http://www.berkeley.edu/news/media/r.../12_mars.shtml

UC Berkeley Press Release

Hundreds of auroras detected on Mars
By Robert Sanders
12 December 2005

BERKELEY - Auroras similar to Earth's Northern Lights appear to be
common on Mars, according to physicists at the University of
California,
Berkeley, who have analyzed six years' worth of data from the Mars
Global Surveyor.

The discovery of hundreds of auroras over the past six years comes as a
surprise, since Mars does not have the global magnetic field that on
Earth is the source of the aurora borealis and the antipodal aurora
australis.

According to the physicists, the auroras on Mars aren't due to a
planet-wide magnetic field, but instead are associated with patches of
strong magnetic field in the crust, primarily in the southern
hemisphere. And they probably aren't as colorful either, the
researchers
say: The energetic electrons that interact with molecules in the
atmosphere to produce the glow probably generate only ultraviolet light
- not the reds, greens and blues of Earth.

"The fact that we see auroras as often as we do is amazing," said UC
Berkeley physicist David A. Brain, the lead author of a paper on the
discovery recently accepted by the journal Geophysical Research
Letters.
"The discovery of auroras on Mars teaches us something about how and
why
they happen elsewhere in the solar system, including on Jupiter,
Saturn,
Uranus and Neptune."

Brain and Jasper S. Halekas, both assistant research physicists at UC
Berkeley's Space Sciences Laboratory, along with their colleagues from
UC Berkeley, the University of Michigan, NASA's Goddard Space Flight
Center and the University of Toulouse in France, also reported their
findings in a poster presented Friday, Dec. 9, at the American
Geophysical Union meeting in San Francisco.

Last year, the European spacecraft Mars Express first detected a flash
of ultraviolet light on the night side of Mars and an international
team
of astronomers identified it as an auroral flash in the June 9, 2005,
issue of Nature. Upon hearing of the discovery, UC Berkeley researchers
turned to data from the Mars Global Surveyor to see if an on-board UC
Berkeley instrument package - a magnetometer-electron reflectometer -
had detected other evidence of auroras. The spacecraft has been
orbiting
Mars since September 1997 and since 1999 has been mapping from an
altitude of 400 kilometers (250 miles) the Martian surface and Mars'
magnetic fields. It sits in a polar orbit that keeps it always at 2
a.m.
when on the night side of the planet.

Within an hour of first delving into the data, Brain and Halekas
discovered evidence of an auroral flash - a peak in the electron energy
spectrum identical to the peaks seen in spectra of Earth's atmosphere
during an aurora. Since then, they have reviewed more than 6 million
recordings by the electron reflectometer and found amid the data some
13,000 signals with an electron peak indicative of an aurora. According
to Brain, this may represent hundreds of nightside auroral events like
the flash seen by the Mars Express.

When the two physicists pinpointed the position of each observation,
the
auroras coincided precisely with the margins of the magnetized areas on
the Martian surface. The same team, led by co-authors Mario H. Acu?a
of
NASA's Goddard Space Flight Center and Robert Lin, UC Berkeley
professor
of physics and director of the Space Sciences Laboratory, has
extensively mapped these surface magnetic fields using the
magnetometer/reflectometer aboard the Mars Global Surveyor. Just as
Earth's auroras occur where the magnetic field lines dive into the
surface at the north and south poles, Mars' auroras occur at the
borders
of magnetized areas where the field lines arc vertically into the
crust.

Of the 13,000 auroral observations so far, the largest seem to coincide
with increased solar wind activity.

"The flash seen by Mars Express seems to be at the bright end of
energies that are possible," Halekas said. "Just as on Earth, space
weather and solar storms tend to make the auroras brighter and
stronger."
Depiction of surface magnetic fields on Mars
These are not boils on Mars, but a way of depicting the surface
magnetic
fields on the planet to emphasize their ability to shield the surface
from the solar wind. The greater the bulge, the stronger and more
protective the magnetic field. Note that most of the remaining magnetic
fields are in the southern hemisphere. (Credit: David Brain/SSL)

Earth's auroras are caused when charged particles from the sun slam
into
the planet's protective magnetic field and, instead of penetrating to
the ground, are diverted along field lines to the pole, where they
funnel down and collide with atoms in the atmosphere to create an oval
of light around each pole. Electrons are a big proportion of the
charged
particles, and auroral activity is associated with a physical process
still not understood that accelerates electrons, producing a telltale
peak in the spectrum of electron energies.

The process on Mars is probably similar, Lin said, in that solar wind
particles are funneled around to the night side of Mars where they
interact with crustal field lines. The ultraviolet light is produced
when the particles hit carbon dioxide molecules.

"The observations suggest some acceleration process occurs like on
Earth," he said. "Something has taken the electrons and given them a
kick."

What that "something" is remains a mystery, though Lin and his UC
Berkeley colleagues lean towards a process called magnetic
reconnection,
where the magnetic field traveling with the solar wind particles breaks
and reconnects with the crustal field. The reconnecting field lines
could be what flings the particles to higher energies.

The surface magnetic fields, Brain said, are produced by highly
magnetized rock that occurs in patches up to 1,000 kilometers wide and
10 kilometers deep. These patches probably retain magnetism left from
when Mars had a global field in a way similar to what occurs when a
needle is stroked with a magnet, inducing magnetization that remains
even after the magnet is withdrawn. When Mars' global field died out
billions of years ago, the solar wind was able to strip the atmosphere
away. Only the strong crustal fields are still around to protect
portions of the surface.

"We call them mini-magnetospheres, because they are strong enough to
stand off the solar wind," Lin said, noting that the fields extend up
to
1,300 kilometers above the surface. Nevertheless, the strongest Martian
magnetic field is 50 times weaker than the field at the Earth's
surface.
It's hard to explain how these fields are able to funnel and accelerate
the solar wind efficiently enough to generate an aurora, he said.

Brain, Halekas, Lin and their colleagues hope to mine the Mars Global
Surveyor data for more information on the auroras and perhaps join with
the European team operating the Mars Express to get complementary data
on the flashes that could solve the mystery of their origin.

"Mars Global Surveyor was designed for a lifetime of 685 days, but it
has been very valuable for more than six years now, and we are still
getting great results," Lin observed.

The work was supported by NASA. Coauthors with Brain, Halekas, Lin and
Acu?a are Laura M. Peticolas, Janet G. Luhmann, David L. Mitchell and
Greg T. Delory of UC Berkeley's Space Sciences Laboratory; Steve W.
Bougher of the University of Michigan; and Henri R?me of the Centre
d'Etude Spatiale des Rayonnements in Toulouse.