Thirty-year-old theory predicted the tail.

But the shoulder and trough came as a surprise, said Bill Sandel of the University of Arizona Lunar and Planetary Laboratory, principal investigator for Extreme Ultraviolet Imager (EUV) on NASA's Imager for Magnetospause-to-Aurora Global Exploration (IMAGE) spacecraaft.

Sandel and other IMAGE scientists report the results in today's issue of Science.

The IMAGE spacecraft was launched in March into a highly elliptical orbit ideal for studying Earth's magnetosphere by imaging techniques. The spacecraft carries six instruments. Three are devoted to neutral atom imaging, one looks at Earth's auroras, and another measures electron densities. Invisible to the human eye, the Earth's magnetosphere dominates our planet's immediate space neighborhood. Here magnetic fields trap energetic particles and produce fast electrons. Some electrons are flung down into the atmosphere and create polar auroras.

The Earth's magnetic field, which stretches for about 10 Earth radii toward the sun and hundreds of Earth radii in the opposite direction, shields the terrestrial atmosphere from being directly bombarded by the solar wind. (Earth's radius is about 6,378 kilometers, or 3,963 miles).

The EUV studies the doughnut-shaped plasmasphere, which shines in extreme ultraviolet light. The instrument was designed, built and tested at the UA Lunar and Planetary Lab by Sandel and his team, which includes Terry Forrester, Robert King, Lyle Broadfoot and Charles Curtis.

"People have been investigating the magnetosphere region by in situ measurements on satellites for 30 years," Sandel said, "But they could only get glimpses of the region, sampling here and there. In order to really understand the structure and dynamics of Earth's magnetosphere, you need images encompassing the entire region in a single exposure. That's what we get with IMAGE, and that's why it's unique.

"With IMAGE, we have been able to verify the presence of features and phenomena that have been confidently predicted on the basis of the measurements over the past three decades. And we also have seen some new things.

"We see how the magnetosphere is constantly squishing and expanding. We see very dramatic changes over the course of a few hours.

"One new and completely unexpected discovery is the phenomenon called a 'shoulder.' It is a sharp change in the position of the edge of the plasmasphere that rotates with the geomagnetic field. It forms on the dayside of the planet and its origin is a big unknown."

Another unexpected phenomenon is the trough in the plasmasphere, which forms on the night side of the Earth and may be related to troughs in the ionosphere, which are regions of lower electron density.

Satellite images also revealed the convection tail. Its existence was predicted in the 1970s on the basis of computer models of the Earth's magnetosphere, but IMAGE photographed it completely for the first time. The tail forms in the magnetosphere and flows in the direction of the sun. It stretches from the dusk side of the plasmasphere toward the magnetopause, the outer boundary of the magnetosphere.

"One of the key questions we are trying to answer is how plasma leaves the plasmasphere and gets transported into the lower-density regions of the magnetosphere," Sandel said. "We know that the source of almost all the plasma in the plasmasphere is Earth's ionosphere. The source is continual so there must be a loss mechanism, since the plasma does not get more and more dense."

New data from IMAGE will help explain events that influence space weather.

The sun is currently close to the maximum of its activity, which means more abrupt and frequent coronal mass ejections. These gigantic blasts of ionized gas from the outer solar atmosphere carry a lot of mass and are highly energetic. When they reach the Earth, they compress the magnetosphere and trigger geomagnetic storms.

Companies with satellites orbiting the Earth are deeply interested in space weather forecasts because space weather affects satellite operation. The effects of geomagnetic storms can wipe out sensitive detectors on a satellite, in some instances causing permanent damage to spacecraft systems.

The storms also affect communications and radio propagation. A large event can induce currents in the surface of the Earth, and those currents often affect power generation, Sandel noted. A geomagnetic storm was credited with the famous power blackout in Quebec on March 10, 1989.

Related websites:

IMAGE EUV Home

IMAGE Spacecraft Home

[Contact: Bill R. Sandel]

26-Jan-2001