The Sun

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The Sun A Typical Star

• The layers of the sun core, radiative zone,
  convective zone, photosphere, chromosphere, and
  corona
• Sunspots and magnetic fields, the sunspot cycle
• Solar activity and how it influences the Earth

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Gravity vs. pressure
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A Star A Balance between Gravity and Pressure

• Self-regulating
• Higher fusion rate would expand star, lowering
  cores self-gravity and thence density,
  pressure, temperature and thus lowering fusion
  rate. And vice versa

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Sun layers
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Layers of the sun
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• Core where temperature exceeds fusion point (10
  million Kelvin)
• Radiative Zone nothing much goes on here. It
  just acts as an obstacle course for the photons
  created in the core and random-walking their way
  upward
• Convection Zone temperature gradient is so
  steep that photon diffusion cant carry the heat
  outward fast enough. The rising temperature
  expands the gas, lowering density and causing it
  to rise (helium-balloon-like) to the surface,
  where it cools, gets denser, and falls back down
  to get reheated and start all over again. Think
  soup cooking on a stove.
• Photosphere visible surface. This is where the
  mean free path now gets so long the material is
  transparent above here.

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Sunspots optical
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Sunspots

• Places where the suns magnetic field is
  concentrated and inhibits the normal convective
  flow of hot material from below. So the material
  sits on the surface and cools off as it radiates
  to the sky.
• Charged particles in a magnetic field feel a
  force sideways to their motion, binding the gas
  to the field.
• Sunspots are like magnetic scabs of gas unable
  to be recirculated to lower, hotter levels. They
  are bound to the magnetic fields in the
  photosphere, cooling as they radiate to the cold
  universe, and hence cool off and darken.
• Vertical structure of a sunspot

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HiRes sunspot
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Sunspot in UV light. Note magnetic field lines
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Earth w/ sunspot
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Marching sunspots
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Sunspot cycle 1760-1965
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Sunspot cycle vs time
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Solar Maxima have been decreasing for decades
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Solar Physics has advanced to the point of being
able to make rough predictions of solar activity
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Sun pic at max,min
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How Does The Solar Cycle Affect Earth?

• Two important ways
• 1. The solar wind creates aurorae (more later)
• 2. Solar luminosity changes during the cycle. We
  have seen that lower solar activity goes with
  lower average temperatures on Earth

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How Does Solar Activity Change Solar Luminosity?

• Higher solar activity produces higher solar
  luminosity.
• Mechanism more surface magnetic field energy,
  which thermalizes (i.e. becomes random kinetic
  energy, as the 2nd Law of Thermodynamics
  requires) and produces more net solar radiation.
• Some of the magnetic field energy directly
  impacts Earth by high speed solar wind particles
  hitting Earth.
• However, the luminosity changes are tiny less
  than 0.1, as measured by satellites above the
  atmosphere.

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Maunder Minimum The Little Ice Age Connected?

• The Maunder Minimum in sunspot numbers
  corresponds to a cool period in climate on Earth.
• The cooler climate is very likely caused in part
  by the lower solar activity, but only in part, as
  the direct solar luminosity changes are too
  small.
• The Little Ice Age also corresponds to a period
  of unusually intense volcanic activity, and also
  to the time when human population dropped by 20
  due to the Plague and mass loss of life in the
  New World due to transported disease leading to
  reforestation which lowered CO2 levels.
• However, the exact apportionment to these
  differing causes is not well constrained by the
  data
• Regardless, solar activity changes cannot be
  responsible for the climate change of the past 60
  years, for multiple reasons well discuss later

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The most intensely spotted star yet discovered
and discovered right here at Cabrillo Observatory!
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chromosphere
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Chromosphere all around
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Diamond ring
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Total eclipse corona
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Eclipse composite
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Total eclipse corona
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Powerful solar flare
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cme
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solarwind
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Sun Halpha big prominence
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Sun halpha big prom later
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EarthOnionMagField
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Earth magnetosphere
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SOHO wide angle
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Aurorae GiNormous Flourescent Lights!

• Caused when high speed solar wind particles
  impact the Earths atmosphere
• Collisionally excites the nitrogen and oxygen
  atoms
• These atoms then de-excite (electrons fall back
  down through the energy levels) giving off
  photons
• Exactly the same as how flourescent lights work!

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greenpurpleAurora
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Aurora westlake
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Aurora lindersen
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Aurora hoffman
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Aurora c/IkeyaZhang
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Aurora ewoldt
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Aurora tricolor
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Aurora from space iss
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Comet NEAT kinky tail
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Long Term Solar Evolution

• As the sun ages, its core collapses as hydrogen
  converts to helium, and this increases the
  gravity and pressure and fusion rate in the core
• So, the sun is getting more luminous on the long
  term
• During the 4.56 billion year life of the solar
  system, the sun has increased in luminosity by
  about 25.
• This will continue, and gradually accelerate

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Long term L,R,T