Scientists have confirmed the existence of and imaged "free spirit" atoms —
atoms not subject to magnetic forces because they are electrically neutral —
in the solar wind for the first time. They hope to use the observations to
better estimate the arrival time of solar storms and to estimate the amount
of dust left over from the solar system’s birth.

The scientific team used the Low Energy Neutral Atom imager (LENA) instrument
on NASA’s Imager for Magnetopause to Aurora Global Exploration (IMAGE)
spacecraft to make the detection and produce the images.

"Our LENA instrument was designed to see neutral atoms coming from our own
planet. It just amazes me that it is also taking us outside the Earth’s
magnetic field to study interplanetary gas and dust, and even outside the
solar system to study the interstellar gas cloud," said Dr. Thomas Moore,
Project Scientist for IMAGE at Goddard Space Flight Center, Greenbelt, Md.
Moore is lead author of a paper describing the neutral atom detection
published in Geophysical Research Letters.

The solar wind is a stream of electrified and magnetic gas that blows
constantly from the Sun at speeds of about 250 miles per second. The gas
is electrified because heat and radiation from the Sun remove electrons
that normally surround the gas atoms and balance their electric charges.
Scientists thought that within this gale of electrified atoms, a few might
manage to obtain a complete complement of electrons and thus be rendered
electrically neutral (the number of surrounding electrons, which are
negatively charged, balances the number of positively charged protons at
the center of the atom). The detection by IMAGE is the first confirmation
of this idea, and the surprising result is that such atoms may be up to
ten times more common than previously thought.

Electrically neutral atoms become the "free spirits" of the solar wind
because they do not feel magnetic forces, and thus can chart their own paths
through the solar system, ignoring the contours of the ubiquitous magnetic
fields in interplanetary space that their electrically charged cousins must
follow. Like free spirits everywhere, neutral atoms are rare; for every
10,000 electrified solar wind atoms, there exist no more than 10 neutral
atoms. Nevertheless, this faint whisper of particles snubs the Earth’s
magnetic field that blocks the electrified solar wind, offering a rare
glimpse of solar wind activity from within our magnetic shield.

Solar scientists have proposed that neutral atoms may help to better estimate
the arrival time of solar storms, which occasionally disrupt high-technology
systems at Earth. Some solar storms are caused by the impact of billion-ton
clouds of electrified gas with the Earth’s magnetic field. Such clouds,
called Coronal Mass Ejections (CME’s), are blasted from near the visible
surface of the Sun at speeds of up to 4.5 million miles per hour.

According to the theory, the CME clouds should carry a higher population
of neutral atoms than the solar wind because they come from near the solar
surface, where it is much cooler than the solar atmosphere, which is the
source of the solar wind. With less heat, fewer atoms will lose their
electrons, and thus more neutral atoms are carried within a CME cloud. As
it races through space, the CME cloud slows down because it plows into the
slower solar wind and its magnetic field. Because they can’t feel magnetic
forces, the CME neutral atoms ignore what the electrically charged majority
is doing and continue to travel at the original speed of the CME.

This leads some solar scientists to propose that there should be a burst of
neutral atoms at Earth shortly before the arrival of an Earth-directed CME
cloud (about 2 – 3 hours before impact, on average). If further observations
with IMAGE show this to be correct, it could greatly benefit space weather
forecasts, because it is difficult to gauge the distance and speed of CME
clouds when they are heading directly towards us.

However, most solar wind neutral atoms are made, not born; that is, they
become neutral by taking electrons from another atom that is electrically
neutral. Like an atomic version of a purse snatching, a fast electrified
atom in the solar wind passes near an existing neutral atom and steals an
electron from it, then races on, blissfully unaware of the magnetic field
that once constrained its motion.

There are three sources of original neutral atoms encountered by the solar
wind as it flows toward Earth: neutrals that are released from the dust
grains left over from the cloud that formed the solar system, neutrals
from other stars (in interstellar gas clouds that the solar system passes
through), and neutrals from the Earth’s upper atmosphere.

The 11-year cycle of violent solar activity will change the amount of
neutrals from Earth, while the amount of interstellar neutrals will vary
as Earth progresses in its orbit around the Sun (from upwind to downwind
in the interstellar gas flow through the solar system). This will cause
the solar wind neutrals detected by IMAGE to vary accordingly. By making
long-term observations and subtracting these variations out, researchers
using IMAGE can estimate the neutral atoms contributed by the solar system
dust (which is constant), and thus get a better estimate on how much dust
remains.

"Estimates on the amount of primordial dust in the inner solar system vary
by a factor of 10,000, so this will really help us get a better picture of
how much dust there is inside of Earth’s orbit," said Dr. Michael Collier,
LENA experiment scientist and co-author of the neutral atom detection paper.

For an image and background information, refer to:
http://www.gsfc.nasa.gov/GSFC/SpaceSci/sunearth/imagena.htm

—
Andrew Yee
ayee@nova.astro.utoronto.ca