THEMIS is an acronym for Time History of Events and Macroscale Interactions during Substorms, and is also named after the Greek goddess of justice.
The Earth’s magnetosphere is a region of ions and electrons surrounding the Earth that exists due to flares and explosions from the Sun (what is called coronal mass ejections). Charged particles are sent from the solar wind, many of which are eventually trapped by the Earth’s magnetic field, which originates in the Earth’s interior. When these particles strike the Earth’s upper atmosphere they release energy (called geomagnetic substorms) that stimulates activities within the northern hemisphere’s aurora borealis and the southern hemisphere’s aurora australis.
The shape of the magnetosphere is not constant—and not really shaped like a sphere, more like a bullet—but varies due to changes in the Earth’s magnetic field, solar wind, and interplanetary magnetic field. Facing the Sun, the boundary of the magnetosphere (the magnetopause) extends roughly out to about 43,500 miles (70,000 kilometers). Alongside the Earth, the distance of the magnetosphere out into space in all directions is about 59,000 miles (95,000 kilometers). On the back side of the Earth away from the Sun (what is called the magnetotail) the magnetosphere extends sometimes over 99,000 miles (160,000 kilometers).
The two-year mission of THEMIS, built by scientists at the University of California, Berkeley (UCB) and Swales Aerospace, is to determine what causes geomagnetic substorms around the Earth. Geomagnetic substorms are periods of violent and rapid activity within the Earth’s magnetic field. They cause auroras to become brighter, release radiation that is dangerous to spacecraft (such as communication satellites) and orbiting astronauts, and may be responsible for failures in Earth-based power grids.
Scientists do not really know what causes substorms. Two theories of thought are prevalent. One side believes the ‘current disruption theory’ in which the solar wind compresses a sheet of accelerating charged particles (the magnetic field lines around the Earth) in the magnetotail about 37,700 miles (60,700 kilometers) away from the Earth, which causes large electrical currents in the magnetosphere. The other side, which supports the ‘magnetic reconnection theory’, thinks that accelerating charged particles between the Sun and the Earth, at about 94,200 miles (151,700 kilometers) from the Earth, compress and expand in sudden events called magnetic reconnections, which then causes charged particles to accelerate towards the Earth.
To find out the real cause, NASA is scheduled to launch all five satellites (the largest number by NASA within one single rocket) aboard a Delta II rocket on February 15, 2007, at 6:08 EST, with a launch window between 6:07 and 6:26. Given a successful launch from the Kennedy Space Center (Cape Canaveral Air Force Station Launch Complex 17), two of the satellites will be placed in orbit about 37,700 miles (60,700 kilometers) from the Earth. A third satellite will be placed in orbit about 75,400 miles (121,400 kilometers) away—about one-third of the way to the Moon—and a fourth satellite will be placed in orbit at 113,200 miles (182,100 kilometers). The fifth satellite will be saved as an emergency replacement in case any of the other four satellites should fail.
After about ten months of coasting in space, the four satellites will be in coordinated positions—once every four days—to monitor the magnetosphere. It is impossible for only one satellite to track a substorm because it is too fast for a single measurement. While these satellites monitor the magnetosphere from their orbits, personnel located at twenty ground stations in North America (starting in Alaska and spread across Canada) will monitor auroras. Together, the mission teams hopes to learn more about how space weather occurs, so that scientists on Earth will have a better chance to understand and predict Earth weather.
THEMIS principal investigator Vassilis Angelopoulos, a research physicist at UCB, says, "THEMIS is so important because the same fundamental physical process is seen around all planets, it happens on the sun in solar flares, and in astrophysical systems such as black holes. It's amazing that being so close to us, here at Earth, it is not understood yet."
The NASA Web page for THEMIS is: http://www.nasa.gov/mission_pages/themis/main/
A simulated launch of THEMIS is found at: http://wm.nasa-global.edgestreams.net/wm.nasa-global/themis/themisL&D.wmv
The Space Sciences Laboratory (University of California at Berkeley) Web page for THEMIS is: http://ds9.ssl.berkeley.edu/themis/flash.html.