Motion of a Coronal Mass Ejection

Here we calculate the velocity and acceleration of a coronal mass
ejection (or CME) based on its position in a series of images from the
LASCO instrument on SOHO.

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 CMEs 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.

Materials:

If you are doing this on paper you will need

SOHO CME IMAGES

ruler

calculator

The following page contains images 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.

1.  Define roles for a cooperative group setting:-

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a.  Someone to take the measurements

b.  Someone to fill in the table and calculate the velocities

c.  Someone to do the graphing

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2.  Measure the position of the purple dot 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 feature as measured on the screen.

s~actual~ is the actual position of the feature.

The diameter of the Sun = 1.4 × 106 (1.4 million) km.

3.  Record the position of the dot in the table given:

4.  Using the position and time, you 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 = (s~2~ - s ~1~)/(t~2~ - t~1~)

where:

s~2~ is the position at time, t~2~.

s~1~ is the position at time, t~1~.

You can record your results in a table:

  ----------------- ----------------- ----------------- -----------------
  TIME              TIME INTERVAL     POSITION          VELOCITY

  8:00                                                  

  8:30                                                  

  9:30                                                  

  10:30                                                 

  11:30                                                 
  ----------------- ----------------- ----------------- -----------------

5.  Now graph you velocity results with time on the x-axis and velocity
    on the y-axis

6.  Describe the slope of the graph -- is the CME velocity increasing or
    decreasing ?

# Further Questions and Activities

Take the slope of the velocity graph to estimate the acceleration or
deceleration of the CME.

Select a feature that you can see in all five images (other than the
purple dot) for instance the outermost extent of the bright structure or
the inner edge of the dark loop shape. Trace this feature, and calculate
the velocity and acceleration. Are they 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 you think this introduces into your calculations?

Compare your velocity results for features other than the purple dot
with those from other groups -- can you estimate the errors associated
with these calculations ?

Describe how the size of the CME changes with time.

Suggestions for use in Elementary Schools

1.  Use images without the time marked

2.  Cut out the images and ask the students to arrange them in order
    based on what they see

3.  Ask the students to describe the changes in CME in each picture

4.  Give the students the times -- simplified to be 8:00, 8:30, 9:30,
    10:30, 11:30

5.  Measure the position of the dot at each time

6.  Calculate velocity

7.  Graph velocity

8.  Describe the shape of the graph without talking about slopes -- is
    the CME speeding up or slowing down ?

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