Berkeley — Among the first pictures captured by the Magnetopause-to-Aurora
Global Exploration, or IMAGE, satellite, launched a year ago to study the
Earth’s magnetic shield, is the first global view of the double aurora —
the pretty curlicues and shimmering curtains of the electron aurora and
the more diffuse proton aurora.

The colored light display most people associate with the aurora or Northern
Lights is produced by electrons crashing into the atmosphere as they
descend along the Earth’s magnetic field lines. Almost equally bright but
less structured are the lights produced by positively charged protons —
hydrogen nuclei — as they ram the atmosphere.

Though scientists have been able to distinguish the electron and proton
auroras from the ground since the 1950s, IMAGE’s far-ultraviolet imager,
built by a team at the University of California, Berkeley’s Space Sciences
Laboratory, has obtained the first pictures from space showing the entire
proton aurora and its relationship to the electron aurora.

The hour-long series of photos, taken every two minutes, suggest that the
proton aurora appears first and may initiate the more spectacular electron

“These pictures show for the first time that the electron and proton
auroras are different and develop differently over time,” said Stephen
B. Mende, an atmospheric physicist and lead investigator of the far-
ultraviolet instrument team. “We’ve looked at the proton light for some
time from the ground, but never seen the proton aurora from a global
perspective like this. That is very exciting.”

The findings will be published along with nine other papers on IMAGE
results in the March 15 issue of Geophysical Research Letters, a
publication of the American Geophysical Union. Some earlier results from
IMAGE were reported in the Jan. 26, 2001, issue of Science.

The far-ultraviolet imaging team includes research physicists Harald U.
Frey and Michael Lampton of UC Berkeley’s Space Sciences Laboratory;
J.-C. Gerard and B. Hubert of the University of Liege, Belgium; S.
Fuselier of Lockheed-Martin Palo Alto Research Laboratories, Calif.;
J. Spann of NASA Marshall Spaceflight Center; and R. Gladstone and
J. L. Burch of the Southwest Research Institute (SWRI) in San Antonio,

Captured last June 28, the auroras were the result of a rather puny
substorm in the Earth’s magnetosphere, the magnetic field region that
enshrouds the Earth and protects it from the Sun’s periodic particle
storms, Mende said. Nevertheless, IMAGE’s far-ultraviolet instrument
was able to capture the first images of the two distinct auroras, like
lopsided halos around the North Pole and slightly offset from one another.

The images show, Frey said, that the diffuse aurora at lower polar
latitudes come from both protons and electrons, while the very pretty,
structured aurora at higher latitudes near the North Pole is due almost
entirely to the electrons.

Substorms are generated when the Earth’s magnetosphere for some reason
gets charged up with protons and electrons and then discharges, sending
ionized particles spiraling along magnetic field lines to where they
converge at the pole. Along the way, they hit atoms in the atmosphere
and emit light, ranging from colorful visible light to the invisible

Substorms, which may last an hour, are distinct from the day-long storms,
which are generated by coronal mass ejections and large flares on the sun,
that disrupt global communications.

What makes the two types of auroras distinct are the different behaviors
of protons and electrons as they enter the atmosphere. Protons quickly
become neutralized as they combine with electrons, and once this happens
they ignore magnetic field lines and fly in all directions. Electrons,
however, remain free and stick to magnetic field lines.

“Electrons spiral tightly around the magnetic field lines, so even after
making many collisions, at the end they’re not far from the original
field line they were attached to,” Mende said. As a result, the light
from their collisions with atmospheric atoms has a structure dictated
by the field lines, typically shimmering curtains of light.

In the June 28 substorm, the proton aurora started at a lower polar
latitude than the electron aurora but gradually moved northward to sit
atop the electron aurora — two concentric ovals some 2,000 miles in
diameter — until it was outshone by the electron aurora. As the electron
aurora continued to expand toward the pole, the proton aurora remained

“The proton aurora is actually very important at the start of the substorm,
but the electron aurora takes over, at least as far as brightness is
concerned,” Mende said. “By studying the two separately we are beginning
to understand the dynamics of the Northern Lights.”

IMAGE was launched March 25, 2000, carrying five suites of camera systems,
among them the far-ultraviolet imager built by Mende’s team. The instrument
observes the aurora in three far-ultraviolet wavelengths: at very short
wavelengths, where mainly hydrogen emissions are seen; at longer
wavelengths, where oxygen atoms are visible; and at even longer
wavelengths, where nitrogen emits.

As IMAGE swings over the North Pole, it is far enough above the Earth —
about seven Earth radii — to be visible from Berkeley. As a result, an
antenna built for the HESSI (High Energy Solar Spectroscopic Imager)
mission, scheduled for launch in the next couple of months, has been able
to download data from the FUV imager directly to the scientific team at
UC Berkeley.

This has allowed the team to put on the Web every two minutes a new
far-ultraviolet picture of the aurora. Assessment of individual storms
and substorms, however, takes months of computer analysis.

The research was sponsored by the National Aeronautics and Space
Administration through SWRI.


NOTE: Stephen Mende can be reached at (510) 642-0876 or Harald Frey can be reached at (510) 643-3323 or

The latest far-ultraviolet image of the electron aurora, taken every two
minutes by IMAGE’s Wideband Imaging Camera, is available on-line at

Pictures of the proton and electron auroras reported in the paper are
available at

For an mpeg movie of a different proton aurora, go to the URL:

An mpeg of an electron aurora is at the URL: