Berkeley – NASA has awarded the University of California, Berkeley, a
$173 million contract to build and operate a fleet of five satellites
to pinpoint the event in Earth’s magnetic neighborhood that triggers
violent but colorful eruptions in the Northern and Southern lights.

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 3-4 hours. Despite
numerous spacecraft in orbit studying the Earth’s magnetosphere and
space weather, 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 – 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,” said Vassilis
Angelopoulos, a research physicist at UC Berkeley’s Space Sciences
Laboratory who will lead the project. Called THEMIS, for the Time
History of Events and Macroscale Interactions during Substorms, the
mission is part of NASA’s Medium-class Explorer (MIDEX) program. The
contract was announced March 20.

“Substorm processes are fundamental to our understanding of space
weather and how it affects satellites and humans in space, as well as
our understanding of fundamental physical processes of plasma
systems,” he said. “Understanding substorms is key to predicting
space weather, but these processes occur whenever a magnetized wind
passes by a magnetized body, such as Mercury or Jupiter.”

Angelopoulos hopes the satellite fleet, to be launched in the summer
of 2006, will quickly confirm one of two 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 said. “We need to
resolve it to make progress.”

The 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 said. Team members include
scientists from Germany, Austria, France, Canada, Netherlands, Japan
and Russia, as well as investigators from UCLA, the University of
Colorado in Boulder, Johns Hopkins University and NASA Goddard Space
Flight Center.

THEMIS also is the name of the Greek goddess of impartial justice,
often depicted in courthouses as “blind justice.” The name is a
reminder that the mission will judge impartially the two competing
theories, Angelopoulos said.

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 said.

Solar wind energy is stored in the long tail and released
unpredictably in bursts of accelerated particles and electron
currents. These bursts of energy, 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 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 tell where exactly the energy gets
released, you need multiple spacecraft aligned along the sun-Earth
line,” Angelopoulos said. “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, like the Iridium satellites, 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 to 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 magnetosphere, 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 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?”
he said. “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 two major hypotheses about the origin of substorms and auroras
pinpoint two distinct regions: the area between 25 and 30 Earth radii
– half the distance to the moon – where magnetic reconnection takes
place; and a closer region, at 10 Earth radii, where current
disruption takes place. The region of magnetic reconnection is where
anti-parallel magnetic field lines break, loop back and reconnect,
snapping like rubber bands and flinging out particles in the process.
Current disruption refers to a sudden arcing from an ion current that
normally runs across the magnetotail. The arcing current abruptly
shoots off along the magnetic field lines toward the poles.

The probes bracket these areas and will be able to tell whether the
burst of energy starts from the region of magnetic reconnection or
from the region of current disruption.

Aside from the five probes – two in identical one-day orbits, a third
in a two-day orbit, a fourth in a four-day orbit, and one spare – 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 correlated with events in
the magnetosphere.

The probes, weighing about 100 kilograms (220 pounds) when loaded
with fuel, each will carry five instruments: a low-frequency magnetic
field detector built by German and Austrian collaborators; a
high-frequency magnetic field detector built in France; and
instruments built at UC Berkeley to measure thermal and super-thermal
ions and electrons, and the electric field.

The five probes will be controlled from UC Berkeley’s Space Sciences
Laboratory, with data downloaded via a radio dish in the Berkeley
hills currently used by the RHESSI satellite. Remote data from the
northern ground stations will be relayed to the lab by a Canadian
satellite service, VSAT.

The spacecraft will be built by Swales Aerospace, Inc., of
Beltsville, Md., a firm with recent experience building NASA’s FUSE
and EO-1 spacecraft. Swales engineers worked closely with Space
Sciences Laboratory engineers and scientists to define a robust and
cost-effective plan to build five satellites in a short period time.

“What excites our SSL and Swales engineers about this is that NASA
has entrusted them with the first scientific Constellation. It is a
big honor, and we are ready to live up to the task,” Angelopoulos
said. NASA’s Constellation missions, planned for the middle of the
next decade, involve swarms of some 30 small spacecraft scattered
around the Earth to answer many outstanding questions – including the
origin of auroral substorms – regarding the magnetosphere and the
aurora.

THEMIS, the fourth mission funded by NASA’s MIDEX program, was chosen
from among 31 MIDEX proposals originally submitted to NASA in October
2001. Five were selected in April 2002 for detailed feasibility
studies, with THEMIS the first to be given the final go-ahead.

“The Explorer program allows the science community to identify the
most compelling science questions and then design the most effective
mission to answer those questions,” said Edward Weiler, associate
administrator for space science at NASA headquarters in Washington,
D.C.
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NOTE: Vassilis Angelopoulos can be reached at (510) 643-1871 or
vassilis@ssl.berkeley.edu.

Information and artist’s concepts of the THEMIS mission are available
on the Internet at http://sprg.ssl.berkeley.edu/themis/.