Solar Rotation

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Solar Rotation

• Lab 3

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Differential Rotation

• The sun lacks a fixed rotation rate
• Since it is composed of a gaseous plasma, the
  rate of rotation is fastest at the equator (24
  days), and decreases as latitude increases (30
  days at polar regions)
• 27 days

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Sidereal Rotation

• At the equator the solar rotation period is 25.38
  days and is called the sidereal rotation period
• Sidereal rotation period time it takes to
  rotate 360 degrees on its axis
• Not to be confused with synodic rotation

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Synodic Rotation

• Sun has a synodic rotation period of 27.2753
  days, which is the time for a fixed feature on
  the sun to rotate to the same apparent position
  as viewed from earth
• The synodic period is longer because the sun must
  rotate for a sidereal period plus an extra amount
  due to the orbital motion of the earth around the
  sun

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Use of Tracers to Measure Rotation

• Sunspots are used as tracers to measure rotation
  rates of Sun
• A tracer is a fixed feature on the Sun
• Sunspots had been observed since ancient times,
  but after Lipperhey/Galileo invented the
  telescope it was seen that they turn with the
  Sun, and thus could define the period of the
  solar rotation

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Sunspots

• magnetic elements
• Every 11 years the number of sunspots seen on
  the Sun increases from 0 to 100, then decreases
  to 0 again as the next cycle starts
• The polarity is reversed also
• So a complete sunspot cycle is 22 years
• Seen in a band, 5-40 latitude above and below
  the solar equator

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What are Sunspots?

• Sunspots appear as dark spots on the surface of
  the Sun
• Sunspots are magnetic regions on the Sun with
  magnetic field strengths thousands of times
  stronger than the Earth's magnetic field
• Temperatures in the dark centers of sunspots drop
  to about 3700 K (compared to 5700 K for the
  surrounding photosphere)
• They typically last for several days, although
  very large ones may live for several weeks
• Sunspots usually come in groups with two sets of
  spots. One set will have positive or north
  magnetic field while the other set will have
  negative or south magnetic field
• The field is strongest in the darker parts of the
  sunspots (umbra), and weaker and more horizontal
  in the lighter part (penumbra)

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Sunspots
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Solar Magnetic Field

• The Suns magnetic field is produced by the flow
  of electrically charged ions and electrons
• Sunspots are places where very intense magnetic
  lines of force break through the Sun's surface
• The sunspot cycle results from the recycling of
  magnetic fields by the flow of material in the
  interior
• The prominences seen floating above the surface
  of the Sun are supported, and threaded through,
  with magnetic fields
• The streamers and loops seen in the corona are
  shaped by magnetic fields
• Magnetic fields are at the root of virtually all
  of the features seen on and above the Sun

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• X-ray image of the sun (corona)

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Corona

• The Sun's outer atmosphere (corona) is hotter
  than 1,000,000ºC
• Usually temperatures fall as distance increases
  from a heat source
• This is true in the Sun's interior right up to
  the visible surface
• BUT THEN, over a relatively small distance, the
  temperature suddenly rises to extremely high
  values
• Areas on the Sun near sunspots often flare up,
  heating material to millions of degrees in just
  seconds (solar flares) and blasting billions of
  tons of material into space (coronal mass
  ejections)

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Other features of the Sun - Faculae

• Faculae are bright areas that are usually most
  easily seen near the limb, or edge, of the solar
  disk
• These are also magnetic areas but the magnetic
  field is concentrated in much smaller bundles
  than in sunspots
• While the sunspots tend to make the Sun look
  darker, the faculae make it look brighter

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Faculae
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Other features of the Sun - Granules

• Granules are small (1000 km across) cellular
  features that cover the entire Sun (except for
  those areas covered by sunspots)
• They are the tops of convection cells, where hot
  fluid rises up from the interior in the bright
  areas, spreads out across the surface, cools, and
  then sinks inward along the dark lanes
• Individual granules last for only 20 minutes
• The granulation pattern continually evolves, old
  granules get pushed aside by new emerging ones
• The flow within the granules can reach supersonic
  speeds of more than 15,000 mph and produce sonic
  "booms" on the Sun's surface

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Granules

• Movie - http//science.nasa.gov/ssl/pad/solar/feat
  ure1.htm

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Other features of the Sun - Supergranules

• Supergranules are much larger versions of
  granules (35,000 km across)
• These features also cover the entire Sun and
  continually evolve
• Individual supergranules last for 1-2 days and
  have flow speeds of 1000 mph
• The fluid flows observed in supergranules carry
  magnetic field bundles to the edges of the cells
  where they produce the chromospheric network

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Supergranules
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Other Features of the Sun - Chromospheric Network
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Other Features of the Sun - Filaments and Plages
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Other Features of the Sun - Prominences
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Prominence eruption

• http//science.nasa.gov/ssl/pad/solar/images/grand
  daddy.mpg

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Other Features of the Sun - Spicules
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Magnetosphere
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Solar Wind

• The Sun is the source of the solar wind a flow
  of gases from the Sun that streams past the Earth
  at speeds of more than a million miles per hour
• Disturbances in the solar wind shake the Earth's
  magnetic field and pump energy into the radiation
  belts
• Solar flares and give off ultraviolet light and
  x-rays that heat up the Earth's upper atmosphere