Deepening insight into solar outbursts.

Imagine billions of tons of gas erupting from the sun's outer atmosphere at speeds greater than 1,250 kilometers per second. These huge upheavals, which are becoming more frequent as the sun enters the most active period of its 11-year cycle, can wreak havoc on Earth.

This type of solar outburst, known as a coronal mass ejection (CME), accelerates interplanetary protons to speeds that enable them to penetrate spacecraft and cripple electronic equipment. A cloud of coronal material colliding headon with Earth's magnetosphere may generate geomagnetic storms that disrupt communication systems and create large-scale power outages.

Relying on computer simulations and new data from the SOHO spacecraft, researchers report that they have developed a deeper understanding of the magnetic forces within the sun that create the fastest, most damaging upheavals. That knowledge, says SOHO investigator Spiro K. Antiochos of the Naval Research Laboratory in Washington, D.C., could lead to better predictions of these catastrophic events.

He described the findings last week at the fall meeting of the American Geophysical Union in San Francisco.

Antiochos began his investigation after viewing SOHO images of high-speed outbursts. In addition to showing magnetized parcels of electrically charged gas emerging from the sun's outer atmosphere, or corona, these images also reveal smaller, neighboring gas blobs threaded by magnetic fields. The smaller blobs remain stationary. Antiochos' computer simulations suggest that the neighbors are not innocent bystanders, as researchers had supposed, but signposts of a magnetic interaction that forces tremendous amounts of energy into some eruptions.

Comparing a coronal ejection to an inflating balloon, Antiochos finds evidence that the magnetic architecture lying above this rising package of gas and intense magnetic field weighs it down. While it remains trapped, the magnetic field soaks up huge amounts of energy and begins expanding upward. Eventually, the overlying fields can no longer contain the swelling gas. The whole kit and caboodle--the magnetized gas and the fields above it--typically moves slowly outward, carried by the sun's wind of charged particles, but under some circumstances, says Antiochos, something more dramatic occurs.

If the magnetic fields associated with the neighboring parcels of gas have an orientation opposing that of the overlying fields, they can merge and cancel each other out in a process known as magnetic reconnection. Then, all restraints on the rising blob of gas abruptly disappear. In this model, coronal outbursts can reach enormous speeds, exemplified by an eruption seen by SOHO on Nov. 6.

The model is important, says SOHO science operations coordinator Piet C. Martens, based at NASA's Goddard Space Flight Center in Greenbelt, Md., because "if you can recognize the magnetic structure that will soon be broken up and will give rise to a CME, you have a means of predicting roughly when it's going to happen."