Background:
Coronal mass ejections (CMEs) events seem to begin with the formation of what are called
helmet streamers.These are otherwise rather stable loops of magnetic fields and
plasma from the solar surface which enter the inner coronal regions
of the Sun. While helmet streamers are long-lived, their demise
often occurs abruptly through one of the most spectacular manifestations
of solar activity.
In the larger coronal mass ejections, coronal material may be
ejected outward at speeds as high as 1000 kilometers per second.
Prior to its disruption, helmet streamers can be visible for a
few days, during which they may show little change in shape or
brightness except for a very gradual rise and/or swelling. But
then whatever balances that exist between the particles and magnetic
fields, comes undone. The helmet streamer begins to swell rapidly
in a matter of a few hours signaling the initial stages of the
ejection. A dark cavity comes into view, within which an erupting
prominence can sometimes be seen. The prominence material is blown
outward along with the original streamer material; the bright,
filamentary structures often seen in the coronal mass ejection
event are, in fact, the remnants of the prominence.
On 6 January 1997, the coronagraph on SOHO observed a "halo" event erupting from the Sun. A coronagraph
is a telescope with a built-in eclipse; the bright Sun is blocked
by a disk so you can view the faint corona outside the Sun. The
halo event looks like a circle around the Sun, because it is moving
along the direction you are looking. Therefore, if it is coming
right at the Earth, and since you are looking straight at it,
it looks like a big, expanding circle coming at you.
The LASCO coronagraph team announced that this halo was a CME, a cloud
of plasma, heading towards Earth, and that it was moving at slightly more
than a million miles per hour, or 1.6 million kilometers per hour.
It crossed the orbit of Mercury in less than a day; by Wednesday
it had passed Venus: an expanding cloud of plasma over 30 million
miles deep, spanning the space between the orbits of Mercury and
Venus. Short of moving the earth out of the way, there was nothing
we could do.The interplanetary coronal storm cloud reached earth
on the 10th of January, slamming into the Earth's magnetosphere. Because the CME had such a large magnetic field, it was able
to literally compress the Earth's magnetic field, causing various
geomagnetic disturbances.
There are more than 200 billion dollars in satellites orbiting
the Earth. These satellites monitor radio, navigations and communications
transmissions here on Earth, so our activities are no longer confined
to the Earth’s surface. One such satellite found high above the
United States, AT&T's Telstar 401 satellite, was busy relaying
television programming between many destinations across the continent.
Public Broadcasting Stations, ABC News and even the Home Shopping
Channel were among its regular paid subscribers for the precious
few channels that the satellite could re-broadcast back to our
home television sets and to cable channel owners on the ground.
Telstar 401 was launched from Cape Canaveral on 13 December 1993
and was the first of a fleet of modern communications satellites
developed by Lockheed-Martin, and equipped with many new technologies.
But on Saturday 11 January 1997 AT&T announced that it was having
some communications difficulties with the satellite.
Despite a number of attempts at diagnosing and repairing the problem
with Telstar 401, on Saturday, 17 January 1997 AT&T gave up the effort and announced
that they had lost the satellite. A $200 million satellite had
been short circuited. A piece of the Sun had momentarily reached
out and touched the Earth, rendering the satellite useless.
The repercussions of this loss were significant. The many paying
subscribers of the satellite’s transponder channels had to quickly
switch to other satellites where space was available to carry
their programming. Some subscribers did not have contingencies
written into their service contracts with AT&T, and were left
in complete blackout. Many newspapers stories were filled with
this event and about space events that can, and will affect us.
Interestingly enough, after everything calmed down, the Sun did
it again. The same region came around one month later (the Sun
fully rotates every 28 days) and let out another blast which had
nearly the same effect. That was on 7 February 1997. 7 March 1997
passed by without incident....
Procedure:
Step 1. Use the glossary to familiarize students with some of the terminology used in
this lesson.
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The sunspot numbers are the yearly mean numbers. CME numbers are
from data archived at the High Altitude Observatory in Boulder,
CO. The HAO is dedicated to the study of the Sun and of the response
of the Earth’s atmosphere to the Sun’s output. Ask students: Is this chart the best way to display the data?
What might give a more accurate display of actual sunspot and
CME events? Yearly mean archival sunspot data can be found at: http://www.ips.gov.au/ Daily archival CME data can be found at: http://www.hao.ucar.edu/public/research
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Investigation Discussion and Questions
Scientists have just begun to keep archival data of coronal mass
ejection events. CMEs were discovered in 1975, but the latest
records only go back to 1980. There is a positive correlation
between coronal mass ejections and the solar cycle, based on observations
that CME activity changes with the solar cycle, increasing at
solar maximum and decreasing at solar minimum.1 . Near sunspot minimum CMEs occur about once a week. At sunspot
maximum there are typically about 2 per day. Dave Webb and Russ
Howard have a fairly recent paper on this in the Journal of Geophysical
Research (Space Physics) vol. 99, pages 4201-4220 (1994). A more
accessible source might be Gosling's article on the solar wind
in the 1996 Annual Review of Astronomy and Astrophysics.2
However, additional research and data collection must take place
for a more definitive pattern of association between the two occurrences
can be shown.
1 Dr. Terry Kucera, NASA/Goddard Space Flight Center
2 Dr. David Hathaway, NASA/Marshall Space Flight Center
Questions
1. Can you determine the year(s) of solar minimum and the year(s)
of solar maximum from the data?
2. Does a pattern exists between the the solar minimum and solar
maximum date (s) and the number of coronal mass ejections? Students’
responses must be supported by examples cited from the data chart.
3. How could we display the data so that a pattern becomes more
visible? Students may choose to do a bar graph or line graph.
Line graphs show trends from which students can extrapolate data
and make predictions based on that data. Bar graphs show quantity,
in this case, the number of sunspots and CMEs in a given year
from 1980 to 1998. The second set of bars are placed next to the
first set. Be sure to tell students how wide you want each bar
to be. This is important for spacing the years equidistant from
each other on the horizontal axis and so that all bars are the
same width. Either graph is appropriate for the data.
Credits:
Linda McClelland