Measuring the Astronomical Unit

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Measuring the Astronomical Unit
Mankind first found the scale of the solar system
using the difference in the appearance of a
transit of Venus from opposite sides of the
Earth, whose size was known (requires accurate
clocks as well).
In modern times we can just bounce radar off
Venus and time the echo. We know c very
precisely.
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Measuring the Sun
Once you know the distance to the Sun (AU), you
can convert its apparent size to a true size.
Kepler/Newton can give you its mass (and thus
density), and you can convert its observed
brightness to an intrinsic luminosity once you
know how far it is.
Distance 1.5x108 km Mass 3.3x105 Earth masses
2x1030 kg Density 1.41 gm/cc Temperature
5800K (at surface) Luminosity 3.8x1026 watts
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The Sun at different wavelengths
Photosphere Visible light 5800K
Chromosphere Ultraviolet light 104-105K
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Parts of the Sun
Energy is produced in the core of the Sun by
nuclear fusion. Initially it flows away from the
core as hot EM radiation. Eventually as the
layers get cooler, they become too opaque. The
energy in the outer 30 flows instead by
convection. At the surface the energy is radiated
into space. There are several surface layers as
well. Surprisingly, the atmosphere gets hotter on
the outside an influence of magnetic fields
(which also produce many interesting features).
Inside
Surface
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Convection
Convection is a mode of energy transport which
crops up often. For example heat flow in
planetary interiors, in our atmosphere, and in
cool stars. Hotter stuff is less dense, and
therefore bouyant. It rises up until it can dump
its energy (radiating it). Then it cools off,
becomes less dense, and sinks back. There is a
circulating pattern set up which carries heat
outwards.
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Granulation in the Photosphere (visible surface)
We see the tops of the convection cells at the
visible surface of the Sun. The main pattern is
called granulation each granule is about the
size of California. The pattern changes in a few
minutes.
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Sunspots
Sometimes the visible surface has a few dark
spots on it. They arent really dark, they are
just cooler than the surroundings (3500K). These
are places where strong magnetic fields emerge
from the solar interior. The fields are very
important to the atmosphere of the Sun, and its
interaction with us.
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The Suns Magnetic Field
The ingredients for a magnetic dynamo are all
present a rotating, conducting, convecting
interior. The Sun renews its fields constantly,
on an 11-year cycle.
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The Chromosphere (hot layer above photosphere)
The chromosphere is the red rim of the Sun which
appears just over the photosphere in an eclipse.
The red light is from a spectral line of
hydrogen. Using a filter for just that line, we
can see the whole chromosphere. It is much more
structured than the photosphere, because at these
heights the magnetic field rules the structure of
the solar gases (they are fully ionized, or
charged).
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Chromospheric Structures
Spicules and fibrils magnetic jets and loops
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Views of the Corona
Above the chromosphere is the corona. Magnetic
heating takes it to 2 million degrees, and the
fields completely define its structure. These 3
images were all made on Aug. 11, 1999. The white
light corona is just reflected sunlight the
real corona is an X-ray gas as seen in the 2
right images from space.
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The real X-ray corona
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Coronal loops
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Prominences
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Sometimes, the fields short out each other
Since loops are constantly rising out of the Sun,
sometimes opposite polarities will meet in the
corona. The field reconnects, simplifying itself,
and the excess energy is released as a huge
explosion flare.
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Flares are best seen in the chromosphere
High energy particles stream down from the
reconnection site, and heat the chromosphere.
They can easily be seen in H-alpha.
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Solar Shock Wave
The flare energies are vastly bigger than
anything we know on Earth. The flare lasts half
an hour. Here the shock wave from the input of
energy to the chromosphere can be seen spreading
out. More commonly, there is a spray of material
upward...
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Coronal Mass Ejections
Huge blasts of high energy particles are followed
by vast amounts of solar plasma travelling
outward at hundreds of km/s. In a day or two they
reach the orbit of the Earth
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The Sun-Earth Connection
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Producing the Aurorae
Some of the high energy particles can get into
the Earths magnetosphere (which otherwise
protects us). They impact our upper atmosphere,
and it glows.
The connections cause particles to stream
preferentially down the magnetic poles, so
normally you have to be at high latitudes to see
them.
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Beautiful Aurorae
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The Magnetic Cycle of the Sun
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The 22-year polarity reversal
The cycle is actually 22 years, with each 11
years having the north-south poles reversed. The
Earth also reverses (more like 50,000 yr).
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Why the Cycle Happens
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Helioseismology