As computer power and speed continues to increase,
scientists are developing ever more sophisticated computer models that can
predict the weather, help design cars and planes, and even evaluate new
medicines. Astronomers use models, too – in order to investigate how solar
systems form, how stars are born, even how the Universe began.

One particularly relevant subject for modeling is our nearest star, the
Sun. The Sun pours out beneficial light and heat, but also dangerous
energetic particles and radiation. Solar physicists Jun Lin
(Harvard-Smithsonian Center for Astrophysics) and Terry G. Forbes
(University of New Hampshire) have developed a state-of-the-art computer
model for the massive solar eruptions that threaten satellites,
communications networks and power grids. Their model matched observations
by the Smithsonian Astrophysical Observatory’s UltraViolet Coronagraph
Spectrometer (UVCS) on the SOHO satellite, which observed a real-world
blast from the Sun in April of 2002, providing hope that one day such
models will predict solar eruptions and space weather.

"By building on four decades of modeling work conducted by many
researchers, we have developed a computer code to describe the entire
development of a solar eruption from beginning to end. By improving our
understanding of the physics behind these blasts, we hope to improve our
ability to predict them," said Lin.

Earthly Effects Of Solar Eruptions

The Sun may appear to be a bright, steadily shining orb, but it is
actually a seething cauldron of hot gases prone to violent eruptions. The
most dramatic eruptions are coronal mass ejections (CMEs), in which giant,
bubble-shaped balloons of plasma and magnetic field lines blast outward at
speeds of up to 1,500 miles per second. If an airplane were able to travel
that fast, a trip across the United States would take only 2 seconds, and
a round-the-world flight would last 20 seconds. CMEs can eject up to 200
billion pounds of matter into interplanetary space.

These bursts of plasma can wreak havoc if they impact the Earth. CMEs have
the potential to disable satellites, disrupt pager and cell phone
networks, and knock out electrical power grids. They also pose a danger to
astronauts, particularly future travelers to Mars.

"An astronaut on Mars, unprotected by a strong magnetic field and thick
atmosphere like we have on Earth, could be exposed to a lethal dose of
radiation and ionized particles. All of these reasons show why it is so
important that we understand, and eventually be able to predict, CMEs,"
said Lin.

A Successful CME Model

The powerful computer model developed by Lin and Forbes simulates the
evolution of coronal mass ejections. Of particular importance, the model
calculates the final configuration of the CME’s magnetic field, which
determines what the effect will be on the Earth – a magnetic field
oriented opposite the Earth’s leads to more dramatic and disruptive

The Lin & Forbes model is the first to predict that a long current sheet
is a key feature of CMEs. The current sheet is a region where oppositely
directed magnetic fields annihilate one another, in a process known as
magnetic reconnection, releasing magnetic energy to accelerate and heat
the CME as it erupts from the Sun’s surface and blasts outward through the
solar corona.

An April 21, 2002 eruption provided an excellent opportunity to gather
data that could be compared to the Lin & Forbes model. A large suite of
instruments on the SOHO, TRACE and RHESSI spacecraft all observed this
eruption in exquisite detail. While TRACE and RHESSI observed the
initiation of the eruption, the UVCS instrument on SOHO observed this
event above the surface in the region of peak acceleration. Its
observations provided direct evidence of the hot gas identified with the
current sheet predicted by the Lin & Forbes model. This is the strongest
evidence yet that the Lin & Forbes model is an accurate description of how
CMEs are produced. UVCS also found that the shock wave did not form until
the CME reached a larger height, and showed the rapid disruption of the
corona as the hot magnetic bubble predicted by the Lin & Forbes model was
accelerated upwards and pushed the coronal gas aside.

Lin and his colleagues expect to continue improving and refining their CME
computer model as more is learned about the physics behind these

NOTE TO EDITORS: An animation and spacecraft image of a coronal mass
ejection are available at:

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian Center
for Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists organized into six research divisions study the origin,
evolution, and ultimate fate of the universe.