Chapter 16: The Sun

1
Chapter 16 The Sun Our Parent Star

• Structure
• Atmosphere
• Activity
• Effect on Earth

2
The Sun
3
Structure of the Sun

• Core-nuclear furnace
• Radiation zone-photons from the core freely pass
  through ionized gas
• Convection zone-area where heated gas transport
  energy to the surface
• Photosphere-the surface where the visible
  radiation we see is emitted
• Atmosphere-chromosphere, transition zone, corona

4
The Core

• The energy of the Sun is produced here by
• Nuclear Fusion
• Mass is converted to energy Emc2
• Mass and energy are always conserved
• Two protons fuse together producing a Deuterium
  atom, a neutrino, and a positron
• Positrons are antimatter the opposite of an
  electron
• When the positron meets an electron they
  annihilate each other, their masses becoming
  energy in the form of a gamma ray

5
The Core
Fusion
Proton-Proton Chain
6
The Radiation Zone

• Above the core, a layer of intensely hot,
  completely ionized gas
• Review of radiation
• Electron are in orbits around the nucleus
• Absorbing a photon kicks the electron into a
  higher more energetic orbit
• The electron radiates a photon of equal energy
  and falls back down to its natural level

7
The Radiation zone

• Ionized means that the electron absorbed enough
  energy to completely leave the nucleus
• In the Radiation zone this is the case
• If there are no electron left in orbits, there is
  nothing to absorb those gamma ray photons from
  the core and they travel straight through

8
Convection Zone

• The further away from the core the cooler it
  gets, eventually electron are rejoined with
  nuclei
• This means that those gamma ray photon get
  absorbed
• Every photon gets absorbed by the time it gets to
  the edge of the radiation zone
• How does the sun transmit energy?
• It is carried to the surface by convection
  currents

9
Convection Zone

• Gas at the bottom gets heated and rises is series
  of cells, the cooler gas at the top of the zone
  falls down, transferring energy from the core
• The energy gets carried up to the photosphere

10
The Photosphere

• At a certain point the gas of the sun gets too
  thin for convection to continue and radiation
  starts again.
• This happens very quickly at the suns surface,
  i.e. the photosphere

Evidence of convection in the granulation of
photosphere
11
Solar Atmosphere

• Chromosphere
• First layer, cooler than the photosphere
• Only visible during eclipses
• Transition zone
• Area where temperature increases greatly
• Corona
• Topmost layer of hot ionized gas
• Mechanism not understood

12
The Corona and Solar Wind

• Corona is so hot it does two things
• Emits X rays
• Generates the solar wind
• The corona is sufficiently hot enough for a
  number of the particles in it to reach escape
  velocity
• These energetic particles, millions of tons a
  second, blow from the sun forming the solar wind

13
The Active Sun

• The Sun is a very violent active place
• Activity on the Sun is primarily driven by its
  powerful magnetic field
• Sunspots
• Flares
• Prominences

14
Solar Magnetic Fields

• An explanation

15
Solar Magnetic Fields

• Created by a dynamo mechanism
• Creation occurs in the convective zone of the
  Sun, which contains turbulent helical 3-D flows
  of plasma
• The movement of the plasma creates electric
  currents, which in turn create magnetic fields,
  which create another current- a cycle of B and E
  field generation

16
Solar Magnetic Fields

• Very energetic fluid motions carry the B field
  with them
• Fluid/Field interactions cause the B field lines
  to act like stretched strings
• The differential rotation of the Sun causes the
  magnetic fields lines to become severely
  distorted giving rise to the 22 year cycle of
  solar activity

17
Magnetic Tube Twist
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Magnetic Loop
171 Å
19
Sunspots

• Areas of cooler gas in the solar atmosphere
• Usually come in pairs, because the occur when a
  kink of the Suns magnetic field sticks out of
  the surface
• Follow the 22 year cycle of the magnetic field

20
The Solar Cycle

• Activity on the Sun follows the 22 year cycle of
  the magnetic fields contortion
• Mostly easily tracked by counting sunspots
• More sunspots greater solar activity

21
Active Regions

• During the period of most activity several
  violent phenomena occur on the sun
• Prominences
• Flares and CME

Active region movie
22
Prominences

• A huge jet of hot gas flowing through a magnetic
  loop
• Measure 100,000 km in extent
• Usually stay in the solar atmosphere for a while
• Prominence movie

23
Solar Flares

• Extremely violent explosions in the lower
  atmosphere
• Only last minutes, but at 100 million K
• Produces a great deal of UV and X radiation

24
Magnetic Reconnection

• Flares seem to be caused by a huge release of
  magnetic potential energy
• When two magnetic loops megre

25
Solar Flares

• Flare are so energetic the particle released
  leave the Sun at high speed
• The energetic particles released in a flare have
  a direct effect on Earth
• Damage Satellites
• Cause power outages
• Disrupt radio communication

26
Luckily this was pointed away from Earth!
27
Coronal Mass Ejections (CMEs)

• A giant magnetic bubble of ionized gas that
  erupts from the corona
• Sometime associated with prominences and flares
• Can be just as disruptive on Earth as flares

CME movie
28
Earth and the Solar Cycle

• The solar cycle appears to have a direct
  relationship with many processes on Earth
• Sunspot number can be determined from
• Tree ring counting
• Animal populations on the Serengeti plains
• Earths atmospheric and oceanic temperatures
• Some historical climate disasters can be
  correlated with solar cycle

29
Maunder Minimum

• There are several different solar cycles besides
  the most apparent 22 year one
• maximum of maxima and minimum of minima
• During the Maunder Minimum there was a year
  without summer, the Thames completely froze, and
  the Greenland Vikings died out

The natural of the relationship between solar
activity and Earth temperatures not well
understood.