The Moon and the Magnetotai

http://science.nasa.gov/headlines/y2008/17apr_magnetotail.htm

The Moon and the Magnetotail
NASA Science News
04.17.2008

April 17, 2008: Behold the full Moon. Ancient craters and frozen lava
seas lie motionless under an airless sky of profound quiet. It's a
slow-motion world where even a human footprint may last millions of
years. Nothing ever seems to happen there.

Right?

Wrong. NASA-supported scientists have realized that something does
happen every month when the Moon gets a lashing from Earth's magnetic
tail.

"Earth's magnetotail extends well beyond the orbit of the Moon and,
once
a month, the Moon orbits through it," says Tim Stubbs, a University of
Maryland scientist working at the Goddard Space Flight Center. "This
can
have consequences ranging from lunar 'dust storms' to electrostatic
discharges."

Yes, Earth does have a magnetic tail. It is an extension of the same
familiar magnetic field we experience when using a Boy Scout compass.
Our entire planet is enveloped in a bubble of magnetism, which springs
from a molten dynamo in Earth's core. Out in space, the solar wind
presses against this bubble and stretches it, creating a long
"magnetotail" in the downwind direction: diagram
<images/magnetotail/magnetotail.jpg>.

Anyone can tell when the Moon is inside the magnetotail. Just look:
"If
the Moon is full, it is inside the magnetotail," says Stubbs. "The
Moon
enters the magnetotail three days before it is full and takes about
six
days to cross and exit on the other side."

It is during those six days that strange things can happen.

During the crossing, the Moon comes in
contact with a gigantic "plasma ***" of hot charged particles
trapped
in the tail. The lightest and most mobile of these particles,
electrons,
pepper the Moon's surface and give the Moon a negative charge.

On the Moon's dayside this effect is counteracted to a degree by
sunlight: UV photons knock electrons back off the surface, keeping the
build-up of charge at relatively low levels. But on the nightside, in
the cold lunar dark, electrons accumulate and voltages can climb to
hundreds or thousands of volts.

Walking across the dusty charged-up lunar terrain, astronauts may find
themselves crackling with electricity like a sock pulled out of a hot
dryer. Touching another astronaut, a doorknob, a piece of sensitive
electronics - any of these simple actions could produce an unwelcome
zap.
"Proper grounding is strongly recommended," advises Stubbs.

The ground, meanwhile, may leap into the sky. There is compelling
evidence (see, e.g., the Surveyor 7 image below) that fine particles
of
moondust, when sufficiently charged-up, actually float above the lunar
surface. This could create a temporary nighttime atmosphere of dust
ready to blacken spacesuits, clog machinery, scratch faceplates
(moondust is very abrasive) and generally make life difficult for
astronauts.

Stranger still, moondust might gather itself into a sort of diaphanous
wind. Drawn by differences in global charge accumulation, floating
dust
would naturally fly from the strongly-negative nightside to the
weakly-negative dayside. This "dust storm" effect would be strongest
at
the Moon's terminator, the dividing line between day and night.

Much of this is pure speculation, Stubbs cautions. No one can say for
sure what happens on the Moon when the magnetotail hits, because no
one
has been there at the crucial time. "Apollo astronauts never landed on
a
full Moon and they never experienced the magnetotail."

The best direct evidence comes from NASA's Lunar Prospector
spacecraft,
which orbited the Moon in 1998-99 and monitored many magnetotail
crossings. During some crossings, the spacecraft sensed big changes in
the lunar nightside voltage, jumping "typically from -200 V to -1000
V,"
says Jasper Halekas of UC Berkeley who has been studying the decade-
old
data.

Above: In 1968, on many occasions, NASA's Surveyor 7 moon lander
photographed a strange "horizon glow" after dark. Researchers now
believe the glow is sunlight scattered from electrically-charged
moondust floating just above the lunar surface.

"It is important to note," says Halekas, "that the plasma *** (where
all the electrons come from) is a very dynamic structure. The plasma
*** is in a constant state of motion, flapping up and down all the
time. So as the Moon orbits through the magnetotail, the plasma ***
can sweep across it over and over again. Depending on how dynamic
things
are, we can encounter the plasma *** many times during a single pass
through the magnetotail with encounters lasting anywhere from minutes
to
hours or even days."

"As a result, you can imagine how dynamic the charging environment on
the Moon is. The Moon can be just sitting there in a quiet region of
the
magnetotail and then suddenly all this hot plasma goes sweeping by
causing the nightside potential to spike to a kilovolt. Then it drops
back again just as quickly."

The roller coaster of charge would be at its most dizzying during
solar
and geomagnetic storms. "That is a very dynamic time for the plasma
*** and we need to study what happens then," he says.

What happens then? Next-generation astronauts are going to find out.
NASA is returning to the Moon in the decades ahead and plans to
establish an outpost for long-term lunar exploration. It turns out
they'll be exploring the magnetotail, too.

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