Transcript Coronal Mass Ejection

Our Star, the Sun
Chapter Eighteen
Guiding Questions
What is the source of the Sun’s energy?
What is the internal structure of the Sun?
How can astronomers measure the properties of the
Sun’s interior?
4. How can we be sure that thermonuclear reactions are
happening in the Sun’s core?
5. Does the Sun have a solid surface?
6. Since the Sun is so bright, how is it possible to see its
dim outer atmosphere?
7. Where does the solar wind come from?
8. What are sunspots? Why do they appear dark?
9. What is the connection between sunspots and the
Sun’s magnetic field?
10. What causes eruptions in the Sun’s atmosphere?
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Source of solar energy
• The sun produces 3.9 * 1026 joules/sec (watts).
• The earth receives 7 * 1017 watts or 2 billionths of sun’s
energy.
• The sun has been shining for about 4.5 billion years.
• Theory of fusion energy source for sun’s radiation was
developed; 1928 – 1938.
• George Gamow, Hans Bethe, F. Houtermans, R.
Atkinson, W. Pauli, W. Heisenberg developed the theory.
• 1968; solar neutinos observed by John N. Bahcall &
Raymond Davis.
Neutrino experiments
• Brookhaven National Laboratory; 100, 000 gallons of
perchloroethylene (C2Cl4); 1968.
– Neutrino strikes Chlorine nucleus, turning it into Argon, can be
separated from fluid.
– Only 1/3 as many neutrinos as predicted.
• Late 1980’s; M. Koshiba
– Kamiokande experiment in Japan
– Showed neutrinos indeed came from the sun.
• 1998 Super Kamiokande experiment
– Showed there were 3 types of neutrinos, explained missing 2/3 of
the neutrinos.
• 1987 Supernova occurs in Magellenic Cloud
– Neutrinos observed in 2 different detectors.
– Product of supernova explosion.
The Sun’s energy is generated by thermonuclear
reactions in its core
• The energy released in a
nuclear reaction
corresponds to a slight
reduction of mass
according to Einstein’s
equation E = mc2
• Thermonuclear fusion
occurs only at very high
temperatures; for example,
hydrogen fusion occurs
only at temperatures in
excess of about 107 K
• In the Sun, fusion occurs
only in the dense, hot core
The Sun’s energy is produced by hydrogen
fusion, a sequence of thermonuclear
reactions in which four hydrogen nuclei
combine to produce a single helium nucleus
Start of hydrogen fusion process in the sun’s interior; 2 protons collide.
Step 2 in the fusion process involves a 3rd proton
In the final step, the end products are helium with 2 of the
original 6 hydrogen atoms recycled.
A theoretical model of the Sun shows how energy
gets from its center to its surface
• Hydrogen fusion takes
place in a core extending
from the Sun’s center to
about 0.25 solar radius
• The core is surrounded by
a radiative zone extending
to about 0.71 solar radius
– In this zone, energy travels
outward through radiative
diffusion
• The radiative zone is
surrounded by a rather
opaque convective zone of
gas at relatively low
temperature and pressure
– In this zone, energy travels
outward primarily through
convection
Astronomers probe the solar interior using
the Sun’s own vibrations
• Helioseismology is
the study of how the
Sun vibrates
• These vibrations have
been used to infer
pressures, densities,
chemical
compositions, and
rotation rates within
the Sun
Note that only 0.8 % of the sun’s volume is < .2 solar radii
Internal solar densities and temperature, note water is 1000 kg/m3
Neutrinos reveal information about the Sun’s
core—and have surprises of their own
• Neutrinos emitted in
thermonuclear
reactions in the
Sun’s core have
been detected, but
in smaller numbers
than expected
• Recent neutrino
experiments explain
why this is so
The photosphere is the lowest of three main layers
in the Sun’s atmosphere
• The Sun’s atmosphere
has three main layers: the
photosphere, the
chromosphere, and the
corona
• Everything below the
solar atmosphere is
called the solar interior
• The visible surface of the
Sun, the photosphere, is
the lowest layer in the
solar atmosphere
Limb darkening because base of photosphere is hotter
than higher layers seen near solar limb.
Convection in the photosphere produces granules
The chromosphere is characterized by spikes
of rising gas
• Above the
photosphere is a
layer of less dense
but higher
temperature gases
called the
chromosphere
• Spicules extend
upward from the
photosphere into the
chromosphere along
the boundaries of
supergranules
Chromospheric Spectrum
• Chromospheric emission spectrum
– Emission lines with some matching wavelengths of
photospheric absorption lines
• Bright yellow line produced by helium (He)
– Chromospheric temperature up to 30,000 K at highest
level
– Gas density is lower than photosphere
• From this, one concludes that temperature
must rise rapidly up through chromosphere
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• The outermost
layer of the solar
atmosphere, the
corona, is made
of very hightemperature
gases at
extremely low
density
• The solar corona
blends into the
solar wind at
great distances
from the Sun
The corona ejects mass into space to form the solar wind
Activity in the corona includes coronal mass ejections and coronal holes
Ultraviolet image taken from SOHO spacecraft.
Sunspots are low-temperature regions in
the photosphere
Temperature in Umbra abt 4400 K, Penumbra abt 5000 K, 30% of light
Tracking the sun’s rotation with sunspots; 25 ¼ days at equator, 28.2
days at latitude 45, 34 days nearer the poles.
Chromospheri
c Flares
• Flares - brief burst
of X-rays and
particle
– Observed in
monochromatic light
– Lifetimes of about
20 minutes
– Size about 30,000
km
– Enhances particle
density in solar
Solar Flare Movie
wind and solar
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Sunspots are produced by a 22-year cycle
in the Sun’s magnetic field
• The Sun’s surface features vary in an 11-year cycle
• This is related to a 22-year cycle in which the surface magnetic field
increases, decreases, and then increases again with the opposite polarity
• The average number of sunspots increases and decreases in a regular cycle
of approximately 11 years, with reversed magnetic polarities from one 11year cycle to the next
• Two such cycles make up the 22-year solar cycle
Variations in solar activity
• 1610 Galileo observed sunspots
• From 1645 to 1715 very few sunspots were observed
– Historical records, Flamsteed in 1674 said that 1st since 1664.
• “Little Ice Age”; 1300 to ~ 1850
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Glaciers advanced in Alps; severe winters
Greenland colony fails
1816, year without a summer follows Mt Tambora eruption
Weather unstable & unpredictable.
• Astronomical evidence that sunspot minima occurs about 20% of the
time in a star like the sun
• 1958, Sunspot activity was the largest ever observed.
• Since 1900, sun is getting hotter, about 1/3 of global warming.
The magnetic-dynamo model suggests that many
features of the solar cycle are due to changes in
the Sun’s magnetic field
These changes are caused by convection
and the Sun’s differential rotation
Rotation of the Solar Interior; center of sun rotates uniformly
The Sun’s magnetic field also produces other
forms of solar activity
• A solar flare is a
brief eruption of hot,
ionized gases from
a sunspot group
• A coronal mass
ejection is a much
larger eruption that
involves immense
amounts of gas from
the corona
Coronal Prominences
• Prominences Chromospheric
material extending
upward into corona
– Seen against photospheric or
chromospheric disk known as
filaments
Prominenc
e
• Properties
– Much cooler than surrounding
corona
– Sizes, if quiescent, height
30,000 km, length 200,000
km, thickness 5000 km
– Exhibit motions associated
with magnetic fields up to
several hundred gauss
– Lifetimes up to 90 days
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• Holes - lower temperature
and much lower density
regions
– Sizes up to hundreds of
thousands of km
– Magnetic field lines open out
to interplanetary space
– Source of solar wind particles
– Changeable in periods of
days to weeks
Soft X-Ray
July 7, 1998
Pole
• Active regions - relatively
hot and dense regions
consisting of magnetic
loop structures
– Sizes up to hundreds of
thousands of km
– Magnetic field lines form
large loop structures
– Occur over chromospheric
plages
Coronal
Hole
Coronal
Quiet
Region
Equato
r
Coronal
Active
Region
• Quiet regions - between
coronal holes and coronal
active regions
– Magnetic fields weak and
roughly in loop structures
Pole
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Coronal Mass Ejection
Coronal mass
ejections send
bursts of energetic
charged particles out
through the solar
system.
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Coronal Mass Ejections
A. Shows relatively quiet corona
a. Black disk blocks photospheric and chromospheric radiation
B. 16 minutes later, huge balloon-shaped volume of high-energy gas is
ejected from corona
C. Ejected material expands at typical velocities of 400 km/s
a. Ejection lasts several hours and contains trillions of tons of matter
b. Often associated with solar flares, but not always
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Coronal mass ejection of 1012 kg mass ( a billion tons)
Solar events also peak at 11 yr cycle
• Numbers of plages, filaments, solar flares and coronal
mass ejects also follow the same 11-yr cycle as
sunspots.
• Coronal mass ejections (CMJ) can occur at any point in
the cycle. Flares are also unpredictable.
• Solar flares and CMJ’s produce showers of charged
particles (electrons and protons).
• Can disrupt electrical grids, radio and TV transmissions.
• Dangerous for astronauts, satellites
• Can produce brilliant auroras, even at low latitudes.
• Space weather – study of variable emission of
high-energy photons, particles, and magnetic
fields and their interaction with the geosphere
• Earth influences
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Van Allen radiation belts
Spacecraft and crews
High-altitude aircraft
Electric power grid
Communications, land and satellite
Major source of natural variability in terrestrial
climate
Space
Weather
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The Big Picture
• The Sun continues to shine, while it radiates away
as its luminosity, by generating energy by
thermonuclear fusion of hydrogen into helium.
• Gravitational and thermal equilibrium determine the
Sun’s internal structure and its rate of energy
generation.
• The Sun’s atmosphere displays its own version of
weather and climate, governed by solar magnetic
fields. Solar weather has important influences on
the Earth.
• The Sun is important not only as our source of light
and heat, but also because it is the only star near
enough for us to study in great detail. In the
coming chapters, we will use what we’ve learned
about the Sun to help us understand other stars. 52
Key Words
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22-year solar cycle
chromosphere
CNO cycle
conduction
convection
convective zone
corona
coronal hole
coronal mass ejection
differential rotation
filament
granulation
granule
helioseismology
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hydrogen fusion
hydrostatic equilibrium
limb darkening
luminosity (of the Sun)
magnetic-dynamo model
magnetogram
magnetic reconnection
negative hydrogen ion
neutrino
neutrino oscillation
photosphere
plage
plasma
positron
prominence