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Two Models: THEMIS Decides Which One
is Right
The THEMIS mission will determine the timing of magnetospheric events causing
the sudden change in aurora: a single motionless, green auroral arc changes to
many colorful (purple, red, green, and white) auroral forms dancing across the
sky. We call this sudden change in aurora, "auroral eruption." This
auroral eruption and its associated magnetospheric events are known together
as "substorm onset."
We know what processes are occurring in the magnetotail when we observe auroral
eruptions, but they occur so fast, and in such a "thin" plane, that
no one has conclusively determined the sequence of events. There are three events
associated with substorm onset: 1) current disruption, 2) auroral eruption, and
3) magnetic reconnection.
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The diagram above shows the locations where the various events occur that relate
to substorm onset, relative to Earth's surface and to the magnetotail.
(See the Sun-Earth
Connection page to learn about the magnetotail.) Earth is
the circle with its white side facing the Sun, and its gray side facing away
from the Sun. The lines represent magnetic field lines in the magnetotail.
Also shown are the locations of the Ground Based Observatories (GBOs) and the
five THEMIS satellites (represented as colored dots), at the time when they
will be appropriately lined up to determine the sequence of events.
Space scientists generally believe that these events occur in one of two sequences
shown in the table below.
Sequence CD:
Current Disruption
Model |
Or |
Sequence MR:
Magnetic Reconnection
Model |
| Time |
Event |
Time |
Event |
0 seconds
|
Current Disruption |
0 seconds |
Magnetic Reconnection |
| 30 seconds |
Auroral Eruption |
90 seconds |
Current Disruption |
| 60 seconds |
Magnetic Reconnection |
120 seconds |
Auroral Eruption |
How will THEMIS determine the answer to
the question of which sequence--or model--is correct? First,
20 All-Sky cameras will be placed
in Canada and in Alaska, along with magnetometers. Ten magnetometers
will be placed in the Northern United States. All of these
instruments will be measuring signatures of the auroral eruption.
Then, five satellites will be placed in a special orbit so that every four days
they will line up in the magnetotail, as shown in the figure above. Each satellite
will be measuring particles and fields at the same time, so scientists can analyze
the data to discover the time history of these events and the resulting substorm
that occurs. Or, in the words of the Principal Investigator, Dr. Vassilis Angelopoulos,
we will discover the Time History of Events and Macroscale Interaction during
Substorms (THEMIS).
THEMIS is named after the Themis, the Goddess
of Justice, because the mission goal is to objectively and justly
determine which of the two models are correct. See the Themis the
Goddess web page
for more information of Themis.
Modern History of Substorms and the Sun-Earth Connection
Understanding substorms and the Sun-Earth Connection
the way we do today has taken about 300 years.
Here is
a quick timeline of when scientists verified key concepts related
to substorms and the Sun-Earth Connection.
References
The Aurora Watcher's handbook by Neil Davis, University
of Alaska Press, 1994
Auroral Physics, Edited by C. –I. Meng, M. J. Rycroft
and L. A. Frank, Cambridge University Press, 1991
Introduction to Space Physics, Edited by M. G. Kivelson and C. T.
Russell, Cambridge University Press, 1997
Majestic Lights: The Aurora in Science, History and the Arts by Robert
H. Eather, American Geophysical Union, 1980
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