Satellite Measurements Reveal Region of Magnetosphere

Earth is protected from the onslaught of solar wind by the magnetosphere, an invisible shield of magnetic fields and electrically charged particles that surrounds our planet. The northern and southern polar lights—the aurora borealis and aurora australis, respectively—are the only visible parts of the magnetosphere, but it is a critical part of Earth’s space environment.

Now analysis of the measurements of five different satellites has revealed the existence of a new region of the magnetosphere that researchers have dubbed the “warm plasma cloak.” The study was conducted by a team of scientists headed by Charles “Rick” Chappell, BA’65, research professor of physics and director of the Dyer Observatory at Vanderbilt University.

“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 that 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 temperature and motion of the upper atmosphere.”

Other regions of the magnetosphere have been known for some time. Chappell and his colleagues pieced together a “natural cycle of energization” that accelerates low-energy ions that originate from Earth’s atmosphere up to the higher energy levels characteristic of the different regions in the magnetosphere. This project brought the existence of the new region into focus.

The warm plasma cloak is a tenuous region that starts on the night side of the planet and wraps around to the day side, 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 is fed by low-energy charged particles that are lifted into space over Earth’s poles, carried behind the Earth in its magnetic tail, but then jerked around 180 degrees by a kink in the magnetic fields that boosts the particles back toward Earth in a region called the “plasma sheet.”

Chappell and his colleagues—Mathew M. Huddleston, MS’01, PhD’03, from Trevecca Nazarene 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 satellite observations 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,” says Chappell. “Ions spiral around in the magnetic field. They bounce and drift. A lot of things can happen, but Dominique developed a mathematical code that can predict where they go.”

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.

The study was published last fall in the Journal of Geophysical Research.

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