The Sun

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The Sun

• Solar Activity

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Prominences

• Dense, relatively cool clouds of gas that look
  dark when viewed with the photosphere as
  background.
• Look either like loops or irregular clouds when
  seen on the limb.
• Can extend tens of thousands or (in rare cases)
  even 200,000 km into the corona.
• Can last as long as 2 or 3 months.
• Most of them erupt and eject material into the
  solar wind.

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Large Eruptive Prominence (HeII Filtergram at
30.4 nm)
24 July 1999
Image from http//sohowww.nascom.nasa.gov/
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Coronal Mass Ejection
http//science.msfc.nasa.gov/ssl/pad/solar/cmes.ht
m

• Sometimes, clouds containing billions of tons of
  matter are ejected from the corona at speeds up
  to 1000 km/s.
• This phenomenon is called a coronal mass
  ejection.
• Coronal mass ejections are usually, but not
  always, associated with prominences or flares
  (see next slide).
• When the sun is least active (solar minimum),
  there may be only one cme per week. When it is
  most active, there may be 2 or 3 per day.

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Solar Flares

• Magnetic field in an active region suddenly
  changes its structure.
• Large numbers of rapidly-moving ions and
  electrons are released.
• Much more energetic eruption than a prominence.
• These energetic particles collide with coronal
  gas and raise its temperature up to 40 million K.
• Contributes up to 1 to the Suns brightness.
• Most of this is X and UV radiation that reaches
  Earth in about 8 minutes.
• Electrons and ions reach Earth in a few hours or
  days.

Click on the picture to play the video.
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Effect of a Solar Flare on Earth

• Ionization of Earths upper atmosphere is
  increased.
• Enhancement of ionosphere disrupts long-range
  radio communications.
• Power surges may occur, causing damage to
  transformers and resulting in blackouts.
• Eventually, ejected coronal matter also reaches
  Earth.
• Unless they are shielded for protection,
  astronauts are endangered by the increased
  radiation.
• The Aurora Borealis and Aurora Australis are
  brighter than usual and are seen farther from the
  poles.

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Sunspots

• Dark spots seen in the photosphere
• 2500 km to 50,000 km in diameter
• Large ones can last for months.
• Occur in groups.
• Associated with magnetic fields
• Spots within a group are paired, onebeing a
  north magnetic pole and theother a south
  magnetic pole.
• Consist of a dark inner part (umbra) and
  asurrounding lighter part (penumbra).
• Are found in active regions along with flares
  and prominences.
• Magnetic fields prevent hot gas from rising
  freely to the surface.
• Tphotosphere 5800 K, Tspot 3800 K.
• Since the spots are cooler, they emit less light
  than the surrounding photosphere.
• Though dark in contrast to the surrounding
  photosphere, they are actually several times
  brighter than the full moon.

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Sunspot Penumbral Filaments (Swedish 1-meter
Solar Telescope at the Roque de los Muchachos
Observatory on La Palma, Canary Islands)
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Zeeman Effect
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The Sunspot Number Cycle

• The number of sunspots reaches a maximum
  approximately every 11 years.
• This cycle is not yet understood well enough to
  make accurate predictions.

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Solar Magnetogram

• Dark areas are north poles and bright areas are
  south poles.
• Magnetic field strength up to 10,000 times as
  large as at Earths surface.

The sunspot polarities switch at the beginning of
a new cycle. In the next cycle, the leading
sunspot in the northern hemisphere will be a
north pole.
http//www.nso.noao.edu/
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The Maunder Minimum

• E. Walter Maunder noticed that there was a period
  from 1645 to 1715 when very few sunspots were
  observed.
• Recent research shows that this corresponded to a
  period of low solar activity.
• It may be the cause of the little ice age, a
  period from 1500 to 1850 when the weather was
  unusually cold in Europe and North America.

Dalton Minimum
Maunder Minimum
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The Maunder Butterfly Diagram

• This is a plot of sunspot latitude as a function
  of date. For each date, a dot is placed on the
  diagram at the latitude of each sunspot observed.
• The first sunspots to appear at the beginning of
  a cycle are about 35º north and south of the
  suns equator. Subsequent spots are formed closer
  to the equator.

http//wwwssl.msfc.nasa.gov/ssl/pad/solar/sunspots
.htm