The magnetosphere provides a barrier between our planet and particles
continually given off by the Sun's corona called the "solar wind." These
particles constitute a plasma - a mixture of electrons (negatively charged) and ions
(atoms that have lost electrons, resulting in a positive electric charge).
Plasma is not a gas, liquid, or solid - it is the fourth state of matter. Plasma
often behaves like a gas, except that it conducts electricity and is affected by magnetic
fields. On an astronomical scale, plasma is common. The Sun is composed of plasma, fire is
plasma, fluorescent and neon lights contain plasma.
"99.9 percent of the Universe is made up of plasma," says Dr. Dennis
Gallagher, a plasma physicist at NASA's Marshall Space Flight Center. "Very little
material in space is made of rock like the Earth."
The plasma
of the magnetosphere has many different levels of temperature and concentration. The
coldest magnetospheric plasma is most often found in the plasmasphere, a donut-shaped
region surrounding the Earth's middle. But plasma from the plasmasphere can be detected
throughout the magnetosphere because it gets blown around by electric and magnetic forces.
Artist's concept of the magnetosphere. The rounded, bullet-like shape represents
the bow shock as the magnetosphere confronts solar winds. The area represented in gray,
between the magnetosphere and the bow shock, is called the magnetopause. The Earth's
magnetosphere extends about 10 Earth radii toward the Sun and perhaps similar distances
outward on the flanks The magnetotail is thought to extend as far as 1,000 Earth radii
away from the Sun.
Gallagher has developed a general model to describe the density of the plasma
surrounding the Earth. His paper, "Global Core Plasma Model," will be published
in the Journal of Geophysical Research. "Core plasma" refers to the low-energy
plasma (zero to 100 electron volts) that makes up the plasmasphere.
Rockets, satellites and the space shuttle have flown in parts of the core plasma
neighborhood. By taking various measurements of this region, scientists have gradually
come to understand the basic nature of the entire plasmasphere.
"We've been flying in plasma for over 40 years and have slowly gained a
statistical picture of what things are like, such as the density and proportion of oxygen,
hydrogen, and helium," says Gallagher.
But our understanding of the plasmasphere is not complete. For one thing, all
the various measurements have resulted in many independent models of specific plasma
regions. By combining previous work, Gallagher's model attempts to describe,
mathematically, a general, complete image of the plasmasphere.
Left:
Animation of the Earth's plasmasphere as it would appear in extreme ultraviolet light
(30.4 nm wavelength). This simulates the view from the IMAGE satellite due to launch in
February 2000. To watch a QuickTime movie of this animation, click here
(6.5MB file).
"This model begins to paint a picture, but it's something of a
Frankenstein's monster," says Gallagher, referring to how his model is pieced
together from several different, dissimilar models. "A significant issue is how you
smooth the stitches."
Gallagher's model combines the International Reference Ionosphere (IRI) model
for low altitudes with higher altitude models. The part of our atmosphere that contains
plasma - the ionosphere - is generally 90 to 1,000 km (54-620 mi.) above the ground.
The shorter wavelengths of sunlight, ranging from the ultraviolet to X-rays,
ionize the Earth's upper atmosphere by tearing electrons off atoms. The ions and electrons
do not readily recombine in the ionosphere because particle collisions are infrequent in
the rarified atmosphere. Ionospheric densities range from a peak of about 1 million
particles/cm3 down to many thousands of particles/cm3. The densities continue to fall as
you move to higher altitudes.
From the equator to the middle latitudes of Earth, the ionosphere joins smoothly
with the plasmasphere. Beyond the outer boundary of the plasmasphere, the densities of
plasma in the magnetosphere can fall as low as 0.01 particles/cm3.
"The
plasma environment around the Earth is a natural extension of Earth's atmosphere, ionized
by the Sun," says Gallagher. "Any planet that has an atmosphere is going to have
energy from the Sun imparted to the atoms. The consequences are that lighter elements
escape. But Earth's magnetic field traps much of this escaping gas. A planet like Mars
that has, at best, a weak magnetic field, also has a very thin atmosphere. Some
researchers have speculated that the Earth's magnetic field may play a role in slowing the
loss of our atmosphere into space."
Our atmosphere provides pressure, proper temperature, and oxygen - fundamental
requirements for life on Earth. Without the atmosphere, one side of our planet would
freeze while the other would broil under intense solar radiation.
Gallagher's model may contribute to our understanding of how the Earth's plasma
affects our quality of life. Radio waves and power lines are affected by the presence of
plasma, as are satellites and the Space Shuttle. Plasma can cause an electric charge to
accumulate on one part of a spacecraft but not another, sometimes resulting in an electric
arc, or discharge. These electric arcs can disrupt or destroy sensitive electronic
components.
Gallagher will be able to refine his model with data from the IMAGE satellite,
due to launch in February 2000. IMAGE will give us a better picture of the Earth's
magnetosphere, and because plasma is bound to magnetic fields, IMAGE should also improve
our understanding of how the plasmasphere and the magnetosphere interact.
Related Links:
Space Plasma Physics
- research on plasma at NASA's Marshall Space Flight Center.
Earth's Solar
Environment - International Space Physics Educational Consortium.
Exploration
of the Earth's Magnetosphere - overview of NASA research on the Earth's environment in
space.
