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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.
Story of THEMIS
This colorful patch tells the story of the THEMIS mission. Let’s take a look at the mission
from a new angle -- a prose-poem!
When You Look At This Patch, What Do You See?
Look at the five yellow dots--
Are they bright, shining stars?
Or five golden spiders hovering in the night?
No, they are five different THEMIS satellites busily collecting information in space.
Look at the blue Earth --
Is that a wreath laid upon it?
Or a glowing green halo?
No, it is the Aurora, known as the Auroral Oval, as it is seen from space.
Look at the big purple violet arcs --
Are they encircling our Earth?
No, the purple rings represent Earth's invisible magnetic field lines, dancing about.
But look at the two smaller purple rings --
Do they represent the invisible magnetic field lines of the moon?
No, the smaller purple rings are also Earth's magnetic field lines. They are dancing in opposite directions and far away from Earth.
And what about that hourglass–shaped space between the two sets of purple arcs?
That's the mysterious “reconnection zone” between the Earth's dancing magnetic fields.
Look at the black background,
Is that the darkest shadow you’ve ever seen?
Or the deepest blackness of space?
Partly correct! This is where the industrious THEMIS satellites will be gathering information late into the night.
Now, look at the big, white letters that read “THEMIS” --
Have you ever seen this name?
Does this refer to the Greek Goddess of Justice, Themis?
That’s one correct answer! Just like Themis, the Goddess of Justice, Wisdom and Good Counsel, the THEMIS mission will seek the truth of the origins of the colorful dancing auroras.
But the THEMIS letters also stand for: Time History of Events and Macroscale Interaction during Substorms (THEMIS). The name comes from the way the five satellites line up to capture the timing of the frenzied dancing lights of auroras as they form.
Look at the flags on the bottom,
Are they the flags of countries with the most medals in the Olympics?
No. They are the flags of countries that are working with this mission.
Can you tell the names of the countries?
From left to right: United States, Austria, Canada, France and Germany.
Look at the letters of NASA --
Do they stand for Nightly Auroras Seem Awesome?
Good idea, but no, it is the National Aeronautics and Space Administration, the funding agency of THEMIS Mission.
The scientific description of the THEMIS Patch:
The THEMIS (Time History of Events and Macroscale Interactions During Substorms) patch represents a satellite mission comprised of five identical micro-satellites. Each is charged with the job of using auroral activity to deduce the causes of the global reconfigurations of the Earth's magnetosphere. THEMIS’ five small satellites, carrying identical suites of electric, magnetic, and particle detectors, will be put in carefully coordinated orbits. The satellite data will be combined with ground-based observations of the aurora from a network of observatories located across the Arctic Circle.
THEMIS science will answer these intriguing Sun-Earth Connection Science questions: How does Earth respond to solar variations? And, how does solar variability affect society? The THEMIS mission is essential for understanding Earth’s space environment and is a key prerequisite to understanding space weather.
For additional information on THEMIS science, please go the About page.
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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