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					Chapter 7A


         The Sun
our Sun
7A.1 The solar atmosphere and
solar interior
 Atmosphere: roughly three
  layers.
 Photosphere (光球層): the
  bottom layer, about 6,000 K.
 Chromosphere (色球層):
  middle layer, may be as high
  as 1,000,000 K.
 Corona (日冕): the outermost
  layer.
7A.1 The solar atmosphere and
solar interior
7A.1 The solar atmosphere and
solar interior
         The photosphere (光球層)
  emits visible light, the photosphere is the
   visible surface of the Sun.
  A gaseous layer of about 500 km in depth.
  Generates most of sunlight we see.
  The temperature ~ 6,000 K.
  Low dense, only 0.1% of atmospheric density at
   the sea level.
7A.1 The solar atmosphere and
solar interior
    Feature I: Granulation (米粒組織)




    shows the convection of gas
    ~ 1,000 km in diameter.
    Lasts for 20 minutes.
7A.1 The solar atmosphere and
solar interior
         Feature I: Granulation
                            hotter




      Colder sinking gas
7A.1 The solar atmosphere and
solar interior
               Feature II: Supergranules
 Groups of about 300 granules        A picture of the chromosphere
  (see also later in chromosphere).
 ~30,000 km in diameter.
 Each lasts for a day or two.
 Very slowly rising hot gases.




                                      At the edge of a supergranule
7A.1 The solar atmosphere and
solar interior
   Feature III: Limb darkening (臨邊昏暗)

 Looking at a steep
  angle when look at the
  limb.
 Evidence of slight
  temperature decrease        looking at the
                              higher altitude
  with height in the
  photosphere.
              cooler region                     Earth
    7A.1 The solar atmosphere and
    solar interior
              The chromosphere (色球層)
 A layer of gas of about 10,000 km
  thick
 Very low dense, only 0.01% of
  photosphere, so you couldn’t see
  it normally, and can be seen during
  solar eclipse!
 at the top of chromosphere,
  extremely low density (about 10-13
  of that of the air we breathe !)
7A.1 The solar atmosphere and
solar interior
        The chromosphere (色球層)

 Flash spectrum: 4,000 K at the top of photosphere
  to 1,000,000 K at a bottom of the corona.
 Why crescent-like?
7A.1 The solar atmosphere and
solar interior
            Feature I: Spicules (針狀體)

 By Ha filter, grass-like
  spicules mark the edges
  of supergranules
  (enormous convective
  cells in the photosphere)
  underneath.
7A.1 The solar atmosphere and
solar interior
                Feature I: Spicules
 Flame-like structures
  extending up to 12,000 km
  above the photosphere.
 Can last for about 5 to 15
  minutes.
 Appear to be cooler
  regions, about 10,000 K in
  temperature, extending
  up into the corona.
7A.1 The solar atmosphere and
solar interior
          Feature I: Spicules
7A.1 The solar atmosphere and
solar interior
              Feature I: Spicules

An Ha filtergram
showing the entire
reddish-pink solar
disk reveals the
structure of the
chromosphere.
7A.1 The solar atmosphere and
solar interior
              Feature II: Solar wind
  Hot gases blowing outward.
  Mainly consists of high-speed (about 400
   km/s) ionized hydrogen.
  Low density, only a few particles per
   cubic centimetre.
  Causes a mass loss of about 107 tons/year,
   a minor loss in the solar mass.
     http://sohowww.nascom.nasa.gov/
7A.1 The solar atmosphere and
solar interior
                 The corona
  The outermost layer.
  Extend out to many (even 39) solar radii.
  Visible only during total solar eclipses.
  500,000 K to 3,500,000 K (not fully
   understood why so hot and so strong
   magnetic field?)
  The density is very low, about a few atoms
   per cubic centimetre.
7A.1 The solar atmosphere and
solar interior
                The corona




    Visible only during total solar eclipses
More about the corona: coronal mass ejection
X-ray picture of the corona
7A.1 The solar atmosphere and
solar interior
                 The corona
 Streamers in the
 solar corona.
 Computer-
 enhanced to
 produce a false-
 colour image.
7A.1 The solar atmosphere and
solar interior
           The solar interior

  Only little is known.
  Helioseismology: a study of solar
   vibrations, may help.
7A.1 The solar atmosphere and
solar interior
               The solar interior

 1 of nearly 10
  million possible
  modes of oscillation
  of the Sun.
 Red regions are
  receding, while blue
  regions are
  approaching.
Sun’s heartbeat
7A.2 Solar activities
7A.2 Solar activities

 There are many violent solar activities.
 We shall discuss sunspots, prominences
  (日珥), and flares (耀斑).
Review and preview
7A.2 Solar activities
              I: Sunspots: Structure

 Umbra: the central
  region. About 4,000 K.
 Penumbra: a bit
  hotter than the umbra
  region but is still
  cooler than the
  photospheric average.
7A.2 Solar activities
       I: Sunspots: Organization and motion

 Move in groups.
 Move from higher
  latitudes to near
  the equator.
7A.2 Solar activities
 I: Sunspots: Physical properties and spot cycle
 Regions of strong magnetic
  fields, about 1000 times
  stronger than the solar
  average.
 Appear in pairs.
 The sunspots in a pair are of
  opposite magnetic polarities.
 Across the equator, polarity
  reverses.
7A.2 Solar activities
I: Sunspots: Physical properties and spot cycle




  An H-alpha filtergram   Iron filings sprinkled
  of a sunspot group      over a bar magnet
7A.2 Solar activities
I: Sunspots: Physical properties and spot cycle

 The number of
  sunspots varies with a
  period of about 11
  years (last maximum
  in 1990), a sunspot
  cycle.
7A.2 Solar activities
I: Sunspots: Physical properties and spot cycle
  The overall polarity reverses from
   cycle to cycle.


     S     N
                                 N      S


      N     S                    S      N
7A.2 Solar activities
   I: Sunspots: Formation — Babcock model
Differential rotation of the Sun
7A.2 Solar activities
   I: Sunspots: Formation — Babcock model
Differential rotation of the Sun

 Helioseismology: the
  equator rotates once
  in 25 days
 Red regions more
  slowly
 Blue regions 35 days.
7A.2 Solar activities
7A.2 Solar activities
    I: Sunspots: Formation — Babcock model
 When the field becomes
  tangled and bursts through the
  surface, sunspot pairs form.

 The strong magnetic fields in a
  sunspot inhibit circulation by
  slowing rising hot gas.

 Hence the sunspot becomes a
  cooler region.
7A.2 Solar activities
             II: Prominences

  Red filaments shooting upward from
   magnetic regions (sunspots).
  Ionized gases trapped in magnetic fields.
  Some last for hours, some for months.
7A.2 Solar activities
7A.2 Solar activities
7A.2 Solar activities
7A.2 Solar activities
                 III: Flares
   More violent eruption rising to
    maximum in minutes.
   Emit X-ray, ultraviolet, visible light,
    and high-energy particles.
   The emitted radiations and particles
    can interrupt communications on the
    Earth and cause auroras near poles.
   Always occur near sunspot groups.
7A.2 Solar activities
7A.2 Solar activities

Events on the sun
can affect the
outer corona. This
picture was taken
during an eclipse.
Computer
processing reveals
twisted streamers.
7A.2 Solar activities
      The concept of the Solar constant

 The total solar energy reaching the
  surface of the Earth, ~ 1360 J/m2s.
 In fact not constant.
 If there is a 1% change in the solar
  constant, there will be 1 - 2C change in
  the Earth’s temperature.
7A.2 Solar activities
      The concept of the Solar constant
  Observation: a long-term decrease of
   0.018% per year in solar constant.
  It is believed that variations in the
   solar constant can cause long-term
   changes in the Earth’s climate, for
   example, ice ages.

				
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posted:10/15/2011
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