• The atmosphere acts like a thermal blanket

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• We will find out in this course that the solar
  constant is not really constant.
• It changes in a number of ways
• Solar effects
• Variations in Earths distance to the Sun

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Terraforming Mars

• Sagan proposed the idea of blackening the polar
  ice caps. This would cause them to melt and
  release water vapor into the atmosphere
• Plants which grow on ice have been proposed as
  the blackening agent
• Would take 100-10,000 years
• Not on NASAs roadmap. Private enterprise?

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Reading For Next Week

• We will continue the discussion of the constant
  Sun
• The Sun as a clock
• Milankovich cycles
• Reading assignment link(s) will be posted this
  afternoon
• Each person should be prepared to come to class
  with a particular discussion-provoking question
  related to the reading.