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Spaces Sciences Lab to build satellite probes
THEMIS project, funded by NASA, will confirm origins of auroral substorms

| 02 April 2003


A substorm in the magnetosphere creates a flashing oval of auroral light around the north pole. The image, which has been superimposed on an outline of the Northern hemisphere, was captured in 2001 by the far-ultraviolet imager built at Berkeley's Space Sciences Laboratory and flown aboard NASA's IMAGE satellite.
Image courtesy Harald Frey, UC Berkeley

NASA has awarded the campus Spaces Sciences Laboratory (SSL) a $173-million contract to build and operate a fleet of five satellites to pinpoint the cause of violent but colorful eruptions in the Northern and Southern lights. The mission, called THEMIS, is part of NASA’s Medium-Class Explorer (MIDEX) program.

The aurora borealis and aurora australis are shimmering light shows that brighten the polar nights, generated by showers of electrons descending along magnetic-field lines onto the poles. These high-speed electrons spark colored lights as they hit the atmosphere, much like a color TV lights up when an electron beam hits the phosphorescent screen.

Though geomagnetic storms generated by outbursts of solar wind often create spectacular light shows, a totally distinct process creates substorms that erupt and wane over periods of three to four hours. Though numerous spacecraft orbit the Earth, studying space weather and the planet’s magnetosphere, they’ve never been able to pinpoint where in the magnetosphere the energy of the solar wind transforms explosively into the electron currents that produce auroral eruptions.

“This question hasn’t been resolved so far because the process is so quick,” says Vassilis Angelopoulos, an SSL research physicist who will lead the THEMIS project. (The acronym stands for Time History of Events and Macroscale Interactions during Substorms.) “The energization starts in space tens to hundreds of thousands of miles away, and 30 seconds later we see the particles coming in the aurora. The whole night-side polar sky lights up within minutes in a global energization called a substorm.”

Substorm processes are fundamental to understanding space weather and how it affects satellites and humans in space, as well as to understanding fundamental physical processes of plasma systems.

‘Mutually exclusive’ hypotheses
Angelopoulos hopes the satellite fleet, to be launched in the summer of 2006, will quickly confirm one of two competing hypotheses about the location of the event causing auroral eruptions.

“We now have two distinct, mutually exclusive hypotheses proposed to explain how and where substorms start, which has resulted in the field diverging in two different directions,” he says. “We need to resolve this fundamental question to make progress.” The name THEMIS refers to the Greek goddess of impartial justice (often depicted in courthouses as “blind justice”) — a deliberate reminder that the mission will judge the two competing theories impartially.

The THEMIS project is a large collaboration involving researchers from NASA, four U.S. universities, and seven foreign nations, some of whom have been studying the substorm problem for more than 30 years. “This is probably the oldest and most important problem to solve in the field, and that is why a very large international magnetospheric community is behind it,” Angelopoulos says.

‘Ducks in a row’
The sun is the source of Earth’s magnetic weather, with coronal mass ejections and other violent explosions propelling pulses of ionized plasma — ionized atoms and electrons intertwined with electric and magnetic fields — through the solar system. When these pulses hit the Earth’s magnetic field, they distort the field and create a downstream tail like a windsock, Angelopoulos says.

Solar wind energy is stored in the long tail and released unpredictably in bursts of accelerated particles and electron currents. These bursts of ener-gy, occurring somewhere along the equatorial plane of Earth’s night side, propagate along magnetic field lines to the two poles, generating simultaneous auroras.

Apart from the beautiful light show, substorms also excite a large portion of the Earth’s ionosphere, interfering with radio signals bouncing off this layer and with radio signals between Earth and orbiting satellites.
“To see this rapid event, and to determine where exactly the energy gets released, you need multiple spacecraft aligned along the sun-Earth line,” explains Angelopoulos. “So, we will align five little probes along the sun-Earth line back on the night side, and they will track that motion of plasma from one probe to the other and tell us where it starts and how it evolves. We’ll have our ducks in a row, so to speak.”

The probes, to be launched from Cape Canaveral aboard a single Delta II rocket, will be inserted into equatorial orbits that bring them in alignment every four days for about 15 hours. Over the mission’s two-year lifetime, Angelopoulos expects, the probes will be perfectly aligned with the Northern hemisphere often enough to catch some 30 substorms, though the first few should answer the team’s major scientific question.

Many other questions remain, however. The solar wind constantly presses on the Earth’s magneto-sphere, creating magnetospheric storms on both the day and night sides. For unknown reasons, storms with embedded substorms create more space weather phenomena and more auroral activity.

“Essentially,” says Angelopoulous, “the aurora is a picture of what happens out in space, and the big question is: Why does it erupt abruptly? Why doesn’t it go on at some low level all the time, instead of waxing and waning? Substorms appear to be critical—they represent some fundamental means by which the solar wind energy gets processed. Our goal is to understand the physics, onset, and evolution of substorms, which is tantamount to understanding why auroras erupt.”

The five probes will be controlled from the SSL, with data downloaded via a radio dish in the Berkeley hills currently used by the RHESSI satellite. In addition, the project will equip 20 ground stations throughout northern Canada and Alaska with automated, all-sky cameras to provide a composite image of the aurora. This way, the aurora can be visually correlated with events in the magnetosphere.