Measuring the 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 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):
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.
- 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:
The diameter of the Sun = 1.4 × 106 (1.4 million) km.
- dscreen is the diameter of the
Sun measured on the screen.
- dactual is the actual diameter of the Sun.
- sscreen is the position of the
mass as measured on the screen.
- sactual is the actual position of the mass.
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)
- s2 is the position at time, t2.
- s1 is the position at time, t1.
The acceleration is the change in time of the velocity:
- a = (v2 - v
2 - t1)
- v2 is the velocity at time, t2.
- v1 is the velocity at time, t1.
- Have student record their results in a table:
||Time Interval ||Position||Average Velocity
|08:05 || _ ||_||_||_
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
Sometimes it can be tough to trace a particular feature.
How much error do they think this introduces into the
Have students determine how the size of the CME changes with time
Discuss: What kind of forces might be acting on the CME? How would
these account for the data? These are important questions in CME
SOHO observes many CMEs and this sort of exercise can be performed on
other sets of images we have on the web.
Data available from
LASCO in the SOHO Gallery and elsewhere usually have a circle in
the center of the coronagraph disk
representing the size and position of the Sun. Using this to find the scale
of the image, you can make calculations similar to the one you
just did for most sequences of LASCO images.
LASCO also observes comets. Students can measure their the velocities and accelerations too.
MEETING THE STANDARDS
How does this exercise meet national education standards?
The following information was taken with authorization from:
Content Knowledge: A Compendium of
Standards and Benchmarks for K 12 Education
John S. Kendall and Robert J. Marzano
Mid-continent Regional Educational Laboratory, Inc. 1996
Mid-continent Regional Educational Laboratory, Inc.
2550 South Parker Road, Suite 500
Aurora, CO 80014
Standards for Mathematics
Understands and applies basic and advanced properties of the
Has a basic understanding of the concept of velocity and how it is
Has a basic understanding of the concept of acceleration and how it
Determines precision and accuracy of measurements
Understands that scale drawings can help one measure distances and
angles that are inconvenient to measure directly
Science and Technology:
Understands the nature of scientific knowledge
Knows that science distinguishes itself from other ways of knowing
from other bodies of knowledge through the use of empirical
logical arguments, and skepticism, as scientists strive for
of their proposed explanations
Knows that scientific explanations must meet certain criteria;
must be consistent with experimental and observational evidence
nature; and they must include a logical structure, rules of
openness to criticism, reporting methods and procedures, and a
commitment to making knowledge public
Knows that because all scientific ideas depend on experimental and
observational confirmation, all scientific knowledge is, in
subject to change as new evidence becomes available; in areas
data, information, or understanding is incomplete, it is normal
scientific ideas to be incomplete, but this is also where the
opportunity for making advances may be greatest
Understands the nature of scientific inquiry
Designs and conducts scientific investigations by identifying and
clarifying the question, method, controls and variables;
and displaying data; revising methods and explanations; presenting
results; and receiving critical response from others
This activity is based on Sun
Centered Physics, a set of lesson plans developed by Linda
Send questions and comments to Terry Kucera or Dennis Christopher.
Last modification: Friday, 09-Nov-2007 11:33:35 EST