Plasma, Plasma, Everywhere
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Plasma, Plasma, Everywhere
Sept. 7, 1999: As photographed
from space, the Earth looks like it is floating in a black void.
But, unseen by our eyes and most cameras, the Earth is actually
surrounded by a complex system of interacting electric and magnetic
fields, electric currents and charged particles called the magnetosphere.
A new model of the plasmasphere
surrounding our world
surrounding our world
Right: If a camera didn't compensate for the bright sunlight in space, stars would be seen in the background and the Earth would appear as a bright white orb. Because distant stars aren't as bright as the Earth, a low photo exposure results in a black background.
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).
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"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."
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.
Left: 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.
The plasmasphere extends out to as little as 2 to 3 Earth radii and, under quiet conditions on the evening side, perhaps more than 6 Earth radii. (Because conditions in space constantly vary and regions never have exact boundaries, plasma physicists measure the plasmasphere relative to the size of Earth: 4,000 miles [6,400 km] is about one Earth radius.) The extent of the plasmasphere depends on space weather activity. High levels of activity erode the plasmasphere; long periods of quiet allow the plasmasphere to expand.
Right: Click the image for a 3D simulation of the magnetosphere's shape. The Sun is off screen to the left. The animation begins showing the Earth, which recedes as the shape and size of the magnetosphere comes into view. The solar wind deforms the magnetosphere into its characteristic shape. Where the magnetosphere and the solar wind meet is the "bow shock," represented in the animation by a faint, translucent bullet shape. Credit: Digital Radiance
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"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,"
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.
Plasma Physics - research on plasma at NASA's Marshall Space
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.
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
Right: Artist's concept of the interaction between the magnetosphere and the Sun. The Earth's magnetic field provides a barrier to the solar wind.
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.
Weather Camera Set for Launch in 2000 - Feb. 16, 1999
Imager for Magnetopause-to-Auroral Global Exploration (IMAGE) - satellite facts and objectives.
Space Physics Textbook - University of Oulu Space Research Group.
More Space Science Headlines - NASA research on the web
NASA's Office of Space Sciencepress releases and other news related to NASA and astrophysics
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