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THEMIS mission overview
Posted: February 12, 2007

Credit: NASA

What does THEMIS mean?
THEMIS is an acronym for Time History of Events and Macroscale Interactions during Substorms. In Greek mythology, Themis is the ancient goddess of justice, wisdom and good counsel and the guardian of oaths. Themis is depicted with a sword and scales, symbolizing both her power and her impartiality. The blindfold she wears depicts "blind justice." This figure is seen in courthouses and law offices.

The THEMIS mission will impartially distinguish between two disparate phenomenological and plasma-physical models of substorm onset in order to solve a tantalizing mystery: Where and when do substorms start in the Earth's magnetosphere?

What is NASA's THEMIS Mission?
THEMIS is NASA's first five identical satellite mission, launched as a constellation, to resolve the tantalizing mystery of what causes the spectacular sudden brightening of the Northern Lights or the aurora borealis - the fiery skies over Earth's northern pole. These lights are the visible manifestations of sudden large energy releases (called substorms) in near-Earth space, out to halfway to the moon. THEMIS will answer the 30-year old question: where and when do substorms start?

What is unique about this mission?
THEMIS will seek to answer the 0-year old question: where and when do substorms start?

THEMIS will impartially decide, like its name for the goddess of justice suggests, between two main competing theories of where and when substorms start in space.

Understanding substorms is a prerequisite to understanding space weather and protecting commercial satellites and humans in space from the adverse effects of particle radiation.

THEMIS is the first five-spacecraft scientific constellation.

What are the mission science goals?
To establish where and when the explosive energy releases that power auroral eruptions called substorms start in Earth's environment.

To determine how the solar wind is affected by its interaction with Earth's bow shock prior to energizing the Earth's magnetosphere.

To determine how the "killer" electrons in the Earth's radiation belts are accelerated.

What are substorms?
Substorms are fundamental modes of explosive energy release in Earth's environment. They are often embedded within large space storms, and can also occur in isolation. Scientists believe some of the most intense space storms - the ones producing the most penetrating radiation - are accompanied by substorms. Understanding substorms is a prerequisite to understanding space weather and protecting commercial satellites and humans in space from the adverse effects of particle radiation.

Are substorms always a factor in space weather?
Most large storms are punctuated by substorms. In some cases repetitive substorms, called sawtooth events, generate intense particle acceleration in Earth's radiation belts. In others, isolated substorms are related with visible auroral streamers rushing towards lower latitudes. Scientists believe substorms may act like storm catalysts, replenishing the radiation belt particles with fresh populations from large distances. The exact relationship of the substorms to the parent storm's severity is still unclear. As an important and visible part of large storms, substorms need to be understood and modeled in order to make progress in understanding and predicting space weather phenomena.

How often to substorms occur?
A substorm is a relatively common and typically benign phenomenon, recurring on average once per four hours, and with frequency that is 50 percent larger during spring than during fall or summer, due to the preferential orientation of Earth's bar-like magnet relative to the magnetic field that emanates from the sun. While substorms take place during both low and high solar activity, and so they are easy to find and study, they are also embedded within large storms - the ones producing intense radiation, damaging satellites and threatening humans in space.

The sun's magnetic field is arranged near the ecliptic plane in a ballerina-like skirt that extends throughout the heliosphere. When Earth goes above or below this skirt, due to crossing one of its folds, it encounters high-speed streams of solar particles that cause recurrent large storms. Those recurrent storms take place typically once per 27 days, the solar rotation period. These are most pronounced in the declining phase of the solar cycle, i.e., as we approach solar minimum.

During solar maximum, the sun's magnetic field near the base of the solar corona, becomes less organized, as sunspots create multiple bar-like magnets near the solar surface. At those times the sun occasionally emits high-speed blobs of strongly magnetized, high-speed particles called coronal mass ejections. If those ejections encounter Earth they may cause severe storms. Damaging radiation produced by interplanetary shocks ahead of the ejections also can cause severe space weather effects.

Both types of storms, the ones recurring during the declining phase of the solar cycle and the random ones at solar max, are accompanied by substorms that may enhance space weather effects. Approximately once per year, a very strong event will be marked by the creation of a new population in the radiation belts, auroras as far south as California and severe effects on the telecommunications and global positioning system (GPS) satellites. The National Oceanic and Atmospheric Administration wants to predict those strong events, both in terms of intensity and when they will occur. Toward that goal, NASA strives to understand what makes them so severe. THEMIS is a stepping stone for reaching that understanding.

How do THEMIS satellites work?
Because a single substorm engulfs the entire near-Earth space within only minutes, the only way to discover their trigger is by coordinated observations with identical instruments distributed over a large region of space.

THEMIS' five identical spacecraft with identical instruments will line up along the sun-Earth line and track the flow of energy from its point of explosive generation in space into the aurora.

After their release from the launch vehicle, the ground controllers will use the spacecraft's own propulsion systems to place them in "resonant" highly elliptical orbits, with periods one, two and four days. Every four days the spacecraft will align at distances ranging from 1/6 to 1/2 the way to the moon. This enables the spacecraft to track the flow of particles and the progression of space currents from one point to another, and identify the elusive substorm point of origin.

What are the two competing theories that THEMIS will address?
Magnetospheric substorms occur in the Earth's magnetic "tail", which extends behind Earth, along its shadow, beyond the moon, deep into interplanetary space. There are two main theories proposed to explain the trigger (onset) of magnetospheric substorms in the magnetotail: The Current Disruption, and the Magnetic Reconnection theory. The first suggests that Current Disruption, which occurs around 50,000 miles (80,000km) in altitude above the equator and is due to electromagnetic turbulence that disrupts the flow of intense space currents, is the substorm trigger mechanism. The second suggests that Magnetic Reconnection, which occurs even further away, at approximately 100,000 miles (160,000km) in altitude above the equator due to spontaneous conversion of magnetic energy into heat and particle acceleration, is responsible for triggering the avalanche of substorm energy. The two competing theories can be distinguished by accurate timing. Timing determines the trigger mechanism and how it sets the entire magnetosphere into motion. Understanding the time history of these events and their macro-scale interactions during substorms is the primary goal of the THEMIS mission.

Explain the two theories in more detail:
The Current Disruption theory states that the cascade of events that constitutes a substorm starts close to Earth, the region where Earth's bar magnet influence starts waning and the solar wind distortion of the magnetosphere starts taking over. That region is a conduit of intense space currents required to flow in the equatorial plane from dawn to dusk. This is possible when the currents are weak, but when the currents get very intense (as is the case under conditions of intense solar wind energy input), that region develops electromagnetic turbulence. The laminar current path, supporting normal space current flow is disrupted. The current finds an easier path, a short circuit, directly through the ionosphere at low altitude. The dissipation of that current causes the aurora to begin brightening. The turbulent heating sets off a local implosion in space, which triggers the substorm. This is the current disruption hypothesis of the substorm onset. This phenomenon happens 1/6 of the way to the moon (roughly 50,000mi or 80,000km above the equator). Two nearby satellites are needed to measure the wave propagation and determine the trigger point of origin and onset time.

The Magnetic Reconnection theory of substorms states that phenomena start further away, where the Earth's magnetic field is stretched into a long magnetotail, which resembles a wind sock. In that environment, Earth's magnetic field lines connected to its two poles are stretched far away, and compressed together like stretched rubber bands. At some point they snap, and re-connect into stretched U-shaped loops, that are then free to contract. The slingshot-like contraction accelerates particles towards Earth and powers the aurora. Two satellites are needed to bracket the re-connection site and determine the precise trigger location and time of onset.

What is the orbit of the satellites and how is it relevant?
The satellites have to be at specific distances from Earth to monitor the two possible trigger locations: two outer satellites must bracket the reconnection site and two inner satellites must be in the current disruption region. Different satellite distances make orbits with different cycles: the farthest probe, halfway to the moon, takes four days to complete an orbit. The second farthest, also used to bracket the reconnection site needs to be one-third of the way to the moon, which puts it in a two-day orbit. The inner probes, about one-sixth of the way to the moon, take only a day to go around Earth. In addition to monitoring the scientifically important regions of space, all satellites simultaneously pass through their highest point in the orbit once per four days. This way they line up every four days and for many hours at a time, along a single line, exactly as necessary to monitor the substorm phenomenon at all four points simultaneously. This unique mission design allows the THEMIS team to obtain its unprecedented measurements of the substorm process in the making.

Do you know which satellite will go where before launch?
THEMIS is unique in that at launch we do not actually know which of the five satellites will go into which orbit. Since all five are identical, we can send any of them into any orbit. One month after launch, engineers and scientists will convene to make the assignment of satellites to orbits based upon how well each is performing.

What are the primary science objectives?
By the winter of 2007-2008, the THEMIS spacecraft will be on the night side of the Earth in positions suitable to address the primary objectives of the mission. Here they will be arrayed at carefully specified locations, where they will pinpoint when and where substorms begin by measuring the abrupt changes in the magnetic field strength and direction, supersonic flows, bursts of charged particles and high-frequency radio waves that occur at substorm onset. The array of ground observatories will map corresponding intensifications of electrical current systems in the ionosphere and generate video displays of the resulting aurora dancing over the Arctic regions of North America. By the end of a second season on the nightside, during the winter of 2008-2009, THEMIS will have observed more than 0 substorms, sufficient to answer once and for all the longstanding mystery concerning the mechanism that drives substorms.

If one of the satellites were to fail could you still accomplish the mission?
The minimum scientific objectives, which call for observing at least 5 substorms in the making, can be accomplished with only four of the five satellites. This can be achieved within 13.5 months from launch, i.e., through the first tail season observations that will take place in January through March of 2008.

How is this mission managed?
THEMIS is a NASA-funded mission managed by the Explorer Program Office at Goddard Space Flight Center in Greenbelt, Md. The principal investigator, Vassilis Angelopoulos, is located at the Space Sciences Laboratory at the University of California at Berkeley (UC Berkeley). UC Berkeley is responsible for overall project management, and for mission design, spacecraft procurement, development of three flight instruments, integration of two non-US flight instruments, instrument data processing unit development, ground based instrument development, mission integration, launch site processing, mission operations and data analysis.

How does the mission use Ground-Based Observatories?
Twenty ground-based observatories (GBOs) equipped with digital cameras and magnetometers have been installed across Alaska and Canada. The cameras view the aurora from the ground up, and compose a mosaic, synoptic view of the dynamic auroral display. This provides the context (location and timing) within which to place THEMIS' spacecraft observations.

The observatories are also equipped with ground magnetometers (super-sensitive compasses) that detect the space currents responsible for the auroral onset and, through modeling, can determine the location and time of substorm onset even during cloudy intervals. The observatories were built at the University of California at Berkeley. Each is comprises an All Sky Imager and a ground magnetometer station from UCLA. They are distributed two per hour of local time sector across the high latitude North American continent. Three are located in Alaska and 7 in Canada.

Why are two NASA missions called THEMIS?
The Thermal Emission Imaging System (THEMIS) is an instrument on board the Mars Odyssey spacecraft. It combines a five-wavelength visual imaging system with a nine-wavelength infrared imaging system. The orbiter launched from Kennedy Space Center on April 7, 2001 and arrived at Mars on October 24, 2001.

The THEMIS mission goes by THEMIS/MIDEX, whereas the instrument is known as the Mars Odyssey/THEMIS instrument. The potential for confusion is minimal.

Since the mission is launching later than planned, how is the science mission impacted?
The mission was originally scheduled to launch on October 9, 2006. Programmatic decisions and difficulties with the second stage of the launch vehicle have delayed the launch four months. The delays offered an opportunity to check and confirm the reliability of both instruments and spacecraft systems, but do not affect the mission's ability to accomplish its scientific objectives.

What are NASA's Strategic Goals for Heliophysics?
Understand Our Home in Space - Discover and understand the response of the Earth and near-Earth space to solar variability

Safeguard the Journey of Exploration - Develop the ability to forecast the space environmental hazards faced by robotic and manned missions as they serve mankind and explore the solar system.

How does THEMIS science fit into NASA's research?
NASA is building a great heliophysics observatory composed of a distributed system of environmental science missions from the sun to the Earth - moon system, to Mars, and spanning the solar system out to the edge of the local interstellar medium. These observatory measurements reveal how the sun and its interactions with the planets drive the space environment throughout the solar system. THEMIS is a major stepping stone towards understanding Earth's space environment and its interaction with the sun. Understanding substorms will allow us to better understand and eventually predict space weather phenomena that may threaten commercial satellites or astronauts on the space station, on their way to the moon, on the moon, and beyond.

How are the THEMIS and STEREO missions related?
STEREO Mission was launched October 25, 2006 and it will allow NASA to track solar eruptions, called coronal mass ejections, from the sun to Earth for the first time.

THEMIS - as part of its primary mission to discover the origins of substorms - will be perfectly positioned to determine the response of the Earth's magnetic field and space environment to these coronal mass ejections. It will explore the flow of energy into the upper atmosphere, where intense auroras occur and also the Earth's radiation belts which affect a variety of national and commercial space systems.

Together STEREO and THEMIS will usher in a new era for NASA's Heliophysics Program and its efforts to understand the Sun - Earth Connection!