By David F. Salisbury
Published: December 12, 2008
E arth's magnetosphere — the invisible bubble of magnetic fields and electrically charged particles that surrounds and protects the planet from the periodically lethal radiation of the solar wind — is divided into six parts.
New insights into the organization and dynamics of this complex structure, which includes a newly identified region called the warm plasma cloak, are described in a paper published last month in the space physics section of the Journal of Geophysical Research. The underlying analysis was performed by a team of scientists headed by Rick Chappell, research professor of physics and director of the Dyer Observatory at Vanderbilt University.
Although the northern and southern polar lights — aurora borealis and aurora australis — are the only parts of the magnetosphere that are visible to the human eye, it is an integral part of Earth's space environment.
"Although it is invisible, the magnetosphere has an impact on our everyday lives,” Chappell says. "For example, solar storms agitate the magnetosphere in ways that can induce power surges in the electrical grid trigger blackouts, interfere with radio transmissions and mess up GPS signals. Charged particles in the magnetosphere can also damage the electronics in satellites and affect the weather by impinging on the upper layers of the atmosphere. These are some practical reasons why it is important to understand the magnetosphere's structure and behavior.”
Despite its importance, the magnetosphere wasn't discovered until 1958 during the International Geophysical Year when the Explorer 1 satellite detected its existence. Before this, scientists knew that magnetic fields and electric currents existed in space but didn't know where they were and why they were there.
Today, scientists have measured the extent of the magnetosphere, and it is huge: five to six Earth diameters on the side facing the Sun, 10 to 12 diameters around and with a tail that streams more than a million miles away from the Sun. This dynamic magnetic structure shields Earth's surface from the solar wind, the stream of charged particles that continuously boils off the Sun's surface. As the strength of the solar wind varies, the magnetosphere expands and contracts. The different regions in the magnetosphere are distinguished by the energy and behavior of the charged particles that they contain. The ions' energy level is measured in electron volts (eV). The typical ion floating around in the air at sea level has an energy level of about one-fortieth of an eV. The energy of ions in the magnetosphere range from a few eV to millions of eV.
Five of the six regions of the magnetosphere have been known for some time. Chappell and his colleagues pieced together a natural cycle that can accelerate the low-energy ions that originate from the ionosphere, a part of the upper atmosphere that is ionized by solar radiation, up to the higher energies characteristic of those that populate different regions in the magnetosphere. This brought the existence of the new region, the warm plasma cloak, into focus.
The warm plasma cloak is a tenuous region that starts on the night side of the planet and wraps around the dayside but then gradually fades away on the afternoon side. As a result, it only reaches about three-quarters of the way around the planet. It consists of warm ions with energies in the 10 eV to 3 thousand electron volts (keV) range.
The other five regions of the magnetosphere are:
The ionosphere, which is also one of the upper levels of the atmosphere. This is a region where ultraviolet light from the sun breaks apart air molecules to form ions of hydrogen, helium, oxygen and nitrogen.
The plasmasphere is just above the ionosphere. It is formed from hydrogen and helium ions that diffuse upward from the ionosphere. It gets its name from the word "plasma” which is the name for gas that contains ions and electrons. The plasmasphere circles the globe, extends out to a distance of about two Earth diameters and moves in synch with the planet's rotation. It contains "cool” or low energy ions.
The Van Allen radiation belts are embedded in the plasmasphere. The inner belt consists of extremely hot, high-energy protons in the one to 10 million electron volt (MeV) range. The outer belt contains ions with somewhat lower energy levels and extends beyond the plasmasphere, where it blends into another distinct region, called the ring current.
The ring current is a belt of hot ions with energies that range from 3 to 50 keV and circle the Earth in the opposite direction to those in the plasmasphere.
The magnetotail streams out from the polar regions and extends back behind the planet to a distance well beyond the Moon's orbit. It consists of two basic parts: the tail lobes and the plasma sheet. The ions in the tail lobes are relatively cool, with energies ranging from 10 to 300 eV, and travel away from the Earth. The plasma sheet lies roughly in the plane of the equator and divides the two tail lobes with a thin layer of plasma made up of ions with energies ranging from 0.5 keV to 5 keV that are moving toward the Earth.
The source of magnetospheric ions has been a matter of some debate. "This is a very vibrant area of discussion that is fundamental to how the whole magnetosphere works,” says Chappell. "Thirty years ago, the conventional wisdom was that all of these particles came from the solar wind. Today, most people think that a large percentage of these particles come from Earth's atmosphere.”
Chappell and his colleagues — Mathew M. Huddleston from Trevecca University, Tom Moore and Barbara Giles from the National Aeronautics and Space Administration, and Dominique Delcourt from the Centre d'etude des Environments Terrestre et Planetaires, Observatoire de Saint-Maur in France — used the observations from instruments on five satellites to measure the properties of the ions in different locations in the magnetosphere.
An important part of their analysis was a computer program developed by Delcourt that can predict how ions move in the earth's magnetic field. "These motions are very complicated. Ions spiral around in the magnetic field. They bounce and drift. A lot of things can happen, but Dominic developed a mathematical code that can predict where they go,” says Chappell.
When the researchers applied this computer code to the satellite observations, some patterns became clear for the first time. One was the prediction of how ions could move upward from the ionosphere to form the warm plasma cloak.
"We have recognized all the other regions for a long time, but the plasma cloak was a fuzzy thing in the background which we didn't have enough information about to make it stand out. When we got enough pieces, there it was!” says Chappell.
The other pattern that became clear was a natural cycle that takes low-energy ions from the ionosphere and boosts them up to the much higher levels found in the ring current, plasma sheet and warm plasma cloak.
The cycle starts with ions that move up into space above the north and south poles, a stream of particles called the polar wind. The researchers determined that the magnetic fields force these particles into the magnetotail. Some of them are carried to the end of the magnetotail where they blend into the solar wind. However, many are deflected down into the plasma sheet. When they reach the plasma sheet, they encounter a kink in the magnetic field caused by the electrical current generated by the solar wind. This kink gives the ions a kick and shoots them back toward Earth along the plasma sheet. The farther back in the tail that the ions travel before hitting the current, the stronger the kick is. When they reach the region around the Earth, the more energetic ions join the ring current, and those with lower energy levels form the warm plasma cloak.
This natural cycle applies for all the regions of the magnetosphere except the Van Allen belts. Scientists think that the radiation belt particles get their million eV energies from high-energy cosmic rays. According to the generally accepted mechanism, the cosmic rays slam into the upper level of Earth's atmosphere, colliding with air molecules. These collisions produce neutrons with high energies that decay into protons that become trapped by the magnetic field.
"We used to think that all these ions came from different places and that a lot of them came from the solar wind, which has more energy than the ionosphere,” says Chappell. "We show that very cold particles from the ionosphere can feed all these different regions, even those which have very high energy. A single particle can circulate around in the magnetosphere, starting as an ionospheric particle, becoming a particle in a tail lobe and the plasma sheet and then finally entering either the ring current or warm plasma cloak.”
The work was supported by a grant from the National Aeronautics and Space Administration.