Lesson: Measuring the Motion of a Coronal Mass Ejection (Grades 9-12)

Teacher information

A coronal mass ejection occurs when a significant amount of relatively cool, dense, ionized gas escapes from the normally closed, confining, low-level magnetic fields of the Sun's atmosphere to streak out into the interplanetary medium, or heliosphere. In other words, a large quantity of mass is accelerated by the magnetic field of the corona and travels through space, sometimes towards the Earth. Eruptions of this sort can produce major disruptions in the near Earth environment, affecting communications, navigation systems and even power grids. SOHO with its uninterrupted view of the Sun, can observe such events continually, and allow us for the first time to get a better understanding of how such violent events occur.

Scientists do not yet really understand why CME occur and how to predict them. One important part of the research is to measure the velocity of the CME and trace its acceleration as it leaves the Sun. This is done by tracing individual features in the CME and measuring their positions as a function of time.

One of the main ways we observe CME's is with coronagraphs. Coronagraphs are telescopes which simulate total solar eclipses by blocking out the disk of the Sun so we can see its fainter outer atmosphere, the corona. On Earth this can be difficult because the Earth's atmosphere scatters the light from the solar disk (that's why the sky is blue). In space, however, this is not a problem. LASCO consists of three coronagraphs with three different occulting disks, each one a different size so we can see a different part of the corona.

This activity cam be done with a pencil and paper or using computer software. We provide additional instructions for those interested in using the free-ware program NIH Image.

Materials (pencil and paper version):

• SOHO CME IMAGES
• ruler
• calculator
• Student activity handout (optional).

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. Have you students 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. Measure its position in each image. Measurements on the screen or page can be converted to kilometers using the simple ratio:

   d_screen/d_actual= s_screen/s_actual

   where:

   d_screen is the diameter of the Sun measured on the screen.

   d_actual is the actual diameter of the Sun.

   s_screen is the position of the mass as measured on the screen.

   s_actual is the actual position of the mass.

   The diameter of the Sun = 1.4 × 10⁶ (1.4 million) km.
2. Using the position and time, students can calculate the average velocity. Velocity is defined as the rate of change of position. Using the change in position and the change in time, the average velocity for the time period can be calculated using the following equation:

   v = (s₂ - s ₁)/(t₂ - t₁)

   where:

   s₂ is the position at time, t₂.

   s₁ is the position at time, t₁.

3. The acceleration is the change in time of the velocity:

   a = (v₂ - v ₁)/(t ₂ - t₁)

   where:

   v₂ is the velocity at time, t₂.

   v₁ is the velocity at time, t₁.

   
   
4. Have student record their results in a table:

   Universal Time	Time Interval	Position	Average Velocity	Average Acceleration
   08:05	_	_	_	_
   08:36	_	_	_	_
   09:27	_	_	_	_
   10:25	_	_	_	_
   11:23	_	_	_	_

5. Have the students select other features, trace it, and calculate the velocity and acceleration.
   Discuss: Are the values different from those for the last feature you selected?
   Which one is "right"?
   Scientists often look at a number of points in different parts of the CME to get an overall idea of what is happening.
   Sometimes it can be tough to trace a particular feature. How much error do they think this introduces into the calculations?
6. Have students determine how the size of the CME changes with time
7. Discuss: What kind of forces might be acting on the CME? How would these account for the data? These are important questions in CME research!

Further work

LASCO also observes comets. Students can measure their the velocities and accelerations too.

MEETING THE STANDARDS