June
22, 2000 -- The arrival from the Sun of billion-ton electrified-gas
clouds that cause severe space storms can now be predicted to
within a half-day, a great improvement over the best previous
estimates of two to five days.
Scientists at the Catholic University of America and NASA's Goddard
Space Flight Center have created a model that reliably predicts
how much time it takes for these clouds, called Coronal Mass
Ejections (CMEs), to traverse the gulf between the Sun and the
Earth, based on their initial speed from the Sun and their interaction
with the solar wind.
Above: This frame from a 350
kb animation shows a coronal mass ejection billowing away
from the Sun on June 6, 2000. The solid-colored blue disk in
the middle is an occulting disk that blocks out the Sun's intense
light to reveal the faint corona, along with background stars
and planets. The white circle shows the true size of Sun. These
images were captured by the wide field coronagraph on board the
orbiting ESA/NASA Solar
and Heliospheric Observatory.
The new model uses recent observations from the European Space
Agency/NASA Solar and Heliospheric Observatory (SOHO) and the
NASA WIND spacecraft. The model has been validated and made more
accurate using historical observations from the Helios-1 (Germany/NASA),
the Pioneer Venus Orbiter (NASA), and the Space Test Program
P78-1 (United States Air Force) spacecraft.
Earth-directed
CMEs cause space storms by interacting with the Earth's magnetic
field, distorting its shape and accelerating electrically charged
particles (electrons and atomic nuclei) trapped within. Severe
solar weather is often heralded by dramatic auroral displays
(northern and southern lights), but space storms are occasionally
harmful, potentially disrupting satellites, radio communications
and power systems.
Above: Click
on the image to see what happens when a coronal mass ejection
strikes our planet's magnetosphere.
"The new model more accurately predicts the arrival of Coronal
Mass Ejections, and will greatly benefit people who operate systems
affected by space storms," said lead author Dr. Natchimuthuk
Gopalswamy of Catholic University, a Senior Research Associate
at the National Academy of Sciences/National Research Council.
"The improved forecasts let operators of sensitive systems
take protective action at the proper time and minimize the unproductive
time when systems are placed in a safe mode to weather the storm."
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Coronal Mass Ejections leave the Sun at various speeds, ranging
from 20 to 2,000 kilometers per second. Only the CMEs directed
at Earth are potentially harmful; estimating when they will arrive
is difficult because their speed changes due to interaction with
the solar wind, a stream of electrically charged gas blowing
constantly from the Sun at about 400 kilometers per second.
Just as a motorboat heading downstream will slow to the speed
of the river's current if its motor is turned off, Coronal Mass
Ejections starting out from the Sun more quickly than the solar
wind eventually are slowed by the drag of this "stream."
If a boat pulls up anchor, it will gradually accelerate until
it is moving at the speed of the current. Similarly, CMEs that
start out more slowly than the solar wind are pulled along until
they match the solar wind's speed.
Using
data from solar-observing spacecraft, Gopalswamy and his team
discovered how much the solar wind sped up or slowed down various
Coronal Mass Ejections according to their initial speeds. If
the initial speed of a CME is known, the new model accurately
accounts for the influence of the solar wind on the CME speed,
and the CME arrival time at Earth can now be precisely estimated.
Above: This prediction curve shows that a CME starting
out near the Sun with a speed of 200 km/s would take 4 1/2 days
to arrive at Earth. On the other hand a fast CME with an initial
speed of 1000 km/s would arrive in 2 1/4 days. Credit: Catholic
University.
Images and more information related to this story can be found
at this URL:
SOHO is a cooperative project between the European Space
Agency (ESA) and NASA. The spacecraft was built in Europe for
ESA and equipped with instruments by teams of scientists in Europe
and the USA. |