Cannibal Coronal Mass Ejections

NASA Space Science News

Fast-moving solar eruptions apparently overtake and often devour their slower-moving kin. This discovery was made by a team of astronomers working with a pair of 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 matched the timing of the radio outbursts to images of solar eruptions consuming each other. The dynamic pictures of the so-called "cannibal coronal mass ejections" were captured by the Solar and Heliospheric Observatory (SOHO) spacecraft from NASA and the European Space Agency (ESA).

Right: A coronagraph on board the ESA-NASA SOHO spacecraft captured this example of CME cannibalism in action on June 6, 2000.

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 the estimated arrival time of Earthbound coronal mass ejections (CMEs).

"Coronal mass ejection cannibalism is the most violent form of interaction between CMEs," says NASA Goddard's 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 one," resulting in a single, complex outward-moving front.

Coronal mass ejections are billion-ton clouds of electrified, magnetic gas that solar eruptions hurl into space at speeds ranging from a few hundred to 2000 km/s. Earth-directed CMEs can trigger magnetic storms when they strike our planet's magnetic field, distorting its shape and accelerating electrically charged particles trapped within.

The researchers believe cannibal eruptions may be 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.

Above: This sequence of images is from a computer animation illustrating an artist's concept of Coronal Mass Ejection (CME) cannibalism. Credit: NASA, Walt Feimer, Max-Q Digital Group, Honeywell

Severe solar weather is often heralded by dramatic auroral displays (also known as Northern and Southern Lights), but magnetic storms are occasionally harmful, potentially affecting satellites, radio communications and power systems. Understanding what happens to CMEs on their way to Earth is important for 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.

Left: This dramatic photo captured on March 24, 2001 by Jan Curtis near Fairbanks, Alaska, shows what can happen when a CME strikes Earth's magnetosphere. To view more images of recent Northern Lights, visit SpaceWeather.com's aurora gallery.

"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 elevated rate of CME interactions 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 solar minimum, only one ejection every few days is common; during maximum, several ejections can 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.