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.
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