Lesson: Measuring the Motion of a Coronal Mass Ejection Modified from a lesson prepared by the SOHO Education and Outreach team

Materials:

• SOHO CME IMAGES
• ruler, pencil and paper
• calculator
• tracing paper

At left is an image taken from one of the coronagraphs on LASCO. To the right of the disk we can see a CME erupting from the Sun. The white circle shows the size and location of the Sun. The black disk is the occulting disk blocking out the disk of the Sun and the inner corona. The tick marks along the bottom of the image mark off units of the Sun's diameter.

Procedure

1. Split into several groups of 4-5 persons each. Have each group independently select a feature that can be seen in all five images: for instance, the outermost extent of the bright structure or the inner edge of the dark loop shape.
2. You should end up with something like the following picture which represents the first image and your tracing paper overlay. X markes the location of an example feature.

3. Measure position of the feature you have selected on the first image.
   • Lay tracing paper over the selected image

   • On the tracing paper, mark off the four corners of the image and the center of the white circle showing where the Sun is on the image: we will call this point, sun-center. The drawing on your tracing paper should look something like this:

   • Using the first image of the CME (the one with time-tag 8:05) line up the corners you drew on your tracing paper with the corner of the image. Is the mark you made for sun-center at the center of the white circle? If not, make sure that all the images are the same size.

   • Make a mark on the tracing paper over the feature you have chosen as your selected feature.

   • Remove the tracing paper and draw a straight line on the tracing paper from sun-center to the feature mark.

   • Print out Table 1. This is the table on which you will record your measurements.

   • Take your ruler and measure the length (in cm) of the line you have drawn, enter this number in Column #3 in the correct row.

   • Before moving on to your next image, overlay the line you drew on your tracing paper with the bottom of the image and count how many tick marks are covered by the length of your line. Write this number in Column #4 in the correct row. [The line will probably not be an exact number of tick marks long. Estimate the additional length as a fraction of a tick mark, e.g. 4.5.]

   • Move on to the 2nd image (the one with time-tag 8:36) and using the same tracing paper line up the corners and sun-center. Notice that your chosen feature has moved further away from the sun-center. Draw a new line from sun-center to the new location and repeat the above procedure of measuring the length of your new line and entering the results in Columns #3 and #4. It is OK if the new line overlaps the old one.

   • Repeat the above for the remaining 3 images.

4. The lengths of the lines you measured are only a few centimeters or tick marks long, depending on the size of image you used and the feature you selected. The Sun is, of course, much bigger than the picture you used to measure the distances recorded in Table 1.

   To change these numbers to real distances from the Sun we have to know the scale of the Sun on the image. This is like looking at a map of your town where, quite often, the scale will be 1 inch to 1 mile or 2cm to 1 km.

   In Column #4 of Table 1 you should have entered the length, in tick marks, for each of the lines you drew from sun-center to your selected feature. This provides the scale from the picture of the CME to the real thing.

   Each tick mark corresponds to a distance equal to the diameter of the Sun, which is 1.4 million km.

   • Multiply each of the numbers in Column #4 by 1,400,000 and enter the result next to it in Column #5.

   You now have a complete set of times and real distances.

5. The speed of an object, like a car or a CME, tells you how far it can travel in a given time. For instance, a car moving with a speed of 30 miles an hour travels 30 miles in 1 hour, 15 miles in half-an-hour, and 10 miles in 20 minutes (1/3 of an hour).

   Column #2 of Table 1 gives the times in hours and minutes that the images were taken. In Column #2 of Table 2 we show the difference in time between two successive images (in hours).

   • In Column #3 of Table 2 enter the difference in distances shown in Column #4 of Table 1.

   • Finally, divide the number in Column #3 of Table 2 by the number in Column #2 and record the result in Column #4.

   You now have a list of speeds for the CME at different times during its escape from the Sun.

Questions

• Are the CME speeds big or small?

• Is the CME faster or slower than a car?, a train?, an aeroplane?

• Were all the speeds you measured the same?

• If not, what do you think this means about the CME?

• Compare your results with the other groups. Did you all get roughly the same answers?

• If not, discuss the reasons why they were different?

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