State-Of-The-Art Solar Model Fits Massive 2002 Eruption

Cambridge, MA - 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 impacts.

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 eruptions.

NOTE TO EDITORS: An animation and spacecraft image of a coronal mass ejection are available at: http://www.cfa.harvard.edu/news/archive/pr0315image.html

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.

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daguilar@cfa.harvard.edu

Christine Lafon
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Harvard-Smithsonian Center for Astrophysics
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