Two Models

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.

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.

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.

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