NASA's IMAGE Spacecraft Reveals Earth's Invisible Magnetic Tail

The tail, which streams from Earth towards the Sun, was spotted by NASA's Imager for Magnetopause to Aurora Global Exploration (IMAGE) spacecraft and is featured on the cover of the Jan. 26 issue of the journal Science. The new imaging technology offers unprecedented views of the transparent, electrified gas trapped within Earth's magnetic field, providing the first visual, global perspective on magnetic storms.

The region laced by Earth's magnetic field, called the magnetosphere, dominates the behavior of electrically charged particles in space near Earth and shields the planet from the solar wind. Explosive events on the Sun can charge the magnetosphere with energy, generating magnetic storms that occasionally affect satellites, communications and power systems.

It's difficult for any one spacecraft, or even a small fleet, to obtain a coherent, large-scale view of activity in this vast region because the magnetosphere extends beyond the Moon on the night side of the Earth. "Imagine trying to track and understand the formation of hurricanes without the view from weather satellites," said Dr. Thomas Moore, IMAGE Project Scientist at NASA's Goddard Space Flight Center, Greenbelt, MD. "Like the first meteorologists with a small number of measuring stations, we had an incomplete and at times misleading view of the magnetosphere and magnetic storms before IMAGE, because we couldn't see the big picture."

"IMAGE is providing for the first time global views of the Earth's charged-particle populations at multiple wavelengths and energies on time scales of a few minutes, which is sufficient to track the dynamics of the magnetosphere," said Dr. James Burch, IMAGE Principal Investigator and lead author of the Science paper at Southwest Research Institute, San Antonio, TX.

The Earth's magnetosphere traps electrified gas, called plasma. The new IMAGE pictures show a tail-like structure in the Earth's own plasma cloud that forms as some of the gas streams toward the Sun. The structure was predicted 30 years ago, but previous spacecraft were unable to confirm its existence.

The tail structure is believed to be a return flow of plasma that occurs when the solar wind (a plasma flow ejected continuously from the solar surface) buffets the magnetosphere and distorts its shape. For example, at first a falling raindrop is roughly spherical. As it falls and gains speed, air resistance causes the droplet to change shape as water is dragged from the bottom (head) to the top (tail). Surface tension prevents most of the water from simply dispersing from the tail, so it is forced to flow within the raindrop and return to the head.

The solar wind distorts the Earth's magnetosphere in a similar way, compressing it on the Earth's day side, like the head of a raindrop. The region is stretched on the night side, like the raindrop tail, forming a teardrop shape.

Plasma near the boundaries of the magnetosphere is dragged with the solar wind, but then is turned around and forced back towards the Sun, moving around the Earth in tail-like flows. Although the plasma tails were expected, IMAGE discovered areas in Earth's plasma cloud that are nearly empty of plasma. The IMAGE team calls these unexpected structures "troughs" and is trying to discover how they form.

IMAGE, launched March 25, 2000, also revealed some surprising activity during magnetic storms, which occur when the solar wind pummels the Earth's magnetosphere, stretching it out on night side. The night-side region of the magnetosphere suddenly snaps back and shoots plasma violently toward Earth. The plasma becomes heated to several hundred million degrees and whirls around Earth in multi-million-amp currents. IMAGE discovered the hot storm-plasma occasionally is most dense on the Earth's day side, which is unexpected. Researchers are currently studying this.

Images and more information are available at: http://www.gsfc.nasa.gov/GSFC/SpaceSci/sunearth/imagescience.htm