Name:
Astronomy 6
The Sun
Objectives
• To learn the basics of magnetic fields
• To learn about sunspots and how they are formed
• To understand differential rotation in the Sun
• To learn about coronal mass ejections
• To observe the Sun safely
XXEquipment
inside: magnets
tiny compasses
laptops
Plastic Ruler
Calculator
outside: telescope
Solar filters
The Basics of Magnetic Fields
Answer the following questions after watching the demonstrations
1. How many magnetic poles are in a “set”?
2. What is the shape of a bar magnet’s magnetic field:
3. How many magnetic poles does Earth have?
4. What is the shape of Earth’s magnetic field?
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SOHO
Many images we will use in this lab come from SOHO, one of the better known space-based
observatories. Go to http://sohowww.nascom.nasa.gov/home.html or link to SOHO through the class
site on SacCT. Go into “about” to answer the following.
5. What does SOHO stand for?
6. Which two agencies run SOHO?
7. When was it launched?
8. How many different measuring instruments are on board?
9. How wide is the satellite when the solar panels are deployed?
10. SOHO’s orbit is unique. It does not orbit the earth but it orbits a point called “L1”. The L1 point is
the gravitationally stable spot between the earth and the sun. Click on “gallery” then “animations”
to find a movie about SOHO’s orbit. Note that the movie is not to scale. SOHO is about 100 times
farther from the sun than it is from earth.
a. Why do you think SOHO was put in orbit around L1, rather than the earth?
b. Sketch a diagram of SOHO’s orbit. Label the earth, sun and SOHO. Include the directions they
spin and revolve (don’t worry about scale).
Sunspots
Sunspots are created when the magnetic field lines of the sun get twisted up and loop out of the
photosphere. The magnetic field lines cool the surface from about 6000K to 4000K these cool spots are
darker and are seen as a set of sunspots. In the gallery heading, go to movies, then animation, watch the
“sunspot formation” movie.
11. How many sunspots appear in a “set”?
On the overhead projector are two images taken at the same time by SOHO. The yellow image is an
MDI continuum image (what you would see with just a solar filter). The other is an MDI magnetogram
where north and south magnetic poles are colored black and white. You may have to get close to see
this.
12. What do you notice about the features on the two images?
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Rotational Period of the Sun
13. On Sac CT there are two files called “Differential Rotation”. You should see the Sun (photosphere) with
a grid overlay. The pages in the file show images taken over 8 days. Notice the sunspots
moving, forming and fading. Three spots are labeled A, B, & C. For each spot, measure the
heliocentric longitude angle - measured along the Sun’s equator, for each day. Be as accurate a
you can.
Spot A Angle Spot B Angle Spot C Angle
Date
(degrees)
(degrees)
(degrees)
June 26
June 27
June 28
June 29
June 30
July 1
July 2
July 3
14. Determine the rotation rate of each spot (degrees per day).
i.e. average the difference in the angles from one day to the next for Spot A and Spot B.
15. How many days would it take each spot to go through one full rotation (360°)?
16. Note that the two spots are at different latitudes (measured N or S of the solar equator). What are the
approximate solar latitudes of spot A and spot B?
17. Look at the file marked “differential rotation” on the class site. Where is the sun’s rotation the
fastest: at the solar equator, or the solar poles?
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Coronal Mass Ejections (CME)
Go to https://www.youtube.com/watch?v=gcn24Qz6zbs which shows a coronal mass ejection.These
images are coronagraphs - meaning a circular mask was used to block thew sun’s brightness so that
we can see the corona. You can run the video and freeze the image at different times to measure the
progress of the CME.
19. Measure the diameter of the white circle in mm.
20. The white circle represents the Sun’s diameter of 1,400,000 km. Divide 1,400,000 by the white
circle’s diameter to get a scale factor that expresses how many km are represented by each
millimeter. This is your scale factor.
21. Start the movie over and go frame by frame until you get to the time frame 4:24. Measure in
millimeters the distance from the edge of the white circle to the outer edge of the emerging CME on
the right. Repeat this for the other times and fill in column 2 in the chart below.
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2
3
4
5
Time
(hr:min)
Distance from white circle edge
to CME outer edge (mm)
min later
Distance Traveled
(mm)
Time Elapsed
(hr)
0
22. Column 4 is the distance traveled by the CME (in mm) since the previous frame. Take your value in
column 2 and subtract the distance from the previous row.
23. Find the average speed of the CME on the screen:
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24. Determine the average speed of the CME in km/hr.
25. The earth is 150,000,000 km from the sun. If a CME were pointed at earth, how long would it take
to get here?
26. Relatively few CMEs actually come toward earth. They can be emitted in any direction so most miss
us by a wide margin. When they do come toward earth, our magnetic field protects us and makes an
amazing light shows called auroras (aurora borealis in the north and aurora australis in the south). In
the SOHO animation gallery watch the CME impacting the magnetosphere and creating aurora and
check out the link CME Earth for another view.
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Telescope views
Sketch and describe what you see in each scope. The shape is the field of view.
Telescope with filter (Photosphere):
Coronado (Chromosphere):
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