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NASA - Two Models: THEMIS Decides Which One is Right
April 20, 2006

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.

The diagram shows the locations where the various events occur that relate to substorm onset, relative to Earth's surface and to the magnetotail.

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:

Sequence MR:
Current Disruption Model
Magnetic Reconnection Model

0 seconds

Current Disruption 0 seconds Magnetic Reconnection
30 seconds Auroral Disruption 90 seconds Current Disruption
60 seconds Magnetic Reconnection 120 seconds Auroral Disruption

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. Below is a quick timeline of when scientists verified key concepts related to substorms and the Sun-Earth Connection.

Sun-Earth Connection Timeline

1716 Sir Edmund Halley Aurora is aligned with Earth's magnetic field...
1741 Anders Celcius and has magnetic disturbances.
1790 Henry Cavendish It's light is produced at 100-130km altitude...
1859 Richard Carrington but is related to Solar eruptions.
1866 Anders Angström Auroral eruptions are self-luminous and…
1896 Kristian Birkeland are due to electric currents from space:
1907 Carl Störmer in fact to field-aligned electrons…
1932 Chapman & Ferraro accelerated in the magnetosphere by…
1950 Hannes Alfvén the Solar-Wind–Magnetosphere dynamo.
1961 James Dungey Magnetic reconnection is important for plasma circulation.
1965-1975 Single satellites Distinct changes in magnetosphere during substorms...
1976 Iijima & Potemra are communicated to ionosphere by currents.
1977 Akasofu Auroral substorms exist and have distinct phases: growth, onset, breakup, and recovery.
1978-79 Multiple satellites Reconnection model for substorm onset is strengthened.
1985 Multiple satellites Current Disruption model for substorm onset is strengthened.
1995-2002 Satellite fleet Solar wind energy is tracked from the Sun to Earth.
2003 NASA The THEMIS mission is selected to determine the sequence of events in a substorm onset: — is it the Reconnection model or the Current Disruption model?


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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Page Last Updated: September 30th, 2013
Page Editor: NASA Administrator