This sequence of images is from a computer animation show Coronal Mass Ejection (CME) cannibalism. In frame one, a CME blasts from the right side of the sun (bright, white area), and as it expands into space, it becomes fainter (frame two). In frame three, a second CME erupts from near the same region on the Sun as the first CME, appearing as another bright burst on the right side of the Sun. The second CME is moving faster than the first, and it overtakes and assimilates the first CME in frames four through five. Credit: NASA, Walt Feimer, Max-Q Digital Group Honeywell

Fast-moving solar eruptions apparently overtake and often devour their slower kin. This discovery was made by a team of astronomers working with tandem NASA spacecraft.

Strange radio fireworks were first heard by the team using NASA's Wind spacecraft. The link to the cosmic collisions came when researchers linked the timing of the radio outbursts to images of solar eruptions consuming each other captured by the Solar and Heliospheric Observatory (SOHO) spacecraft from NASA and the European Space Agency (ESA).

Solar eruptions directed toward Earth are potentially harmful to advanced technology, including communications and power systems, and this cannibalistic behavior may result in longer magnetic storms. These collisions change the speed of the eruption, which is important for space weather prediction because it alters estimated arrival time of Earthbound coronal ejections.

"Coronal Mass Ejection cannibalism is the most violent form of interaction between CMEs," said Dr. Natchimuthuk Gopalswamy, lead author of a research paper presented today during a meeting of the European Geophysical Society in Nice, France. "This happens when a slow CME is expelled before a fast one from the same general region on the Sun. The fast CME simply gobbles up the slow CME, resulting in a single CME beyond the region of interaction." Gopalswamy, a research professor with The Catholic University of America, Washington, DC, is stationed at NASA's Goddard Space Flight Center, Greenbelt, MD. He presented the research with his colleagues from Goddard, Catholic University and the Naval Research Laboratory.

Coronal mass ejections are clouds of electrified, magnetic gas, weighing billions of tons, ejected from the Sun and hurled into space at speeds of 12 to 1,250 miles per second. Depending on the orientation of the magnetic fields carried by the ejection cloud, Earth-directed eruptions cause magnetic storms by interacting with the Earth's magnetic field, distorting its shape and accelerating electrically charged particles trapped within.

The researchers believe cannibal eruptions may be the source of "complex ejecta" CME clouds, larger and more complex in structure than typical eruptions. These traits cause complex ejecta CMEs to trigger protracted magnetic storms when they envelop the Earth.

Severe solar weather is often heralded by dramatic auroral displays (northern and southern lights), but magnetic storms are occasionally harmful, potentially affecting satellites, radio communications and power systems. Understanding what happens to ejection clouds on their way to Earth is important in assessing their impact on the near-Earth space environment.

Observations from Wind's Radio and Plasma Wave experiment revealed occasional intense bursts of emission originating far away from the Sun. When Gopalswamy and his colleagues were searching for the source of these radio outbursts, they discovered the ejection interaction, which produces high- energy electrons and cause the radio outbursts. After the initial discovery, 21 cannibalistic ejections have been identified since April 1997. There may be even more events that aren't detected because they are less energetic and do not produce a radio outburst, according to the researchers.

"Collisions between CMEs may be more common than previously thought and may play a key role in determining the interplanetary traffic of CMEs," Gopalswamy added.

The astronomers expect an increased rate of ejection interaction during the current peak in the 11-year cycle of violent solar activity, called solar maximum, because more ejections are expelled in quick succession during a solar maximum. During solar minimum, only one ejection per every other day is common; during maximum, several ejections occur in a day.

The cooperative SOHO project is part of NASA's and ESA's Solar Terrestrial Science Program (STSP), comprising of SOHO and CLUSTER. SOHO was launched Dec. 2, 1995. The SOHO spacecraft was built in Europe, and instruments were provided by European and American scientists.