Magnetic
Sun in 3D
One of the first things you learn about magnets is
that like poles attract and unlike poles repel.
The Sun has a magnetic field like the Earth. It has a pole at the top and one at the
bottom and a field in-between. Also,
like the Earth, the Sun spins on its axis once about every month. But unlike the Earth, the Sun is not a solid
body. The equator of the Sun revolves
faster than its poles. This
‘differential’ rotation winds up the Sun’s magnetic field until it bursts out
of the side of the star as loops of magnetic field. Sometimes these loops are so concentrated
that they rip holes in the Sun’s surface; these are sunspots.
SEEING
IN 3D
By having two eyes with a gap between them, each of
our eyes give slightly different views of nearby objects. Our brain interprets this difference to give
us a sense of depth – the third dimension.
Astronomical objects are too far away to give us any detectable
difference between the views our eyes get, so that they appear flat.
THE
CHANGING VIEW
Luckily as the Sun spins on its axis we get different
view of its changing features. 3-6
hours of spin gives enough difference to give a 3D view. Its not an ideal way of getting a 3D
picture of the Sun as some of its features can be very dynamic. Very active magnetic loops can expand during
those few hours and spoil the 3D effect.
You may find that it even appears as though the loops dive down into the
Sun rather than out of it. Tracking
down an effective stereo pair is mainly a matter of trial and error.
MAKING
A STEREO IMAGE
Inside your eye you have three
types of colour sensors; red, green and blue. These sensors combine to give you
full colour vision. For example, yellow
light stimulates both red a green sensors and the resulting dual signal is
interpreted by your brain as yellow.
If you cover an eye with a blue green or red filter only that colour
light reaches your retina and only that colour sensors will be stimulated. What a stereo image processing program does
is change each view into a primary colour image and combine them together. By looking at the resulting image through
the same coloured filters each eye only sees one of the combined images. In the past red and green or red and blue
filters were used. But now cheap ‘cyan’ filters that let both blue and green but not
red through are available. These can
still be viewed with red/green and red/blue filters but give the advantage of
colour stereo viewing with cyan. Our
recommended program for making 3D images ‘Anaglyph Maker’ gives you the option
for any of these codings. For this
exercise we find that, for
Go
here for to get the software via the ‘Your Software Tools’ page
What
you will be seeing
The
Extreme Ultraviolet Imaging Telescope (EIT)
The EIT isolates emissions from ions in the corona of
the Sun which is composed of an ionised gas called plasma. 171,195 and 284 look at emissions from
different ions on Iron , whilst 304 looks at an emission from ionised
Helium.
Each wavelength
is mainly emitted at different temperature ranges.
|
EIT wavelength |
Temperature (oK) |
|
304 |
60.000-80,0000 |
|
171 |
1 million |
|
195 |
1.5 million |
|
284 |
2 million |
As a general rule the plasma of the corona increases
with height. Scientist are researching
how this could be. Current theories are
based on the changing magnetic fields transferring energy to the upper levels
of the corona. Think of it a bit like a
microwave oven transferring energy to your food without heating the air around
the food.
By clicking each of the following buttons you can
download a pair of selected EIT images which show good 3D features. They have already been changed into jpegs
for you and can be saved and opened in Anaglyph Maker. Offsetting the anaglyph to the left will
bring the Sun’s globe out of the screen.
This tends to improve the view of features on the side of the Sun. Offsetting right makes the globe recede into
the screen and improves the view of feature in the centre of the Sun.
RESULTS
Experiment with a pair of bar magnets under a piece of
paper and a sprinkling of iron filings on top.
How do you need to arrange the poles of the magnets to get a looped
field?
What can you say about the polarity either end of a
solar magnetic loop?
Iron filings make the bar magnet field visible. What is interacting with the solar magnetic
field to make it visible?
Click here
for a SOHO MDI magnetogram taken at the
same time as the EIT 171 image. White is north magnetic polarity and black is
south. Does this confirm your
theory?
Click here for an inverted black and white copy of
the EIT 171 image which you can print out and mark on the polarities.
WHAT
NEXT?
Now you have mastered making 3D images of the Sun you
are ready to do your own 3D Sun investigation.
Use the near-real-time images and recent image log on the
The most powerful magnetic loops expand very fast and
hurl vast amounts of the corona into space.
These are called coronal mass
ejections (CMEs). They tend to expand
too fast to get a 3D view due to the Sun’s spin. However you can sometimes get an impression
of the 3D nature of CMEs from the difference caused by expansion. Click here to see an example.
More
information about how scientists are using 3D imaging to understand the Sun: